Introduction

Comets have been known for thousands of years; Chinese records dating from before 240BC mention "Broom Stars" which graced the sky for days or perhaps weeks, before disappearing. There are records of comets so bright that they could be seen during the daytime (as has been known in more recent times), and comets with tails that stretched halfway across the sky. The comet came to be regarded as an omen - the bright sword-like appearance seemed to herald impending disaster. The Bayeux Tapestry (above) shows Halley's comet appearing above a depiction of King Harold, who reputedly later recieved a terminal injury from an arrow in the eye at the 1066 Battle of Hastings. As recently as the 1910 return of Halley's comet (and even the 1986 apparition in some unfortunate cases) the newspapers were full of horror stories, forecasting disaster on Earth from poisons within the cometary tail, or from impacts with its debris.

Astronomers have also studied comets from a more scientific viewpoint, and our understanding of these fascinating objects has steadily grown. In particular, Halley's comet, undoubtedly the most famous of all, has seen at least two historical scientific milestones: the first comet to have an apparition predicted by the new laws of motion and gravitation formulated by Newton (the predicted return occurred in 1759), and more recenty in 1986, the first comet to be visited by spacecraft from Earth. When Halley next returns in the year 2062, our understanding of comets will have advanced yet further.

Halley's comet - a brief scientific history of comets

The name "comet" comes from the Greek, kometes, which means "head of hair". The greek philosopher Aristotle believed that comets existed in the Earth's atmosphere. However in 1577 the Danish astronomer Tycho Brahe managed to prove that they were much more distant than this; comets belonged in space, not the Earth. But the objects still appeared and disappeared without warning - they seemed to be unpredicatable, and people remained fearful of their presence in the skies.

The most crucial advance in the understanding of comets came over one hundred years after Brahe's work. The English astronomer Edmond Halley (right) was very interested in the comet which he had seen in 1682. A close friend of Isaac Newton, Halley persuaded Newton to publish his work on gravitation; in fact, Halley paid for Newton's writings to be published. He then used Newton's theories of planetary motion to work out the motions of several different comets which had been seen in past years, and noticed that three of the comets - those of 1531, 1607 and the one he'd seen in 1682 - had very similar orbital paths. Halley believed that these three objects were just seperate appearances of one comet, and he predicted that, if his theory was correct, the same comet would return once again in 1759. Unfortunately, Halley died on January 14th, 1742, before he could put his prediction to the test. But on Christmas day, 1758, a German amateur astronomer called Palitzsch spotted a comet in the sky, just as Halley had predicted. So the work of Edmond Halley and Isaac Newton had finally shown that comets are not just mysterious objects which appear randomly in the sky - they follow orbits, similar to those of the planets.

From this point onwards, each appearance of Halley's comet - every 76.1 years - has been eagerly awaited, and observations of the comet carefully planned. In the 1910 appearance, astronomers all over the world cooperated with each other and took a library of photographs of the comet, which were carefully studied for years afterwards, to find out what the comet was made of, and how it behaved. These photographs were subsequently used to plan the encounters between Halley's comet and the fleet of spacecraft which intercepted it in 1986. Amongst these spacecraft was the European probe Giotto (above, courtesy of NSSDC). This mission revolutionised our understanding of comets, and we look at the findings of Giotto later on.

The origins of comets

Just like the planets, comets orbit the Sun. Halley used the laws discovered by Kepler in the 1600's, to investigate "his" comet, as observed in 1682. He found that for the comet to appear for as long as it did in the sky, and then disappear, its orbit must be like a "squashed circle" or ellipse. The orbits of all the planets are ellipses, but are more circular in shape than those of comets. This is one of the reasons why we can see planets equally well all of the time, whilst comets are only visible for part of their orbit. At its most distant, Halley's comet can lie out near the distance of the planet Uranus.

These strange orbits, which astronomers call highly eccentric, take the comets far out beyond the earth, and in many cases beyond the most distant of the known planets, Pluto. In fact, observations of some of the comets seen in our skies in recent times have shown that many comets have orbits which are so stretched out that they will not return to the region of space around Earth again for millions of years. These comets we call long period comets, whereas comets like Halley, which return to Earth in a few hundred years or less, are called periodic comets. The orbits of the comets are "scattered" randomly, so that they don't occupy a thin plane like the planets. Instead, comets orbit the Sun from all angles - about half of the comets orbit the Sun in the opposite direction to the planets.

