INTRODUCTION

Some of the strangest objects in the Solar System are comets. These are balls of rock, ice and dust which orbit the Sun in elliptical orbits (oval-shaped orbits, not round). They can go very close to the Sun, and then they glide across the galaxy, past the nine planets and go millions of miles beyond the furthest reaches of the Solar System before returning to spin around the Sun again.

Comets are recognisable because of their tails. This tail is only visible when the comet is close enough to the Sun, usually when it is at about the same distance from Earth as Mars is. This is because heat from the Sun warms up the comet, causing its ice and dust to melt and burn away and leave a tail. It is similar to when a meteorite enters the Earth's atmosphere and burns up, leaving a short, twinkling tail and appearing like a "Shooting Star." The tail of a comet can be up over 1 million kilometres long!

The most recent comet seen recently from Earth was the Hale-Bopp comet in 1997 (pictured above and at the bottom of the page). This comet was visible from all over the planet and looked like a star with a blurry tail. After spinning around the Sun, it is now (February 2000) nearly half way between the orbits of Jupiter and Saturn. This comet won't be seen again on Earth for thousands of years. However, one comet which makes regular appearances is Halley's comet. This comet goes as far away from the Sun as almost Neptune, but comes almost as close as Mercury to the Sun. This planet is visible from Earth every 76 years, meaning that the average person can see it once during his/her lifetime. Its next appearance will be in 2061, after last appearing in 1986. The picture below is of Halley's Comet from 1986.

Comets orbit the Sun by crossing the Solar System. This means that they cross the paths of the planets. Ocassionally, a planet may also be in the way as the comet crosses its path. This happened in 1994 when the comet, Shoemaker-Levy 9 (known as the Shoemaker-Levy Comet) collided with Jupiter. This is the first time a comet has ever been seen to collide with a planet, and satellites everywhere were pointed at the planet to watch the event.

Comets are usually discovered about 2 to 3 months before they are clearly visible from Earth. This is because they are so small and hardly glow at all when they are at a great distance from Earth. The Hale-Bobb comet, discovered by Alan Hale in July 1995, is an exception. It would not be seen at its maximum brightness from Earth for nearly two more years. The comet was accidently discovered because it was so big and would, Hale believed, leave a tail four times wider than a full moon.

Missions planned at the moment involve sending probes into the tails of comets and bringing back dust particles from them. It is believed that these particles will tell us something about what is contained at the furthest reaches of the Solar System and maybe more about its formation. And, it gives scientists something different to do when they get bored of staring at pictures of Mars' surface. Here is a summary of the space missions happening in the recent past or in the near future involving comets:

6th February 1999: Stardust (USA). Designed to return samples from the comet Coma

July 2002: Contour (USA): A flyby of three comet nuclei (the central parts)

23rd January 2003: Rosetta (European Space Agency): Designed to orbit a comet

May 2003: Champollion.DS4: (USA) Return of samples from a comet

 

 

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