Tuesday, December 11, 2007
Can these Doomsday messengers be stopped?
Asteroid Deflection Strategies.
Asteroid deflection strategies are methods by which near-Earth objects could be diverted, preventing potentially catastrophic impact events. A sufficiently large impact would cause massive tsunamis and/or, by placing large quantities of dust into the stratosphere blocking sunlight, an impact winter. A collision between the earth and a ~10 km object 65 million years ago is believed to have produced the Chicxulub Crater and the extinction of the majority of species preserved in the fossil record.
While in theory the chances of such an event are no greater now than at any other time in history, recent astronomical events (such as Shoemaker-Levy 9) have drawn attention to such a threat, and advances in technology have opened up new options.
Early detection
Almost any deflection effort requires years of warning, allowing time to build a slow-pusher or explosive device to deflect the object.
A number of potential threats have been identified, such as 99942 Apophis (previously known by its provisional designation 2004 MN4), which had been given an impact probability of ~3% for the year 2029. This probability has been revised to zero on the basis of new observations.[1]
An impact by a 10 km asteroid on the Earth is widely viewed as an extinction-level event, likely to cause catastrophic damage to the biosphere.[citation needed] Depending on speed, objects as small as 100 m in diameter are historically extremely destructive. There is also the threat from comets coming into the inner Solar System. The impact speed of a long-period comet would likely be several times greater than that of a near-Earth asteroid, making its impact much more destructive; in addition, the warning time is unlikely to be more than a few months.[citation needed]
Finding out the material composition of the object is also necessary before deciding which strategy is appropriate. Missions like the 2005 Deep Impact probe have provided valuable information on what to expect.
Popular strategies
Nuclear weapons
One of the often proposed solutions is firing nuclear missiles at the oncoming asteroid to vaporize all or most of it. While today's nuclear weapons are not powerful enough to destroy a 1 km asteroid theoretically, thermonuclear weapons can be scaled up to any size so long as enough raw materials are available. If not completely vaporized, the resulting reduction of mass from the blast combined with the radiation blast could produce positive results. The largest problem with this solution is that if the asteroid breaks into fragments, any fragment larger than 35 m across would not burn up in the atmosphere and itself could impact Earth. Tracking of the thousands of fragments that could result would prove daunting.
Another proposed solution is to detonate a series of smaller nuclear devices alongside the asteroid, far enough away as to not fracture the object. Providing this was done far enough in advance, the relatively small forces from any number of nuclear blasts could be enough to alter the object's trajectory enough to avoid an impact. This is a form of nuclear pulse propulsion. In 1967, students at the Massachusetts Institute of Technology designed a system using nuclear explosions to prevent a hypothetical impact on Earth by the asteroid 1566 Icarus.This design study was later published as Project Icarus.
1) Kinetic Impact
An alternative means of deflecting an asteroid is to attempt to directly alter its momentum by sending a spacecraft to collide with the asteroid.
The European Space Agency is already studying preliminary design of a space mission able to demonstrate this futuristic technology. The mission, named Don Quixote will be the first real asteroid deflection mission ever designed.
In the case of 99942 Apophis it has been demonstrated by ESA's Advanced Concepts Team that deflection could be achieved by sending a simple spacecraft weighing less than one ton to impact against the asteroid. During a trade-off study one of the leading researchers argued that a strategy called 'kinetic impactor deflection' was more efficient than others.
Asteroid gravitational tractor.
The major alternative to explosive deflection is to move the asteroid slowly over a period of time. Tiny constant thrust accumulates to deviate an object sufficiently from its predicted course. Edward T. Lu and Stanley Love have proposed using a large heavy unmanned spacecraft hovering over an asteroid to gravitationally pull the latter into a non-threatening orbit. The spacecraft and the asteroid mutually attract one another. If the spacecraft counters the force towards the asteroid by, e.g., a nuclear electric rocket, the net effect is that the asteroid is accelerated towards the spacecraft and thus slightly deflected from its orbit. While slow, this method has the advantage of working irrespective of the asteroid composition or spin rate — rubble pile asteroids would be difficult or impossible to deflect by means of nuclear detonations while a pushing device would be hard or inefficient to mount on a fast rotating asteroid. A gravity tractor would likely have to spend several years beside the asteroid to be effective.
2) Use Of Focused Solar Energy.
H. Jay Melosh proposed to deflect an asteroid or comet by focusing solar energy onto its surface to create thrust from the resulting vaporization of material, or to amplify the Yarkovsky effect. Over a span of months or years enough solar radiation can be directed onto the object to deflect it.
Other proposals
a) Setting up an automated mass driver machine on the asteroid to eject material into space thus giving the object a slow steady push and decreasing its mass.
b) Wrapping the asteroid in a sheet of reflective plastic such as aluminized PET film, or dusting the object with titanium dioxide to alter its trajectory via radiation pressure.
c) Dusting the object with soot to alter its trajectory via the Yarkovsky effect.
d) Attaching a large enough solar sail directly to the object, thus using solar pressure to shift the object's orbit.
e) If the asteroid gets too close to Earth, a last-minute strategy could involve directing it towards the moon in the hope that its gravity would pull the asteroid into it, removing the asteroid.
f) An array of laser satellites to melt parts of the asteroid which in turn provides exhaust to alter its directional path.
Deflection technology concerns
Carl Sagan, in his book Pale Blue Dot, expressed concerns about deflection technology: that any method capable of deflecting impactors away from Earth could also be abused to divert non-threatening bodies toward the planet. Considering the history of genocidal political leaders and the possibility of the bureaucratic obscuring of any such project's true goals to most of its scientific participants, he judged the Earth at greater risk from a man-made impact than a natural one. Sagan instead suggested that deflection technology should only be developed in an actual emergency situation.
Analysis of the uncertainty involved in nuclear deflection shows that the ability to protect the planet does not imply the ability to target the planet. A nuclear bomb which gave an asteroid a delta v of 10 meters/second (plus or minus 20%) would be adequate to push it out of an earth-impacting orbit. However, if the uncertainty of the velocity change was more than a few percent, there would be no chance of directing the asteroid to a particular target.
3) Planetary Defense Timeline.
a) In the 1980s NASA studied evidence of past strikes on planet Earth, and the risk of this happening at our current level of civilization. This led to a program that maps which objects in our solar system both cross Earth's orbit and are large enough to cause serious damage if they ever hit.
b) In the 1990s, US Congress held hearings to consider the risks and what needed to be done about them. This led to a US$3 million annual budget for programs like Spaceguard and the near-earth object program, as managed by NASA and USAF.
c) In 2005 the world's astronauts published an open letter through the Association of Space Explorers calling for a united push to develop strategies to protect Earth from the risk of a cosmic collision.
d) It is currently believed that there are approximately 1500 objects capable of crossing Earth's orbit and large enough to warrant concern. The odds are that within a 1,000 year period, some of them will collide with Earth, unless preventative measures are undertaken. It is now anticipated that by year 2008, all such objects that are 1 km or more in diameter will have been identified and will be monitored.
Saturday, December 1, 2007
COMETS
A comet is a small body in the solar system that orbits the Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail — both primarily from the effects of solar radiation upon the comet's nucleus, which itself is a minor body composed of rock, dust, and ice. Comets' orbits are constantly changing; their origins are in the outer solar system, and they have a propensity to be highly affected (or perturbed) by relatively close approaches to the major planets. Some are moved into Sun-grazing orbits that destroy the comets when they near the Sun, while others are thrown out of the solar system forever.
A new comet may be discovered photographically using a wide-field telescope or visually with binoculars. However, even without access to optical equipment, it is still possible for the amateur astronomer to discover a Sun-grazing comet online by downloading images accumulated by some satellite observatories such as SOHO(Solar and Heliospheric Observatory).
Most comets are believed to originate in a cloud (the Oort cloud) at large distances from the Sun consisting of debris left over from the condensation of the solar nebula; the outer edges of such nebulae are cool enough that water exists in a solid (rather than gaseous) state. Asteroids originate via a different process, but very old comets which have lost all their volatile materials may come to resemble asteroids.
The word comet came to the English language through Latin cometes. From the Greek word komē, meaning "hair of the head," Aristotle first used the derivation komētēs to depict comets as "stars with hair." The astronomical symbol for comets accordingly consists of a disc with a tail of hair.
PHYSICAL CHARACTERISTICS
Long-period comets are believed to originate in a distant cloud known as the Oort cloud (after the Dutch astronomer Jan Hendrik Oort who hypothesised its existence).[3] They are sometimes perturbed from their distant orbits by gravitational interactions, falling into extremely elliptical orbits that can bring them very close to the Sun. One theory holds that as a comet approaches the inner solar system, solar radiation causes part of its outer layers, composed of ice and other materials, to melt and evaporate, but this has not been proven, due to its distance.
The streams of dust and gas thus released form a huge, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the sun. The streams of dust and gas each form their own distinct tail, pointed in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail. At the same time, the ion tail, made of gases, always points directly away from the Sun, as this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory. While the solid body of comets (called the nucleus) is generally less than 50 km across, the coma may be larger than the Sun, and ion tails have been observed to extend 1 astronomical unit (150 million km) or more."
Both the comet and tail are illuminated by the Sun and may become visible from Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly and the gases glowing from ionisation. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible with the naked eye. Before the invention of the telescope, comets seemed to appear out of nowhere in the sky and gradually vanish out of sight. They were usually considered bad omens of deaths of kings or noble men, or coming catastrophes. From ancient sources, such as Chinese oracle bones, it is known that their appearances have been noticed by humans for millennia. One very famous old recording of a comet is the appearance of Halley's Comet on the Bayeux Tapestry, which records the Norman conquest of England in AD 1066.
Surprisingly, cometary nuclei are among the darkest objects known to exist in the solar system. The Giotto probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects only 2.4% to 3% of the light that falls on it; by comparison, asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing.
In 1996, comets were found to emit X-rays.[6] These X-rays surprised researchers, because their emission by comets had not previously been predicted. The X-rays are thought to be generated by the interaction between comets and the solar wind: when highly charged ions fly through a cometary atmosphere, they collide with cometary atoms and molecules. In these collisions, the ions will capture one or more electrons leading to emission of X-rays and far ultraviolet photons.
ORBITAL CHARACTERISTICS
Comets are sometimes classified according to the length of their orbital periods. Short-period comets, also called periodic comets, have orbital periods of generally 40 years or less (even though some take the very arbitrary figures of 50, 100, or even 200 years), while long-period comets have longer orbital timespans but remain gravitationally bound to the Sun by definition (those comets that are ejected from the solar system due to close passes by major planets are no longer properly considered as having "periods"), and main-belt comets orbit within the asteroid belt.
Single-apparition comets have parabolic or hyperbolic orbits which will cause them to permanently exit the solar system after passing the Sun once.
Early observations have revealed a few genuinely hyperbolic orbits, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of kilometres per second). If such objects entered the solar system, they would have positive total energies, and would be observed to have genuinely hyperbolic orbits. A rough calculation shows that there might be 4 hyperbolic comets per century, within Jupiter's orbit, give or take one and perhaps two orders of magnitude.
On the other extreme, the short period Comet Encke has an orbit which never places it farther from the Sun than Jupiter. Short-period comets are thought to originate in the transneptunian region (called by some the "Kuiper belt"), whereas the source of long-period comets is thought to be the Oort cloud.
Since their elliptical orbits frequently take them close to the giant planets, cometary orbits are often perturbed. Short period comets display a tendency for their aphelia to coincide with a giant planet's orbital radius, with the Jupiter family of comets being the largest, as the histogram shows. It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined, in addition to being the swiftest of the giant planets.
A number of periodic comets discovered in earlier decades or previous centuries are now "lost." Their orbits were never known well enough to predict future appearances. However, occasionally a "new" comet will be discovered and upon calculation of its orbit it turns out to be an old "lost" comet. An example is Comet 11P/Tempel-Swift-LINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001.
COMET NOMENCLATURE.
The names given to comets have followed several different conventions over the past two centuries. Before any systematic naming convention was adopted, comets were named in a variety of ways. Prior to the early 20th century, most comets were simply referred to by the year in which they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680" (Kirch's Comet), the "Great September Comet of 1882," and the "Daylight Comet of 1910" ("Great January Comet of 1910"). After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759, that comet became known as Comet Halley. Similarly, the second and third known periodic comets, Comet Encke and Comet Biela, were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their apparition.
were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their apparition.
Increasing numbers of comet discoveries made this procedure awkward, and in 1994 the International Astronomical Union approved a new naming system. Comets are now designated by the year of their discovery followed by a letter indicating the half-month of the discovery and a number indicating the order of discovery (a system similar to that already used for asteroids), so that the fourth comet discovered in the second half of February 2006 would be designated 2006 D4. Prefixes are also added to indicate the nature of the comet, with P/ indicating a periodic comet, C/ indicating a non-periodic comet, X/ indicating a comet for which no reliable orbit could be calculated (generally, historical comets), D/ indicating a comet which has broken up or been lost, and A/ indicating an object that was mistakenly identified as a comet, but is actually a minor planet. After their second observed perihelion passage, periodic comets are also assigned a number indicating the order of their discovery.[15] So Halley's Comet, the first comet to be identified as periodic, has the systematic designation 1P/1682 Q1. Comet Hale-Bopp's designation is C/1995 O1. Comets which first received a minor planet designation keep the latter, which leads to some odd names such as P/2004 EW38 (Catalina-LINEAR).
Early observations and thought.
Historically, comets were thought to be unlucky, or even interpreted as attacks by heavenly beings against terrestrial inhabitants. Some authorities interpret references to "falling stars" in Gilgamesh, the Book of Revelation and the Book of Enoch as references to comets, or possibly bolides.
