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stunning spacecraft seeks comet. Should be small and highly active, and reachable by launch from Earth within the next 2.5 years. Rendezvous must require no major engineering works, and incur few additional costs. Technically risky options need not apply.

Europe's comet scientists are this week struggling to determine a new route — and probably a new target — for Rosetta, the world's largest and most ambitious comet mission, whose postponed launch last week saw it miss the January window that would have allowed the probe to land on the comet Wirtanen in 2010 (see Nature 421, 198; 200310.1038/421198b).

The scientists now face a formidable challenge if they are to realize Rosetta's exacting goals. The truck-sized craft must be lobbed into the outer reaches of the Solar System, and accelerated to the 9 kilometres per second needed to catch up with its tiny target, a dirty snowball barely a few kilometres across. The probe then has to orbit the comet, plant a lander on it, and hang on while the comet arcs around the Sun.

The sooner Rosetta is launched, the less the postponement will cost the European Space Agency (ESA) — most costs are derived from retaining the design and development teams until the launch. But most of the earliest launch possibilities for Rosetta carry major technical risks.

Two dates later this year would use Venus, instead of Mars, to provide a gravitational 'catapult' that would enable Rosetta to catch up with its original target, Wirtanen. But these options are unlikely to be selected, according to Gerhard Schwehm, who leads the core Rosetta project team from ESA's European Space Research and Technology Centre (ESTEC) in Noordwijk, the Netherlands. The probe was designed for cold space, rather than the cauldron of the inner Solar System. “The heat load is too high for some of the spacecraft's subsystems to survive,” he says. David Southwood, ESA's director of science agrees: “You can't just install a fan,” he quips.

An explosion of the Ariane launcher caused the Rosetta comet probe to miss its launch window. Credit: J. AMIET/AFP

A January 2004 launch window, on the other hand, would retain both the Mars pass and the 2010 Wirtanen rendezvous. This particular flight plan would call for a rocket launch velocity that is higher than previously planned, however. That would leave mission planners facing the unenticing prospect of using the Ariane 5 'heavy lifter' that blew up in December — the incident that created the problem in the first place. Schwehm prefers to stick with the original launcher, especially as there is no guarantee that the souped-up model will be flying safely by January 2004.

That leaves scientists with what might be their best bet: shooting for another comet from the Jupiter family, so called because its members' orbits cross that of Jupiter and are influenced by the planet's gravitational pull. The likely target will be in a phase and orbit where it is producing a lot of hot gas and dust, says Schwehm.

This will involves reviewing reams of existing observations, and making new ones. Problems with the various options include possible design changes needed to the probe, and how the size and surface of the target comet will affect the lander's descent plan.

Rosetta's planners currently seem to favour a spring 2004 launch that would allow targeting of the comet Churyumov–Gerasimenko; other possible windows would allow visits to the comets Howell, Finlay, Schwassmann–Wachmann 2 and Wild-2. Launch dates for these options cannot be firmly defined until scientists at ESA's Operations Centre in Darmstadt, Germany, take ESTEC's proposals and work out the various possible trajectories to get the craft to its potential targets. But these targets might not be reachable until 2015 — a painful delay for those involved in a mission for which planning began in 1993.

Rosetta teams will meet on 13 February to consider possible solutions to the problem, before presenting them to ESA's Science Programme Committee on 26 February. But the crunch will come in May, when ESA will select one of the options — and decide how to pay for it. Southwood estimates that the delay will inflate the mission's €1-billion (US$1-billion) price tag by a further €50 million–100 million.

ESA may ask its member states to make a one-off contribution to cover this, although with national space budgets under pressure, this may not be forthcoming (see previous page). That could force the agency to make cuts elsewhere in its science programme — possibly including the cancellation of Venus Express, ESA's planned 2005 mission to Venus.