INFOMAN

COMETS

A blazing streak of fire scorching through the heavens and lighting the sky in broad daylight is most likely a comet. Traditionally, comets were thought to be "exhalations" of the atmosphere, as Aristotle described them, and since they were thought to travel in straight lines rather than the ellipses that Newton later demonstrated, few measurements of their positions were made. Aristotle’s idea that comets were merely an upper-air phenomenon held for two thousand years, until Tycho Brahe, having made accurate measurements of the comet of 1577, demonstrated that a comet moved in an orbit far beyond the moon. Donati made the first spectroscopic observation of a comet in 1864, and sketched the three bands of molecular emission typifying them.

There are normally three parts to a comet: the coma, nucleus and tail. The tail consists principally of gas, dust and small particles and has a fuzzy appearance, especially at the coma, a nebulous cloud surrounding the nucleus of a comet, which forms the bulk of their volume. The Great Comet of 1811 had a coma larger than the Sun. As a comet approaches the Sun and becomes warmer, the part of the nucleus consisting of ice changes to water vapour. It is this presence of water which distinguishes a comet from an asteroid, and causes them to be known as "dirty snowballs", a term stemming from Whipple in 1951. The nucleus, which is now known to be extremely small, mainly consists of volatile ices, mainly water, dust and hydrocarbons. Solar wind then causes it to form a tail pointing away from the Sun, and this can be up to 205 million miles in length, like the Great Comet of 1843.

Kometes means ‘hairy star’, a reference to the dusty tail of a comet. When a ‘long haired’ comet appeared over Rome in 79 AD, the emperor Vespasian (b. 18.11.9 AD, Reate) remarked, "it doesn’t scare me, I’m bald. Let the King of the Parthians worry about it, he’s hairy" (He died 23 June that year).

There are two types of cometary tails: the dust tail, which is yellow in colour consisting of billions of micron-sized particles riding on sunbeams through space; and the plasma tail, where trapped gas particles create a blue hue stimulated by a magnetic flux.

Comets are unstable, and gradually disintegrate, distributing particles and debris in their wake. These contain iron, nickel, copper, sodium, carbon, oxygen, potassium and calcium - many of the compounds essential to life and preservation. Therefore, comets are seen as messengers of materials, shedding their compounds and meteors upon the planets they pass. The Earth passes regularly through cometary debris, or swarms, and these cause the meteor storms, such as the Leonid shower of shooting stars, we observe as tiny particles burn up in the upper atmosphere. Earth may be being continually pelted by small comet-like objects, raining down by the millions each day. The rate at which meteors strike the Earth varies throughout the year, peaking in November and hitting a low in January.

If a small comet falls into our atmosphere, friction breaks it up and it vaporises to water at a height of 300-800 miles. Ironically, it could have been cometary collisions that led to the deposit of water, ice crystals in noctilucent clouds from cometary debris, on the young planet Earth, which in turn made possible the evolution of life through the organic chemical ingredients they held that enabled life to evolve. A 2lb meteorite that crashed in Monahans TX in March 1988 has been found to contain pockets of fluid inside crystals of rock salt, dating back to the start of the Solar System.

Fossilised stromatolites discovered in Australia have been dated to 3.6 billion years old, and showed that two varieties of prokaryotic bacteria must have existed here around 4 billion years ago. Their sudden appearance only one half billion years after the formation of Earth was probably due to panspermia, or accidental seeding, from organisms found in the water that was deposited here by comets or asteroids. Earth’s second life form, eukaryotes, date from 2 billion years, and were the first life forms to have a nucleus.

Sir Fred Hoyle (b. 24 June 1915, Bingley; d. 21 August 2001, Bournemouth) was an early proponent of the role of the comet in the model of life, and confidently predicted an outbreak of influenza in the 1985/1986 winter due to the influx of alien materials from Halley’s Comet.

