NASA鈥檚 Stardust spacecraft, which recently returned samples from Comet Wild 2, could be called back into action to study the comet hit hard by the Deep Impact mission in July 2005. Stardust could pick up where the other craft left off 鈥� by imaging the crater produced by Deep Impact, revealing the comet鈥檚 interior structure.
One of Deep Impact鈥檚 main goals was to study the crater gouged out by its 370-kilogram projectile, which slammed into Comet Tempel 1 on 4 July 2005. But the impact generated more dust than expected, and that 鈥� along with an out-of-focus camera 鈥� prevented the mother ship from imaging the newly formed crater.
鈥淭he reason for doing the Deep Impact experiment was to find out how comets are put together on the inside,鈥� says Deep Impact team member Joe Veverka of Cornell University in Ithaca, New York, US.
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In addition to revealing the comet鈥檚 interior composition, studies of the crater should shed light on the comet鈥檚 structure and density. 鈥淚f the impactor hit something that was very hard, it would produce a smaller crater than if it hit something very soft,鈥� Veverka told New 杏吧原创.
Observations of how long particles ejected by the Deep Impact collision remained near the comet suggest Tempel 1 is actually quite soft and spongy. 鈥淏ut it would be nice to verify that by looking at the crater directly,鈥� Veverka says, adding that such information could help scientists plan how to deflect any comets that might be discovered on a collision course with Earth.
Valentine鈥檚 Day rendezvous
Now, Veverka and colleagues are proposing that NASA image the crater with the Stardust spacecraft in a project called ScarQuest. Stardust captured dust from the wake of Comet Wild 2 in 2004 and dropped a capsule full of the prized particles to Earth in January 2006.
The main spacecraft is still orbiting the Sun, moving away from the Earth. But it has enough fuel to fire its thrusters in September 2007 to return back towards Earth in January 2009.
That Earth flyby will put it on course to reach Tempel 1 on 14 February 2011, when the crater produced by Deep Impact will be oriented towards the spacecraft. 鈥淭hings will have settled down and cleared up, so now if we come back, it鈥檒l be easy to see the surface,鈥� says Veverka.
鈥淢ission of opportunity鈥�
By the time it reaches Tempel 1, the comet will have orbited the Sun once since the Deep Impact encounter. The comet is thought to lose about a million tonnes of water vapour through evaporation on each orbit. So scientists can compare images of its surface taken by Deep Impact as it neared the comet during one orbit with those from Stardust during the next.
鈥淪o for the first time, we can see how much of the surface changes as material evaporates,鈥� says Veverka. 鈥淭he question is: is material removed uniformly from everywhere? Or does it come from specific areas 鈥� and if so, what is unusual about those areas?鈥�
Veverka estimates that ScarQuest will cost between $20 million and $30 million. The proposal will be submitted by 5 April as a low-cost 鈥渕ission of opportunity鈥� in NASA鈥檚 Discovery programme. These are designed to use existing spacecraft 鈥� or spacecraft parts 鈥� for new missions. 鈥淲hat鈥檚 neat about them is you can do them for a fraction of the cost 鈥� 10% or less 鈥� of a new mission,鈥� Veverka says.
