Exactly 15 years ago today, the European Space Agency’s Giotto spacecraft lifted off from Kourou
at the start of an epic adventure – a rendezvous with the most famous chunk of dirty ice in the
Universe, Halley’s Comet.
Without Giotto’s pioneering flybys of Halley in 1986 and Comet Grigg-Skjellerup in July 1992, ESA’s Rosetta mission to orbit Comet Wirtanen would not have been possible. Indeed, some of those same scientists who participated in the Giotto mission are now playing a leading role in the Rosetta programme. Even today, they remember the accomplishments of the little probe with awe and pride.
One of these is Rosetta Project Scientist Gerhard Schwehm, who acted as Deputy Project Scientist for Giotto in 1985 and later took over as Project Scientist for the Grigg-Skjellerup encounter. Schwehm considers that our mental image of what comets are like was conditioned by the gallery of images returned by gallant Giotto on what was expected to be a suicide mission.
“The only close-up images we have of a comet nucleus, even today, were returned by Giotto,” said “Everyone still has these images in mind, 15 years later.”
However, Schwehm also believes that Giotto’s impact stretched far beyond revolutionising our knowledge of cosmic dirty snowballs.
“It was the European Space Agency’s first planetary mission,” he explained. “This gave a great boost to planetary science in Europe and created a very strong scientific community, a heritage from which Rosetta is benefiting. And, of course, other missions have also gained from what we learned.” Other leading members of the European planetary science community whose careers span both Giotto and Rosetta, are Hans Balsiger from the Physikalisches Institut at the University of Bern (Principal Investigator for the ROSINA experiment on Rosetta), Jochen Kissel from the Max-Planck-Institut f¸r Extraterrestrische Physik (Principal Investigator for the COSIMA experiment), and Uwe Keller from the Max-Planck-Institut f¸r Aeronomie (MPAe) in Germany, who is responsible for the OSIRIS experiment.
Back in the 1980’s, Uwe Keller headed the development of the multi-colour camera on Giotto that provided the first glimpses of Halley’s nucleus – a black, potato-shaped object spewing bright jets of dust and gas from its sunlit side.
“Giotto showed us that the old picture of comets as icy objects was wrong,” said Keller. “We now know that a comet is mostly composed of dust in which ice is embedded.”
Significant as Giotto’s discoveries were, they also left many unanswered questions.
“Less than 1% of a comet’s surface may be active, but somehow the activity persists,” Keller said. “Is there a solid crust on top of the active ingredients, and how does this layer subdue jet activity? We just don’t know, but with Rosetta, we should be able to unravel the great mystery of what happens to the ice in a comet as it approaches the Sun.”
“Rosetta will allow us to observe and interpret the physical processes that are taking place as it flies with a comet all the way from inactivity to full activity,” he explained.
One of the main instruments that will monitor this changing behaviour is the new OSIRIS camera that is being built by a European consortium led by Dr. Keller and is currently undergoing electrical testing at the European Space Technology Centre in The Netherlands.
One of the main differences between the camera system on Giotto and the next generation instrument, OSIRIS, is that the earlier version had to take crisp images from a spacecraft that was continually spinning. On Rosetta and most other planetary missions, the camera is located on a three axis stabilised spacecraft that is very stable and can be easily aimed in the required direction.
“Giotto’s multicolour camera was an extremely complex instrument,” explained Keller. “It had more onboard software than the spacecraft and the other instruments put together so that it could acquire the comet and follow it.”
Unfortunately, Giotto obtained very limited coverage of Halley’s nucleus due to the illumination conditions and the loss of the camera due to dust impacts just prior to closest approach. Its groundbreaking images will be greatly surpassed when Rosetta slips into orbit around the nucleus of Comet Wirtanen in 2011. The entire surface of Comet Wirtanen will then be photographed with a resolution of one metre or less under varying lighting conditions.
“From a height of about 1 km, the narrow angle camera on OSIRIS will be able to resolve objects only 4 cm across on the surface of Wirtanen’s nucleus,” said Keller. “Because we will be so much closer, the resolution will be up to 1000 times better than we obtained from the camera on Giotto. Rosetta will also carry a wide angle camera to image the coma of dust and gas, something Giotto was not able to do.”
