Comets are suddenly in vogue in space research. ESA is getting ready to send its comet chaser Rosetta in January 2003 to rendezvous with Comet Wirtanen and study it in immense detail. Rosetta aims to physically drop a lander on a comet for the first time. Before that, however, NASA will dispatch its CONTOUR spacecraft to fly past at least two comets, and it has two other small comet missions planned.

What makes comets special is that they contain raw materials left over from the birth of the Sun and the planets. Finding out what comets are made of gives scientists priceless clues to both the origin of the Earth and the origin of life. It is also important for planning possible defences, if a comet should threaten to collide with the Earth, as Comet Shoemaker-Levy 9 did with the planet Jupiter in 1994.

Comets have always been attractive to scientists and to the general public. However, they are elusive objects and catching one is very difficult. For this reason, different space projects have different aims. In 1986, two Japanese, two Soviet and one European spacecraft flew past Halley's Comet. ESA's Giotto went closest to the nucleus of the comet. It sent back wonderful pictures and data for scientists to analyse. Although damaged by Halley's dust, Giotto went on to fly even closer to Comet Grigg-Skjellerup in 1992.

This artist's impression shows Rosetta, its lander and Comet Wirtanen. Credit: ESA

In the follow-up, ESA has one major project, Rosetta, and NASA three small ones, Stardust, CONTOUR, and Deep Impact. Stardust is already on its way to gather dust from close to Comet Wild and return it to the Earth. CONTOUR, leaving shortly, will make fast but very close fly-bys of Comets Encke and Schwassmann-Wachmann 3, investigating why comets can be so different from one another. Deep Impact, due for launch in January 2004, will shoot a large copper ball into the nucleus of Comet Tempel 1. Its fireworks show, on 4 July 2005, will scatter subsurface comet matter into space for analysis by telescopes back at the Earth.

ESA's Rosetta, however, is the one milestone mission that comet scientists have wanted since the Space Age began. It will fly past Comet Wirtanen, go into orbit around its nucleus, and drop an instrumented lander on it. Named after the famous stone with inscriptions that held the key to understanding ancient Egyptian civilisation, Rosetta will cruise alongside the comet for 17 months while Wirtanen nears the Sun. Unlike previous brief impressions, Rosetta promises to give us the first complete picture of a comet's composition and behaviour.

Comet scientists on both sides of the Atlantic are already cooperating fully. For example, Jochen Kissel of Germany's Max-Planck-Institut for extraterrestrische Physik is responsible for the comet dust analysers on CONTOUR and Stardust, as well as on Rosetta. He is a veteran of the Soviet and Giotto missions to Halley's Comet. The CONTOUR science team has Gerhard Schwehm, who is also ESA's project scientist for Rosetta.

"We're all after the same knowledge," Schwehm comments "What we learn from the NASA missions will help us to be even better prepared for our big task at Comet Wirtanen. So all of us in ESA's Rosetta team say, 'Bon voyage, CONTOUR, and happy comet chasing!' "