In the first book of his Meteorology, Aristotle propounded the view of comets that would hold sway in Western thought for nearly two thousand years. He rejected the ideas of several earlier philosophers that comets were planets, or at least a phenomenon related to the planets, on the grounds that while the planets confined their motion to the circle of the Zodiac, comets could appear in any part of the sky.[16] Instead, he described comets as a phenomenon of the upper atmosphere, where hot, dry exhalations gathered and occasionally burst into flame. Aristotle held this mechanism responsible for not only comets, but also meteors, the aurora borealis, and even the Milky Way.[17]
A few later classical philosophers did dispute this view of comets. Seneca the Younger, in his Natural Questions, observed that comets moved regularly through the sky and were undisturbed by the wind, behavior more typical of celestial than atmospheric phenomena. While he conceded that the other planets do not appear outside the Zodiac, he saw no reason that a planet-like object could not move through any part of the sky, humanity's knowledge of celestial things being very limited.[18] However, the Aristotelian viewpoint proved more influential, and it was not until the 16th century that it was demonstrated that comets must exist outside the earth's atmosphere.
In 1577, a bright comet was visible for several months. The Danish astronomer Tycho Brahe used measurements of the comet's position taken by himself and other, geographically separated, observers to determine that the comet had no measurable parallax. Within the precision of the measurements, this implied the comet must be at least four times more distant from the earth than the moon.
ORBITAL STUDIES
Although comets had now been demonstrated to be in the heavens, the question of how they moved through the heavens would be debated for most of the next century. Even after Johannes Kepler had determined in 1609 that the planets moved about the sun in elliptical orbits, he was reluctant to believe that the laws that governed the motions of the planets should also influence the motion of other bodies—he believed that comets travel among the planets along straight lines. Galileo Galilei, although a staunch Copernicanist, rejected Tycho's parallax measurements and held to the Aristotelian notion of comets moving on straight lines through the upper atmosphere.
The first suggestion that Kepler's laws of planetary motion should also apply to the comets was made by William Lower in 1610.[19] In the following decades other astronomers, including Pierre Petit, Giovanni Borelli, Adrien Auzout, Robert Hooke, Johann Baptist Cysat, and Giovanni Domenico Cassini all argued for comets curving about the sun on elliptical or parabolic paths, while others, such as Christian Huygens and Johannes Hevelius, supported comets' linear motion.
he matter was resolved by the bright comet that was discovered by Gottfried Kirch on November 14, 1680. Astronomers throughout Europe tracked its position for several months. In 1681, the Saxon pastor Georg Samuel Doerfel set forth his proofs that comets are heavenly bodies moving in parabolas of which the sun is the focus. Then Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of his inverse square law of universal gravitation must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.[21]
In 1705, Edmond Halley applied Newton's method to twenty-four cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758–9.[22] (Earlier, Robert Hooke had identified the comet of 1664 with that of 1618,[23] while Jean-Dominique Cassini had suspected the identity of the comets of 1577, 1665, and 1680.[24] Both were incorrect.) Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy.[25] When the comet returned as predicted, it became known as Comet Halley or Halley's Comet (its official designation is 1P/Halley). Its next appearance will be in 2061.
Among the comets with short enough periods to have been observed several times in the historical record, Comet Halley is unique in consistently being bright enough to be visible to the naked eye. Since the confirmation of Comet Halley's periodicity, many other periodic comets have been discovered through the telescope. The second comet to be discovered to have a periodic orbit was Comet Encke (official designation 2P/Encke). Over the period 1819–1821 the German mathematician and physicist Johann Franz Encke computed orbits for a series of cometary apparitions observed in 1786, 1795, 1805, and 1818, concluded that they were same comet, and successfully predicted its return in 1822.[26] By 1900, seventeen comets had been observed at more than one perihelion passage and recognized as periodic comets. As of April 2006, 175 comets have achieved this distinction, though several have since been destroyed or lost.
Studies of physical characteristics.
saac Newton described comets as compact, solid, fixed, and durable bodies: in other words, a kind of planet, which move in very oblique orbits, every way, with the greatest freedom, persevering in their motions even against the course and direction of the planets; and their tail as a very thin, slender vapour, emitted by the head, or nucleus of the comet, ignited or heated by the sun. Comets also seemed to Newton absolutely requisite for the conservation of the water and moisture of the planets; from their condensed vapours and exhalations all that moisture which is spent on vegetations and putrefactions, and turned into dry earth, might be resupplied and recruited; for all vegetables were thought to increase wholly from fluids, and turn by putrefaction into earth. Hence the quantity of dry earth must continually increase, and the moisture of the globe decrease, and at last be quite evaporated, if it have not a continual supply. Newton suspected that the spirit which makes the finest, subtilest, and best part of our air, and which is absolutely requisite for the life and being of all things, came principally from the comets.
As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion.[27] In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor in the 1835 apparition of Comet Halley, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit and argued that the non-gravitational movements of Comet Encke resulted from this mechanism.[28]
However, another comet-related discovery overshadowed these ideas for nearly a century. Over the period 1864–1866 the Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteors, and based on orbital similarities, correctly hypothesized that the Perseids were fragments of Comet Swift-Tuttle. The link between comets and meteor showers was dramatically underscored when in 1872, a major meteor shower occurred from the orbit of Comet Biela, which had been observed to split into two pieces during its 1846 apparition, and never seen again after 1852.[29] A "gravel bank" model of comet structure arose, according to which comets consist of loose piles of small rocky objects, coated with an icy layer.
By the middle of the twentieth century, this model suffered from a number of shortcomings: in particular, it failed to explain how a body that contained only a little ice could continue to put on a brilliant display of evaporating vapor after several perihelion passages. In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[30] This "dirty snowball" model soon became accepted. It was confirmed when an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) flew through the coma of Halley's comet in 1986 to photograph the nucleus and observed the jets of evaporating material. The American probe Deep Space 1 flew past the nucleus of Comet Borrelly on September 21, 2001 and confirmed that the characteristics of Comet Halley are common on other comets as well.
Although comets formed in the outer Solar System, radial mixing of material during the early formation of the Solar System is thought to have redistributed material throughout the proto-planetary disk,[31] so comets also contain crystalline grains which were formed in the hot inner Solar System. This is seen in comet spectra as well as in sample return missions.
Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
The Stardust spacecraft, launched in February 1999, collected particles from the coma of Comet Wild 2 in January 2004, and returned the samples to Earth in a capsule in January 2006. Claudia Alexander, a program scientist for Rosetta from NASA's Jet Propulsion Laboratory who has modeled comets for years, reported to space.com about her astonishment at the number of jets, their appearance on the dark side of the comet as well as on the light side, their ability to lift large chunks of rock from the surface of the comet and the fact that comet Wild 2 is not a loosely-cemented rubble pile.[32]
Forthcoming space missions will add greater detail to our understanding of what comets are made of. In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. And in 2014, the European Rosetta probe will orbit comet Comet Churyumov-Gerasimenko and place a small lander on its surface.
Rosetta observed the Deep Impact event, and with its set of very sensitive instruments for cometary investigations, it used its capabilities to observe Tempel 1 before, during and after the impact. At a distance of about 80 million kilometres from the comet, Rosetta was the only spacecraft other than Deep Impact itself to view the comet.
Notable comets
While hundreds of tiny comets pass through the inner solar system every year, very few are noticed by the general public. About every decade or so, a comet will become bright enough to be noticed by a casual observer — such comets are often designated Great Comets. In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, the Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions, while the appearance of Comet Hale-Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult. To most people, however, a great comet is simply a beautiful spectacle.
Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions. Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it will have a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular, but failed to do so. Comet West, which appeared three years later, had much lower expectations (perhaps because scientists were much warier of glowing predictions after the Kohoutek fiasco), but became an extremely impressive comet.[35]
The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession — Comet Hyakutake in 1996, followed by Hale-Bopp, which reached maximum brightness in 1997 having been discovered two years earlier. The first great comet of the 21st century was Comet McNaught, which became visible to naked eye observers in January 2007. It was the brightest in over 40 years.
The Great Comet of 1882, is a member of the Kreutz group
A Sungrazing comet is a comet that passes extremely close to the Sun at perihelion, sometimes within a few thousand kilometres of the Sun's surface. While small sungrazers can be completely evaporated during such a close approach to the Sun, larger sungrazers can survive many perihelion passages. However, the strong tidal forces they experience often lead to their fragmentation.
About 90% of the sungrazers observed with SOHO are members of the Kreutz group, which all originate from one giant comet that broke up into many smaller comets during its first passage through the inner solar system,[36] The other 10% contains some sporadic sungrazers, but four other related groups of comets have been identified among them: the Kracht, Kracht 2a, Marsden and Meyer groups. The Marsden and Kracht groups both appear to be related to Comet 96P/Machholz, which is also the parent of two meteor streams, the Quadrantids and the Arietids.
Unusual comets
Of the thousands of known comets, some are very unusual. Comet Encke orbits from outside the main asteroid belt to inside the orbit of Mercury while Comet 29P/Schwassmann-Wachmann orbits in a nearly circular orbit entirely between Jupiter and Saturn.[38] 2060 Chiron, whose unstable orbit keeps it between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[39] Similarly, Comet Shoemaker-Levy 2 was originally designated asteroid 1990 UL3.[40] Some near-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.
Some comets have been observed to break up during their perihelion passage, including great comets West and Comet Ikeya-Seki. Comet Biela was one significant example, breaking into two during its 1846 perihelion passage. The two comets were seen separately in 1852, but never again after that. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A lesser meteor shower, the Andromedids, occurs annually in November, and is caused by the Earth crossing Biela's orbit.[41]
Another very significant cometary disruption was that of Comet Shoemaker-Levy 9, which was discovered in 1993. At the time of its discovery, the comet was in orbit around Jupiter, having been captured by the planet during a very close approach in 1992.[42] This close approach had already broken the comet into hundreds of pieces, and over a period of 6 days in July 1994, these pieces slammed into Jupiter's atmosphere — the first time astronomers had observed a collision between two objects in the solar system.[43] It has also been suggested that the object likely to have been responsible for the Tunguska event in 1908 was a fragment of Comet Encke.
Sungrazing comets
A Sungrazing comet is a comet that passes extremely close to the Sun at perihelion, sometimes within a few thousand kilometres of the Sun's surface. While small sungrazers can be completely evaporated during such a close approach to the Sun, larger sungrazers can survive many perihelion passages. However, the strong tidal forces they experience often lead to their fragmentation.
About 90% of the sungrazers observed with SOHO are members of the Kreutz group, which all originate from one giant comet that broke up into many smaller comets during its first passage through the inner solar system,[36] The other 10% contains some sporadic sungrazers, but four other related groups of comets have been identified among them: the Kracht, Kracht 2a, Marsden and Meyer groups. The Marsden and Kracht groups both appear to be related to Comet 96P/Machholz, which is also the parent of two meteor streams, the Quadrantids and the Arietids.
Unusual comets
Of the thousands of known comets, some are very unusual. Comet Encke orbits from outside the main asteroid belt to inside the orbit of Mercury while Comet 29P/Schwassmann-Wachmann orbits in a nearly circular orbit entirely between Jupiter and Saturn.[38] 2060 Chiron, whose unstable orbit keeps it between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[39] Similarly, Comet Shoemaker-Levy 2 was originally designated asteroid 1990 UL3.[40] Some near-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.
Some comets have been observed to break up during their perihelion passage, including great comets West and Comet Ikeya-Seki. Comet Biela was one significant example, breaking into two during its 1846 perihelion passage. The two comets were seen separately in 1852, but never again after that. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A lesser meteor shower, the Andromedids, occurs annually in November, and is caused by the Earth crossing Biela's orbit.[41]
Another very significant cometary disruption was that of Comet Shoemaker-Levy 9, which was discovered in 1993. At the time of its discovery, the comet was in orbit around Jupiter, having been captured by the planet during a very close approach in 1992.[42] This close approach had already broken the comet into hundreds of pieces, and over a period of 6 days in July 1994, these pieces slammed into Jupiter's atmosphere — the first time astronomers had observed a collision between two objects in the solar system.[43] It has also been suggested that the object likely to have been responsible for the Tunguska event in 1908 was a fragment of Comet Encke.
A new comet may be discovered photographically using a wide-field telescope or visually with binoculars. However, even without access to optical equipment, it is still possible for the amateur astronomer to discover a Sun-grazing comet online by downloading images accumulated by some satellite observatories such as SOHO(Solar and Heliospheric Observatory).
Most comets are believed to originate in a cloud (the Oort cloud) at large distances from the Sun consisting of debris left over from the condensation of the solar nebula; the outer edges of such nebulae are cool enough that water exists in a solid (rather than gaseous) state. Asteroids originate via a different process, but very old comets which have lost all their volatile materials may come to resemble asteroids.
The word comet came to the English language through Latin cometes. From the Greek word komē, meaning "hair of the head," Aristotle first used the derivation komētēs to depict comets as "stars with hair." The astronomical symbol for comets accordingly consists of a disc with a tail of hair.
PHYSICAL CHARACTERISTICS
Long-period comets are believed to originate in a distant cloud known as the Oort cloud (after the Dutch astronomer Jan Hendrik Oort who hypothesised its existence).[3] They are sometimes perturbed from their distant orbits by gravitational interactions, falling into extremely elliptical orbits that can bring them very close to the Sun. One theory holds that as a comet approaches the inner solar system, solar radiation causes part of its outer layers, composed of ice and other materials, to melt and evaporate, but this has not been proven, due to its distance.