As comets lose material at perihelion and other times, they must be relatively short-lived, and several comets have been seen to disintegrate. Biela’s Comet used to have a period of 6.75 years, but broke in two on its 1845 return. Both parts were seen in 1852, but never since. In 1979 the brilliant long-period Comet Howard-Kooman-Michaels came to perihelion and was destroyed. It is likely that many Sun-grazing comets are fragments of larger comets that have broken up. Some believe that the Earth-grazing Apollo-type asteroids are in fact comet nuclei. The closest known approach of a comet was on 1 July 1770, when Lexell’s Comet came within 1½ million miles of Earth, at a speed of 23.9 miles per second relative to the Sun. Most appear in the Northern Hemisphere, and are generally succeeded by an unseasonable period of warmth.

Most comets have extremely elongated orbits; some have periods of thousands or even millions of years, with an orbital eccentricity near to a value of one (which would be a straight line, stretching to infinity). Comets may come from any direction, direct or retrograde, or even at right angles to the Ecliptic. Their orbits can be elliptical or hyperbolic. It was thought that some had parabolic orbits but this has recently been found not to be the case.

Those with ellipses are probably members of our Solar System, having definite return periods. These include Halley, Olbers, D’Arrest, Pons-Brook and Westphal-Delevan’s Comets. Those with hyperbolic orbits may be visitors from interstellar space; captured by the Sun to become part of our Solar System, or returning to the Infinite from whence they came. Some may have been members of our System whose ellipses have become hyperbolic as a result of approaching too close to the gravitational pull of a planet or nearby star, and thus are doomed eventually to become lost to our Solar System forever.

There are currently six separate classification classes of comet, with a nucleus size range of 1-300 kilometres. The first class is the long-period comet, whose orbit period exceeds two hundred years. 100,000 million long-period comets exist in a cometary debris-ridden swarm of dust and gas, or Oort Cloud. This is a spherical cloud that surrounds the Sun up to two light years away (12 million-million miles), or 1,000 times further away than Pluto at its greatest distance, near the ecliptic plane, halfway to Proxima Centauri, our nearest star. The cloud marks the furthest extent of our Solar System. We are discovering that these comets can encroach on the orbits of Uranus, Neptune and Pluto. All have orbits highly inclined to the Ecliptic, often retrograde. When this cloud is given a gravitational jog by the movement of a star, planet or dust-cloud nearby, some fragments will become speeded up and ejected from the cloud into deep space, while some will slow down and travel inwards towards the Sun, developing double-century or longer orbital paths. Some may conversely evolve into short-period comets.

The Oort Cloud was named after its first proponent, the Dutch astronomer Jan Oort (b. 28.4.1900, 0830 hr LT, Franekur, Friesland; d. 12.11.1992, Leiden, Holland). Comets from the Oort cloud may have their origins in the "solar nebula", no further out than Uranus or Neptune, at the formation of the Solar System, along with many other planetisimals, contributing to the formation of the moons, asteroids and rocky cores of existing planets. Millions were ejected outwards to form the Oort Cloud. 

However, the initial mass of planetisimals needed to deliver the amount of material estimated to exist in the Oort Cloud would need to be 2 to 5 times greater for Neptune and Uranus to have been able to eject them, so it is now thought likely that the comets originated at their present distances. This would explain why no interstellar comet has ever been observed, though it is not yet understood how cometary nuclei were able to aggregate in such a low-density region as the early solar nebula.

Short-period comets are thought also to mainly originate in the Edgeworth-Kuiper Belt, a wide band of debris that starts beyond the orbit of Neptune. Objects have started being discovered within the Kuiper Belt only in the last few years, with Pluto and Charon being by far the two largest of these. Short-period comets are generally found close to the plane of the Solar System since they are derived from the same materials as the planets, and were formed in the proto-planetary disk around the young Sun. It is now thought, from their composition, that all comets were formed as planetisimals, in the Kuiper Belt. The short-period type is subdivided into five sub-classes:

If a comet or asteroid hits the Earth’s surface, the result is known as a meteorite. There are four kinds:

Astrology

Comets have always been given astrological significance, usually regarded with foreboding and considerable awe, since they are thought to often portend death and calamity. The earliest records of comets date from the 8th century BC in China and most relate some significant simultaneous event. In 44 BC, for example, a comet appeared on the Ides of March to carry Caesar’s soul to heaven. Claudius Ptolemy says of comets, "They operate effects like those of Mars and Mercury; exciting wars, heated and turbulent dispositions in the atmosphere, and in the constitutions of men, with all their evil consequences." Shakespeare wrote, "When beggars die, there are no comets seen: The heavens themselves blaze forth the death of princes." To him, a comet was a disorder in the heavens, "importing changes of time and state." A contemporary of Newton, Rev William Whiston, predicted that Earth would eventually be destroyed by collision with a comet.

