Three decades after the first Apollo landing on the moon, the debate between proponents of manned and unmanned space missions has not changed a great deal. But many space scientists who work with robotic satellites, including me, have gradually moved from opposing human spaceflight to a more moderate position. In special situations, we now realize, sending people into space is not just an expensive stunt but can be more cost-effective than sending robots. Mars exploration is one of those cases.

The basic advantage of astronauts is that they can explore Mars in real time, free of communications delays and capable of following up interesting results with new experiments. Robots, even after decades of research to make them completely autonomous, cannot manage without people in the loop. But the question arises: Where should the astronauts be? The obvious answer--on the surface of Mars--is not necessarily the most efficient. At the first "Case for Mars" conference in 1981, one of the more provocative conclusions was that the Martian moons, Phobos and Deimos, could serve as comparatively inexpensive beachheads.

   
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