Rebecca Coss, of Nga Tawa School, asks :-

What caused the comet Shoemaker-Levy 9 to collide with Jupiter?

Rod Austin, a newspaper photolithographer and an amateur astronomer who has discovered three comets, all of which are named after him, responded.

A comet can be considered to be something like a 'dirty snowball'; a nucleus consisting of a loose conglomeration of frothy ice (mainly water ice) and very fine dust, a bit like the frost inside a freezer.

Probably more than twenty years ago, this comet, some 20 kilometres across, approached the Sun from deep space and passed very close to Jupiter, the most massive planet in the solar system. The immense gravity of the planet swung the orbit of the comet so that it lost orbital energy and entered a long and very ellipitical orbit about Jupiter. The sun's heat causes outgassing from a comet and therefore jet effects, rather like the retro rockets on a Space Shuttle. This had the effect of slowing down the comet in its orbit about Jupiter, causing it to fall lower and lower in its orbit.

When a comet comes within about 2.4 times the radius of a planet, the difference of gravity of the planet across a comet nucleus causes great strain and finally breaks the nucleus into pieces.

By the time of last close approach in July 1992, comet Shoemaker-Levy was taking about two years to make one orbit about Jupiter, with the comet then passing only 16,000 kilometres, about a quarter of Jupiter's radius, above the top of Jupiter's clouds. The effect was quite small at first, but with time the pieces finally drifted apart, releasing more dust and gas which caused the comet to slow even further, slowly lowering the point of closest approach to the planet to a distance less than the radius of the planet.

The comet was then doomed.