The streams of dust and gas thus released form a huge, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the sun. The streams of dust and gas each form their own distinct tail, pointed in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail. At the same time, the ion tail, made of gases, always points directly away from the Sun, as this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory. While the solid body of comets (called the nucleus) is generally less than 50 km across, the coma may be larger than the Sun, and ion tails have been observed to extend 1 astronomical unit (150 million km) or more."
Both the comet and tail are illuminated by the Sun and may become visible from Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly and the gases glowing from ionisation. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible with the naked eye. Before the invention of the telescope, comets seemed to appear out of nowhere in the sky and gradually vanish out of sight. They were usually considered bad omens of deaths of kings or noble men, or coming catastrophes. From ancient sources, such as Chinese oracle bones, it is known that their appearances have been noticed by humans for millennia. One very famous old recording of a comet is the appearance of Halley's Comet on the Bayeux Tapestry, which records the Norman conquest of England in AD 1066.
Surprisingly, cometary nuclei are among the darkest objects known to exist in the solar system. The Giotto probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects only 2.4% to 3% of the light that falls on it; by comparison, asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing.
In 1996, comets were found to emit X-rays.[6] These X-rays surprised researchers, because their emission by comets had not previously been predicted. The X-rays are thought to be generated by the interaction between comets and the solar wind: when highly charged ions fly through a cometary atmosphere, they collide with cometary atoms and molecules. In these collisions, the ions will capture one or more electrons leading to emission of X-rays and far ultraviolet photons.
ORBITAL CHARACTERISTICS
Comets are sometimes classified according to the length of their orbital periods. Short-period comets, also called periodic comets, have orbital periods of generally 40 years or less (even though some take the very arbitrary figures of 50, 100, or even 200 years), while long-period comets have longer orbital timespans but remain gravitationally bound to the Sun by definition (those comets that are ejected from the solar system due to close passes by major planets are no longer properly considered as having "periods"), and main-belt comets orbit within the asteroid belt.
Single-apparition comets have parabolic or hyperbolic orbits which will cause them to permanently exit the solar system after passing the Sun once.
Early observations have revealed a few genuinely hyperbolic orbits, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of kilometres per second). If such objects entered the solar system, they would have positive total energies, and would be observed to have genuinely hyperbolic orbits. A rough calculation shows that there might be 4 hyperbolic comets per century, within Jupiter's orbit, give or take one and perhaps two orders of magnitude.
On the other extreme, the short period Comet Encke has an orbit which never places it farther from the Sun than Jupiter. Short-period comets are thought to originate in the transneptunian region (called by some the "Kuiper belt"), whereas the source of long-period comets is thought to be the Oort cloud.
Since their elliptical orbits frequently take them close to the giant planets, cometary orbits are often perturbed. Short period comets display a tendency for their aphelia to coincide with a giant planet's orbital radius, with the Jupiter family of comets being the largest, as the histogram shows. It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined, in addition to being the swiftest of the giant planets.
A number of periodic comets discovered in earlier decades or previous centuries are now "lost." Their orbits were never known well enough to predict future appearances. However, occasionally a "new" comet will be discovered and upon calculation of its orbit it turns out to be an old "lost" comet. An example is Comet 11P/Tempel-Swift-LINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001.
COMET NOMENCLATURE.
The names given to comets have followed several different conventions over the past two centuries. Before any systematic naming convention was adopted, comets were named in a variety of ways. Prior to the early 20th century, most comets were simply referred to by the year in which they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680" (Kirch's Comet), the "Great September Comet of 1882," and the "Daylight Comet of 1910" ("Great January Comet of 1910"). After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759, that comet became known as Comet Halley. Similarly, the second and third known periodic comets, Comet Encke and Comet Biela, were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their apparition.
were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their apparition.
Increasing numbers of comet discoveries made this procedure awkward, and in 1994 the International Astronomical Union approved a new naming system. Comets are now designated by the year of their discovery followed by a letter indicating the half-month of the discovery and a number indicating the order of discovery (a system similar to that already used for asteroids), so that the fourth comet discovered in the second half of February 2006 would be designated 2006 D4. Prefixes are also added to indicate the nature of the comet, with P/ indicating a periodic comet, C/ indicating a non-periodic comet, X/ indicating a comet for which no reliable orbit could be calculated (generally, historical comets), D/ indicating a comet which has broken up or been lost, and A/ indicating an object that was mistakenly identified as a comet, but is actually a minor planet. After their second observed perihelion passage, periodic comets are also assigned a number indicating the order of their discovery.[15] So Halley's Comet, the first comet to be identified as periodic, has the systematic designation 1P/1682 Q1. Comet Hale-Bopp's designation is C/1995 O1. Comets which first received a minor planet designation keep the latter, which leads to some odd names such as P/2004 EW38 (Catalina-LINEAR).
Early observations and thought.
Historically, comets were thought to be unlucky, or even interpreted as attacks by heavenly beings against terrestrial inhabitants. Some authorities interpret references to "falling stars" in Gilgamesh, the Book of Revelation and the Book of Enoch as references to comets, or possibly bolides.
In the first book of his Meteorology, Aristotle propounded the view of comets that would hold sway in Western thought for nearly two thousand years. He rejected the ideas of several earlier philosophers that comets were planets, or at least a phenomenon related to the planets, on the grounds that while the planets confined their motion to the circle of the Zodiac, comets could appear in any part of the sky.[16] Instead, he described comets as a phenomenon of the upper atmosphere, where hot, dry exhalations gathered and occasionally burst into flame. Aristotle held this mechanism responsible for not only comets, but also meteors, the aurora borealis, and even the Milky Way.[17]
A few later classical philosophers did dispute this view of comets. Seneca the Younger, in his Natural Questions, observed that comets moved regularly through the sky and were undisturbed by the wind, behavior more typical of celestial than atmospheric phenomena. While he conceded that the other planets do not appear outside the Zodiac, he saw no reason that a planet-like object could not move through any part of the sky, humanity's knowledge of celestial things being very limited.[18] However, the Aristotelian viewpoint proved more influential, and it was not until the 16th century that it was demonstrated that comets must exist outside the earth's atmosphere.
In 1577, a bright comet was visible for several months. The Danish astronomer Tycho Brahe used measurements of the comet's position taken by himself and other, geographically separated, observers to determine that the comet had no measurable parallax. Within the precision of the measurements, this implied the comet must be at least four times more distant from the earth than the moon.
ORBITAL STUDIES
Although comets had now been demonstrated to be in the heavens, the question of how they moved through the heavens would be debated for most of the next century. Even after Johannes Kepler had determined in 1609 that the planets moved about the sun in elliptical orbits, he was reluctant to believe that the laws that governed the motions of the planets should also influence the motion of other bodies—he believed that comets travel among the planets along straight lines. Galileo Galilei, although a staunch Copernicanist, rejected Tycho's parallax measurements and held to the Aristotelian notion of comets moving on straight lines through the upper atmosphere.
The first suggestion that Kepler's laws of planetary motion should also apply to the comets was made by William Lower in 1610.[19] In the following decades other astronomers, including Pierre Petit, Giovanni Borelli, Adrien Auzout, Robert Hooke, Johann Baptist Cysat, and Giovanni Domenico Cassini all argued for comets curving about the sun on elliptical or parabolic paths, while others, such as Christian Huygens and Johannes Hevelius, supported comets' linear motion.
he matter was resolved by the bright comet that was discovered by Gottfried Kirch on November 14, 1680. Astronomers throughout Europe tracked its position for several months. In 1681, the Saxon pastor Georg Samuel Doerfel set forth his proofs that comets are heavenly bodies moving in parabolas of which the sun is the focus. Then Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of his inverse square law of universal gravitation must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.[21]
In 1705, Edmond Halley applied Newton's method to twenty-four cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758–9.[22] (Earlier, Robert Hooke had identified the comet of 1664 with that of 1618,[23] while Jean-Dominique Cassini had suspected the identity of the comets of 1577, 1665, and 1680.[24] Both were incorrect.) Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy.[25] When the comet returned as predicted, it became known as Comet Halley or Halley's Comet (its official designation is 1P/Halley). Its next appearance will be in 2061.
Among the comets with short enough periods to have been observed several times in the historical record, Comet Halley is unique in consistently being bright enough to be visible to the naked eye. Since the confirmation of Comet Halley's periodicity, many other periodic comets have been discovered through the telescope. The second comet to be discovered to have a periodic orbit was Comet Encke (official designation 2P/Encke). Over the period 1819–1821 the German mathematician and physicist Johann Franz Encke computed orbits for a series of cometary apparitions observed in 1786, 1795, 1805, and 1818, concluded that they were same comet, and successfully predicted its return in 1822.[26] By 1900, seventeen comets had been observed at more than one perihelion passage and recognized as periodic comets. As of April 2006, 175 comets have achieved this distinction, though several have since been destroyed or lost.
Studies of physical characteristics.
saac Newton described comets as compact, solid, fixed, and durable bodies: in other words, a kind of planet, which move in very oblique orbits, every way, with the greatest freedom, persevering in their motions even against the course and direction of the planets; and their tail as a very thin, slender vapour, emitted by the head, or nucleus of the comet, ignited or heated by the sun. Comets also seemed to Newton absolutely requisite for the conservation of the water and moisture of the planets; from their condensed vapours and exhalations all that moisture which is spent on vegetations and putrefactions, and turned into dry earth, might be resupplied and recruited; for all vegetables were thought to increase wholly from fluids, and turn by putrefaction into earth. Hence the quantity of dry earth must continually increase, and the moisture of the globe decrease, and at last be quite evaporated, if it have not a continual supply. Newton suspected that the spirit which makes the finest, subtilest, and best part of our air, and which is absolutely requisite for the life and being of all things, came principally from the comets.
As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion.[27] In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor in the 1835 apparition of Comet Halley, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit and argued that the non-gravitational movements of Comet Encke resulted from this mechanism.[28]
However, another comet-related discovery overshadowed these ideas for nearly a century. Over the period 1864–1866 the Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteors, and based on orbital similarities, correctly hypothesized that the Perseids were fragments of Comet Swift-Tuttle. The link between comets and meteor showers was dramatically underscored when in 1872, a major meteor shower occurred from the orbit of Comet Biela, which had been observed to split into two pieces during its 1846 apparition, and never seen again after 1852.[29] A "gravel bank" model of comet structure arose, according to which comets consist of loose piles of small rocky objects, coated with an icy layer.
By the middle of the twentieth century, this model suffered from a number of shortcomings: in particular, it failed to explain how a body that contained only a little ice could continue to put on a brilliant display of evaporating vapor after several perihelion passages. In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[30] This "dirty snowball" model soon became accepted. It was confirmed when an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) flew through the coma of Halley's comet in 1986 to photograph the nucleus and observed the jets of evaporating material. The American probe Deep Space 1 flew past the nucleus of Comet Borrelly on September 21, 2001 and confirmed that the characteristics of Comet Halley are common on other comets as well.
Although comets formed in the outer Solar System, radial mixing of material during the early formation of the Solar System is thought to have redistributed material throughout the proto-planetary disk,[31] so comets also contain crystalline grains which were formed in the hot inner Solar System. This is seen in comet spectra as well as in sample return missions.
Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
The Stardust spacecraft, launched in February 1999, collected particles from the coma of Comet Wild 2 in January 2004, and returned the samples to Earth in a capsule in January 2006. Claudia Alexander, a program scientist for Rosetta from NASA's Jet Propulsion Laboratory who has modeled comets for years, reported to space.com about her astonishment at the number of jets, their appearance on the dark side of the comet as well as on the light side, their ability to lift large chunks of rock from the surface of the comet and the fact that comet Wild 2 is not a loosely-cemented rubble pile.[32]
Forthcoming space missions will add greater detail to our understanding of what comets are made of. In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. And in 2014, the European Rosetta probe will orbit comet Comet Churyumov-Gerasimenko and place a small lander on its surface.
Rosetta observed the Deep Impact event, and with its set of very sensitive instruments for cometary investigations, it used its capabilities to observe Tempel 1 before, during and after the impact. At a distance of about 80 million kilometres from the comet, Rosetta was the only spacecraft other than Deep Impact itself to view the comet.
Notable comets
While hundreds of tiny comets pass through the inner solar system every year, very few are noticed by the general public. About every decade or so, a comet will become bright enough to be noticed by a casual observer — such comets are often designated Great Comets. In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, the Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions, while the appearance of Comet Hale-Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult. To most people, however, a great comet is simply a beautiful spectacle.
Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions. Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it will have a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular, but failed to do so. Comet West, which appeared three years later, had much lower expectations (perhaps because scientists were much warier of glowing predictions after the Kohoutek fiasco), but became an extremely impressive comet.[35]
The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession — Comet Hyakutake in 1996, followed by Hale-Bopp, which reached maximum brightness in 1997 having been discovered two years earlier. The first great comet of the 21st century was Comet McNaught, which became visible to naked eye observers in January 2007. It was the brightest in over 40 years.
The Great Comet of 1882, is a member of the Kreutz group
A Sungrazing comet is a comet that passes extremely close to the Sun at perihelion, sometimes within a few thousand kilometres of the Sun's surface. While small sungrazers can be completely evaporated during such a close approach to the Sun, larger sungrazers can survive many perihelion passages. However, the strong tidal forces they experience often lead to their fragmentation.
About 90% of the sungrazers observed with SOHO are members of the Kreutz group, which all originate from one giant comet that broke up into many smaller comets during its first passage through the inner solar system,[36] The other 10% contains some sporadic sungrazers, but four other related groups of comets have been identified among them: the Kracht, Kracht 2a, Marsden and Meyer groups. The Marsden and Kracht groups both appear to be related to Comet 96P/Machholz, which is also the parent of two meteor streams, the Quadrantids and the Arietids.
