But with U.S. federal spending teetering on the “fiscal cliff,” the space agency's White House overseers are not ready to commit to a mission that is pointless without an expensive follow-on to develop a way to get the samples back.

Instead, NASA is setting up a “science definition team” to decide how to use the proposed Curiosity-derived rover when it reaches the surface of Mars in 2020. The instruments that will ride on the big rover will be chosen in an open competition based on that team's work, starting with an announcement of opportunity next summer. Like Curiosity, the new rover will again use the “sky crane” technique to touch down.