ASHINGTON, Dec. 8 - Contradicting NASA policy in a long and bitter debate over science and safety, a panel of the National Academy of Sciences said Wednesday that the agency should send astronauts to repair the Hubble Space Telescope rather than rely on a robotic device.
After six months of study conducted at Congress's request, the committee of 21 experts said that a robotic mission would hold too many uncertainties, that it would probably be ready too late to extend the telescope's life and that it might actually damage the instrument.
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As for the risk to a space shuttle crew, the panel said, there is a "very small" difference in safety between a mission to the telescope and a typical mission to the International Space Station. More than two dozen journeys to the station are planned over the next decade.
"A shuttle servicing mission is the best option for extending the life of the Hubble telescope," said Dr. Louis J. Lanzerotti, a physics professor at the New Jersey Institute of Technology and chairman of the panel, assembled by the academy's National Research Council. "It is highly unlikely that the science life of Hubble would be extended with a robotic mission."
There was no immediate response from the National Aeronautics and Space Administration, which touched off an outcry from astronomers and lawmakers last January when its administrator, Sean O'Keefe, announced that he was canceling a shuttle mission to repair the telescope because it would be too dangerous to the crew. Over the summer, Mr. O'Keefe called for the development of a robot that could perform the repairs, without which the telescope is expected to die in orbit in 2007 or 2008.
While the recommendations in the new report are not binding, they will put pressure on NASA to reconsider its position. Representative Sherwood Boehlert, Republican of New York and chairman of the House Science Committee, noted that the findings were "diametrically opposite to those reached by NASA" and said he would hold hearings early next year on the report and all options for the telescope.
Members of the National Academy's panel briefed Mr. O'Keefe and senior NASA managers on their conclusions on Tuesday.
On Wednesday, Robert Mirelson, an agency spokesman, said NASA would need more time to study the report.
"In the meantime, NASA is continuing to examine a variety of methods for extending the Hubble's life," Mr. Mirelson said. The agency will continue to plan for a robotic mission, he said, but will not do anything to preclude a possible shuttle mission.
Earlier in the day, Dr. Lanzerotti, who is also a consultant with Bell Laboratories and Lucent Technologies, said at a news conference that the telescope was one of the greatest achievements of the space program and that keeping it in orbit should be a priority so it could continue making significant discoveries about the universe.
A robotic mission for saving the telescope presents many technical risks because nothing like it has ever been done, requiring the development and testing of new technology.
Astronauts have serviced the telescope four times since it was launched in 1990, and a shuttle mission could be accomplished sooner and with a higher probability of success than one using untried robotic measures, the 135-page report said.
Astronomers said they were delighted by the experts' findings. Dr. Steven Beckwith, director of the Space Telescope Science Institute in Baltimore, which supervises the Hubble's research operations, said of the telescope: "They have affirmed that the future of its science is very bright. It's not an old telescope whose time is gone. If anything, it is becoming more valuable."
Dr. John Bahcall, a physicist at the Institute for Advanced Study in Princeton, N.J., who has been involved with the Hubble program from its beginning, called the new study "a wonderful report" and said of its dismissal of the robotic mission, "Finally, somebody told the king he didn't have any clothes."
Several astronomers said they had long been worried about the effects of a costly robotic mission on other science projects and about the possibility of resulting friction between astronomers and astrophysicists. Estimates for a robotic mission have been as high as $2.6 billion, which would come out of NASA's science budget, perhaps delaying other projects like the study of black holes and dark energy.
Dr. Donald Lamb, a University of Chicago astrophysicist, said, "The tremendous cost of the robotic repair mission would really mean a broad array of other space science missions would be very unlikely to happen."
On Capitol Hill, Senator Barbara A. Mikulski, the Maryland Democrat who helped secure $300 million for NASA's current budget to use for servicing the telescope, said: "NASA has the experience, the technology, and now it has the money. It's time to fix Hubble."
Mr. O'Keefe, the NASA administrator, said last winter that he considered a manned Hubble mission riskier than shuttle flights to the space station. He said a strict interpretation of the findings of the board that investigated the loss of the shuttle Columbia and its crew of seven made him believe that the risk of sending astronauts to the observatory was too high.
But the academy's report notes that NASA is planning 25 to 30 more shuttle missions to the space station and says the risks faced by a crew on a mission to the telescope would be similar to the risk of any single mission to the station.
Roger E. Tetrault, a member of the committee who also served on the Columbia investigation board, said at the news conference that all space travel was risky and that the nation had accepted a certain level of risk with the human program.
"We are saying that going to Hubble as opposed to going to the space station is worth that incremental risk, which is not large," Mr. Tetrault said.
Warren E. Leary reported from Washington for this article, and Dennis Overbye from New York.
