Tight budgets are forcing NASA to consider turning off one or more planetary science projects that have completed their primary missions but still have life left in them. In an unprecedented move to save money for other mission extensions, the agency has already proposed killing the Opportunity rover on Mars, which has been hobbled by malfunctioning equipment, and a spacecraft that has been orbiting and mapping the moon. But officials are giving the two teams a chance to make the case for saving their science. The competition that a review panel is considering is tough: It includes the Cassini orbiter around Saturn, the Curiosity rover on Mars, and the two spacecraft orbiting Mars. Congress could extend all the missions if it were to provide more money, but tight spending caps could prevent that.