Five years after losing half its instruments battling through the tail of Halley's comet, the Giotto spacecraft is limping towards an extra rendezvous. The financially troubled European Space Agency has finally agreed to pay for Giotto to visit comet Grigg-Skjellerup next July.
Giotto got within 600 kilometres of Halley's nucleus in 1986, giving the first close-up information on a big comet. But in the final seconds of the encounter, dust and rocks in the tail, moving at 240 000 kilometres an hour, smashed through the spacecraft's shield. The collision knocked out the camera, battery and other key components.
Now, Giotto is spending the last gasps of the gas it uses for manoeuvring to edge towards the Sun and its second encounter with a comet.
'It is battered but unbowed,' says Alan Johnstone who designed one of Giotto's instruments at the Mullard Space Science Laboratory in Dorking, Surrey. 'Instruments projecting out of the spacecraft took a beating. But it is still quite operable.'
Still working are three instruments for looking at charged particles, two dust detectors, and a magnetometer for measuring turbulence in the comet's magnetic field.
Giotto was built for a single mission to Halley's comet. But an unexpectedly accurate launch put the craft into the right orbit leaving it with an excess of manoeuvring gas. As Giotto homed in on Halley's comet in 1986, the ESA decided to try and use the spare gas to fly through the tail and on to Grigg-Skjellerup.
Two seconds before reaching the point where it came closest to Halley, Giotto went dead, the result - the ESA believes - of a flash of radiation caused by a heavy collision. Twenty-one seconds later, the spacecraft, or what was left of it, was operating again. Only 600 grams of the craft was missing, but half the instruments were out of action.
Since 1986 the ESA's finances have been squeezed, putting a question mark over Giotto's added mission. Although the craft is free, technical support on the ground will cost £3 million - money the ESA has now approved. Grigg-Skjellerup is smaller than Halley, with less dust and gas. It orbits the sun every five years. The ESA wants to see how its tail compares with Halley's twin tails of blue plasma and white dust.
On its second rendezvous on 10 July 1992 Giotto will probably suffer further damage. Its shield of aluminium and the light-weight composite, kevlar, is riddled with holes. The shield will also be pointing in the wrong direction in order to allow its solar panels to face the Sun. Without a battery, experiments and transmissions are dependent on power from the panels.
As Giotto flies out of Grigg-Skjellerup's tail, ground control will direct its remnants towards a final resting place, a stable solar orbit.
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