An artist's impression of Giotto studying Halley's Comet. Photo: ESA

Almost exactly 15 years ago, during the night of March 13/14, 1986, ESA's Giotto spacecraft made history by obtaining the first close-up pictures of a comet's black, icy nucleus. After surviving a battering from grains of comet dust, Giotto went on to become the first spacecraft to visit a second comet. The flyby of Comet Grigg-Skjellerup in July 1992 is still the closest encounter ever achieved with one of these cosmic icebergs.

At a recent two-day 'Giotto Heritage' symposium held in The Science Museum, London, scientists and engineers who worked on this pioneering deep space mission came together to reminisce about past triumphs and to look forward to the next generation of comet explorers.

Knowledge gained from Giotto, ESA's first 'deep space' probe, is currently being used to develop the Rosetta comet probe, which is scheduled for launch in January 2003.

Project scientist Gerhard Schwehm, who also played a leading role in the success of Giotto, explained to the audience how Rosetta will make its own unique contributions to Solar System exploration. Most notably, it will become the first spacecraft to orbit a comet at close quarters, and the first to deploy a lander onto the surface of a comet nucleus.

Dr Schwehm began by explaining a little of Rosetta's historical background. The mission was born in Zurich on May 22, 1985, when it was accepted as a Cornerstone science project by the ESA Space Science Advisory Committee. It was then known as the Comet Nucleus Sample Return mission (CNSR) and was envisaged as a collaboration with NASA. After a few years NASA's priorities changed, so the highly ambitious sample return mission evolved into a slightly less complex ESA-only mission dedicated to a comet rendezvous and two asteroid flybys.

At the end of 1987, the project received the name 'Rosetta', after the famous Rosetta Stone that now lies in the British Museum in London. In the same way that the Rosetta Stone enabled historians to decipher the mysterious hieroglyphic writing of ancient Egypt, so Rosetta will enable scientists to learn about the most primitive matter in the Solar System - the building blocks from which the planets were created.

"Comets are the messengers that give us access to the early Solar System and the way it evolved," said Dr Schwehm. "They are believed to have survived almost unaltered from the primitive protosolar nebula. This, in turn, contained material derived from the interstellar medium and giant molecular clouds."

So, 15 years after the Halley flyby, what is the relevance of the Giotto mission for Rosetta?

"Rosetta was a logical progression from the initial reconnaissance by Giotto," explained Dr Schwehm. "Giotto showed us for the first time what a comet nucleus looked like. Now, instead of a short fly past of the nucleus, Rosetta will be able to study the comet from close range for more than one and a half years and land a small spacecraft on its surface."

In this way, Rosetta will not only tell us about the origin and evolution of comets, but it may contribute to the debate over the origins of planetary atmospheres and the evolution of life.

Continuing on the theme of the Giotto heritage, Dr Schwehm explained some of the similarities between the two missions.

"Like Giotto, we have a fixed launch window - 20 days in the case of Rosetta," he said. "If we miss this, we will not be able to reach our target, Comet Wirtanen, so we would have to wait one or two years before another suitable comet came along."

"We also know that we will have to put Rosetta into hibernation for long periods during the eight-year journey to Wirtanen," he said, "but we are confident that we can do this because of our previous experience with Giotto."

"The Giotto heritage is important in other ways," he added. "The community of planetary scientists in Europe was quite small back in the eighties, so it is not surprising that many of those who worked on the earlier mission are also involved in Rosetta. However, Giotto encouraged scientists from other fields, such as plasma physics, into planetary science."

"Furthermore, the instruments on Rosetta have a lot of common elements with their Giotto predecessors," he said. "And last but not least, there is a European industrial heritage linking the two generations of comet probe," said Dr Schwehm. "Despite corporate amalgamations and changes of name, a similar industrial team has been involved in both missions, since Astrium includes many of the companies that worked on Giotto."

"This broad heritage increases our confidence that Rosetta will follow in Giotto's footsteps and become a tremendous success," he concluded.