For the past decade, a firewall has existed within NASA's budget, separating the agency's science programme from its astronaut programme. Thanks partly to congressional supervision, NASA has not raided its science account to pay for the space shuttle or the space station. Spending on science has actually gone up, while the shuttle and space-station budgets have stayed flat or declined.

Scientists believe, with good reason, that this is based largely on merit. The Hubble Space Telescope, the Wilkinson Microwave Anisotropy Probe and the Mars Exploration Rovers have returned solid results. The shuttle and space station have not.

Now, however, NASA faces more fiscal pressure than at any time in recent memory. Fixing the shuttle will, predictably, be more expensive than NASA first thought. The latest estimate is $1.15 billion, plus the cost of added safety once the vehicle is flying again. Administrator Sean O'Keefe's decision to bar astronauts from visiting Hubble again has pinned the telescope's fate to an expensive robotic repair mission. Cost estimates vary wildly but start at about $1.3 billion.

Looming even larger is President Bush's ‘Vision for space exploration’, which would send astronauts back to the Moon and on to Mars. No one is sure what this will cost, but the Congressional Budget Office recently guessed at $127 billion by 2020. NASA wants to start next year, with $438 million requested for the Crew Exploration Vehicle.

The White House and NASA sold the exploration programme on the basis that it would require no large increase in funding — the money would come from retiring the shuttle and the space station. It is a sensible plan but may already be unravelling. A report released last week by the National Academy of Sciences showed how just one area of science — solar and space physics — could be affected by the “more constrained funding climate” that would accompany a Moon–Mars programme. A sequence of missions could delay high-priority projects such as the proposed Solar Probe for years.

Fortunately, Congress is watching. Last week the Senate appropriations committee cut NASA's $16.2 billion request by $664 million, including projects associated with the exploration programme. An additional $800 million in emergency spending would go to solve the shuttle and Hubble problems. More importantly, the committee directed NASA not to upset the balance of its science programme to fund the Moon–Mars effort. The House appropriations committee, which made even deeper cuts to the exploration programme this summer, was more blunt, writing: “While the Committee is supportive of the exploration aspect of NASA's vision, the Committee does not believe it warrants top billing over science and aeronautics.”

Bush's Vision has failed to wow either the media or the general public. NASA should stick to its original plan to pay for the Moon–Mars programme from within its existing spaceflight account. If the shuttle or space station have financial setbacks, the Vision should be scaled back or delayed. But hands off the science programme, which today is doing the real exploring in space.