Unusual comets
Of the thousands of known comets, some are very unusual. Comet Encke orbits from outside the main asteroid belt to inside the orbit of Mercury while Comet 29P/Schwassmann-Wachmann orbits in a nearly circular orbit entirely between Jupiter and Saturn.[38] 2060 Chiron, whose unstable orbit keeps it between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[39] Similarly, Comet Shoemaker-Levy 2 was originally designated asteroid 1990 UL3.[40] Some near-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.
Some comets have been observed to break up during their perihelion passage, including great comets West and Comet Ikeya-Seki. Comet Biela was one significant example, breaking into two during its 1846 perihelion passage. The two comets were seen separately in 1852, but never again after that. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A lesser meteor shower, the Andromedids, occurs annually in November, and is caused by the Earth crossing Biela's orbit.[41]
Another very significant cometary disruption was that of Comet Shoemaker-Levy 9, which was discovered in 1993. At the time of its discovery, the comet was in orbit around Jupiter, having been captured by the planet during a very close approach in 1992.[42] This close approach had already broken the comet into hundreds of pieces, and over a period of 6 days in July 1994, these pieces slammed into Jupiter's atmosphere — the first time astronomers had observed a collision between two objects in the solar system.[43] It has also been suggested that the object likely to have been responsible for the Tunguska event in 1908 was a fragment of Comet Encke.
Sungrazing comets
A Sungrazing comet is a comet that passes extremely close to the Sun at perihelion, sometimes within a few thousand kilometres of the Sun's surface. While small sungrazers can be completely evaporated during such a close approach to the Sun, larger sungrazers can survive many perihelion passages. However, the strong tidal forces they experience often lead to their fragmentation.
About 90% of the sungrazers observed with SOHO are members of the Kreutz group, which all originate from one giant comet that broke up into many smaller comets during its first passage through the inner solar system,[36] The other 10% contains some sporadic sungrazers, but four other related groups of comets have been identified among them: the Kracht, Kracht 2a, Marsden and Meyer groups. The Marsden and Kracht groups both appear to be related to Comet 96P/Machholz, which is also the parent of two meteor streams, the Quadrantids and the Arietids.
Unusual comets
Of the thousands of known comets, some are very unusual. Comet Encke orbits from outside the main asteroid belt to inside the orbit of Mercury while Comet 29P/Schwassmann-Wachmann orbits in a nearly circular orbit entirely between Jupiter and Saturn.[38] 2060 Chiron, whose unstable orbit keeps it between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[39] Similarly, Comet Shoemaker-Levy 2 was originally designated asteroid 1990 UL3.[40] Some near-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.
Some comets have been observed to break up during their perihelion passage, including great comets West and Comet Ikeya-Seki. Comet Biela was one significant example, breaking into two during its 1846 perihelion passage. The two comets were seen separately in 1852, but never again after that. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A lesser meteor shower, the Andromedids, occurs annually in November, and is caused by the Earth crossing Biela's orbit.[41]
Another very significant cometary disruption was that of Comet Shoemaker-Levy 9, which was discovered in 1993. At the time of its discovery, the comet was in orbit around Jupiter, having been captured by the planet during a very close approach in 1992.[42] This close approach had already broken the comet into hundreds of pieces, and over a period of 6 days in July 1994, these pieces slammed into Jupiter's atmosphere — the first time astronomers had observed a collision between two objects in the solar system.[43] It has also been suggested that the object likely to have been responsible for the Tunguska event in 1908 was a fragment of Comet Encke.
Wednesday, November 14, 2007
SETI
SETI is an acronym for Search For Extra-Terrestrial Intelligence. It was created in an effort to track any kind of extra-terrestrial intelligence. A number of efforts with "SETI" have been organized, including projects funded by the United States Government. The general approach of SETI projects is to survey the sky to detect the existence of transmissions from a civilization on a distant planet, an approach widely endorsed by the scientific community as hard science.
n 1960, Cornell University astronomer Frank Drake performed the first modern SETI experiment, named "Project Ozma", after the Queen of Oz in L. Frank Baum's fantasy books. Drake used a 25-meter-diameter radio telescope at Green Bank, West Virginia, to examine the stars Tau Ceti and Epsilon Eridani near the 1.420 gigahertz marker frequency. A 400 kilohertz band was scanned around the marker frequency, using a single-channel receiver with a bandwidth of 100 hertz. The information was stored on tape for off-line analysis. He found nothing of great interest.
The first SETI conference took place at Green Bank in 1961. The Soviets took a strong interest in SETI during the 1960s and performed a number of searches with omnidirectional antennas in the hope of picking up powerful radio signals. TV host and American astronomer Carl Sagan and Soviet astronomer Iosif Shklovskii together wrote the pioneering book in the field, Intelligent Life in the Universe which was published in 1966 [2].
In the March 1955 issue of Scientific American, Dr. John Kraus, Professor Emeritus and McDougal Professor of Electrical Engineering and Astronomy at the Ohio State University, described a concept to scan the cosmos for natural radio signals using a flat-plane radio telescope equipped with a parabolic reflector. Within two years, his concept was approved for construction by the Ohio State University. With $71,000 total in grants from the National Science Foundation, construction of the first Kraus-style radio telescope began on a 20-acre plot in Delaware, Ohio. The 360-feet wide, 500-feet long, and 70-feet high telescope was powered up in 1963. This Ohio State University radio telescope was called Big Ear. Later, it began the world's first continuous SETI program, called the Ohio State University SETI program.
In 1971, the U.S. National Aeronautics and Space Administration (NASA) funded a SETI study that involved Drake, Bernard Oliver of Hewlett-Packard Corporation, and others. The resulting report proposed the construction of an Earth-based radio telescope array with 1,500 dishes known as "Project Cyclops". The price tag for the Cyclops array was $10 billion USD. Cyclops was not built.
The "Wow!" signal
The OSU SETI program gained fame on August 15, 1977 when Dr. Jerry R. Ehman, a project volunteer, witnessed a startlingly strong signal received by the telescope. He quickly circled the indication on a printout and scribbled the phrase “Wow!” in the margin. This signal, dubbed the Wow! signal, is considered by some to be the most likely candidate from an artificial, extraterrestrial source ever discovered, but it has not been detected again in several additional searches. These sort of signals do not appear today as the ability to verify if signals represent man made objects have been improved , it is therefore highly unlikley that the "WOW!" signal was of Alien origin.
Arecibo message
n 1974, a largely symbolic attempt was made to send a message to other worlds. It was sent towards the globular star cluster M13, which is 25,000 light years from Earth.
Sentinel, META, and BETA
In 1980, Carl Sagan, Bruce Murray, and Louis Friedman founded the U.S. Planetary Society, partly as a vehicle for SETI studies.
In the early 1980s, Harvard University physicist Paul Horowitz took the next step and proposed the design of a spectrum analyzer specifically intended to search for SETI transmissions. Traditional desktop spectrum analyzers were of little use for this job, as they sampled frequencies using banks of analog filters and so were restricted in the number of channels they could acquire. However, modern integrated-circuit digital signal processing (DSP) technology could be used to build autocorrelation receivers to check far more channels. This work led in 1981 to a portable spectrum analyzer named "Suitcase SETI" that had a capacity of 131,000 narrow band channels. After field tests that lasted into 1982, Suitcase SETI was put into use in 1983 with the 26-meter Harvard/Smithsonian radio telescope at Harvard, Massachusetts. This project was named "Sentinel", and continued into 1985.
Even 131,000 channels weren't enough to search the sky in detail at a fast rate, so Suitcase SETI was followed in 1985 by Project "META", for "Megachannel Extra-Terrestrial Assay". The META spectrum analyzer had a capacity of 8.4 million channels and a channel resolution of 0.05 hertz. An important feature of META was its use of frequency doppler shift to distinguish between signals of terrestrial and extraterrestrial origin. The project was led by Horowitz with the help of the Planetary Society, and was partly funded by movie maker Steven Spielberg. A second such effort, META II, was begun in Argentina in 1990 to search the southern sky. META II is still in operation, after an equipment upgrade in 1996.
The follow-on to META was named "BETA", for "Billion-channel ExtraTerrestrial Assay", and it commenced observation on October 30, 1995. The heart of BETA's processing capability consisted of 63 dedicated FFT engines, each capable of performing a 2^22-point complex fast Fourier transform in two seconds, and 21 general-purpose PCs equipped with custom digital signal processing boards. This allowed BETA to receive 250 million simultaneous channels with a resolution of 0.5 hertz per channel. It scanned through the microwave spectrum from 1.400 to 1.720 gigahertz in eight hops, with two seconds of observation per hop. An important capability of the BETA search was rapid and automatic re-observation of candidate signals, achieved by observing the sky with two adjacent beams, one slightly to the east and the other slightly to the west. A successful candidate signal would first transit the east beam, and then the west beam and do so with a speed consistent with the earth's sidereal rotation rate. A third receiver observed the horizon to veto signals of obvious terrestrial origin. On March 23, 1999 the 26-meter radio telescope on which Sentinel, META and BETA were based was blown over by strong winds and seriously damaged. This forced the BETA project to cease operation.
[edit] MOP and Project Phoenix
In 1992, the U.S. government funded an operational SETI program, in the form of the NASA "Microwave Observing Program (MOP)". MOP was planned as a long-term effort, performing a "Targeted Search" of 800 specific nearby stars, along with a general "Sky Survey" to scan the sky. MOP was to be performed by radio dishes associated with the NASA Deep Space Network, as well as a 43-meter dish at Green Bank and the big Arecibo dish. The signals were to be analyzed by spectrum analyzers, each with a capacity of 15 million channels. These spectrum analyzers could be ganged to obtain greater capacity. Those used in the Targeted Search had a bandwidth of 1 hertz per channel, while those used in the Sky Survey had a bandwidth of 30 hertz per channel.
MOP drew the attention of the U.S. Congress, where the program was strongly ridiculed, and was canceled a year after its start. SETI advocates did not give up, and in 1995 the nonprofit SETI Institute of Mountain View, California, resurrected the work under the name of Project "Phoenix", backed by private sources of funding. Project Phoenix, under the direction of Dr. Jill Tarter, previously Project Scientist for the NASA project, is a continuation of the Targeted Search program, studying roughly 1,000 nearby Sun-like stars. Seth Shostak also worked on Project Phoenix. From 1995 through March 2004, Phoenix conducted observing campaigns at the 64-meter Parkes radio telescope in Australia, the 140 Foot Telescope of the National Radio Astronomy Observatory in West Virginia, USA, and the Arecibo Observatory in Puerto Rico. The project observed the equivalent of 800 stars over the available channels in the frequency range from 1200 to 3000 MHz. The search was sensitive enough to pick up transmitters with power output equivalent to airport radars to a distance of about 200 light years.
Allen Telescope Array.
The SETI Institute is now collaborating with the Radio Astronomy Laboratory at UC Berkeley to develop a specialized radio telescope array for SETI studies, something like a mini-Cyclops array. The new array concept is named the "Allen Telescope Array" (ATA) (formerly, One Hectare Telescope [1HT]) after the project's benefactor Paul Allen. Its sensitivity will be equivalent to a single large dish more than 100 meters in diameter. The array is being constructed at the Hat Creek Observatory in rural northern California.
The full array is planned to consist of 350 or more Gregorian radio dishes, each 6.1 meters (20 feet) in diameter. These dishes are the largest producible with commercially available satellite television dish technology. The ATA was planned for a 2007 completion date, at a very modest cost of $25 million USD. The SETI Institute provides money for building the ATA while UC Berkeley designs the telescope and provides operational funding. Berkeley astronomers will use the ATA to pursue other deep space radio observations. The ATA is intended to support a large number of simultaneous observations through a technique known as "multibeaming", in which DSP technology is used to sort out signals from the multiple dishes. The DSP system planned for the ATA is extremely ambitious.
The first portion of the array became operational in October 2007 with 42 antennas. Completion of the full 350 element array will depend on funding and the technical results from the 42 element sub-array.
SETI Net
SETI Net is a private search system created by a single individual. It is closely affiliated with the SETI League and is one of the project Argus stations (DM12jw).
The SETI Net station consists of off the shelf, consumer grade electronics to minimize cost and to allow this design to be replicated as simply as possible. It has a 3 Meter parabola antenna that can be directed in azimuth and elevation, a LNA that covers the 1420 MHz spectrum a receiver to produce the wideband audio and a standard pc computer for control and for the detection algorithms.
The antenna can be pointed and locked to one sky location enabeling the system to integrate on it for long periods. Currently the Wow! signal area is being monitored when it is above the horizon but all search data is collected and made available on the internet archive.
SETI Net started operation in the early 80’s as a way to learn about the science of the search and has developed several software packages for the amateur SETI community. It has provided an astronomical clock, a file manager to keep track of SETI data files, a spectrum analyzer optimized for amateur SETI, remote control of the station from the internet and other packages.
Optical SETI experiments
While most SETI sky searches have studied the radio spectrum, some SETI researchers have considered the possibility that alien civilizations might be using powerful lasers for interstellar communications at optical wavelengths. The idea was first suggested in a paper published in the British journal Nature in 1961, and in 1983 Charles Townes, one of the inventors of the laser, published a detailed study of the idea in the US journal Proceedings of the National Academy of Sciences. Most SETI researchers agreed with the idea. The 1971 Cyclops study discounted the possibility of optical SETI, reasoning that construction of a laser system that could outshine the bright central sun of a remote star system would be too difficult. Now some SETI advocates, such as Frank Drake, have suggested that such a judgment was too conservative.
