Giotto, which skimmed past Halley's Comet six years ago, is due to make another cometary encounter at 4.30pm British Summer Time this afternoon. It will pass by the comet Grigg- Skjellerup, a mile-wide lump of ice and dust orbiting the sun.
Scientists at the European Space Agency's control room in Darmstadt, Germany, yesterday sent the final instructions to the Giotto probe to prepare it for a last mission. Some of the scientific instruments on Giotto, a cylinder of electronics 1.84m (1 yard 2ft) in diameter and 1.6m (1 yard 1ft) long, were damaged during the Halley encounter but enough are functioning for researchers at Darmstadt to learn more about the structure and make-up of comets such as Grigg-Skjellerup.
Professor Alan Johnstone of the Mullard Space Science Laboratory at University College, London, said that studying comets was important because they represented the earliest phases of the birth of the solar system.
'They've spent most of their lives far away from the sun and so are in much the same form as they were 4 to 5 billions years ago. They are little remnants from the birth of the solar system.'
Two British instruments, funded by the Science and Engineering Research Council, will analyse the comet's cloud of dust and charged particles that stream out from the icy core. Professor Johnstone said events taking place around the comet would help physicists understand the processes involved in nuclear fusion. 'What happens when you inject material into a fusion test reactor is a direct analogy of a comet.'
One difference between today's encounter and that with Halley six years ago is that scientists will be unable to see the action because of a defective camera. Comet Grigg-Skjellerup, named after the two astronomers who detected it early this century, is one of the Jovian comets because its orbit is frequently upset by the gravitational pull of Jupiter.
The last time this occurred, in 1964, the path of the comet was disturbed enough for it to pass just inside the Earth's orbit, which also meant it would pass close to Giotto.
Scientists reactivated Giotto in May and yesterday made last- minute adjustments that would bring it to within about 220km (354 miles) of the comet. Professor Johnstone said: 'There is about a one in 50,000 chance of it hitting the comet's nucleus.'
Ten minutes before the encounter, Giotto is expected to detect the shock wave caused by the movement of the comet through the solar wind. 'There is a possibility the spacecraft will suffer a power failure,' Professor Johnstone said.
The encounter will not be visible from Britain.