Three times in the space age, Americans have placed flowers on the graves of astronauts, but under such different circumstances that they might have happened in three different countries. The Apollo 1 launchpad fire in 1967 was a tragic setback, but it failed to deter a determined nation in its race to reach the Moon. The 1986 Challenger disaster, perhaps the most unsettling of the three, struck hard at the psyche of a less confident people. And now we have Columbia: coming between the terrorist outrages of 11 September 2001 and a looming war in the Middle East, it feels sadly familiar, the latest in a series of grim developments.

In the downtime forced by the accident investigation, it is appropriate to ask where NASA should go from here. After the Apollo and Challenger accidents, both of which came early in their respective programmes, it was clear that the answer — once the safety issues had been addressed — was to keep working towards much the same goals as before. But the space shuttles have been flying for more than two decades now, and the loss of Columbia comes at a time when NASA was already rethinking its approach to human space flight.

Last autumn, the agency unveiled a new space transportation plan that calls for a small Orbital Space Plane to be built. Launched on a conventional rocket, it would provide an alternative means of reaching space. The plan comes after a series of failed attempts to develop a larger, fully re-usable space plane, and after NASA bailed out on a mini-shuttle concept that it had been developing as a 'lifeboat' for the International Space Station.

Critics charge that the Orbital Space Plane will take so long to develop that it could serve little useful purpose with regard to the space station, and that NASA should have gone ahead with its mini-shuttle instead. But at least it represents an attempt to get on with building a new space vehicle for the first time in decades, instead of simply producing artists' concepts.

When NASA administrator Sean O'Keefe took over the space agency in December 2001, he was criticized for lacking vision and focusing on accounting. Yet the budget for 2004 that he unveiled on Monday (see page 565) — without much fanfare, given the sombre mood of the moment — includes the seeds of a vision that could see astronauts venturing beyond the confines of Earth orbit.

Bold vision

With the blessing of the White House, NASA has revived the old idea of nuclear-powered rockets, which most people in the space business agree is the only way to push ahead with ambitious space exploration. For years, discussion of nuclear propulsion was forbidden at NASA, for fear of a negative public reaction. But O'Keefe's agency seems unafraid of this debate.

Also downplayed in recent years was the possibility of sending astronauts beyond Earth orbit. There have always been a few engineers, tucked away in the deepest corners of NASA centres, who were allowed to think about missions to the Moon, Mars and elsewhere. But their work was outside the mainstream, and wasn't attached to any larger programme. In this week's budget proposal, NASA has requested $39 million for a new Human Research Initiative, with a planned budget of $347 million over five years. Its unabashed goal is to find safe ways for people to spend extended periods in space.

By themselves, such moves are hardly enough to accomplish the objective of sending people beyond Earth orbit. But marking them out as specific initiatives, for everyone to see, is an important first step towards this goal.

Nature has been critical of human space flight in the past, particularly when it relies spuriously on science to justify projects that have difficulty standing on their own merits. If the Human Research Initiative isn't linked to longer-term plans for human space exploration, then this same objection will hold, as it would be difficult to see any point to the endeavour other than continuing to justify the space station's existence.

NASA might therefore seize the moment, and honour its fallen astronauts, by further encouraging its visionaries to come out from their hiding places. They have plenty of ideas that could be executed in the next decade or so. One is to send astronauts as field geologists to investigate near-Earth asteroids. Another is to have them service telescopes at the L2 lagrangian point, more than one-and-a-half million kilometres from Earth, where the successors to the Hubble Space Telescope will be located (see Nature 419, 666; 2002). In the longer term, of course, Mars beckons.

Common goal

Sending astronauts beyond Earth orbit again will be tremendously expensive. But if America's political leaders do decide that exploration should be NASA's goal, then all of the agency's workforce could work towards a common purpose, much as it did in the Apollo era. As it stands now, space scientists and engineers developing concepts for future crewed spacecraft have little to do with each other.

Space scientists who use orbiting observatories to study the cosmos or send robotic probes to explore the planets might be wary of any such coming together. It is they, after all, who have been doing most of the real exploring since the Apollo programme. And after seeing many of their projects delayed after the Challenger accident, they might be reluctant to hitch their fortunes to human space flight once again.

There are positive examples of cooperation between NASA's two 'camps', however. Notable among these are the Hubble repair missions, which first salvaged the telescope after it was launched with a faulty mirror, and which have subsequently continued to upgrade this historic scientific asset.

The immediate aftermath of the Columbia tragedy, which reminded us of the fallibility of our spacecraft and engineers, may seem like an odd time to talk about heading off on some far more ambitious venture. Then again, it may be just the right time.

The shuttle is just a means to an end, rather than a destination in itself. The same goes for the space station. What excites the public, and what inspired the astronauts who gave their lives last week, is exploration. A space programme that doesn't dare to explore may ultimately be doomed to wither.