Chinese interest in comets historically was mainly astrological. Wen Yin, the 2nd century AD writer, commented in the Han-shu, "The three types of stars are po (bushy star), hui (broom star) and ch’ang (long star). Their astrological significance is similar but with slightly different size and shape. The rays of bushy stars are short; their light rays bush out in all directions. The light rays of broom stars are long and tufted like a broom. Long stars have one straight light ray, sometimes it stretches across the sky, sometimes it is 10 chang [about 100°], sometimes it is 3 chang [about 30°] and sometimes it is 2 chang [about 20°], without any regularity. According to the Ta-fa, bushy stars and broom stars are mostly for sweeping away with the old and spreading the new, or for fire disasters. Long stars mostly represent warfare." On 15 February 1997, a meteorite hit the Shangdong Province of China, turning both Earth and sky red in a flash of light. This, only the fifth known to have hit communist China, was naturally seen as a portent of dramatic change, and Deng Xiao Ping died four days later.

In medieval astrology the position of a comet in the Zodiac at first appearance and its relation to the ecliptic were two of the factors determining its interpretation, since a comet traditionally expressed the sign in which it appeared; and most major comets were extensively discussed in literature. The comet of 1528 was "so horrible and dreadful, and engendered such terror in the minds of men, that they died from fear alone." However, years in which remarkable comets appear are supposed to provide better-flavoured grapes for wine production, and the superior wines of these comet years are known as Comet Wines. A comet could indicate a change of political leadership, an epidemic, war, or other dramatic event.

The difficulty of finding ephemerides of the comets, which stems from the difficulties of calculating their orbits, plus the fact that they travel far outside the Zodiac belt, has discouraged their use in modern astrology, though the transit of a comet over a natal point is thought to be significant. Comet Kohoutek reached perihelion in 1974 at the same degree of Capricorn as Richard Nixon’s Sun, just before Watergate.

A natural astrological technique would be for any natal planet transited by a comet to be regarded as reinforcing the mineral and vitamin associations of that planet. The comet would bring an influx of these substances to the being as well. Under a cometary transit there would be an acceleration of awareness and consciousness, which would add to the need to consume these substances.

The ascending north node of a comet is also said to provide a sense of the purpose of the comet, or the implications of the destiny of the current passage. The nodal degree could be compared with the Sabian degree interpretation, for example. Individuals with contacts to a comet’s nodes could be called upon to work with the comet to ensure its success, but most individuals could feel the influence of a comet as it transits their planets as dramatic but short-lived impulses.

EXAMPLES OF COMETS

Delevan’s Comet of 1914 is not expected to return for 24 million years. Others are short-period comets, with periods of as little as 3.3 years (Encke’s Comet) and less eccentric orbits, i.e. not inclined steeply to the Ecliptic. These include Jupiter’s family of about 40 comets, which have been captured by the planet so their present aphelia lies at about the distance of the orbit of Jupiter. Some are occasionally visible to the naked eye. Those most frequently observed are Schwassmann-Wachmann I, Kopff and Oterma. Most of these short-period comets never develop appreciable tails because their tail-forming matter has been exhausted in previous passages around the Sun.

