But it's not clear whether a new administrator will change the space agency's current preference for a robotic mission to save the orbiting observatory.
O'Keefe's departure "will provide some opportunity for reconsideration," said Howard McCurdy, a space historian at American University in Washington. But the decision "could go either way."
The administration's key priorities are to complete the International Space Station and get on with President Bush's ambitious plans to return astronauts to the moon and send them on to Mars. Hubble isn't on that short list, McCurdy said, and "Nobody wants to fly one more [shuttle] mission than is necessary."
Safety concerns raised by the Columbia Accident Investigation Board spurred O'Keefe's decision 11 months ago to cancel a fifth and final shuttle mission to service and upgrade the orbiting observatory.
The move effectively doomed Hubble to an early demise, because some of the observatory's critical electrical and guidance systems are expected to fail as early as 2007.
The decision was closely watched in Baltimore, where the Space Telescope Science Institute employs nearly 400 scientists, engineers and other personnel. When scientists, politicians and many in the general public objected to the cancellation, O'Keefe asked NASA engineers to develop a proposal for doing the same work with a robotic mission.
But last week, a prestigious panel of experts, assembled by the National Research Council at the request of Congress, concluded that a robotic mission would take too long and be too likely to fail.
A manned shuttle mission to Hubble, the panel found, offered the greatest hope for a success, with only marginally more risk to a human crew than a mission to the space station.
NASA officials declined to speculate on Hubble's future. For now, "we're moving forward on a robotic servicing option," said Bob Jacobs, a spokesman at NASA headquarters in Washington. A preliminary design review is due in March, with a final decision expected in September.
In his resignation letter yesterday, O'Keefe offered to remain at NASA until his successor is named, Jacobs said. At least until then, "we've got no reason to think there would be a change in the agency's direction in servicing Hubble."
At the same time, he said, the space agency continues work on returning the remaining shuttles to flight. Nothing will be done to preclude a manned mission to Hubble, and there's still time to train astronauts for the job. "We will keep our options open," he said.