The night sky: Stargazers get ringside seat to comet's last days

The northwestern sky at 7pm tonight. Use it to locate Mars and Saturn.

Astronomers around the world are witnessing the destruction of a comet. Such events are rarely seen in such spectacular detail.

Comets are small bodies, typically less than 15km across. They are composed mainly of water ice but include dusty material as well - hence their common characterisation as "dirty snowballs".

Most comets orbit the sun at great distances (1000 times further than Neptune) but occasionally stray into the inner solar system where they can interact with the sun and the large planets.

Some comets become trapped within the inner solar system, and their destruction is then inevitable.

As comets are warmed by the sun, their frozen surface layers sublimate and eject material into the surrounding space. The pressure of the solar wind blows on this light material to produce the comet's tail.

Comet 73P was discovered in 1930 by two German astronomers, Schwassmann and Wachmann. Its orbital period was then estimated at about 5.45 years but the comet was lost until rediscovered in 1979 by astronomers at Perth Observatory.

It was next observed in 1990 when it passed within 50 million km of Earth. In 1995 it was seen again and brightened unexpectedly as it proceeded to break into at least four pieces as it rounded the sun. Although in 2001 it never got very close to Earth, the fragmentation continued.

On Saturday the comet will pass within just 11 million km of Earth, giving astronomers their best-ever view of the disintegrating comet.

Recent observations still show four major pieces but they are each shedding smaller lumps almost daily.

The resulting dusty cloud of debris has already been linked by Canadian astronomers to a weak meteor shower, activated as the Earth occasionally ploughs through the thin material.

The orbit of Comet 73P is strongly influenced by Jupiter. Jupiter, currently in Libra, reached its closest point to Earth this year last Friday and can be seen shining brightly in the east during the early evening. Venus is the brilliant white object in the eastern sky in the few hours before dawn. Saturn is due north at sunset and sets around 10pm. Mars now sets at 9pm in the northwest.

* Grant Christie is an astronomy researcher and columnist for the Herald.