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The first sample return mission from a comet has been achieved by NASA's Stardust spacecraft, and the first sample return mission from an asteroid has been achieved by JAXA's Hayabusa spacecraft. What are the main challenges and difficulties for such missions, and how much do these types of missions differ from each other regarding difficulties encountered (for example spacecraft different velocities, technical or technological features)?
Hayabusa spacecraft, JAXA.
There is a lot that the two missions you mentioned--Stardust and Hayabusa--have taught us about sample return.
However, despite these precautions the Stardust spacecraft outgassing was sufficient to degrade camera operations, and the aerogel capture media was significantly contaminated during manufacture. We also never completely solved the problem of defining useful limits for organic contaminants of spacecraft hardware, which haunts us as we rather unexpectedly captured primitive cometary organics.
Landing Conditions and Sample Recovery: The landing site for Stardust in the Utah Test and Training Range was unexpectedly wet and the lack of a canister seal for the samples became an issue.
Sample Curation: The necessary preparations for managing the returned sample take a long time--longer than expected:
More than two full years were required to prepare curation facilities for Stardust. Despite this seemingly adequate lead time, major changes to curation procedures were required once the actual state of the returned samples became apparent. Two years of Curation preparation are insufficient. The Stardust sample database was not fully implemented before sample return –we did not adequately think through all of the possible sub-sampling and analytical activities before settling on a database design. Remote storage of a sample subset is critical, for Stardust the remote samples are in a vault in New Mexico.
We didn't know what Itokawa's surface would be like before the spacecraft actually arrived there. Indeed, the assignment given to us was quite broad. We were to build a device that, no matter what the surface was like, would grab samples, ranging from monolithic bedrocks to powdery regolith, in the brief instant of each of the spacecrfat's [sic] touchdowns.
Because the sampler horn is attached to the edge of the spacecraft, if the horn were to contact the surface of the asteroid for too long, the spacecraft would start tumbling on its center of mass. In other words, if the spacecraft did not ascend right after the touchdown, it would lose its balance and fall on its rear. Thus samples would have to be collected in less than one second, by firing a bullet at a speed of 300 meters per second.
Sources:
To get samples from a comet, the spacecraft may fly through the tail to collect some tiny dust parts. To get samples from an asteroid, the spacecraft must land, collect samples and then leave the orbit of the asteroid and fly back to earth. A lot of fuel is needed for landing and accelerating again to get back to earth. High precision navigation is necessary for a smooth landing.
