A balloon-shaped robot explorer that one day could search for evidence that water existed on other planets has survived some of the most trying conditions on planet Earth during a 70-kilometer (40-mile), wind-driven trek across Antarctica. The tumbleweed rover, which is being developed at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., left the National Science Foundation’s Amundsen-Scott South Pole Station on Jan. 24, and spent the next eight days rolling across Antarctica’s polar plateau.