The last blog entry looked at overall priorities recommended for asteroid and comet missions by the white papers sponsored by the Small Bodies Analysis Group (SBAG), and scientific group that advises NASA. The three separate white papers recommended Discovery class missions ($425M) as their highest mission priorities so that the diversity of these worlds could be explored.
This blog entry quotes from each of the relevant white papers to recommended list the mission priorities within each mission class.
For main belt and Trojan asteroids, the top two priorities for Discovery missions are:
"D/P-type Asteroid Rendezvous: Spectroscopically, these unusual asteroids are very similar to many outer solar system objects (e.g., comets, Trojans, irregular satellites, Kuiper belt objects). According to recent dynamical models, perhaps all of these objects came from a disk of comet-like objects originally located beyond the Jovian planets. D/P-types may be transplanted Kuiper belt objects now located within relatively easy reach of our spacecraft to understand in detail the mineralogy and processes of these primitive objects.
Themis Family and the MB Comets Rendezvous: The Themis family, produced by one of the largest disruption events in MB history, is filled with primitive objects. Family members display C-, D-, and B- type surfaces that could provide a window into the compositional stratification in the parent body and the aqueous alteration processes that may have been common in the early evolution of primitive bodies."
For near-Earth asteroids, the top Discovery mission priorities are:
"Reconnaissance of the population of NEOs: Asteroid rendezvous (which can include a flyby component) missions with imaging and surface-modification capabilities could study the several NEOs in detail, and should be able to connect asteroid taxonomic types with meteorite classes, as did the NEAR mission.
NEO Search from a space-based platform: A space-based discovery platform has clear performance advantages over the same-size ground-based telescope due to the lack of atmospheric absorption (particularly in the IR), weather, continuous operation, and diffraction-limited optical performance. Space-based platforms also have more limited lifetimes, higher costs, and more restricted operating modes than do ground-based systems."
For comets, the priorities are less specific: "Discovery class missions are a vital component of any strategy to understand the nature of comets, as they can address the diversity of comets and their activity mechanisms through flybys of multiple targets and low-cost rendezvous investigations of specific targets, including the characterization and reconnaissance of the best CNSR targets."
New Frontiers Priorities
For main belt and Trojan asteroids, the top two priorities are:
"Trojan Rendezvous: The Trojan asteroids are important targets because they i) represent primitive mineralogies that are not in the meteorite collection; ii) have potential to provide constraints on dynamical models of the early solar system; and iii) preserve evidence of early solar system volatile-rich processes... A Trojan rendezvous with spacecraft equipped for geochemical remote sensing and in-situ surface probes/landers can significantly advance our knowledge of this link between the terrestrial planets and the outer solar system.
Multiple Flybys of MB: Some of the science goals of a rendezvous mission such as shape, surface processes, and mineralogy, and evolution of these objects may be at least partially achieved via a fly-by mission that targets multiple objects representing a range of poorly understood spectral types. While the spacecraft may be relatively simple, scope, length and complexity of the operations would put it into the NF class."
Sample return: "The highest priority is a sample return from a volatile-rich object not known to be represented in meteorite collections. A P-, D-, or W-class object would be optimal, followed by the other “wet” classes such as B, C and G. For targets that are well-characterized by ground-based spectroscopy and imaging, as well as in-orbit characterization before sampling, existing technologies will be able to return samples."
Grand Tour: "The second priority is a Grand Tour mission to rendezvous with a number of NEOs of a variety of classes. Only a spacecraft mission is capable of elucidating the distinctions between the various compositional classes and providing the imaging detail needed to understand the details of formation of difference physical types. Such a spacecraft could have several (perhaps three or four) penetrators, microlanders, or similar low-cost easily-deployed surface exploration modules. This mission could be at a somewhat lower cost than the sample return mission."
"We reiterate the finding of the previous Decadal Survey that the return to Earth for analysis of a sample from the surface of a comet’s nucleus remains a critical component of NASA’s systematic investigation of comets. The complexity of such a mission pushes it into the New Frontiers class, but the potential scientific return justifies this larger investment. In particular, a Comet Surface Sample Return (CSSR) mission will reveal the complexity of cometary organics, and whether comets could have provided pre-biological material to the Earth and other planets." [In this mission concept, the samples are not kept frozen, so any ices will melt and possibly undergo chemical change during the return to Earth.]
Only one flagship (>$1B) mission is a priority, a comet sample return that keeps the collected ices frozen on their return to Earth. "The holy grail of cometary spacecraft missions is the return to Earth of a cryogenic sample extracted from deep (> 1 m; the deeper, the better) within a nucleus, which is referred to generically as the Cryogenic Nucleus Sample Return (CNSR) mission. Owing to the complexity and technical challenges associated with CNSR, it is commonly assumed that such a mission will fall into the NASA Flagship class...We strongly recommend that NASA invest in a detailed study of the technical feasibility and cost of a CNSR mission during the next decade (2011-2020), with a goal of enabling such a mission in the following decade (2021-2030)."
You can find all the SBAG white papers and white papers on these bodies submitted by the rest of the science community at http://www.psi.edu/decadal/
Note: The main belt-Trojan asteroids white paper listed a number of additional lower priority missions for both the Discovery and New Frontiers class missions. Please check out that white paper if you are interested.