Near Earth Objects | The Discovery of the Asteroid Belt
Comets as Harbingers of Momentous Events | Evidence of Meteorite Impacts
Asteroids and Meteorites - Mankind's Fate?

Since time immemorial, man has been fascinated by the heavens, watching and recording the regular cycles of the Sun and the Moon. At night he identified certain patterns in the stars onto which he superimposed pictures, perhaps of familiar objects or animals. The ancient Greeks, in particular, superimposed figures representing mythological persons and events. These groupings are now known as constellations.

Many ancient cultures, including the Babylonians, Egyptians, Chinese and Hindus, kept elaborate records of astronomical observations. This was thousands of years before the Christian era, thus making astronomy the oldest of the sciences.

From these records, man realised that the heavens followed certain regular patterns and he was able to use this information, for example, to determine the proper times for sowing of crops and for the celebration of religious festivals. It also made it possible to predict such regularly recurring phenomena as solar eclipses and lunar eclipses.

However, from time to time something totally unexpected would happen, such as the appearance of an object we would now know to be a comet. This must have come as rather a shock and, perhaps understandably, people came to regard such appearances as presages of some momentous event. As such events were generally calamitous in nature, people came to fear these manifestations. Some peoples believed that the heavens were about to fall down.

This early fear of comets may not be without good reason. It is quite possible that one hit the Earth in our distant past, and the memory of it has percolated through folktales into today's legends.

We are very fortunate that comets in ancient times received such close attention, for frequently when one appeared both its track and its physical appearance were recorded. The ancient Chinese, in particular, thought that comets were 'celestial ambassadors', and because of this made very careful records of their appearances and positions in the heavens. These records have subsequently been of immense service to modern-day astronomers in tracing solar comets back through many centuries.

What are Comets?

There is confusion in the minds of many between asteroids and comets. Both asteroids and comets may be considered to be leftovers from the creation of the solar system. When the Sun and planets formed from the coalescence of dust and gas, there was some material left over.

Material that was relatively close to the Sun formed asteroids, or minor planets - rocky bodies which move around the Sun between the orbits of Mars and Jupiter. Asteroids are, typically, a few miles across.

Material that was further from the Sun formed comets: colossal conglomerations of dirty, dust-filled grains of ice and rock, which are thought to originate in the Oort Cloud.

Both asteroids and comets (collectively called bolides) have, in the past, collided with the Earth, and will continue to do so in the future.

The name 'asteroid' was given by Sir William Herschel on account of their star-like¹ appearance when viewed through a telescope.

Modern Description of Comets

A comet is a large ball of rock and ice, typically from 100m to about 5km in diameter, that orbits the Sun with an extremely elliptical orbit. When it is close to the Sun it gets hot; some of the ice melts and its water vapour and dust is lit up in the Sun's rays, visible as a 'tail' behind it, pointing away from the Sun. Indeed, the word 'comet' comes from the Greek 'kometes', meaning 'long-haired'. Because the orbit is elliptical, it means that the comet travels very quickly when close to the Sun, and very slowly when far away from it, usually out beyond the orbit of Pluto.

Early References and Folklore

References to 'fires from the skies' prevail in the myths and legends of most cultures. Some of these involve winged serpents battling in the sky before one crashes to Earth.

In 500BC, the Persian prophet Zoroaster predicted that the world would come to an end with Satan hurling a comet at the Earth and causing a 'huge conflagration'.

In the Bible, the Book of Revelation describes a vast burning mountain falling from the sky, dropping hail and fire on Earth while the Sun and the Moon are darkened; while in the Old Testament, Sodom and Gomorrah were destroyed by a rain of fire and brimstone from Heaven. Fireballs also featured heavily in Babylonian astrology.

Cometary icons were apparently widespread throughout early civilisations, including the 'omega' symbol found throughout the Near East. Dr Bill Napier, astronomer at the Armagh Observatory, and others have also suggested that the swastika, a symbol with roots in Asia stretching back to at least 1400BC, could be an artist's rendering of a comet, with jets spewing material outward as the head of the comet points earthward. An ancient Chinese manuscript found during the 1970s at Mawangdui shows 29 images of comets, and each image has a distinctively different tail. One ancient comet evidently had four tails, making it look like a progenitor of the swastika (the Buddhist symbol, not the reversed Nazi version).

