A NASA-ordered report by the nation's top scientists yesterday urged the space agency not to rule out a potentially risky space shuttle mission to service the Hubble Space Telescope, which it described as "arguably the most important telescope in history."

The interim report from a committee of the National Research Council buoyed shuttle mission advocates and could reopen debate on NASA's unpopular decision to let the telescope wear out and tumble into the ocean -- unless the agency can figure out a way to service it with a robotic spacecraft.

The report challenged statements made by NASA Administrator Sean O'Keefe over the past six months, in which he repeatedly ruled out a shuttle-servicing mission for safety reasons.

Instead, the committee said a shuttle mission "is not inconsistent" with recommendations made after last year's Columbia disaster. The committee added that the proposed robotic alternative would present daunting levels of "complexity, sophistication and technological maturity."

Although NASA should "vigorously" pursue the robotic option, it "should take no actions that preclude a space shuttle servicing mission," the report said.

The report also challenged NASA's interest in sending a robotic mission simply to replace Hubble's batteries and gyroscopes or to steer it into the ocean. It said NASA should make every effort to install new instruments.

"The Hubble Space Telescope is arguably the most important telescope in history," it said, and new instruments would ensure a new generation of stunning images.

Despite the differences between the report and O'Keefe's statements, committee Chairman Louis J. Lanzerotti said in a telephone news conference, "I do not think [there is] necessarily a gap between our report and the administrator." He did not elaborate, adding only that "we look forward to the opportunity to discuss it further."

In a statement, O'Keefe was equally noncommittal and did not address the possibility of a shuttle-servicing mission. Under President Bush's new space initiative, the shuttles are to serve exclusively as cargo carriers for the international space station when they resume flying and would be retired in 2010, when NASA says the station will be completed.

"The challenges of a robotic mission are under examination," O'Keefe said, praising the committee for its "thoughtful diligence." He promised to "continue our exhaustive and aggressive efforts to assess innovative servicing options."

Elsewhere, the committee's recommendations were welcomed. In Baltimore, Steven V.W. Beckwith, director of the Space Telescope Science Institute, which manages Hubble's science, said in a telephone interview that he is "pleased that [committee members] think there are two potential options for servicing Hubble."

Sen. Barbara A. Mikulski (D-Md.), whose state is also home to NASA's Goddard Space Flight Center, which services Hubble, was much less restrained. "Essentially, the [council] is saying: Have a servicing mission and keep your options open, not only [to] keep Hubble alive, but to extend its abilities," Mikulski said. "I was holding my breath, but now I can breathe a lot easier."

House Science Committee Chairman Sherwood L. Boehlert (R-N.Y) also gave the report an enthusiastic reception, underscoring Hubble's bipartisan popularity.

"I wholeheartedly endorse its recommendations," Boehlert said in a statement. "We will continue to work with NASA to see that the agency keeps all its options open . . . and sets aside the funding needed to carry out any mission, whether manned or robotic."

The Hubble telescope, launched in 1990, orbits 360 miles above Earth. It has been serviced by shuttle astronauts four times, and each time new instruments have been added. A new ultraviolet and near-infrared camera, as well as an ultraviolet spectrograph, are awaiting installation.

On Jan. 16, O'Keefe triggered a national outpouring of dismay when he announced that NASA was canceling an upcoming space shuttle mission to the Hubble because the agency had neither the time nor the money to outfit a shuttle with the equipment needed to comply with safety recommendations by the Columbia Accident Investigation Board.

Yesterday's report, however, noted that the board "did not prescribe operational constraints" on a shuttle mission to Hubble, even though such a mission would pose "a somewhat greater difficulty" than servicing the space station.

O'Keefe asked the council to examine the Hubble problem in March. The council, a division of the National Academies of Science, convenes panels of nationally known experts to study scientific questions posed by government agencies.

In issuing the interim report, the committee said it understood "there is some urgency" for NASA to have findings in time to make decisions about repair options. NASA must have a scheme in place for servicing or de-orbiting by the end of this year in order to launch in 2007. That is when NASA estimates that Hubble's batteries will no longer be able to hold a charge, and at that point the telescope will die.