Tuesday, September 29, 2009

Comet Coma Rendezvous Sample Return (CCRSR) Mission Concept

This blog entry continues both at looking at white papers on missions to the solar system's small, primitive bodies and to continue the occasional series of mission concepts in development to use ASRG plutonium power supplies. In the former series, you'll remember that the number one priority mission for comet exploration in the coming decade is a warm sample return (more on the warm part, later). In the latter series, NASA has developed a new electrical power supply system that uses much less plutonium that the old RTGs or MMRTGs. The agency is ea gar to test the ASRGs on a flight mission, and has funded 12 mission concept studies to explore what types of Discovery missions ($450M) these power supplies would enable.

Comets are believed to be the least altered bodies in the solar system, preserving both the mineral (in dust form) and volatiles present at the birth of our solar system. Returning samples to be studied in terrestrial laboratories has been a high priority for the scientific community. The Stardust mission partially fulfilled that goal by collecting dust particles during a high velocity flyby of comet Wild 2. The brief nature of the encounter and the high velocity limited the number of dust particles collected and the types of particles that could survive the high speed impact with the sample collectors.

Ultimately, the goal is to land on a comet, collect a sample containing both dust and volatiles, and return the frozen sample to year. Frozen is the key. Volatiles that melt will undergo various chemical reactions that will alter the samples. Unfortunately, such a mission is bedeviled by questions of both how to collect the samples (what is the surface a comet like and once we land on one, is that a good guide to all comets?) and of how to keep samples frozen well below the freezing point of water within a small re-entry capsule (even during the descent through Earth's atmosphere). This class of mission has been put off to the following decade with a goal of advancing the technology in the coming decade.

An alternative mission would collect a sample and allow the volatiles to warm during the return voyage and plummet through Earth's atmosphere. The dust particles would be unaffected by the warming, and the melted and altered volatiles would still provide clues. Such a mission might be possible in the New Frontiers program ($650M).

CCRSR takes a different approach. It would not land on a comet, but would instead make multiple passes through a comet's coma and jets. The encounter with dust particles would be at low speeds, preserving fragile samples. No volatiles would be collected, but a mass spectrometer would measure their composition in real time. By sampling different jets, the mission may be able to sample different portions of the comet's interior. Multiple collectors will be used so samples from specific jets can be identified. In addition to the mass spectrometer, adust detector (to estimate amounts of samples collected) and wide and narrow angle cameras would be flown.

Editorial Thoughts: This proposal gets around a key problem of sampling any small body: given the wide range of surface materials and surface densities possible, how do you intelligently design a collection system? It falls short of the hoped for goal for this decade to return both dust and volatiles (even if warmed over). Discovery mission opportunities, however, are more frequent than New Frontiers opportunities, so this mission could meet much of the goal at a lower cost and with better chance of selection.

A key goal of the next Discovery mission selection may be to test the ASRG system. Because CCRSR returns to Earth, it would have to jettison its ASRG's in deep space and make the return voyage using solar panels. The white paper suggests that the mission might be possible with only solar panels -- a possible knock in the coming selection.

Link: The Comet Coma Rendezvous Sample Return (CCRSMission Concept - The Next Step Beyond Stardust

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