Until now, only two comets have been examined at close quarters by instrument-laden spacecraft. In September 1985, in the first encounter of its kind, the International Cometary Explorer flew through the tail of Giacobini- Zinner, a very old, quiet comet. The spacecraft carried no camera and simply measured electrical and magnetic fields.
In March 1986, Giotto swooped past Halley's Comet, barely surviving prolonged exposure to an intense rain of fast-moving particles as it sent back the first close-up pictures of a comet's nucleus. On Friday this redoubtable explorer, which was built by British Aerospace and a team of contractors from nine other countries, will be subjected to a second, probably fatal, dose of the same medicine.
The latest target for Giotto's instruments will be Comet Grigg- Skjellerup, named after its co-discoverers. It was first seen by John Grigg, a New Zealander, in 1902 but was subsequently lost until May 1922, when it was recorded by a South African, John F Skjellerup. When it was realised that the two sightings were of the same object, the work of both men was given equal recognition and the comet duly acquired its ungainly name.
Comets are of particular interest because they are thought to contain the primeval raw materials from which the planets were formed four-and-a-half billion years ago.
Just like moths attracted to a flame, these small balls of snow and ice shorten their active lives each time they return to the infernal regions near the Sun. Their surface layers evaporate, creating wispy tails that stream millions of kilometres into space.
However, Giotto's two cometary targets could hardly be more different. Grigg-Skjellerup approaches the inner solar system every five years, while Halley's much-trumpeted visits to our skies occur once every 76 years. Whereas Halley's much larger, relatively pristine nucleus still retains sufficient icy material to present magnificent celestial illuminations, Grigg-Skjellerup has lost its youthful vigour.
According to Andrew Coates of the Mullard Space Science Laboratory in Surrey, it is much smaller and older than either of the comets so far visited. Its mile-wide nucleus is only hidden by a small coma of dust and gas because most of its volatile ingredients have long since boiled away.
Although Dr Coates expects Grigg-Skjellerup to develop small versions of the dust and plasma tails normally associated with its more active brethren, he believes that its interaction with the surrounding environment, especially with the solar wind of charged particles emitted by the Sun, will be much less marked.
The nature of its orbit also makes Grigg-Skjellerup different. It belongs to a fairly small group of about 100 comets that orbit the Sun in the same direction as the planets. This means that Giotto will be able to overhaul its target at the relatively leisurely pace of nine miles per second.
In contrast, Halley's Comet travels in the opposite, or retrograde, direction around the Sun. Giotto and Halley's Comet hurtled past each other at a velocity of 42 miles per second, like two supersonic aircraft.
Nevertheless, Giotto's unique vantage point, 375 miles from Halley's nucleus, enabled it to unveil the black, encrusted surface and brilliant gaseous eruptions taking place at the heart of the comet. For the first time, the true nature of one of the solar system's building blocks was revealed.
Not surprisingly, few scientists expected the craft to survive the ordeal as it was bombarded by dust particles travelling 50 times faster than the speed of a bullet. These fears seemed to have been borne out when the screen at mission control went blank one minute before the closest approach. In fact, Giotto was knocked spinning by an impact with a 'large' dust particle of about a gram in mass. More than half an hour passed before reliable contact was re-established.
After a preliminary check of the spacecraft's condition, it was put into hibernation until its orbit would carry it back to the vicinity of Earth and a decision could be made about targeting it towards a second comet. Contact was re-established on 19 February 1990 when Giotto was about 60 million
miles from home. Ground controllers were able to turn the on- board main antenna towards the Earth in order to increase the signal strength, and used Giotto's telemetry data to check out its instruments.
Considering the pounding undergone by the craft, scientists at the European Space Operations Centre, in Darmstadt, Germany, were pleasantly surprised by the results of their health check, although damage to the thermal blankets and much of the outer shell had produced a marked increase in spacecraft temperature. Sadly, the baffle on the colour camera seemed to have been dislodged, preventing it from sending back any more spectacular images. However, plenty of fuel remained to manoeuvre the craft for a second cometary flyby, and seven of the eleven scientific experiments were still fully or partially operational.
The opportunity was too good to miss. European Space Agency member states were invited to contribute dollars 12m ( pounds 6.3m), in return for which they would be able to take advantage of a rare, low-cost opportunity to study a comet at close quarters. Approval for the attempt was given in June 1991.
Giotto completed another historic first on 2 July 1990 when it passed within 14,500 miles of the Earth and became the first spacecraft to utilise our planet's gravity to change orbit. As a result of this gravitational assist, Giotto was flung towards Comet Grigg- Skjellerup, then returned to hibernation.
The final reawakening came on 7 May 1992. With the aid of the most powerful radio telescopes available - the 70m (230ft) diameter antennae of the American Deep Space Network - contact with Giotto was re-established at a distance of almost 140 million miles.
Despite the loss of the camera system, scientists hope to gain insights into the nature of the dirty snowball and its enveloping coma. The dust impact detection system and the optical probe experiment will measure dust densities, size and distribution. Analysis of the solar wind and the various other charged particles in the coma will be carried out by several instruments, including a plasma analyser provided by the Mullard laboratory, while the magnetometer will study the interaction between the comet and the surrounding environment.
If all goes according to plan, Europe's first interplanetary probe will end its operational life in a blaze of glory as it passes within 300 miles of the comet's nucleus. Giotto's suicide mission will pave the way for an even more ambitious quest soon after the turn of the century: to land on a comet and retrieve a sample for analysis back on Earth.