It follows opposition to Nasa's chief, Sean O'Keefe's decision, that servicing missions should be cancelled because of astronaut safety concerns.

New rules, following the loss of space shuttle Columbia last year, do not allow a Hubble visit by astronauts.

The US space agency has said that if it is not serviced, Hubble will probably last only a few more years.

Unpopular decision

"There is life beyond Hubble, as much as I hate to admit that," said Ed Weiler, Nasa's head of space science, responding to critics.

Weiler, and other officials, took issue with press reports and leaked Nasa documents that argued it was no riskier for astronauts to pay a service call to Hubble than it was for them to build the International Space Station (ISS).

The reports, written by Nasa engineers who declined to be named, argued against the unpopular decision to forgo a scheduled shuttle mission to repair and upgrade Hubble in 2005 or 2006.

Without that mission, which would repair failing gyroscopes and upgrade detectors, the telescope will eventually stop functioning and will need to be nudged out of orbit toward Earth in a controlled descent.

During a news conference, Weiller, along with Bill Readdy, head of space flight at Nasa, and John Grunsfeld, the agency's chief scientist, expressed regret at the expected end of the Hubble project.

No safe haven

Readdy, a former shuttle astronaut, said Nasa had already analysed the question of whether to send astronauts to fix Hubble, and determined that it was unsafe.

He added that Hubble offers no "safe haven" for astronauts seeking refuge from a damaged shuttle, while the ISS does.

"The documents (from the engineers) really did not go into the kind of depth and detail that we already had," Readdy said, who faulted the two engineers' reports for their "superficial" analysis.

Regarding a possible shuttle mission to Hubble, Readdy said schedule pressures and logistics would be formidable, especially if Nasa were required to ready two shuttles simultaneously - one to service Hubble and a second to wait on an adjacent launch pad if a rescue was needed.

Grunsfeld, a former shuttle astronaut who was on the last Hubble servicing mission aboard shuttle Columbia in 2002, had personal feelings about the telescope.

"I'm in the position of being the last person ever to hug Hubble," Grunsfeld said. "I know we've allowed the American public to fall in love with Hubble, for good reason".

But he said that the decision not to service Hubble was based on "a good rationale".

"So we have a family member that has now a timetable for when that family member's going to depart and we all feel bad," he said. "I think the most important thing is to make sure that the time we have left is quality time."