THE space shuttle’s return to orbit has been postponed by at least two months because of enduring safety fears, casting fresh doubt on President Bush’s spaceflight programme.

Nasa announced yesterday that the planned launch of the Discovery orbiter has been put back from mid-May to mid- July at the earliest, because of the danger that ice could fall from its fuel tank and trigger another accident similar to that which befell Columbia.

The decision has heightened concerns that it remains too risky to fly 2½ years after Columbia crashed and that Nasa’s schedule for future launches is unrealistic.

Its problems threaten the completion of the International Space Station (ISS) and Mr Bush’s plans to send astronauts back to the Moon and to Mars.

Michael Griffin, Nasa’s new administrator, confirmed yesterday that a launch window had now been set between July 13 and 31. “We are not going to rush to flight, and we want it to be right, so we’re doing what we need to do to ensure that,” he said.

The delay will give engineers time to install a heater to prevent ice building up on a pipe on the shuttle’s external fuel tank. If ice broke off during launch, it could damage thermal tiles that protect the shuttle on re-entry. Similar damage from a suitcase-sized piece of insulation foam was blamed for the loss of Columbia, which broke up over Texas on February 1, 2003, killing its seven astronauts.

Though the problem should be fixed in the next few weeks, Nasa has to wait until July because an earlier date would have required Discovery to lift off at night, which is forbidden under post-Columbia safety measures. Good light is needed to allow the shuttle to be filmed as it lifts off, so that any falling debris can be spotted.

The delay makes it increasingly unlikely that Nasa will finish the ISS by its 2010 target. A further 28 missions are needed and the deadline would require each of the remaining three shuttles to fly twice a year.

Andre Balogh, Professor of Space Physics at Imperial College, London, said: “This schedule was not realistic even before Columbia. Major delays to the space station build-up are now highly likely.”

These troubles will have a knock-on effect for other Nasa activities. The Hubble Space Telescope is already being scrapped prematurely, as a servicing mission has been cancelled, and other programmes are expected to be cut as the shuttle eats more resources.

Scientists are particularly concerned about the Jupiter Icy Moons Orbiter, a £4.5 billion (£2.36 billion) probe scheduled for launch in 2011 that aims to repeat the success of Cassini’s Saturn exploration.

The “Moon to Mars” initiative announced last year by Mr Bush is also likely to suffer. The aim of using the ISS as a staging post for lunar and Martian missions is threatened by the shuttle’s difficulties.

Lori Garver, a former Nasa associate administrator, told New Scientist magazine this week: “Nasa is being asked to return the shuttles to flight, safely fly 28 times to complete the ISS and retire the shuttles in 2010 even as it creates new space vehicles and begins conducting missions to the Moon.

“This is simply not possible without causing serious damage to existing programmes.”

Serious safety fears also remain. Only eight of the fifteen recommendations made by the Columbia Accident Investigation Board (CAIB) have been implemented so far, and some scientists think that even these cannot compensate for the shuttle’s antiquated design.

The Bush Administration and leading figures at Nasa are committed to putting the shuttle into orbit again and proceeding with the ISS as a precursor to manned expeditions to the Moon and to Mars. Supporters of manned missions see it as the best way to enthuse the public about spaceflight, but many scientists question whether it is worth it.

Each shuttle launch cost $400 million even before the Columbia crash, and the safety measures imposed since have increased this. The final cost of the ISS is expected to exceed $100 billion. On top of that, the Challenger and Columbia accidents have cost 14 lives.

Critics such as James Van Allen, one of the architects of the Apollo missions, would like Nasa to cut spending on human spaceflight and invest in cheaper, longer-range robotic missions. They cite the success of the Mars rovers Spirit and Opportunity — each of which cost the equivalent of a shuttle launch — and the Cassini mission to Saturn. Cassini cost $2 billion, 50 times less than the ISS.

Douglas Osheroff, of Stanford University, a member of the CAIB, said the risk of a serious failure is between 1 and 2 per cent a launch, or between 24 and 43 per cent over the 28 missions still planned.

Professor Balogh said. “It is always a matter of judgment about what level of risk Nasa is willing to accept. The debate about that is quite a lively one.”

Times Recommends

• Zardari still has to tackle tough questions
• Material evidence 'debunks' Turin Shroud
• BBC criticised over Uganda gay execution poll