After the Columbia catastrophe, Senator John McCain said, "Space exploration is a mission the U.S. will not abandon." I hope he is right. But (and here Senator McCain might differ), such exploration in the next decade or two could perhaps be better done with unmanned vehicles. We are already sending robots to Mars. We have plans to bring back samples. Some day we may even send astronauts there. But let's not be in a hurry to do that. New telescopes and unmanned instruments will tell us more about space than orbiting astronauts.

There is a future for humans in space. Eventually hypersonic flight will be perfected, and we will be able to ride airplanes into orbit. But nobody knows when the required "scramjet" engines will be deployable; they are still in the R&D stage, with strong emphasis on the R. Tests reveal critical new physics with each added Mach number, and hypersonic prototypes have barely reached Mach 5. Orbit requires greater than Mach 18.

Maybe a different approach will succeed first. The development of ultra-strong carbon fibers may make the dream of a space elevator (formerly called "skyhook") real, allowing us to ride slowly and safely up and down. But the science isn't here yet. In 20 years-who knows?

Many in NASA believe that the future of their agency depends on continuing and extending human space flight. Without astronauts, the argument goes, the public will lose interest in space. I think this argument is wrong. Few Americans even knew the Space Shuttle was in orbit-until the astronauts were killed. When the students I know at Berkeley put space posters on their walls, they are not posters of astronauts. They are images of regions where stars are being born, of exploding stars, of extremely distant and hauntingly beautiful fields of galaxies. They were all taken with instruments that didn't need humans to get into space, and required their absence in order to take stable pictures.

The Space Shuttle is not safe and it will not be safe in the foreseeable future. Is using it worth the lives lost? We must be honest. Perhaps the Shuttle missions should continue, but if they do, let us do it in full public realization that the chance of death on each mission is about 2 percent. Soldiers going into battle often accept risks even higher than that. The astronauts always knew this danger, and they chose to accept it. Can the public accept such a high level? I don't know. But whatever the decision is, it should be made in candor and in truth. The Space Shuttle is big engineering; it is the dream of man in space; it is an adventure. But it is not safe, it cannot be made safe, and it is not big science.