George W. Bush seems driven to complete his father's unfinished business in space, as in Iraq. But much has changed. The cold war, which provided the initial motivation for our space program, is long gone. And technological progress has superseded human space exploration. Remotely controlled instruments have become natural extensions of frail human bodies.

Much of what we yearn to discover in space is inaccessible to humans. Astronauts on Mars, locked in their spacesuits, could not venture far from shelter amid the constant bombardment of energetic particles that are unscreened by the thin atmosphere. Beyond Mars, there is no place humans can go in the foreseeable future. The great adventure of the 21st century will be to explore where no human can possibly set foot. The great quest is to find life to which we are not related. Could nature have solved the problem of life in some other way, in some other place? When we find out, we will know much more about ourselves.

Two mechanical geologists, Spirit and Opportunity, are doing this even now, by searching for evidence of water on opposite sides of Mars. They don't break for lunch or complain about the cold nights, and they live on sunshine. They've been at it for nearly two years, yet their mission costs less than sending a shuttle to the International Space Station. The brains of Spirit and Opportunity are the brains of geologists back on Earth.

Progress in society is measured by the extent to which work that is dangerous or menial is done by machines. The benefits we enjoy from the space program -- weather satellites, communications satellites and global positioning -- come from robotic spacecraft. Few scientists are calling for a human mission to the Moon or Mars. Human space exploration is essentially over. It is too expensive and provides too little return. But politicians know that the American public identifies progress in space with human astronauts.

The Bush administration's solution is to create an impossibly expensive and pointless program for some other administration to cancel, thus bearing the blame for ending human space exploration. The return to the moon is not a noble quest. It is a poison pill.