There are two problems with optical SETI. The first problem is that lasers are highly "monochromatic", that is, they emit light only on one frequency, making it troublesome to figure out what frequency to look for. However, according to the uncertainty principle, emitting light in narrow pulses results in a broad spectrum of emission, with the spread in frequency becoming higher as the pulse width becomes narrower, making it easier to detect an emission.
The other problem is that while radio transmissions can be broadcast in all directions, lasers are highly directional. This means that a laser beam could be easily blocked by clouds of interstellar dust, and Earth would have to cross its direct line of fire by chance to receive it.
Optical SETI supporters have conducted paper studies[8] of the effectiveness of using contemporary high-energy lasers and a ten-meter focus mirror as an interstellar beacon. The analysis shows that an infrared pulse from a laser, focused into a narrow beam by a such a mirror, would appear thousands of times brighter than the Sun to a distant civilization in the beam's line of fire. The Cyclops study proved incorrect in suggesting a laser beam would be inherently hard to see.
Such a system could be made to automatically steer itself through a target list, sending a pulse to each target at a constant rate. This would allow targeting of all Sun-like stars within a distance of 100 light-years. The studies have also described an automatic laser pulse detector system with a low-cost, two-meter mirror made of carbon composite materials, focusing on an array of light detectors. This automatic detector system could perform sky surveys to detect laser flashes from civilizations attempting contact.
In the 1980s, two Soviet researchers conducted a short optical SETI search, but turned up nothing. During much of the 1990s, the optical SETI cause was kept alive through searches by Stuart Kingsley, a dedicated British amateur living in the US state of Ohio.
Several optical SETI experiments are now in progress. A Harvard-Smithsonian group that includes Paul Horowitz designed a laser detector and mounted it on Harvard's 155 centimeter (61 inch) optical telescope. This telescope is currently being used for a more conventional star survey, and the optical SETI survey is "piggybacking" on that effort. Between October 1998 and November 1999, the survey inspected about 2,500 stars. Nothing that resembled an intentional laser signal was detected, but efforts continue. The Harvard-Smithsonian group is now working with Princeton to mount a similar detector system on Princeton's 91-centimeter (36-inch) telescope. The Harvard and Princeton telescopes will be "ganged" to track the same targets at the same time, with the intent being to detect the same signal in both locations as a means of reducing errors from detector noise.
The Harvard-Smithsonian group is now building a dedicated all-sky optical survey system along the lines of that described above, featuring a 1.8-meter (72-inch) telescope. The new optical SETI survey telescope is being set up at the Oak Ridge Observatory in Harvard, Massachusetts.
The University of California, Berkeley, home of SERENDIP and SETI@home, is also conducting optical SETI searches. One is being directed by Geoffrey Marcy, an extrasolar planet hunter, and involves examination of records of spectra taken during extrasolar planet hunts for a continuous, rather than pulsed, laser signal. The other Berkeley optical SETI effort is more like that being pursued by the Harvard-Smithsonian group and is being directed by Dan Werthimer of Berkeley, who built the laser detector for the Harvard-Smithsonian group. The Berkeley survey uses a 76-centimeter (30-inch) automated telescope and an older laser detector built by Werthimer.
Probe SETI and SETA experiments
The possibility of using interstellar messenger probes in the search for extraterrestrial intelligence was first suggested by Ronald N. Bracewell in 1960 (see Bracewell probe), and the technical feasibility of this approach was demonstrated by the British Interplanetary Society's starship study Project Daedalus in 1978. Starting in 1979, Robert Freitas advanced arguments [9] [10] [11] for the proposition that physical space-probes are a superior mode of interstellar communication to radio signals.
Subsequently, in a September 2004 paper featured on the cover of Nature [12], Christopher Rose and Gregory Wright showed that inscribing a message in matter and transporting it to the destination is vastly more energy efficient than communication using electromagnetic waves if the message can tolerate delivery delay beyond light transit time [13] [14] [15]. Thus, a solarcentric Search for Extraterrestrial Artifacts (SETA) [16] would seem to be favored over the more traditional radio or optical searches.
Much like the "preferred frequency" concept in SETI radio beacon theory, the Earth-Moon or Sun-Earth libration orbits [17] might therefore constitute the most universally convenient parking places for automated extraterrestrial spacecraft exploring arbitrary stellar systems. A viable long-term SETI program may be founded upon a search for these objects.
In 1979, Freitas and Valdes conducted a photographic search of the vicinity of the Earth-Moon triangular libration points L4 and L5, and of the solar-synchronized positions in the associated halo orbits, seeking possible orbiting extraterrestrial interstellar probes, but found nothing to a detection limit of about 14th magnitude.[17] The authors conducted a second, more comprehensive photographic search for probes in 1982 [18] that examined the five Earth-Moon Lagrangian positions and included the solar-synchronized positions in the stable L4/L5 libration orbits, the potentially stable nonplanar orbits near L1/L2, Earth-Moon L3, and also L2 in the Sun-Earth system. Again no extraterrestrial probes were found to limiting magnitudes of 17-19th magnitude near L3/L4/L5, 10-18th magnitude for L1/L2, and 14-16th magnitude for Sun-Earth L2.
In June 1983, Valdes and Freitas [19] used the 26-m radiotelescope at Hat Creek Radio Observatory to search for the tritium hyperfine line at 1516 MHz from 108 assorted astronomical objects, with emphasis on 53 nearby stars including all visible stars within a 20 light-year radius. The tritium frequency was deemed highly attractive for SETI work because (1) the isotope is cosmically rare, (2) the tritium hyperfine line is centered in the SETI waterhole region of the terrestrial microwave window, and (3) in addition to beacon signals, tritium hyperfine emission may occur as a byproduct of extensive nuclear fusion energy production by extraterrestrial civilizations. The wideband- and narrowband-channel observations achieved sensitivities of 5-14 x 10-21 W/m²/channel and 0.7-2 x 10-24 W/m²/channel, respectively, but no detections were made.
Where are they? (Fermi Paradox)
Italian physicist Enrico Fermi suggested in the 1950s that if technologically advanced civilizations are common in the universe, then they should be detectable in one way or another. (According to those who were there[20], Fermi either asked "Where are they?" or "Where is everybody?")
The Fermi paradox can be stated more completely as follows:
The size and age of the universe incline us to believe that many technologically advanced civilizations must exist. However, this belief seems logically inconsistent with our lack of observational evidence to support it. Either the initial assumption is incorrect and technologically advanced intelligent life is much rarer than we believe, our current observations are incomplete and we simply have not detected them yet, or our search methodologies are flawed and we are not searching for the correct indicators.
Possible explanations for the paradox suggest, for example, that while simple life may well be abundant in the universe, intelligent life may be exceedingly rare. In 2000, Peter Ward, professor of Biology and of Earth and Space Sciences at the University of Washington authored a book claiming the Rare Earth hypothesis. In short, the theory claims that the emergence of complex multicellular life (metazoa) on Earth required an extremely unlikely combination of astrophysical and geological events and circumstances. This hypothesis contradicts the principle of mediocrity, which SETI takes as an assumption.
Another suggestion, made by astrophysicist Ray Norris in 1999 in Acta Astronautica (and subsequently in Allen Tough's book When SETI Succeeds: The Impact of High-Information Contact - ISBN 0-9677252-2-4) was that gamma-ray burst events are sufficiently frequent to sterilize vast swaths of galactic real-estate. This idea was subsequently popularized by physicist Arnon Dar, and described in the PBS Nova show 'Death Star'.
Science writer Timothy Ferris has posited that since galactic societies would most likely be only transitory, then an obvious solution is an interstellar communications network, or type of library consisting mostly of automated systems. They would store the cumulative knowledge of vanished civilizations and communicate that knowledge through the galaxy. Ferris calls this the "Interstellar Internet", with the various automated systems acting as network "servers". If such an Interstellar Internet exists, the hypothesis states, communications between servers are mostly through narrow-band, highly directional radio or laser links. Intercepting such signals is, as discussed earlier, very difficult. However, the network could maintain some broadcast nodes in hopes of making contact with new civilizations. Although somewhat dated feeling in terms of "information culture" arguments, not to mention obvious technological problems of a system that could work effectively for billions of years and requires multiple lifeforms agreeing on certain basics of communications technologies, this hypothesis is actually testable (see below).
Others believe that intelligent life would or will communicate through an obvious medium. Mostly this is based on experiential supposition.
Criticism of SETI.
As various SETI projects have continued, some have criticized early claims by researchers now seen to be too "euphoric" or "optimistic." For example, Peter Schenkel, while remaining a supporter of SETI projects, has written that "[i]n light of new findings and insights, it seems appropriate to put excessive euphoria to rest and to take a more down-to-earth view ... We should quietly admit that the early estimates - that there may be a million, a hundred thousand, or ten thousand advanced extraterrestrial civilizations in our galaxy - may no longer be tenable." [3].
SETI has also occasionally been the target of criticism by those who suggest that it is a form of pseudoscience. In particular, critics allege that no observed phenomena suggest the existence of extraterrestrial intelligence, and furthermore that the assertion of the existence of extraterrestrial intelligence has no good Popperian criteria for falsifiability [4]. Science fiction writer Michael Crichton, in a 2003 lecture at Caltech, stated that "The Drake equation cannot be tested and therefore SETI is not science. SETI is unquestionably a religion." [5].
In response, SETI advocates note, among other things, that the existence of intelligent life on Earth is a plausible reason to expect it elsewhere, and that individual SETI projects have clearly defined "stop" conditions. The collection and processing of data, the first order of business, and the refining of those data streams, in the case of SETI through algorithm optimization, has not been considered by many of these detractors. Concerning the latter argument, the justification for SETI projects doesn't necessarily require an acceptance of the Drake equation. Science proceeds through hypothesis. If one were only to take what was at face value observable, many many scientific phenomena never would have been discovered. In addition it should be noted that the Drake equation by itself is not an hypothesis and hence it is not even supposed to be testable. The equation can serve as a tool in formulating testable hypotheses.
The search for extra-terrestrial intelligence is not an assertion that extra-terrestrial intelligence exists, and conflating the two can be seen as a straw man argument. There is an effort to distinguish the SETI projects from UFOlogy, the study of UFOs considered to be pseudoscience by many. In Skeptical Inquirer, Mark Moldwin explicitly made the distinction between the two projects, arguing that an important discriminator was the acceptance of SETI by the mainstream scientific community and that "[t]he methodology of SETI leads to useful scientific results even in the absence of discovery of alien life."
Is "active" SETI dangerous?
Positive SETI (also known as "active SETI" or as METI = "messages to extraterrestrial intelligence") consists of directing signals into space in the hope that they will be picked up by an alien intelligence. Some feel that this activity contains improbable but real dangers and ought to be discussed more broadly before it is undertaken. But some consider these anxieties as panic and irrational superstition, see: Sending and Searching for Interstellar Messages
The concern over SETI was raised by the science journal Nature in an editorial in October 2006, which commented on a recent meeting of the International Academy of Astronautics SETI study group. The editor said, “It is not obvious that all extraterrestrial civilizations will be benign, or that contact with even a benign one would not have serious repercussions”. (Nature Vol 443 12 Oct 06 p 606). Astronomer and science fiction author David Brin has expressed similar concerns.
As was suggested by Richard Carrigan, a particle physicist at the US Fermi National Accelerator Laboratory in Illinois, 'passive' SETI could also be dangerous in style of computer viruses.
n 1960, Cornell University astronomer Frank Drake performed the first modern SETI experiment, named "Project Ozma", after the Queen of Oz in L. Frank Baum's fantasy books. Drake used a 25-meter-diameter radio telescope at Green Bank, West Virginia, to examine the stars Tau Ceti and Epsilon Eridani near the 1.420 gigahertz marker frequency. A 400 kilohertz band was scanned around the marker frequency, using a single-channel receiver with a bandwidth of 100 hertz. The information was stored on tape for off-line analysis. He found nothing of great interest.
The first SETI conference took place at Green Bank in 1961. The Soviets took a strong interest in SETI during the 1960s and performed a number of searches with omnidirectional antennas in the hope of picking up powerful radio signals. TV host and American astronomer Carl Sagan and Soviet astronomer Iosif Shklovskii together wrote the pioneering book in the field, Intelligent Life in the Universe which was published in 1966 [2].
In the March 1955 issue of Scientific American, Dr. John Kraus, Professor Emeritus and McDougal Professor of Electrical Engineering and Astronomy at the Ohio State University, described a concept to scan the cosmos for natural radio signals using a flat-plane radio telescope equipped with a parabolic reflector. Within two years, his concept was approved for construction by the Ohio State University. With $71,000 total in grants from the National Science Foundation, construction of the first Kraus-style radio telescope began on a 20-acre plot in Delaware, Ohio. The 360-feet wide, 500-feet long, and 70-feet high telescope was powered up in 1963. This Ohio State University radio telescope was called Big Ear. Later, it began the world's first continuous SETI program, called the Ohio State University SETI program.
In 1971, the U.S. National Aeronautics and Space Administration (NASA) funded a SETI study that involved Drake, Bernard Oliver of Hewlett-Packard Corporation, and others. The resulting report proposed the construction of an Earth-based radio telescope array with 1,500 dishes known as "Project Cyclops". The price tag for the Cyclops array was $10 billion USD. Cyclops was not built.
The "Wow!" signal
The OSU SETI program gained fame on August 15, 1977 when Dr. Jerry R. Ehman, a project volunteer, witnessed a startlingly strong signal received by the telescope. He quickly circled the indication on a printout and scribbled the phrase “Wow!” in the margin. This signal, dubbed the Wow! signal, is considered by some to be the most likely candidate from an artificial, extraterrestrial source ever discovered, but it has not been detected again in several additional searches. These sort of signals do not appear today as the ability to verify if signals represent man made objects have been improved , it is therefore highly unlikley that the "WOW!" signal was of Alien origin.
