Comets: Ghostly Wanderers

Portents of Doom?

In the spring of 1997 we were lucky enough to be treated to the dramatic spectacle of comet Hale-Bopp slowly making its way across the night skies. Discovered in 1995 by Alan Hale and Thomas Bopp, it has been one of the brightest comets of this century.

As a result of their ability to awe and their apparent unpredictibility, great importance used to be attached to comets. They came to be viewed as omens, usually bad ones. In particular, Halley's comet has been associated with the destruction of Jerusalem in A.D. 70 and the Norman Invasion of England in 1066.

In mediaeval times it was believed that comets were an atmospheric effect. However, in 1577 the astronomer Tycho Brahe compared observations from different points on Earth and proved that the comet being observed lay well outside the atmosphere.

Astronomers now study comets from a scientific viewpoint. Although much is now known about them, they remain one of the most mysterious yet beautiful members of the solar system.

Comet Hale-Bopp

Comet Halley in the Bayeux Tapestry

In from the darkness

Most bodies in the solar system move around the Sun in nearly circular orbits in the plane of the solar system. However, comets break all the rules - their orbits are stretched out to form elongated oval shapes (called ellipses), can be at any angle to the solar system, and can last for anything from a few years to many millennia. Indeed, some comets that have been seen are on paths that will send them flying out of the solar system for ever.

So where do they come from? According to one popular theory, they come from a huge spherical cloud of comets that stretches over a thousand times further from the Sun than far-off Pluto. Known as the Oort cloud (after the astronomer who first proved its existence) it is suggested that occasional disturbances, such as a passing star, might disrupt the cloud through the influence of its gravity. When this happens, some comets could find themselves nudged into new orbits that would take them into the inner parts of the solar system. Once within the realm of the planets, the paths of the comets could be affected by other gravitational affects, especially those of giant Jupiter. Some get thrown out of the solar system forever, whereas others end up on yet smaller orbits, visiting the inner solar system fairly frequently - such as comet Halley.

Icebergs in Space - the Structure of Comets

Comets are traditionally thought of being dirty snowballs - mostly frozen water with some dust. This forms the nucleus, which is typically several kilometres across. When a comet moves into the inner solar system, this is heated by the Sun, and the ices turn into gas which forms the head or coma of the comet, which may be hundreds of thousands of kilometres across and visible from Earth.

The most striking part of many active comets is the tail. A common fallacy is that the tail of a comet trails behind the nucleus as it journeys through the solar system. This is not the case. The tail actually consists of material that the force of light and particles streaming out of the Sun - the solar wind - has pushed away from the coma. Hence, the tail always points away from the Sun.

Comets generally have at least two tails - one consisting of dust that has been pushed away by sunlight and generally stays in the path of the comet's orbit, and straighter tail resulting from the effects of charged particles from the Sun, known as ions.

Encounter with Halley

Halley's comet is undoubtably the most famous comet that graces our skies. Visiting the inner solar system every 76 years or so, its appearances have been well documented well into antiquity. However, until the 17th century no-one realised that it was the same comet coming back time after time. The English astronomer Edmund Halley studied it in 1682, and using the new theories of gravitation of his friend Isaac Newton, realised that it had a similar orbit to two previous comets - those of 1531 and 1607. He predicted its return in 1759, and on Christmas Night, 1758, it was duly discovered.

The long awaited return of Halley's comet in 1986 signified another major first in astronomy. It was visited by spacecraft from Earth, most notably the European Space Agency's Giotto. Encountering Halley on March 13th 1986, it approached to within 600km of the comet's nucleus, carrying many scientific instruments including cameras and dust detectors.

Giotto found that the nucleus of the comet measured about 16 by 8 by 7.5 km - roughly the same size as the Isle of Wight. It also discovered that the comet was indeed a "dirty snowball" made of water ice and dust. The nucleus showed a considerable amount of detail, with the shape being continuously altered by jets of gas coming off the surface and forming the coma and tail. At the time of encounter the comet was losing 50 tonnes of material every second in this manner - but it will not run out for a while yet!

Inside Halley

Into the celestial frying pan

Normally after a comet passes closest to the Sun - its perihelion - it heads back out to the outer solar system, cooling back down and becoming a rather unexciting "dirty snowball". However, sometimes a comet may not be so lucky. Occasionally it may pass so close to the Sun that it crashes into the star and is destroyed.

On rarer occasions a comet may pass close to a planet, usually Jupiter, and the intense gravitational forces may tear it apart. The result of this - comets broken up into several pieces - has been observed a few times. The most spectacular example of this was comet Shoemaker-Levy 9, which was torn into a number of fragments during a close encounter with Jupiter in 1992. It then went into orbit around Jupiter, and most dramatic of all, crashed into Jupiter in July 1994.

Observatories the world over watched this unique event, along with the Hubble Space Telescope. In particular the best view was to be had from the Galileo spacecraft, en route to Jupiter at the time.

Fragments of the broken comet were tracked as they plunged into the gaseous atmosphere of the giant planet, releasing their energy in explosions equivalent to the detonation of millions of nuclear warheads. Afterwards, the marks from the encounter were clearly visible on the surface of Jupiter.

Comet Crash on Jupiter