The era of human exploration of the solar system is over. It ended 28 years ago with the return to Earth of Apollo 17. It had lasted less than four years and never reached beyond the Earth’s own moon.

Since that time, no human has been any farther from Earth than Baltimore is from New York. As if by mutual agreement, the contest between the United States and Russia for human supremacy in space was played out in the relative safety of low-Earth orbit.

The contest finally petered out last August, when Russia abandoned space station Mir after 14 years of near-continuous occupation. Although Mir's accomplishments were slight, the U.S. and Russia are now committed to assembling a new space station. There's not much else for humans to do in low-Earth orbit.

In the space program's early days, a permanent space station seemed like an inevitable step in the conquest of space. From such a platform the crew could make astronomical observations free of distortion by Earth’s atmosphere, monitor weather systems, relay communications around the globe, provide navigational assistance to ships and planes and monitor military operations.

But today, all of these functions are being carried out by robot satellites. What’s more, the robots are doing the job far better and more cheaply than would ever have been possible with humans.

The American Society of Cell Biology went so far as to call for scrapping plans to grow protein crystals in space -- one of the ISS' major selling points. Protein crystals grown at great cost on the shuttle have made no serious contribution to anything. Indeed, NASA was forced to retract the claim that a protein crystal used in developing a new flu drug was grown in space. The crystal, it turned out, was grown in Australia.

There's always been a public expectation, carefully nourished by NASA, that all this is preparation for the eventual return of astronauts to the task of exploring the solar system. The only realistic destination would be Mars. Anyplace else is either too hot, too distant, or the gravity is too strong, or the radiation levels are too high.

Indeed, we already have. The whole world saw Mars through the eyes of Sojourner, the cocker spaniel-sized robot that roamed the martian surface in the summer of 1997. Sojourner even had a "nose," an atomic sniffer that analyzed the composition of rocks.

Sojourner was a tele-robot. Its brain was the human brain of its handler on Earth. In spite of recent setbacks, we will return to Mars with more sophisticated tele-robots, possessing whatever senses we choose to build into them. And they will never break for lunch or complain about the cold nights.

Tele-robots are the natural extension of our frail human bodies, and we can send them to places where no human could survive. It is the scientists who control the robots that will explore Mars, having become virtual astronauts.

In November 1998, John Glenn returned to space as a space shuttle crew member. Discovery was a far cry from the cramped Mercury capsule he rode in 1962, but it traveled only 80 miles deeper into space. America’s astronauts, confined to low-Earth orbit, are like passengers waiting beside an abandoned stretch of track for a train that will never come, bypassed by the advance of science.