Arecibo message
n 1974, a largely symbolic attempt was made to send a message to other worlds. It was sent towards the globular star cluster M13, which is 25,000 light years from Earth.
Sentinel, META, and BETA
In 1980, Carl Sagan, Bruce Murray, and Louis Friedman founded the U.S. Planetary Society, partly as a vehicle for SETI studies.
In the early 1980s, Harvard University physicist Paul Horowitz took the next step and proposed the design of a spectrum analyzer specifically intended to search for SETI transmissions. Traditional desktop spectrum analyzers were of little use for this job, as they sampled frequencies using banks of analog filters and so were restricted in the number of channels they could acquire. However, modern integrated-circuit digital signal processing (DSP) technology could be used to build autocorrelation receivers to check far more channels. This work led in 1981 to a portable spectrum analyzer named "Suitcase SETI" that had a capacity of 131,000 narrow band channels. After field tests that lasted into 1982, Suitcase SETI was put into use in 1983 with the 26-meter Harvard/Smithsonian radio telescope at Harvard, Massachusetts. This project was named "Sentinel", and continued into 1985.
Even 131,000 channels weren't enough to search the sky in detail at a fast rate, so Suitcase SETI was followed in 1985 by Project "META", for "Megachannel Extra-Terrestrial Assay". The META spectrum analyzer had a capacity of 8.4 million channels and a channel resolution of 0.05 hertz. An important feature of META was its use of frequency doppler shift to distinguish between signals of terrestrial and extraterrestrial origin. The project was led by Horowitz with the help of the Planetary Society, and was partly funded by movie maker Steven Spielberg. A second such effort, META II, was begun in Argentina in 1990 to search the southern sky. META II is still in operation, after an equipment upgrade in 1996.
The follow-on to META was named "BETA", for "Billion-channel ExtraTerrestrial Assay", and it commenced observation on October 30, 1995. The heart of BETA's processing capability consisted of 63 dedicated FFT engines, each capable of performing a 2^22-point complex fast Fourier transform in two seconds, and 21 general-purpose PCs equipped with custom digital signal processing boards. This allowed BETA to receive 250 million simultaneous channels with a resolution of 0.5 hertz per channel. It scanned through the microwave spectrum from 1.400 to 1.720 gigahertz in eight hops, with two seconds of observation per hop. An important capability of the BETA search was rapid and automatic re-observation of candidate signals, achieved by observing the sky with two adjacent beams, one slightly to the east and the other slightly to the west. A successful candidate signal would first transit the east beam, and then the west beam and do so with a speed consistent with the earth's sidereal rotation rate. A third receiver observed the horizon to veto signals of obvious terrestrial origin. On March 23, 1999 the 26-meter radio telescope on which Sentinel, META and BETA were based was blown over by strong winds and seriously damaged. This forced the BETA project to cease operation.
[edit] MOP and Project Phoenix
In 1992, the U.S. government funded an operational SETI program, in the form of the NASA "Microwave Observing Program (MOP)". MOP was planned as a long-term effort, performing a "Targeted Search" of 800 specific nearby stars, along with a general "Sky Survey" to scan the sky. MOP was to be performed by radio dishes associated with the NASA Deep Space Network, as well as a 43-meter dish at Green Bank and the big Arecibo dish. The signals were to be analyzed by spectrum analyzers, each with a capacity of 15 million channels. These spectrum analyzers could be ganged to obtain greater capacity. Those used in the Targeted Search had a bandwidth of 1 hertz per channel, while those used in the Sky Survey had a bandwidth of 30 hertz per channel.
MOP drew the attention of the U.S. Congress, where the program was strongly ridiculed, and was canceled a year after its start. SETI advocates did not give up, and in 1995 the nonprofit SETI Institute of Mountain View, California, resurrected the work under the name of Project "Phoenix", backed by private sources of funding. Project Phoenix, under the direction of Dr. Jill Tarter, previously Project Scientist for the NASA project, is a continuation of the Targeted Search program, studying roughly 1,000 nearby Sun-like stars. Seth Shostak also worked on Project Phoenix. From 1995 through March 2004, Phoenix conducted observing campaigns at the 64-meter Parkes radio telescope in Australia, the 140 Foot Telescope of the National Radio Astronomy Observatory in West Virginia, USA, and the Arecibo Observatory in Puerto Rico. The project observed the equivalent of 800 stars over the available channels in the frequency range from 1200 to 3000 MHz. The search was sensitive enough to pick up transmitters with power output equivalent to airport radars to a distance of about 200 light years.
Allen Telescope Array.
The SETI Institute is now collaborating with the Radio Astronomy Laboratory at UC Berkeley to develop a specialized radio telescope array for SETI studies, something like a mini-Cyclops array. The new array concept is named the "Allen Telescope Array" (ATA) (formerly, One Hectare Telescope [1HT]) after the project's benefactor Paul Allen. Its sensitivity will be equivalent to a single large dish more than 100 meters in diameter. The array is being constructed at the Hat Creek Observatory in rural northern California.
The full array is planned to consist of 350 or more Gregorian radio dishes, each 6.1 meters (20 feet) in diameter. These dishes are the largest producible with commercially available satellite television dish technology. The ATA was planned for a 2007 completion date, at a very modest cost of $25 million USD. The SETI Institute provides money for building the ATA while UC Berkeley designs the telescope and provides operational funding. Berkeley astronomers will use the ATA to pursue other deep space radio observations. The ATA is intended to support a large number of simultaneous observations through a technique known as "multibeaming", in which DSP technology is used to sort out signals from the multiple dishes. The DSP system planned for the ATA is extremely ambitious.
The first portion of the array became operational in October 2007 with 42 antennas. Completion of the full 350 element array will depend on funding and the technical results from the 42 element sub-array.
SETI Net
SETI Net is a private search system created by a single individual. It is closely affiliated with the SETI League and is one of the project Argus stations (DM12jw).
The SETI Net station consists of off the shelf, consumer grade electronics to minimize cost and to allow this design to be replicated as simply as possible. It has a 3 Meter parabola antenna that can be directed in azimuth and elevation, a LNA that covers the 1420 MHz spectrum a receiver to produce the wideband audio and a standard pc computer for control and for the detection algorithms.
The antenna can be pointed and locked to one sky location enabeling the system to integrate on it for long periods. Currently the Wow! signal area is being monitored when it is above the horizon but all search data is collected and made available on the internet archive.
SETI Net started operation in the early 80’s as a way to learn about the science of the search and has developed several software packages for the amateur SETI community. It has provided an astronomical clock, a file manager to keep track of SETI data files, a spectrum analyzer optimized for amateur SETI, remote control of the station from the internet and other packages.
Optical SETI experiments
While most SETI sky searches have studied the radio spectrum, some SETI researchers have considered the possibility that alien civilizations might be using powerful lasers for interstellar communications at optical wavelengths. The idea was first suggested in a paper published in the British journal Nature in 1961, and in 1983 Charles Townes, one of the inventors of the laser, published a detailed study of the idea in the US journal Proceedings of the National Academy of Sciences. Most SETI researchers agreed with the idea. The 1971 Cyclops study discounted the possibility of optical SETI, reasoning that construction of a laser system that could outshine the bright central sun of a remote star system would be too difficult. Now some SETI advocates, such as Frank Drake, have suggested that such a judgment was too conservative.
There are two problems with optical SETI. The first problem is that lasers are highly "monochromatic", that is, they emit light only on one frequency, making it troublesome to figure out what frequency to look for. However, according to the uncertainty principle, emitting light in narrow pulses results in a broad spectrum of emission, with the spread in frequency becoming higher as the pulse width becomes narrower, making it easier to detect an emission.
The other problem is that while radio transmissions can be broadcast in all directions, lasers are highly directional. This means that a laser beam could be easily blocked by clouds of interstellar dust, and Earth would have to cross its direct line of fire by chance to receive it.
Optical SETI supporters have conducted paper studies[8] of the effectiveness of using contemporary high-energy lasers and a ten-meter focus mirror as an interstellar beacon. The analysis shows that an infrared pulse from a laser, focused into a narrow beam by a such a mirror, would appear thousands of times brighter than the Sun to a distant civilization in the beam's line of fire. The Cyclops study proved incorrect in suggesting a laser beam would be inherently hard to see.
Such a system could be made to automatically steer itself through a target list, sending a pulse to each target at a constant rate. This would allow targeting of all Sun-like stars within a distance of 100 light-years. The studies have also described an automatic laser pulse detector system with a low-cost, two-meter mirror made of carbon composite materials, focusing on an array of light detectors. This automatic detector system could perform sky surveys to detect laser flashes from civilizations attempting contact.
In the 1980s, two Soviet researchers conducted a short optical SETI search, but turned up nothing. During much of the 1990s, the optical SETI cause was kept alive through searches by Stuart Kingsley, a dedicated British amateur living in the US state of Ohio.
Several optical SETI experiments are now in progress. A Harvard-Smithsonian group that includes Paul Horowitz designed a laser detector and mounted it on Harvard's 155 centimeter (61 inch) optical telescope. This telescope is currently being used for a more conventional star survey, and the optical SETI survey is "piggybacking" on that effort. Between October 1998 and November 1999, the survey inspected about 2,500 stars. Nothing that resembled an intentional laser signal was detected, but efforts continue. The Harvard-Smithsonian group is now working with Princeton to mount a similar detector system on Princeton's 91-centimeter (36-inch) telescope. The Harvard and Princeton telescopes will be "ganged" to track the same targets at the same time, with the intent being to detect the same signal in both locations as a means of reducing errors from detector noise.
The Harvard-Smithsonian group is now building a dedicated all-sky optical survey system along the lines of that described above, featuring a 1.8-meter (72-inch) telescope. The new optical SETI survey telescope is being set up at the Oak Ridge Observatory in Harvard, Massachusetts.
The University of California, Berkeley, home of SERENDIP and SETI@home, is also conducting optical SETI searches. One is being directed by Geoffrey Marcy, an extrasolar planet hunter, and involves examination of records of spectra taken during extrasolar planet hunts for a continuous, rather than pulsed, laser signal. The other Berkeley optical SETI effort is more like that being pursued by the Harvard-Smithsonian group and is being directed by Dan Werthimer of Berkeley, who built the laser detector for the Harvard-Smithsonian group. The Berkeley survey uses a 76-centimeter (30-inch) automated telescope and an older laser detector built by Werthimer.
Probe SETI and SETA experiments
The possibility of using interstellar messenger probes in the search for extraterrestrial intelligence was first suggested by Ronald N. Bracewell in 1960 (see Bracewell probe), and the technical feasibility of this approach was demonstrated by the British Interplanetary Society's starship study Project Daedalus in 1978. Starting in 1979, Robert Freitas advanced arguments [9] [10] [11] for the proposition that physical space-probes are a superior mode of interstellar communication to radio signals.
Subsequently, in a September 2004 paper featured on the cover of Nature [12], Christopher Rose and Gregory Wright showed that inscribing a message in matter and transporting it to the destination is vastly more energy efficient than communication using electromagnetic waves if the message can tolerate delivery delay beyond light transit time [13] [14] [15]. Thus, a solarcentric Search for Extraterrestrial Artifacts (SETA) [16] would seem to be favored over the more traditional radio or optical searches.
Much like the "preferred frequency" concept in SETI radio beacon theory, the Earth-Moon or Sun-Earth libration orbits [17] might therefore constitute the most universally convenient parking places for automated extraterrestrial spacecraft exploring arbitrary stellar systems. A viable long-term SETI program may be founded upon a search for these objects.
In 1979, Freitas and Valdes conducted a photographic search of the vicinity of the Earth-Moon triangular libration points L4 and L5, and of the solar-synchronized positions in the associated halo orbits, seeking possible orbiting extraterrestrial interstellar probes, but found nothing to a detection limit of about 14th magnitude.[17] The authors conducted a second, more comprehensive photographic search for probes in 1982 [18] that examined the five Earth-Moon Lagrangian positions and included the solar-synchronized positions in the stable L4/L5 libration orbits, the potentially stable nonplanar orbits near L1/L2, Earth-Moon L3, and also L2 in the Sun-Earth system. Again no extraterrestrial probes were found to limiting magnitudes of 17-19th magnitude near L3/L4/L5, 10-18th magnitude for L1/L2, and 14-16th magnitude for Sun-Earth L2.
In June 1983, Valdes and Freitas [19] used the 26-m radiotelescope at Hat Creek Radio Observatory to search for the tritium hyperfine line at 1516 MHz from 108 assorted astronomical objects, with emphasis on 53 nearby stars including all visible stars within a 20 light-year radius. The tritium frequency was deemed highly attractive for SETI work because (1) the isotope is cosmically rare, (2) the tritium hyperfine line is centered in the SETI waterhole region of the terrestrial microwave window, and (3) in addition to beacon signals, tritium hyperfine emission may occur as a byproduct of extensive nuclear fusion energy production by extraterrestrial civilizations. The wideband- and narrowband-channel observations achieved sensitivities of 5-14 x 10-21 W/m²/channel and 0.7-2 x 10-24 W/m²/channel, respectively, but no detections were made.
Where are they? (Fermi Paradox)
Italian physicist Enrico Fermi suggested in the 1950s that if technologically advanced civilizations are common in the universe, then they should be detectable in one way or another. (According to those who were there[20], Fermi either asked "Where are they?" or "Where is everybody?")
The Fermi paradox can be stated more completely as follows:
The size and age of the universe incline us to believe that many technologically advanced civilizations must exist. However, this belief seems logically inconsistent with our lack of observational evidence to support it. Either the initial assumption is incorrect and technologically advanced intelligent life is much rarer than we believe, our current observations are incomplete and we simply have not detected them yet, or our search methodologies are flawed and we are not searching for the correct indicators.