In the 1950s, the Dutch astronomer Jan Oort investigated the orbital paths of comets, and suggested that comets come from a cloud of debris surrounding the solar system - a cloud which we now call the Oort cloud. Oort originally suggested that the cloud was so large that it reached almost one third of the way to the nearest star. However, today astronomers believe that the main part of this cloud is much closer, lying at a distance of between 1000 and 30,000 astronomical units. (An astronomical unit is a measure of distance, and is the average distance between the Earth and the Sun - about 149,600,000 kilometres). This cloud may act as a "reservoir" of comets. Very occasionally, a piece of debris may be pulled out of the cloud - this could be caused by collisions with other pieces of debris, or by the gravitational pull of a passing star. When this happens, the object falls towards the sun, and begins its life as a true comet. As the object passes the planets, and particularly if it approaches the strong gravitational field of Jupiter, the path of the comet may be altered, and this effect changes the "period" of the comet (i.e. how long it takes to complete one orbital revolution).

More recently, studies of comets with short periods has suggested that they originate not from the Oort cloud, but from another collection of objects closer in to the Sun. This inner cloud is believed to be disc shaped, and exists between 30 and 50 astronomical units from the Sun, beyond the planet Neptune (some astronomers believe the small planet Pluto and its moon Charon, may belong to this cloud). This collection of debris is called the Kuiper Belt, and may contain a similar amount of mass to that found in the Earth (the Oort cloud is proposed to contain around 100 times more mass).

Kuiper Belt schematic

 

The image on the left (courtesy of David Jewitt) shows an object detected by the University of Hawaii 2.2 metre telescope in October 1995. The two frames show how the object moved over the course of three hours. This object is estimated to measure around 270 km across, and is about 40 astronomical units distant. It is believed to be one of the objects in the Kuiper belt, and was the 29th such object to be discovered.

 

Explaining the appearance of comets

The images of small, distant objects look very unlike the spectacular, tailed comets which can grace our skies, and yet they are the same object. It is the comet's elliptical orbit, bringing the object close to the sun, which holds the explanation for the remarkable transition. By studying many comets, (and sending probes to Halley) we now have a very good idea of what a comet is made of. The most popular description of what a comet is made of is a "dirty snowball", a phrase introduced by the American astronomer Fred Whipple, who studied comets.

The center or nucleus is believed to consist of dust and rock particles held together with ices such as water ice and carbon dioxide ice. Far away from the Sun, the comet is cold, and to an observer close by it would appear as nothing more than a fairly small dead chunk of material. However, as the comet approaches the Sun, it begins to heat up. The ices start to melt, and gases begin to escape from the body, surrounding the nucleus with a glowing cloud called a coma, which may measure more than 100,000 km across (in comparison to the nucleus which may only be a few km in diameter). This coma is in turn surrounded by a halo of hydrogen, again from the gas which up until this point have been locked up inside the cometary ice. These gases glow, because they absorb sunlight and then re-emit more light.

As the comet draws closer in, the energy from the Sun, including a stream of particles called the solar wind, begins to push some of the gas away to form the long tail - which always points away from the Sun, as illustrated on the diagram, left (courtesy of Cambridge University Press) . Often a comet will show two tails. One is very straight and is caused by electrically charged particles, and the other is made of dust, curving away from the nucleus. So, the comet loses material when it enters the inner solar system. Some estimates from the Giotto - Halley encounter suggest that the comet lost over 50 tons of material every second when it was closest to the sun.

Disappearing comets

After the comet passes closest to the Sun, it begins its journey back to the outer solar system - perhaps well beyond the orbit of Pluto. As it does so, it begins to cool, and the gases stop escaping so that once again the comet appears as nothing more than a "dirty snowball". However, some comets don't have such a lucky escape. Occasionally a comet may pass so close to the Sun that it crashes into the star, never to be seen again. Such events have been recorded, and the movie presented here (courtesy of NASA) shows a comet disappearing behind the Sun, as observed by the Solar Maximum spacecraft. Instruments on board were designed to block out the disc of the Sun, and the comet which can be seen disappearing into this region, was never seen to re-appear, suggesting that the comet may have been "boiled away" by the Sun.

Comets can also be destroyed by colliding with other solar system inhabitants, such as the planet Jupiter. In July 1994, the comet Shoemaker-Levy 9 (named after the people who discovered it, and pictured left) did just that. Observatories all over the world, and the orbiting Hubble Space Telescope watched the events unfold. Fragments of the broken-up comet were tracked as they entered the gaseous atmosphere of the giant planet, and the marks from the encounter were clearly visible afterwards (NASA).

The results of Giotto

Several spacecraft encountered Halley's comet in 1986, but the most significant was the European Space Agency's Giotto. Launched on July 2nd 1985, it encountered Halley on March 13th 1986, approaching to within 600km of the comet's nucleus. Giotto carried many scientific instruments including cameras and dust detectors.

Giotto found that the nucleus of the comet measured around 16 by 8 by 7.5 km, and that the comet was indeed the "dirty snowball" which Fred Whipple had suggested years previously. The nucleus showed hills and "valleys", although the shape was being altered continuously by the jets of gas coming through the surface and forming the tail and coma. Giotto's view of the nucleus of comet Halley is shown on the left (courtesy of the European Space Agency).