The best scientific payoffs require venturing to the harshest environments, where blistering volcanoes or subarctic icescapes could instantly transform multimillion-dollar machines into scrap metal.

One bold proposal comes from a team led by planetary geologist Ellen Stofan at the technical consulting firm Proxemy Research. The Titan Mare Explorer, or TiME, is a 500-pound robot that would take to the seas of Saturn's giant moon Titan, the only solar system body other than Earth that has liquid on its surface. The probe, powered by a newly developed battery made of plutonium, would have a protective outer shell to shield it while it bobs along in a -300 degree F lake of liquid natural gas. Titan's gentle winds will push TiME along as it snaps photos and measures the moon's rich mix of organic compounds. "A lot of the chemistry on early Earth involved the same chemicals, but Titan has them in a deep freeze," Stofan says.

NASA will decide TiME's fate next year. Meanwhile, the agency is mulling missions to the slope of a volcano on Venus and an ocean miles beneath the icy surface of Jupiter's moon Europa.