Comet Shoemaker-Levy 9 was discovered on 24 March 1993 by Carolyn Shoemaker (b. 24.6.1929, Gallup NM) and Eugene Shoemaker (b. 28.4.1928, Los Angeles CA, d. 18.7.1997, Alice Springs, Australia, in a car accident, while studying impact craters) and their colleague David Levy. It had already been split by Jupiter’s gravity into a string-of-pearl formation of 20 or more mountain-sized rocks and was on a collision course with it, due to self-destruct in sixteen months. One by one the fragments would come up from below Jupiter, as seen from Earth, and pass behind the planet before striking its centre. As the collisions would occur on the night side of Jupiter, they would not be seen from Earth, though several remote space bodies including the Galileo satellite and Hubble telescope were to be trained on the phenomenon. Because it was heading for a point near Jupiter’s centre, it was unlikely to be swallowed up by its atmosphere, so a big bang was expected.

Shoemaker-Levy 9 may have been born from another comet, itself broken up by Jupiter after being ensnared into its orbit 200 years earlier, along with Gehrels 3, Helin Roman-Crockett and Kowal (seen in 1975). Unexplained flashes seen on Io in July 1983 could have been comet remnants hitting Jupiter in the same way as Shoemaker-Levy 9.

The first Shoemaker-Levy 9 fragment, Fragment A, two-thirds of a mile across or greater, was due to hit Jupiter at about 2053 hr BST on 16 July 1994, and actually impacted at 2120 hr BST, at 138,000 mph. Once it had passed through Jupiter’s ammonia crystal clouds, it collapsed under the force of its own shock waves, creating an explosion estimated to be equivalent to 10 million megatons of TNT, or millions of Hiroshima bombs. According to Gene Shoemaker each comet piece was made of ice, rocks, and organic material mixed into a fizzing, tarry gunk. It created a 20-minute fireball, making a 620-mile plume lasting 9 minutes. 90 minutes later, when the impact site had rotated into view, Jupiter was seen to have a new "hot spot", reaching temperatures of 30,000°C. Fragment F hit Jupiter at 0128 hr BST on 18 July 1994.

"It’s such a rare night when nature calls you on the phone and says, ‘I’m going to drop 20 comets on Jupiter...all I want you to do is watch’," said David Levy at the time "We are watching with everything we’ve got tonight, and nature winked at us." The event was a unique opportunity to learn much about both the natures of comets and Jupiter, collecting data that would take months and years to process.

The biggest explosion witnessed in the Solar System occurred later on 18 July 1994 when G, its largest fragment at 6 miles diameter, hit Jupiter at 0828 hr BST. This caused a 250-million megaton explosion, 2 million million miles wide, as bright as Jupiter itself, overwhelming and saturating the Keck Telescope at Mount Mauna Kea in Hawaii. The new black spot that resulted was more intense than Jupiter’s Giant Red Spot and was equal in size to Earth. Fragment R hit Jupiter at 0622 hr and Fragment S at 1600 hr on 21 July 1994 (as Tony Blair was being declared the new leader of the Labour Party, with John Prescott as his deputy). The final fragment hit Jupiter just after 0900 hr BST on 22 July 1994. These collisions could leave semi-permanent features on Jupiter’s surface and change its weather patterns. Intense media coverage included some astrological speculation, but this was mostly of the general "It will herald the awakening of a raising in human awareness and idealism; a new dawn for Man" variety.

Encke’s Comet, first identified in 1786, returns every 1,206 days, the shortest established period of return. It moves in an orbit resembling that of the asteroid Icarus, from inside Mercury’s orbit out towards the region of Jupiter. All of its subsequent 53 returns have been observed by astronomers. Increasingly faint, the comet was expected to have died by February 1994.

On 30 June 1908 at 0017 hr 11 sec GMT there was the equivalent of a 12½-megaton explosion, the Tunguska Event, at the basin of Podkamennaya Tunguska River (60N55 101E57), 40 miles north of Vanavara in Siberia. This devastated an area of about 1,500 square miles, and the shock was felt over 600 miles away. Nights were unduly bright around the world for several days afterwards. Thousands of reindeer were incinerated though, miraculously there were no human fatalities. Had the object crashed a few hours later it would have struck a major city and killed tens of thousands of people. The phenomenon is thought to have been caused by a meteor, stony debris, perhaps from Encke’s Comet falling to Earth from its orbit at only 20,000 ft. Travelling at 25km per second, it is calculated to have fallen at a 30° trajectory, 115° from North. Studies from plant life in the basin after the impact have shown marked chromosomal anomalies in cell division, far greater even than those found in areas of dense pollution. Although it flattened trees, it left no visible impact crater as it exploded prior to impact 6-8km above ground, at a temperature of 100,000°.

