NASA said it has not ruled out sending astronauts to the Hubble Space Telescope for one final service mission but made it clear the agency's preference is to do the job robotically.

"We agree that servicing by the shuttle should not be ruled out," NASA spokesman Glenn Mahone said Wednesday, "although we are aggressively pursuing robotic servicing options."

On Tuesday, the chairman of a blue-ribbon panel tasked early this year to evaluate options for extending Hubble's life wrote NASA Administrator Sean O'Keefe urging him to keep both human and robotic servicing options open.

The panel, which expects to complete its final report by early fall, also urged NASA to commit to adding to Hubble two science instruments already developed for flight -- the Wide Field Camera 3 and the Cosmic Origins Spectrograph -- regardless of how the telescope ultimately is serviced.

Prior the February 2003 loss of the Space Shuttle Columbia, NASA had planned to fly a final Hubble servicing mission -- dubbed SM4 -- to install the two instruments and replace the telescope's batteries and gyroscopes. NASA cancelled that mission in January, touching off a national furor.

U.S. lawmakers requested NASA to get a second opinion on Hubble's future before condemning the cherished observatory to an earlier than necessary demise. O'Keefe agreed in March to seek out the National Research Council's advice on the matter provided that the council would also give full consideration to robotic options. Within hours of agreeing to get an outside opinion, O'Keefe told reporters that he remained steadfastly opposed to putting astronauts at risk to save Hubble and that nothing was likely to change his mind.

"Could we do this and take the risk? Sure, but somebody else has to make that decision, not me, because I'm not going to," O'Keefe said during a March 11 media roundtable at the agency's headquarter's here. "I will not, under any circumstances, authorize the conduct of a mission like that, that is not in compliance with the recommendations of the Columbia Accident Investigation Board."

Mahone said Wednesday that O'Keefe's statement to reporters in March still stands, noting that NASA is working very aggressively to find a robotic solution to keeping Hubble up and operating.

"We intend to accomplish as many of the objectives [of SM-4] as possible with a robotic mission currently envisioned at the end of 2007 or 2008," Mahone said.

NASA asked the aerospace industry in June to submit proposals for conducting a robotic servicing mission that would at minimum attach a de-orbit module to Hubble and replace its aging batteries and worn gyroscopes. Those proposals are due in this month, but NASA officials said they have already gotten enough input from industry to know that the mission would not be cheap.

Craig Steidle, NASA's associate administrator for exploration systems, said in a recent interview that he anticipates bids as high as $300 million to $1 billion for the robotic mission. "Those ranges that you hear go from doing the de-orbiting module only to doing science as well," Steidle said.

Hubble has no propulsion system. To keep the massive telescope from reentering the Earth's atmosphere on its own and possibly hurting someone, NASA has been planning since before the Columbia accident to launch some type of robotic mission around the end of the decade to grab Hubble and drop it safely into the ocean.

Steidle said that still remains the top priority but that the agency is also looking for ways to extend Hubble's service life and possible augment its science capability by adding new instruments. He said he has been asked to present O'Keefe by fall with a range of robotic options, what each would cost, and what other projects NASA might have to cancel to make it all fit within its budget.

Prior to the Columbia accident, sending a shuttle to service Hubble would have cost a small fraction of what NASA now predicts is might cost to overhaul the telescope robotically. NASA officials have not said what they think a shuttle servicing mission would cost today in light of a mandate from the Columbia Accident Investigation Board to be able to inspect and repair a damaged shuttle on orbit and comply with other safety protocols. NASA is already anticipating significantly larger space shuttle operations bills -- knowledgeable sources say hundreds of millions of dollars more each year -- once it returns the three orbiter fleet to flight.

Pro-Hubble lawmakers met the National Research Council panel's interim recommendations with praise, but most stopped short of pledging to save Hubble no matter the cost. Congressional aides have told SPACE.com however that many lawmakers would be inclined to find extra money for Hubble even if that meant getting NASA's proposed space exploration vision off to a slower start.

Sen. Barbara Mikulski (D-Md.), a Hubble champion who has tremendous influence over NASA's budget as a senior member of the Senate Appropriation Committee, welcomed the panel's letter.

"This preliminary assessment strongly recommends another servicing mission to Hubble to extend its life and to extend its vision into space" she said in a statement.  "It urges NASA not to preclude the use of the shuttle for this mission, while encouraging robust study of a robotic mission. I support these recommendations."

Rep. Sherwood Boehlert (R-NY), the chairman of the House Science Committee, said he "wholeheartedly endorse(d)" the panel's recommendations and said his committee would work to ensure NASA keeps its options open and sets aside the money needed to do any mission, whether human or robotic.

The U.S. General Accounting Office, the investigative arm of the Congress, is currently looking into what it would cost to do a human or robotic servicing mission.  It is expected to report its findings to Congress in September or October.

An appropriations staffer, who asked not to be identified by name, said some lawmakers are skeptical of NASA's $1 billion estimates for full-up robotic servicing and want better data before making budget decisions for the agency. This staffer said any NASA proposal to pay for the mission -- whether its done by astronauts or a robot -- by cutting other science programs would not be looked upon favorably, however.

"I think any proposal that involves cutting other science programs to pay for it is not going to be acceptable to Congress," the appropriations staffer said. "Their budget already has cuts to science."

A spokesman for Rep. James Walsh (R-NY), the chairman of the House appropriations subcommittee that oversees NASA's budget, said the congressman is already on the record urging NASA to take the National Research Council recommendation's seriously before making any final decisions about Hubble.

He said Walsh subcommittee is planning to take up the NASA budget bill on July 20, but would not say whether the bill would address the Hubble mission.