On Valentine's Day 2012, NASA will revisit a comet it already injured once. The Stardust probe will rendezvous with comet Tempel 1, into which NASA crashed the Deep Impact probe on Independence Day five years ago. Stardust already encountered Comet Wild 2 and collected some samples, returning them to Earth in 2006.
Cassini is the size of a small school bus. Imagine a Robot that big wheeling (or walking around mars). You could shove every instrument you ever needed on it. one step at a time I suppose. This new rover looks promising.