Some Well-known Comets

Halley's Comet

The most famous comet, documented by the astronomer Edmund Halley and popularly called Halley's Comet, appears in our skies roughly every 76 years. It is pictured in the Battle of Hastings portion of the Bayeux Tapestry, which means it was around in 1066; it was also with us in 1985 - 86.

Hale-Bopp

Hale-Bopp was discovered almost simultaneously (within 20 minutes of each other) in 1995 by Alan Hale, a space scientist from New Mexico, and Thomas Bopp, an amateur astronomer from Arizona.

Observations from the Hubble Space Telescope have shown that the comet has an unusually large nucleus, reaching about 40km in diameter (25 miles) - ten times that of the average comet and seven times larger than Halley's comet. This makes it one of the largest comets ever recorded.

Comet Hale-Bopp came within less than a million miles of Earth's orbit in early 1997. Fortunately the Earth was further along its journey round the Sun, so it missed us by about 122 million miles. It did give a spectacular display, however, and its dust and plasma tails were clearly visible from the northern hemisphere for a few months. Comet Hale-Bopp was the brightest since the Daylight Comet of 1910, and was clearly visible from even the most brightly-lit towns.

Its orbit is very irregular compared to the other bodies in our solar system - it is tipped on its side, and will probably not be visible to us again from Earth. Hale-Bopp was previously visible from Earth during the Bronze Age in 214BC.

Hyakutake

Comet Hyakutake was first sighted in January 1996, by a Japanese astronomer using binoculars. It was visible to the naked eye from Britain in March 1996. It has a period of 18,000 years and was the brightest comet since Comet Westin 1976.

Shoemaker-Levy 9

In July 1994, astronomers around the world had the chance to watch something that may never have been seen before, and probably (fingers crossed) will never be seen again. A comet named Shoemaker-Levy 9, after its joint discoverers, decided to plunge straight into the side of Jupiter, with dramatic effect.

As the comet entered the considerable gravitational field of the gas giant², it fragmented into some 20 discernible chunks, each of about 2km in size. One by one, these huge rocks plummeted through the Jovian atmosphere at about 60km/s. Because Jupiter is a gas giant, and has no discernible solid surface, the results of the impacts were not as spectacular as they might have been, but they still created huge gas plumes thousands of kilometres high, and left dark 'scars' in the planet's atmosphere for many weeks. You can read all about it on NASA's website, which also includes some great pictures.

The implications of this are immediately obvious - what if it had hit the Earth instead of Jupiter? Well, for a start, you probably wouldn't be reading this... However, fortunately for us, because of its huge size Jupiter acts as an immense magnet which attracts any comets which enter our solar system, thus preventing a great many from hitting the Earth.

Chronological Catalogue of Catastrophes Caused by Comets

Demise of the Dinosaurs

A comet six miles (9.6km) across, hitting the Earth at the Gulf of Mexico 65 million years ago, is thought to have caused the demise of the dinosaurs.

The evidence for this is the so-called 'Chicxulub Structure' in the Yucatan Peninsular in Mexico, which was discovered in 1978 by a geophysicist working for the Mexican national oil company. This looks just like a crater on the Moon; circular with a series of concentric ripples radiating outwards from its centre. This crater is between 180-280km across and is the largest known on Earth. It is buried under about 3km of more recent sedimentary rock.

Supporting evidence for this comes from all over the world. In Haiti, on the opposite side of the Gulf of Mexico, geologists have found minerals that appear to have been thrown there after being violently heated. Others seem to have been dumped there by a tsunami³, thrown up by the meteor's impact.

The most persuasive piece of evidence is that, at the same geological levels all over the world, geologists have found a layer rich in iridium - an element rare on Earth, but more common in meteorites and comets. This is called the K-T Boundary.

Furthermore, in locations from Europe to New Zealand, scientists have discovered a layer of soot, apparently formed by gigantic forest fires that swept the Earth following a meteor strike. The debris from the strike, together with the smoke from forest fires would have been sufficient to block out sunlight for tens of years, and thus lead to the mass extinction of life forms on Earth, including the dinosaurs.

Could a Comet Have Caused the Last Ice Age?

Ice ages or 'glacial periods' are periods in the Earth's history when significant, extended cooling of the atmosphere and oceans occur. During such times, the polar ice-caps have occupied a much greater area than they do at present.