Craters were found, however, at Clearwater Lake East (56N05 74W07), Mistastin (55N53 63W18), Lake Bosumtwi (06N32 01W25) and the Manicougan Crater (51N23 68W42), for example. It is thought a giant Earth-crossing comet broke up within the last 20,000 years, as debris that could have come from it is frequently becoming discovered. The orbit of this object would have been similar to that of the Taurid meteor stream, the asteroidal Oljato and Encke’s Comet.

The fact that it exploded in the air could suggest that this was no ordinary meteorite. Some Russian scientists including Vladimir Rubtsov, director of the Research Institute on Anomalous Phenomena, have concluded that the cause was as first postulated by the Soviet engineer and science fiction writer Alexander Kazantsev in 1945. He claimed it was due to "an extraterrestrial spaceship that had met with disaster due to a malfunction at the final stage of its space voyage."

Comet Swift-Tuttle also hit the headlines because it could collide with Earth. Sighted on 26 September 1992 for the first time since its "discovery" in 1862, it crosses Earth’s orbit approximately every 130 years (it is now known to be the same comet as Comet Kegler, observed in China in 1737). It reached its perihelion on 12 December 1992, ten years overdue. Earth next crossed its orbit on 11/12 August 1993, but at its next approach for the first time its orbit will have swung close enough for it to collide with Earth. There was thought to be a one in 400 chance of the comet, which is six miles in diameter, hitting Earth on 14 August 2126, travelling at a speed of 130,000 mph. The probability of this world-wide devastating effect has since been downgraded to a less sensational one in several million!

Comet Hale-Bopp was discovered separately 22/23 July 1995, by Alan Hale at the Southwest Institute for Space Research, Cloudcroft NM, 35N03 106W36, at 0010 CST, 23 July; and about 10 minutes earlier by Thomas Bopp, an amateur comet-finder, working in Stanfield AZ, 32N53 111W58, approx. 2300 MST, 22 July. It passed near the sun on 19 February 1997 and became visible to the naked eye; the day that Deng Xiao Ping died. 18 days after that, a total eclipse of the Sun left north China and most of the landmass of Asia, in darkness for two minutes. On 15 February 1997 a meteorite had struck the Shandong province of China with a flash of bright light turning earth and sky red. Hale-Bopp reached perihelion on 6 March 1997 at 2000 GMT.

On March 26 1997 38 Heaven’s Gate cult members under the direction of Marshall Applewhite (b. 17.5.1931, Spur TX, 0320 CST), committed suicide in their home at Rancho Santa Fe (33N01 117W12), near San Diego CA. They were to rendezvous with a spacecraft they believed to be travelling behind Hale-Bopp following reports by an amateur Texan astronomer named Chuck Shramek. Four days earlier, bodies of 5 members of the Solar Temple cult were found in a burnt-out building in Saint Casimir, Quebec. According to the Talmud, "should a comet pass across the face of Orion, the Earth will be destroyed". Fortunately Hale-Bopp passed uneventfully across that constellation on 23 April 1997.

Comet Hyakutake became one of the brightest comets of the 20^th century, briefly outshining Hale-Bopp in March 1996, two months before it reached perihelion. At perihelion its tail was 3.8 AU (568 million kilometres) long, the greatest recorded tail. The comet was discovered (and classified C/1996 B2) by Yuji Hyakutake on 30 January 1996. It was the first comet found to be a source of X-rays, and was soon discovered to have a fragmenting nucleus. Cometary scientists got a lot of mileage from their studies of Hyakutake, as it was also found to be the cause of strange data returned from the Ulysses spacecraft. It had inadvertently crossed its path in April 1996, causing a drop of solar wind protons to be recorded by the magnetometer - a puzzle it took four years to solve.

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Last updated: January 14, 2002