NASA Administrator Sean O'Keefe said yesterday the space agency was soliciting proposals for a robotic mission to refurbish the Hubble telescope, but the first priority would be to ensure the telescope's safe descent to a watery grave.

"There may be . . . options for extending the Hubble's useful work -- good options that are looking more promising as we've examined them more closely," O'Keefe said. "Our confidence is growing that robots can do the job."

O'Keefe's remarks earned cheers at the American Astronomical Society meeting in Denver, and fueled hopes that the popular telescope might be saved, or its life at least prolonged. On Jan. 16, O'Keefe triggered a national paroxysm of outrage by canceling a scheduled space shuttle servicing mission, deemed too risky after last year's Columbia disaster. Hubble, it appeared, would be allowed to wear out and die.

O'Keefe's new optimism, however, did little to mute doubts about a robotic mission's ability to do an adequate job, a view reinforced last week in a petition signed by 27 astronauts and endorsed by Sen. Kay Bailey Hutchison (R-Tex.) in a letter to President Bush.

Robotic servicing "would only be able to accomplish a portion of the tasks . . . and would have a lower probability of success," the astronauts' petition said. "Consequently a shuttle servicing mission would be more cost-effective."

Steven V.W. Beckwith, director of the Space Telescope Science Institute, which sets Hubble's scientific agenda, said he was "tremendously encouraged" by O'Keefe's wish to service Hubble "by any means possible."

But he noted in a telephone interview from the Astronomical Society meeting that O'Keefe had "made it clear that it would certainly be acceptable if all that could be done [by a robotic mission] was to de-orbit the telescope."

Space shuttle astronauts have serviced Hubble four times since putting it in orbit in 1990, adding new instruments that have produced fresh generations of spellbinding images.

Cancellation of the fifth servicing mission, scheduled for 2006, appeared to be a death sentence, because the telescope's handlers expect its batteries to wear out by the end of 2007. Eventually, the telescope will fall out of orbit and plunge to Earth.

At a minimum, NASA must develop a spacecraft that can dock with the telescope and then guide it to a safe descent -- preferably in the open ocean. But by April, enthusiasm had begun to grow that a robotic servicing mission might be possible -- to replace batteries and gyroscopes, and even perhaps install new instruments.

O'Keefe's comments yesterday reflect these priorities: NASA has asked for proposals both to de-orbit and service the telescope, but has emphasized that a de-orbiting plan must be included in all submissions. NASA officials have said that a de-orbiting scheme, with or without a servicing component, must be in place by the end of the year to ensure a 2007 launch.

The prospect of a robotic mission that replaces batteries and gyros has won some acceptance, but Beckwith, among others, has noted that astronauts have added instruments to improve the telescope every time they serviced it. He expressed hope yesterday that NASA will be able to do the same with robots "so as not to just leave Hubble on life support."

The astronauts' petition, however, warned that a robotic mission "will require both the development of new and unproven technology and flying a first time ever, unmanned rendezvous and docking mission to the Hubble."

By contrast, Hutchison added in her May 26 letter to Bush, shuttle servicing would be a matter of "simple repairs." Signers of the petition included Mercury astronaut Scott Carpenter; Eugene A. Cernan, the last man to walk on the moon; and Robert L. Crippen, pilot of the first shuttle mission.

"Frankly we use robotics whenever we can," said Apollo astronaut Walter Cunningham, who drafted the petition. "But very few people feel that robotics are sophisticated enough to do what the shuttle servicing mission would accomplish."

And as for risk, Cunningham noted that Columbia failed because of damage to its heat shielding, "and the shuttle isn't going to fly until that problem's been solved." Since 1961, NASA has undertaken 90 missions similar to a Hubble flight, he added, "and now they're saying that these were too risky? Astronauts know what they're biting off when they get in one of these machines."