Sir

Your far-sighted Editorial (Nature 421, 559, 2003) on the future of human space exploration in the wake of the Columbia accident is a welcome contrast to some of your more sceptical comments about this subject in the past. Your call for vision, and clear endorsement of the exploratory value of human spaceflight, was a breath of fresh air. I would like to make two points.

First, you identify three possible future roles for humans in space: as geologists exploring near-Earth asteroids; as servicing crews for telescopes at L2; and as explorers on Mars. There are indeed good reasons for believing that all these activities, and more, would benefit from a human presence in space, but we must not forget the Moon.

The Apollo missions pioneered the use of astronauts as field geologists, and I shudder to think what the textbooks would now have to say about the early history of the Solar System had Apollo not taken place. Even today, one can scarcely attend a scientific meeting on the subject without seeing geochemical and isotope analyses of Apollo samples presented in one context or another. Yet Apollo, quite literally, only scratched the surface of the Moon, and there is so much more to learn. Moreover, while Mars does indeed beckon, we should probably learn to operate successfully on the Moon before attempting this much greater challenge.

Second, your Editorial implied that human space exploration is solely NASA's responsibility. But if such exploration is worthwhile, as I believe it is, we should all share in the costs and the risks as well as in the benefits. The pre-eminent value of the much-maligned International Space Station is not so much the science to be performed on it, valuable though that is, but the model it provides for international cooperation in space. We should aim to build on this experience to develop a global human spaceflight infrastructure from which science can only benefit — an international Moon base being the obvious next step.