ASRG. These power systems use a quarter the plutonium of previous designs, allowing NASA to consider using is small supply of plutonium for small missions. Previous entries summarized the Io Volcano Observer and the Venus VALOR balloon mission.
The Comet Hopper (CHopper) is a study proposal led by Dr. Jessica Sunshine of the University of Maryland. (Perhaps not the best name for an investigator proposing to use a plutonium power supply instead of panels that utilize sunshine. :> ). This proposal addresses two key issues regarding comets: (1) the nature of their surface changes across the comet nucleus and (2) the rates (and perhaps, types) of activity they display changes as they move to different positions in their orbit around the sun. The ESA Rosetta mission will partially address these issues. It, however, will place a lander only at a single location on the nucleus and will follow its comet for only a portion of its solar orbit (1.5 years out of a 6.5 year orbit).
Because it uses plutonium as its power source, CHopper would have two advantages over Rosetta. First, a single craft can be both a lander and an orbital craft. If the mission used solar panels, it would be virtually impossible for it to do both. (Think about managing repeated landings on a comet with Rosetta's 14 m solar panels extending from either side of the craft!) Second, because it doesn't need solar power, it can operate when the craft and comet are far from the sun.
The CHopper team has published a short summary of their mission. I'm reproducing the introduction here (using their words minimizes my mistakes), but I encourage you to read the entire one page summary.
"The Comet Hopper (CHopper) mission explores the compositional and morphologic heterogeneity of a comet. Recent cometary flybys (Deep Impact at P/Tempel 1, Stardust at 81P/Wild 2, and Deep Space 1 at 19/P Borrelly) have revealed great diversity among comets as well as significant variation within individual bodies. Understanding the inherentdiversity of a comet nucleus and the origins thereof are now a clear objective of future cometary exploration. CHopper is an instrumented lander that will build upon the results of these recent missions. With a 2012-2013 launch Chopper will examine in detail the inner coma and surface of comet P/McNaught 2 (P/2004 R1).
"CHopper observes the comet while formation flying over one full orbital period obtaining measurements during the descent to, and on the surface of, the nucleus. CHopper takes advantage of the low cometary gravity field to take off and land (“hop”) multiple times during each descent to the surface. ... six “sorties” to the surface are envisaged to investigate changes with heliocentric distance."
My take on this proposal is that it is a clever design that is enabled by an ASGR power source. I would like to see this mission fly. How well it does in the proposal competition (if NASA opens the next Discovery competition to ASRG designs) will depend on the details of the design: What is the design risk? Do the instruments address the key science questions? Can the mission be implemented within a Discovery program budget?