Possible explanations for the paradox suggest, for example, that while simple life may well be abundant in the universe, intelligent life may be exceedingly rare. In 2000, Peter Ward, professor of Biology and of Earth and Space Sciences at the University of Washington authored a book claiming the Rare Earth hypothesis. In short, the theory claims that the emergence of complex multicellular life (metazoa) on Earth required an extremely unlikely combination of astrophysical and geological events and circumstances. This hypothesis contradicts the principle of mediocrity, which SETI takes as an assumption.
Another suggestion, made by astrophysicist Ray Norris in 1999 in Acta Astronautica (and subsequently in Allen Tough's book When SETI Succeeds: The Impact of High-Information Contact - ISBN 0-9677252-2-4) was that gamma-ray burst events are sufficiently frequent to sterilize vast swaths of galactic real-estate. This idea was subsequently popularized by physicist Arnon Dar, and described in the PBS Nova show 'Death Star'.
Science writer Timothy Ferris has posited that since galactic societies would most likely be only transitory, then an obvious solution is an interstellar communications network, or type of library consisting mostly of automated systems. They would store the cumulative knowledge of vanished civilizations and communicate that knowledge through the galaxy. Ferris calls this the "Interstellar Internet", with the various automated systems acting as network "servers". If such an Interstellar Internet exists, the hypothesis states, communications between servers are mostly through narrow-band, highly directional radio or laser links. Intercepting such signals is, as discussed earlier, very difficult. However, the network could maintain some broadcast nodes in hopes of making contact with new civilizations. Although somewhat dated feeling in terms of "information culture" arguments, not to mention obvious technological problems of a system that could work effectively for billions of years and requires multiple lifeforms agreeing on certain basics of communications technologies, this hypothesis is actually testable (see below).
Others believe that intelligent life would or will communicate through an obvious medium. Mostly this is based on experiential supposition.
Criticism of SETI.
As various SETI projects have continued, some have criticized early claims by researchers now seen to be too "euphoric" or "optimistic." For example, Peter Schenkel, while remaining a supporter of SETI projects, has written that "[i]n light of new findings and insights, it seems appropriate to put excessive euphoria to rest and to take a more down-to-earth view ... We should quietly admit that the early estimates - that there may be a million, a hundred thousand, or ten thousand advanced extraterrestrial civilizations in our galaxy - may no longer be tenable." [3].
SETI has also occasionally been the target of criticism by those who suggest that it is a form of pseudoscience. In particular, critics allege that no observed phenomena suggest the existence of extraterrestrial intelligence, and furthermore that the assertion of the existence of extraterrestrial intelligence has no good Popperian criteria for falsifiability [4]. Science fiction writer Michael Crichton, in a 2003 lecture at Caltech, stated that "The Drake equation cannot be tested and therefore SETI is not science. SETI is unquestionably a religion." [5].
In response, SETI advocates note, among other things, that the existence of intelligent life on Earth is a plausible reason to expect it elsewhere, and that individual SETI projects have clearly defined "stop" conditions. The collection and processing of data, the first order of business, and the refining of those data streams, in the case of SETI through algorithm optimization, has not been considered by many of these detractors. Concerning the latter argument, the justification for SETI projects doesn't necessarily require an acceptance of the Drake equation. Science proceeds through hypothesis. If one were only to take what was at face value observable, many many scientific phenomena never would have been discovered. In addition it should be noted that the Drake equation by itself is not an hypothesis and hence it is not even supposed to be testable. The equation can serve as a tool in formulating testable hypotheses.
The search for extra-terrestrial intelligence is not an assertion that extra-terrestrial intelligence exists, and conflating the two can be seen as a straw man argument. There is an effort to distinguish the SETI projects from UFOlogy, the study of UFOs considered to be pseudoscience by many. In Skeptical Inquirer, Mark Moldwin explicitly made the distinction between the two projects, arguing that an important discriminator was the acceptance of SETI by the mainstream scientific community and that "[t]he methodology of SETI leads to useful scientific results even in the absence of discovery of alien life."
Is "active" SETI dangerous?
Positive SETI (also known as "active SETI" or as METI = "messages to extraterrestrial intelligence") consists of directing signals into space in the hope that they will be picked up by an alien intelligence. Some feel that this activity contains improbable but real dangers and ought to be discussed more broadly before it is undertaken. But some consider these anxieties as panic and irrational superstition, see: Sending and Searching for Interstellar Messages
The concern over SETI was raised by the science journal Nature in an editorial in October 2006, which commented on a recent meeting of the International Academy of Astronautics SETI study group. The editor said, “It is not obvious that all extraterrestrial civilizations will be benign, or that contact with even a benign one would not have serious repercussions”. (Nature Vol 443 12 Oct 06 p 606). Astronomer and science fiction author David Brin has expressed similar concerns.
As was suggested by Richard Carrigan, a particle physicist at the US Fermi National Accelerator Laboratory in Illinois, 'passive' SETI could also be dangerous in style of computer viruses.
The Arecibo Observatory.
The Arecibo Observatory is part of the National Astronomy and Ionosphere Center (NAIC), a national research center operated by Cornell University under a cooperative agreement with the National Science Foundation (NSF). The NSF is an independent federal agency whose aim is to promote scientific and engineering progress in the United States. NSF funds research and education in most fields of science and engineering. Additional support is provided by the National Aeronautics and Space Administration (NASA)
The Observatory operates on a continuous basis, 24 hours a day every day, providing observing time, electronics, computer, travel and logistic support to scientists from all over the world. All results of research are published in the scientific literature which is publicly available.
As the site of the world's largest single-dish radio telescope, the Observatory is recognized as one of the most important national centers for research in radio astronomy, planetary radar and terrestrial aeronomy. Use of the Arecibo Observatory is available on an equal, competitive basis to all scientists from throughout the world. Observing time is granted on the basis of the most promising research as ascertained by a panel of independent referees who review the proposals sent to the Observatory by interested scientists. Every year about 200 scientists visit the Observatory facilities to pursue their research project, and numerous students perform observations that lead to their master and doctoral dissertations.
The Observatory had its origins in an idea of Professor William E. Gordon, then of Cornell University, who was interested in the study of the Ionosphere. Gordon's research during the fifties led him to the idea of radar back scatter studies of the Ionosphere. Gordon's persistence culminated in the construction of the Arecibo Observatory which began in the Summer of 1960. Three years later the Arecibo Ionospheric Observatory (AIO) was in operation under the direction of Gordon. The formal opening ceremony took place on November 1, 1963.
From the beginning there were certain requirements for the site. It had to be near the equator, since there, a radar capable of studying the ionosphere could also be used to study nearby planets which pass overhead. The Arecibo site offered the advantage of being located in Karst terrain, with large limestone sinkholes which provided a natural geometry for the construction of the 305 meter reflector.
In addition an Optical Laboratory with a variety of instrumentation used for the passive study of terrestrial airglow is located at the Observatory. A lidar (Light Detection And Ranging) together with a Fabry-Perot interferometer is primarily used to measure neutral winds and temperatures of the middle atmosphere This capability complements that of the incoherent scatter radar, and gives Arecibo a unique capability in the world in terms of aeronomic research.
On October 1, 1969 the National Science Foundation took over the facility from the Department of Defense and the Observatory was made a national research center. On September 1971 the AIO became the National Astronomy and Ionosphere Center (NAIC).
In 1974 a new high precision surface for the reflector (the current one) was installed together with a high frequency planetary radar transmitter. The second and major upgrade to the telescope was completed in 1997. A ground screen around the perimeter of the reflector was installed to shield the feeds from ground radiation. The gregorian dome with its subreflectors and new electronics greatly increases the capability of the telescope. A new more powerful radar transmitter was also installed.
About 140 persons are employed by the Observatory providing everything from food to software in support of the operation. A scientific staff of about 16 divide their time between scientific research and assistance to visiting scientists. Engineers, computer experts, and technicians design and build new instrumentation and keep it in operation. A large maintenance staff keeps the telescope and associated instrumentation as well as the site in optimal condition. A staff of telescope operators support observing twentyfour hour per day.
The Observatory operates on a continuous basis, 24 hours a day every day, providing observing time, electronics, computer, travel and logistic support to scientists from all over the world. All results of research are published in the scientific literature which is publicly available.
As the site of the world's largest single-dish radio telescope, the Observatory is recognized as one of the most important national centers for research in radio astronomy, planetary radar and terrestrial aeronomy. Use of the Arecibo Observatory is available on an equal, competitive basis to all scientists from throughout the world. Observing time is granted on the basis of the most promising research as ascertained by a panel of independent referees who review the proposals sent to the Observatory by interested scientists. Every year about 200 scientists visit the Observatory facilities to pursue their research project, and numerous students perform observations that lead to their master and doctoral dissertations.
The Observatory had its origins in an idea of Professor William E. Gordon, then of Cornell University, who was interested in the study of the Ionosphere. Gordon's research during the fifties led him to the idea of radar back scatter studies of the Ionosphere. Gordon's persistence culminated in the construction of the Arecibo Observatory which began in the Summer of 1960. Three years later the Arecibo Ionospheric Observatory (AIO) was in operation under the direction of Gordon. The formal opening ceremony took place on November 1, 1963.
From the beginning there were certain requirements for the site. It had to be near the equator, since there, a radar capable of studying the ionosphere could also be used to study nearby planets which pass overhead. The Arecibo site offered the advantage of being located in Karst terrain, with large limestone sinkholes which provided a natural geometry for the construction of the 305 meter reflector.
In addition an Optical Laboratory with a variety of instrumentation used for the passive study of terrestrial airglow is located at the Observatory. A lidar (Light Detection And Ranging) together with a Fabry-Perot interferometer is primarily used to measure neutral winds and temperatures of the middle atmosphere This capability complements that of the incoherent scatter radar, and gives Arecibo a unique capability in the world in terms of aeronomic research.
On October 1, 1969 the National Science Foundation took over the facility from the Department of Defense and the Observatory was made a national research center. On September 1971 the AIO became the National Astronomy and Ionosphere Center (NAIC).
In 1974 a new high precision surface for the reflector (the current one) was installed together with a high frequency planetary radar transmitter. The second and major upgrade to the telescope was completed in 1997. A ground screen around the perimeter of the reflector was installed to shield the feeds from ground radiation. The gregorian dome with its subreflectors and new electronics greatly increases the capability of the telescope. A new more powerful radar transmitter was also installed.
About 140 persons are employed by the Observatory providing everything from food to software in support of the operation. A scientific staff of about 16 divide their time between scientific research and assistance to visiting scientists. Engineers, computer experts, and technicians design and build new instrumentation and keep it in operation. A large maintenance staff keeps the telescope and associated instrumentation as well as the site in optimal condition. A staff of telescope operators support observing twentyfour hour per day.
Monday, October 29, 2007
1950DA
History of Observation
Asteroid (29075) 1950 DA was discovered on 23 February 1950. It was observed for 17 days and then faded from view for half a century. Then, an object discovered on 31 December 2000 was recognized as being the long-lost 1950 DA. (Note this was New Century's Eve and exactly 200 years to the night after the discovery of the first asteroid, Ceres.)
Radar observations were made at Goldstone and Arecibo on 3-7 March 2001, during the asteroid's 7.8 million km approach to the Earth (a distance 21 times larger than that separating the Earth and Moon). Radar echoes revealed a slightly asymmetrical spheroid with a mean diameter of 1.1 km. Optical observations showed the asteroid rotated once every 2.1 hours, the second fastest spin rate ever observed for an asteroid its size.
Detection of A Potential Hazard
When high-precision radar meaurements were included in a new orbit solution, a potentially very close approach to the Earth on March 16, 2880 was discovered to exist. Analysis performed by Giorgini et al. and reported in the April 5, 2002 edition of the journal Science ("Asteroid 1950 DA's Encounter With Earth in 2880: Physical Limits of Collision Probability Prediction") determined the impact probability as being at most 1 in 300 and probably even more remote, based on what is known about the asteroid so far. At its greatest, this could represent a risk 50% greater than that of the average background hazard due to all other asteroids from the present era through 2880, as defined by the Palermo Technical Scale (PTS value = +0.17). 1950 DA is the only known asteroid whose hazard could be above the background level.
Understanding the Risk
However, these are maximum values. The study indicates the collision probability for 1950 DA is best described as being in the range 0 to 0.33%. The upper limit could increase or decrease as we learn more about the asteroid in the years ahead.
Expressing the risk as an interval is necessary because not enough is known about the physical properties of the asteroid. For example, radar data suggests two possible directions for the asteroid's spin pole. If one pole is correct, solar radiation acceleration could significantly cancel thermal emission acceleration. Collision probability would then be close to the maximum 0.33%. If the spin pole is instead near the other possible solution, there would be little chance of collision. There are other factors also.
The situation is similar to knowing you have a coin that is biased so one side will land up 80% of the time -- but you don't know which side. You can only say that when you flip the coin, the chance of heads is 80% or 20%.
Results of the Study
Whether or not the impact hazard of 1950 DA is excluded at some later date, results of the case have significance beyond the impact issue:
A) Physical knowledge of asteroids is required for long-term predictions, especially for objects gravitationally encountering planets. Regardless of how accurate the position and velocity measurements of an asteroid, it's properties and environment affect the trajectory.
Arecibo Radar Movie
B) Asteroid deflection can be made easy and low-tech by modifying the surface properties of asteroids, given enough warning time. The required warning time for the method may vary from years to centuries, depending on the gravitational encounters along the way, which can amplify the effect.
C) Repetitive patterns of gravitational interactions (called "resonances") can help preserve our ability to predict orbits into the future by constraining the growth of orbit statistical uncertainties.