Three such periods are recognised in geological history, separated by much longer periods of warm, uniform climate. The most recent such period began about 1.6 million years ago, at the beginning of the Quaternary period, and ended in North America and Europe about 10,000 years ago, at the end of the Pleistocene epoch. Some scientists believe that this most recent ice age could well have been caused by a comet.

A study, published in Science in 1998, showed that a previously unknown impact from an asteroid or comet, impacting in what is now south-eastern Argentina, coincided with the disappearance of 35 different types of ancient mammals and a flightless bird 3.3 million years ago. This impact may have directly caused the regional extinctions or triggered a climate change that led to the disappearance of the animals.

The scientific team studied an 18-mile-long narrow layer of greenish glass and red brick-like materials, called escoria found in the high ocean cliffs of south-eastern Argentina. The origin of this material had puzzled scientists ever since it was first described in 1865.

Chemical analysis of the glass produces all the right impact signatures for a comet impact: unusually high levels of magnesium oxide and calcium oxide, significant amounts of iridium and chromium, and only the tiniest traces of water.

The study showed that the glass occurs just below a layer of dusty deposits containing fossil evidence of a three-million-year-old disappearance of 36 local types of animals. Extinct species include large armadillo-like creatures, ground sloths, hoofed groups of related mammals and a flightless carnivorous bird. Other fauna later appeared in their place.

Collapse of Bronze Age Civilisations

Several advanced civilisations are known to have vanished or begun a rapid decline during the early Bronze Age, about 4000 years ago. These included the Old Kingdom of ancient Egypt, the Sumerian civilisation of Mesopotamia and the Harappan civilisation of the Indus Valley. Forty cities around the world are believed to have collapsed around this time.

Evidence has been mounting that this may have been due to a giant comet which swept past the Earth about 4000 years ago, breaking up as it did so, resulting in multiple impacts and possibly a rain of other smaller meteors and dust. Dr Bill Napier (Armagh Observatory) estimates that the dust and meteors could have cooled the planet by 5° C for several years, thus causing widespread crop failures, leading to mass starvation and the collapse of societies. Napier has tied this in with findings from scientists at Queen's University, Belfast. By studying patterns of ancient tree rings from around the world they have found that the planet did indeed cool suddenly between 2354BC and 2345BC.

Dr Napier believes that comet Encke, discovered in 1786, and the Taurid meteor shower, are remnants of this larger comet.

In support of a cometary event at this time, archaeologists working in northern Syria have found evidence of a catastrophic event which caused mud-brick buildings to collapse at around this time. The most exciting new evidence comes from Dr Marie-Agnès Courty, a French expert in the microscopic study of soils and sediments. She has found that samples from three regions of the Middle East, taken from levels corresponding to the period around 2200 BC when there were abrupt climatic changes, contain tiny spheres of a calcite material unknown on Earth but found in meteorites.

It is highly likely that a catastrophic event such as this, occurring as it did within recordable human history, entered folklore (including Greek mythology), and the Bible (see above).

Consequently, the appearance of comets came to be associated with events of huge significance. As most such events are 'disasters', comets (see also Messier Objects) came to be seen as augurs of doom, being associated with catastrophic events such as war, pestilence, the deaths of kings and the fall of nations. Indeed, even the word 'disaster' is derived from the Latin - astre meaning 'star' (dis-astra, or 'out of sync with the stars').

Jonathan Swift wrote:

Old men and comets have been reverenced for the same reason; their long beards, and the pretences to foretell events.
- Thoughts on Various Subjects, 1728.

The Tunguska Incident

The greatest cometary impact of the 20th Century occurred in a remote region of Russia on 30 June, 1908, at Tunguska in central Siberia. With no warning, a small comet or meteor, estimated to be just 30 metres in diameter, struck the Earth and laid waste to an area of forest more than 30 miles across. The impact had a force of 20 million tonnes of TNT - equivalent to 1,000 Hiroshima bombs. It is estimated that 60 million trees were destroyed over an area of 2,200km². Had this explosion occurred over London or Paris, hundreds of thousands of people would have been killed.

For a more sceptical interpretation of this event, see The Tunguska Incident.

