Sitting at a three-dimensional computer display in mission control for the Mars rover, engineer Ashitey Trebi-Ollennu is as close as any human can get right now to working on the surface of another world. Even as President Bush announced plans last week to send humans to the moon and Mars, the successes of scientists controlling the robotic probe Spirit offered perhaps the strongest argument to date that sending humans on long missions of exploration may be not only expensive, but also unnecessary.