D) Radar measurements allow us to predict trajectories 5-10 times further into the future than with optical telescopes only,
The paper explored the physical factors limiting such long-term predictions. It was found the most significant factor affecting its future long-term motion was the way heat radiates off the asteroid into space. Others factors discussed in the paper include: solar radiation pressure, uncertainties in the masses of the planets, gravitational tugging by thousands of other asteroids, the shape of the Sun, galactic tides due to other stars, solar particle wind and computer hardware imprecision.
Asteroid 1950 DA
The case of 1950 DA differs from previous hazard predictions. For past cases, a risk was detected based on a few days or weeks of data for a newly discovered object.
The uncertainty region that surrounds an object then is large, sometimes spanning a significant part of the inner solar system. Additional measurements made a few days or weeks later shrink the region such that the Earth falls out of it and the risk goes to zero.
Although other currently unknown asteroids may pose a risk before 2880, the situation with 1950 DA is unique. It is based on observations spanning 51 years, has high-precision radar data, and has a favorable orbit geometery. All these factors together allow us to predict far into the future and explore the physical limits of such collision probability predictions.
Predictions so far in the future require knowledge of the physical nature of the asteroid. How it spins in space, what it is made of, its mass, and the variations in the way it reflects light affect the way it moves though space over time. Such detailed knowledge of 1950 DA does not exist at present and may not be available for years, decades or longer.
However, because of the long-time span involved (878 years -- 35 generations!), there is plenty of time to improve our knowledge. If it is eventually decided 1950 DA needs to be diverted, the hundreds of years of warning could allow a method as simple as dusting the surface of the asteroid with chalk or charcoal, or perhaps white glass beads, or sending a solar sail spacecraft that ends by collapsing its reflective sail around the asteroid. These things would change the asteroids reflectivity and allow sunlight to do the work of pushing the asteroid out of the way.
There is no reason for concern over 1950 DA. The most likely result will be that St. Patrick's Day parades in 2880 will be a little more festive than usual as 1950 DA recedes into the distance, having passed Earth by.
UPDATE NOTES
2007-Jul-20:
Results of a new study (Busch et al.) combining the 2001 Goldstone and Arecibo radar data with optical lightcurves are presented in the journal Icarus. Shape, spin state and surface structure of 1950 DA are estimated. New observations intended to resolve the prograde/retrograde spin issue were inconclusive, therefore two distinct shape models are presented. One rotates in a prograde sense and is roughly spheroidal with a mean diameter of 1.16 +/- 0.12 km. The other rotates in a retrograde sense, is oblate, and about 30% larger. Both models suggest a nickel-iron or enstatite chondritic composition.
2005-Apr-22:
On the cultural frontier, a Scottish heavy-metal band has adopted the asteroid's designation, "1950 DA", as their name. "Stomping, groove-laden metal" is their chosen path. A more main-stream group, "Monster Movie", released a CD ("To The Moon") in 2004, including a pop song about asteroid impacts titled "1950 DA".
2005-Mar-02:
The relative effect of error source and certain known and unknown dynamics on the nominal along-track position intersecting the Earth are shown below, normalized in units of numerical integration noise. This expands on Table 3 of the published paper.
Parameter Relative Along-track Effect
----------------------------------------------- -----------------------------------
Solar particle wind 0.001
Galilean satellites -0.333
Galactic tide -0.833
Numerical integration error (128-bit vs. 64-bit) -1.000 (9900 km, 12 min)
Solar mass loss +1.333
Poynting-Robertson drag -2.333
Solar oblateness [ +4.08, +1.75]
Sun-barycenter relativistic shift +81.0 (inc. in nominal)
61 most perturbing "other" asteroids -144
Planetary mass uncertainty [ +132, -156]
Solar radiation pressure -1092
Yarkovsky effect [+1152, -6924]
Numbers in brackets indicate a range of possible values due to poorly known physical parameters. These factors together reduce prediction window extent from 2880 to 2860 (-20 years, or -2.3%)
2003-May-16:
Results of a study simulating the impact of a 1950 DA-like object in the northern Atlantic ocean were published (Ward & Asphaug, UCSD, June issue of the Geophysical Journal International). The same impact velocity and general impact region were used, but a less massive (thus rigid) object with less energy dissipation was assumed. The actual mass of 1950 DA is unknown. It was found waves propagate throughout the Atlantic Ocean and the Caribbean. Two hours after impact, 400-foot waves reach beaches from Cape Cod to Cape Hatteras. Four hours after impact, the entire East Coast experiences waves at least 200 feet high. It takes 8 hours for the waves to reach Europe, where they come ashore at heights of about 30 to 50 feet.
2003-Jan-04:
New positional measurements were reported by the Desert Moon Observatory (448) in Las Cruces, New Mexico (MPEC 2003-A22). These were the first new measurements of 29075 (1950 DA) reported since 2001-Oct-17. No statistically significant deviation from the predicted trajectory was observed.
2002-Apr-05:
Formal paper published in the journal Science: ("Asteroid 1950 DA's Encounter With Earth in 2880: Physical Limits of Collision Probability Prediction")
2001-Jun-11:
Initial 1950 DA results were first reported at the "Asteroids 2001: from Piazzi to the 3rd Millennium" conference in Palermo, Sicily June 11-16: J.D. Giorgini et al., "Asteroid 1950 DA: Long Term Prediction of its Earth Close Approaches" Asteroids 2001, Palermo, Italy, June 2001.
References
Wikipedia, NASA.
Asteroid (29075) 1950 DA was discovered on 23 February 1950. It was observed for 17 days and then faded from view for half a century. Then, an object discovered on 31 December 2000 was recognized as being the long-lost 1950 DA. (Note this was New Century's Eve and exactly 200 years to the night after the discovery of the first asteroid, Ceres.)
Radar observations were made at Goldstone and Arecibo on 3-7 March 2001, during the asteroid's 7.8 million km approach to the Earth (a distance 21 times larger than that separating the Earth and Moon). Radar echoes revealed a slightly asymmetrical spheroid with a mean diameter of 1.1 km. Optical observations showed the asteroid rotated once every 2.1 hours, the second fastest spin rate ever observed for an asteroid its size.
Detection of A Potential Hazard
When high-precision radar meaurements were included in a new orbit solution, a potentially very close approach to the Earth on March 16, 2880 was discovered to exist. Analysis performed by Giorgini et al. and reported in the April 5, 2002 edition of the journal Science ("Asteroid 1950 DA's Encounter With Earth in 2880: Physical Limits of Collision Probability Prediction") determined the impact probability as being at most 1 in 300 and probably even more remote, based on what is known about the asteroid so far. At its greatest, this could represent a risk 50% greater than that of the average background hazard due to all other asteroids from the present era through 2880, as defined by the Palermo Technical Scale (PTS value = +0.17). 1950 DA is the only known asteroid whose hazard could be above the background level.
Understanding the Risk
However, these are maximum values. The study indicates the collision probability for 1950 DA is best described as being in the range 0 to 0.33%. The upper limit could increase or decrease as we learn more about the asteroid in the years ahead.
Expressing the risk as an interval is necessary because not enough is known about the physical properties of the asteroid. For example, radar data suggests two possible directions for the asteroid's spin pole. If one pole is correct, solar radiation acceleration could significantly cancel thermal emission acceleration. Collision probability would then be close to the maximum 0.33%. If the spin pole is instead near the other possible solution, there would be little chance of collision. There are other factors also.
The situation is similar to knowing you have a coin that is biased so one side will land up 80% of the time -- but you don't know which side. You can only say that when you flip the coin, the chance of heads is 80% or 20%.
Results of the Study
Whether or not the impact hazard of 1950 DA is excluded at some later date, results of the case have significance beyond the impact issue:
A) Physical knowledge of asteroids is required for long-term predictions, especially for objects gravitationally encountering planets. Regardless of how accurate the position and velocity measurements of an asteroid, it's properties and environment affect the trajectory.
Arecibo Radar Movie
B) Asteroid deflection can be made easy and low-tech by modifying the surface properties of asteroids, given enough warning time. The required warning time for the method may vary from years to centuries, depending on the gravitational encounters along the way, which can amplify the effect.
C) Repetitive patterns of gravitational interactions (called "resonances") can help preserve our ability to predict orbits into the future by constraining the growth of orbit statistical uncertainties.
D) Radar measurements allow us to predict trajectories 5-10 times further into the future than with optical telescopes only,
The paper explored the physical factors limiting such long-term predictions. It was found the most significant factor affecting its future long-term motion was the way heat radiates off the asteroid into space. Others factors discussed in the paper include: solar radiation pressure, uncertainties in the masses of the planets, gravitational tugging by thousands of other asteroids, the shape of the Sun, galactic tides due to other stars, solar particle wind and computer hardware imprecision.
Asteroid 1950 DA
The case of 1950 DA differs from previous hazard predictions. For past cases, a risk was detected based on a few days or weeks of data for a newly discovered object.
The uncertainty region that surrounds an object then is large, sometimes spanning a significant part of the inner solar system. Additional measurements made a few days or weeks later shrink the region such that the Earth falls out of it and the risk goes to zero.
Although other currently unknown asteroids may pose a risk before 2880, the situation with 1950 DA is unique. It is based on observations spanning 51 years, has high-precision radar data, and has a favorable orbit geometery. All these factors together allow us to predict far into the future and explore the physical limits of such collision probability predictions.
Predictions so far in the future require knowledge of the physical nature of the asteroid. How it spins in space, what it is made of, its mass, and the variations in the way it reflects light affect the way it moves though space over time. Such detailed knowledge of 1950 DA does not exist at present and may not be available for years, decades or longer.
However, because of the long-time span involved (878 years -- 35 generations!), there is plenty of time to improve our knowledge. If it is eventually decided 1950 DA needs to be diverted, the hundreds of years of warning could allow a method as simple as dusting the surface of the asteroid with chalk or charcoal, or perhaps white glass beads, or sending a solar sail spacecraft that ends by collapsing its reflective sail around the asteroid. These things would change the asteroids reflectivity and allow sunlight to do the work of pushing the asteroid out of the way.
There is no reason for concern over 1950 DA. The most likely result will be that St. Patrick's Day parades in 2880 will be a little more festive than usual as 1950 DA recedes into the distance, having passed Earth by.
UPDATE NOTES
2007-Jul-20:
Results of a new study (Busch et al.) combining the 2001 Goldstone and Arecibo radar data with optical lightcurves are presented in the journal Icarus. Shape, spin state and surface structure of 1950 DA are estimated. New observations intended to resolve the prograde/retrograde spin issue were inconclusive, therefore two distinct shape models are presented. One rotates in a prograde sense and is roughly spheroidal with a mean diameter of 1.16 +/- 0.12 km. The other rotates in a retrograde sense, is oblate, and about 30% larger. Both models suggest a nickel-iron or enstatite chondritic composition.
2005-Apr-22:
On the cultural frontier, a Scottish heavy-metal band has adopted the asteroid's designation, "1950 DA", as their name. "Stomping, groove-laden metal" is their chosen path. A more main-stream group, "Monster Movie", released a CD ("To The Moon") in 2004, including a pop song about asteroid impacts titled "1950 DA".
2005-Mar-02:
The relative effect of error source and certain known and unknown dynamics on the nominal along-track position intersecting the Earth are shown below, normalized in units of numerical integration noise. This expands on Table 3 of the published paper.
Parameter Relative Along-track Effect
----------------------------------------------- -----------------------------------
Solar particle wind 0.001
Galilean satellites -0.333
Galactic tide -0.833
Numerical integration error (128-bit vs. 64-bit) -1.000 (9900 km, 12 min)
Solar mass loss +1.333
Poynting-Robertson drag -2.333
Solar oblateness [ +4.08, +1.75]
Sun-barycenter relativistic shift +81.0 (inc. in nominal)
61 most perturbing "other" asteroids -144
Planetary mass uncertainty [ +132, -156]
Solar radiation pressure -1092
Yarkovsky effect [+1152, -6924]
Numbers in brackets indicate a range of possible values due to poorly known physical parameters. These factors together reduce prediction window extent from 2880 to 2860 (-20 years, or -2.3%)
2003-May-16:
Results of a study simulating the impact of a 1950 DA-like object in the northern Atlantic ocean were published (Ward & Asphaug, UCSD, June issue of the Geophysical Journal International). The same impact velocity and general impact region were used, but a less massive (thus rigid) object with less energy dissipation was assumed. The actual mass of 1950 DA is unknown. It was found waves propagate throughout the Atlantic Ocean and the Caribbean. Two hours after impact, 400-foot waves reach beaches from Cape Cod to Cape Hatteras. Four hours after impact, the entire East Coast experiences waves at least 200 feet high. It takes 8 hours for the waves to reach Europe, where they come ashore at heights of about 30 to 50 feet.
2003-Jan-04:
New positional measurements were reported by the Desert Moon Observatory (448) in Las Cruces, New Mexico (MPEC 2003-A22). These were the first new measurements of 29075 (1950 DA) reported since 2001-Oct-17. No statistically significant deviation from the predicted trajectory was observed.
2002-Apr-05:
Formal paper published in the journal Science: ("Asteroid 1950 DA's Encounter With Earth in 2880: Physical Limits of Collision Probability Prediction")
2001-Jun-11:
Initial 1950 DA results were first reported at the "Asteroids 2001: from Piazzi to the 3rd Millennium" conference in Palermo, Sicily June 11-16: J.D. Giorgini et al., "Asteroid 1950 DA: Long Term Prediction of its Earth Close Approaches" Asteroids 2001, Palermo, Italy, June 2001.
References
Wikipedia, NASA.
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