Association of Comets with War, Pestilence and Fall of Nations

The very first known record of a comet was by scribes during a war between two rival Chinese kings, Wu-Wang and Chou, around 1059BC. They describe an object (comet) with an eastward-pointing tail, which dominated the morning sky. Chinese recorders eventually noted two types of comet - the po and the hui; the po, or bushy star comet, generally meant a comet with a large fuzzy coma or atmosphere, usually without a tail; the hui, or broom star comet, had a tail. The Greek philosopher Aristotle called them fringed and bearded stars, respectively.

A comet (now known to be the first recorded sighting of Halley's Comet) appeared in 239BC, towards the end of the First Punic War.

Comets are thought to have accompanied the deaths of the Roman General Agrippa (12BC), Attila the Hun (453AD) and Emperor Valentinian (455AD). A comet is often said to have accompanied the death of Charlemagne (814AD), but this appears to have been a complete fabrication on the part of chroniclers who considered the appearance of comets to be de rigueur for the death of mighty rulers!

The biography of Julius Caesar by the ancient author Suetonius says that a comet was seen over Rome just after the assassination of Caesar in 44BC, which was believed to be his soul ascending to heaven. Thus, in Shakespeare's Julius Caesar the Roman empress Calpurnia reminded her husband that:

When beggars die, there are no comets seen; The heavens themselves blaze forth the death of princes.
- William Shakespeare, Julius Caesar.

When a comet hovered over Jerusalem in 66AD, it prompted the historian Josephus to warn that it heralded the destruction of Jerusalem, which did indeed happen four years later.

William the Conqueror saw a bright comet, which we now know to be Halley's comet, in 1066. This was perceived by King Harold as a dire omen, but as a favourable one by William. Indeed, William's battle cry came: 'A new star, a new King'. Several months later, Harold was indeed killed at the Battle of Hastings and William had an image of the comet embroidered in the Bayeux Tapestry. A group of terrified Saxons is seen looking up at it; a legend reads, 'They are in awe of the stars.'

In 1301 Halley's Comet again appeared - after which the Italian painter Giotto di Bondone immortalised it by depicting it as The Star of Bethlehem in his painting the Adoration of the Magi in the Arena Chapel, Padua.

A 'black comet' was said to have preceded the Black Death in 1665 and another comet was said to precede the Great Fire of London in 1666.

The appearance of a comet was also associated with the Great Constantinople Earthquake of 1556.

In 1456 opposing armies of Turks and Christians faced each other at the Battle of Belgrade, when Halley's comet appeared. Its tail, shaped like an avenging sword, pointed towards the Turks; the Christians won. At this time the Turks had become masters of Constantinople and were threatening to advance into Europe. Hence the appearance of Halley's comet was regarded by Christendom with superstitious dread. Ecclesiastical authority saw Halley's comet as an agent of the devil and led to the myth that the pope had excommunicated it. At this time a prayer was added to the Ava Maria: 'Lord save us from the devil, the Turk, and the comet.' (Chambers Encyclopaedia, 1887).

In 1607 Halley's comet was sighted by American colonists, who were subsequently plagued by rampant diseases, hostile Indians and near-starvation.

The penultimate appearance of Halley's comet in 1910 coincided with the death of King Edward VII and the accession of George V. By this time most people had lost their superstitious dread of comets, but other fears had arisen. An astronomer reported that spectroscopic analysis showed that the comet's tail, through which the Earth would pass, contained a poisonous gas, and charlatans made a small fortune selling 'anti-comet pills', guaranteed to protect people from harm.

In 1997 fantasists believed that a Saturn-shaped spaceship was hiding behind Comet Hale-Bopp, ready to invade Earth. This led to the mass suicide of 39 members of the Heaven's Gate cult!

Comets in Literature

Andrew Marvell's poem The Mower to the Glo-Worms (see below) was published in 1681, and contrasts the welcome light of glow worms to the ominous presence of a comet.

The Mower to the Glo-Worms

by Andrew Marvell (1681)

Ye living lamps, by whose dear light
The nightingale does sit so late,
And studying all the summer night,
Her matchless songs does meditate.

Ye country comets, that portend
No war, nor prince's funeral,
Shining unto no higher end
Than to presage the grass's fall;

Ye glo-worms, whose officious flame
To wandering mowers shows the way,
That in the night have lost their aim
And foolish fires do stray;

Your courteous lights in vain you waste
Since Juliana here is come
For she my mind hath so displaced
That I shall never find my home.

Duncan Steel, a British astronomer and expert on comets and meteors, believes that a passage in Coleridge's 'Rime of the Ancient Mariner' may have been inspired by the passage of Comet Temple-Tuttle in 1797. The debris of comet Temple-Tuttle is known to produce the spectacular annual meteor shower known as the Leonids, every November.

The Rime of the Ancient Mariner

by Samuel Taylor Coleridge

The upper air burst into life!
And a hundred fire-flags sheen.
To and fro they were hurried about!
And to and fro, and in and out,
The wan stars danced between.

And the coming wind did roar more loud,
And the sails did sigh like sedge;
And the rain poured down from one black cloud;
The Moon was at its edge.

In 1812, the comet mentioned by Tolstoy in War and Peace coincided with a particularly fine vintage of port, leading its merchants to call it 'Comet Port'. Subsequently, a notion has prevailed that the grapes in comet years are better in flavour than in other years, either because the weather is warmer and ripens them better, or because the comets themselves exercise some chemical influence on them. So as well as 1812, the wines produced in the years 1826, 1839, 1845, 1852, 1858, 1861, etc are considered to be particularly fine. (Brewer's Dictionary of Phrase and Fable, 1894).

What about the Star of Bethlehem?

Several theories have been propounded to explain the Star of Bethlehem.

The idea that it might have been a comet was first proposed as early as 248AD by the theologian and writer, Origenes Adamantius, better known as Origen. This also is the favoured theory of Professor Chandra Wickramasinghe, Professor of Applied Mathematics and Astronomy at Cardiff University. However, he thinks the comet was in a more spectacular form known as a fireball. This is a meteorite, or fragment of a comet, pulled into the Earth by gravity, which then explodes in the atmosphere or when it strikes the ground. The Russian newspaper Sibir reported of the Tunguska event:

Early in the ninth hour of the morning of June 30, a very unusual natural phenomenon was observed here. In the village of Nizhne-Karelinsk the peasants saw a body shining very brightly, indeed too bright for the naked eye, with a blue-white light.

Wickramasinghe compares this with another medieval account of a fireball over Germany and the Dutch coast:

A very terrifying apparition and sign of wonder has been seen in Bamburg and Liechenfels. In the year 1560 on the 28 December, 1560 this apparition was seen in the sky which first had its beginning over Eberssberg in Franconia and rose directly over Zeyl, and then moved towards the town called Elpmann, and stopped still there for a long time.

Therefore, this fulfils the requirements of something new, bright, unusual, spectacular, and which moves from village to village before appearing to stop still.

Wickramasinghe says that such a comet would need to approach the Earth at a glancing angle, otherwise the fireball would come and go too quickly. In order to slow down, the comet would need to have been pulled into Earth's gravitational field and then travel at the same speed as the Earth so as to remain stationary. As it slowed it would have burned brightly, scattering showers of sparks - meteoroids - which might have been interpreted by watching shepherds as a 'host of angels'.

The Spaceguard Project

Due to the fact that approximately three-quarters of the Earth's surface is sea, most cometary impacts are likely to be over the oceans (or on relatively uninhabited land). However, it is inevitable that at some time an asteroid or comet will collide with the Earth in a populated area, with potentially calamitous consequences. Thus an asteroid measuring only 1km in diameter would cause a global catastrophe, releasing more energy in its impact than all the world's arsenals put together.

Since 1990 well over 100 asteroids have been discovered with orbits that intersect the Earth's orbit. Dr Duncan Steel estimates that an asteroid large enough to kill a quarter and maybe as much as half of the world's population strikes on average once every 100,000 years. Given the number killed, this means that the risk of dying in this way to the average person is one in 5,000 - four times the chances of being killed in an air crash!

For this reason The Spaceguard Foundation was officially set up on 26 March, 1996 with the aim of 'protection of the Earth's environment against bombardment of objects of the solar system (asteroids and comets)'.

Acknowledgements

'Halley's Comet is Coming' - Blake Clark. Reader's Digest, December 1983.

'Is this the signal that tragedy is about to rock mankind?' - Julian Champkin. Daily Mail, 20th March 1997.

'Comet guided Coleridge to the Ancient Mariner' - Robert Matthews, Science Correspondent for the Sunday Telegraph.