With the International Space Station and possibly the Hubble Space Telescope waiting for astronauts to make critical upgrades and repairs, NASA engineers are working feverishly to get the shuttle fleet flying again as early as May.
Although significant problems remain, space agency officials and outside experts say that progress has been substantial and that a return to flight could come on schedule.
"In my heart, I believe we can do this," said John Casper, space shuttle integration manager.
With a National Research Council panel recommending that NASA send a manned shuttle instead of a robot to fix the Hubble telescope, the agency's ability to get the shuttle fleet back in action soon could become even more important.
The first mission since the Columbia disaster in 2003 - and the only one with an assigned launch window - would send the shuttle Discovery into space between May 12 and June 3.
The shuttle would fly with a crew of seven to the International Space Station. It would also test critical in-flight inspection and repair procedures demanded by the Columbia Accident Investigation Board.
The CAIB report called for four key fixes before the shuttles return to space:
A cure for the problem of hard foam insulation breaking off the shuttle's external fuel tank. Damage to Columbia's left wing from a chunk of foam enabled superheated gases to enter the wing during re-entry Feb. 1, 2003, causing the shuttle to break up, with the loss of all seven aboard.
A system for recording images of the shuttle during its ascent and then inspecting it for damage in orbit.
Technology for the in-orbit repair of damaged carbon heat shields and ceramic tiles.
Provision of a "safe haven" for astronauts - either in the space station or in another spacecraft - in the event damage to their shuttle makes it impossible for it to return them safely to Earth.
John Logsdon, director of the Space Policy Institute at George Washington University, said questions about whether the shuttle fleet will be ready in time to reach the Hubble Space Telescope before its systems begin to fail in 2007 are secondary.
"If they're flying at all, they can do the Hubble mission," he said. A Hubble repair flight would not come until after the first six or seven missions to the space station, whose construction is far behind schedule.
The more fundamental question, he said, is whether the National Aeronautics and Space Administration can get the aging shuttles flying on schedule at all.
NASA officials insist they will not be driven by the calendar. "We are going to fly when we have determined the vehicle is ready to fly, when it is safe to fly," said deputy shuttle program manager Wayne Hale.
The technical obstacles have been formidable, and nature has seemed bent on adding its own.
This summer's barrage of hurricanes in Florida caused considerable damage at the Kennedy Space Center, interrupting work on the shuttle and pushing the first post-Columbia launch into May.
Sean O'Keefe, the NASA administrator, set a high bar for a return to flight, pledging that the agency would adhere to all 15 of the accident board's recommendations and two dozen "observations."
Key among the CAIB recommendations is a change in NASA's organizational "culture," which investigators blamed as much as any technical failures for the Columbia accident.
"Before Columbia, [NASA's] vision of the shuttle was that it was far more reliable than it actually was," said Howard McCurdy, a space historian at American University in Washington.
If the agency appears to be running scared, McCurdy said, "there's nothing that improves safety better than fright. ... People who get alert make fewer mistakes."
But NASA engineers must also fix the mechanical things that went wrong on Columbia. At a news conference this week, Hale reported real progress in reducing the danger from falling insulation during launch.
With a redesign of the huge fuel tank, he said, any chunks of insulation shaken loose should be significantly lighter than half an ounce - the weight at which significant damage becomes a worry.
While some debris may still break away, he said, "we expect to see much less damage in the tile and no critical damage that would require repair. That's our goal, and it's beginning to look very positive that we can reach that level of control."
NASA has also made progress with surveillance systems that astronauts in flight and engineers on the ground can use to visually inspect and identify damage anywhere on the shuttle's exterior.
The missions will launch with a system of cameras and robotic arms on board the shuttle as well as on the exterior tank and solid rocket boosters. Specialized cameras and radar on the ground, on aircraft and ships, and on the International Space Station will also be watching.
NASA will limit its first group of new missions to hours when the launch and the critical separation of the external fuel tank - on the other side of the world - can be viewed in daylight.
Shuttle program manager William Parsons said this week that the camera boom, which includes sensors to help astronauts reach and detect damage on the shuttle's underside, is not ready for flight.
More problematically, engineers are also struggling with a shuttle repair kit - patching goo, tools and procedures that will enable spacewalking astronauts to repair damage to the shuttle's carbon heat shields and ceramic tiles.
"They are having difficulty," said Roger E. Tetrault, a former member of the CAIB, a fact, he said, that "would not be surprising to anybody on CAIB."
Crew members will need a patching material they can apply in weightlessness and the vacuum of space, under extreme conditions of alternating heat and cold. And the patch job will have to withstand re-entry temperatures of 5,300 degrees Fahrenheit for 15 minutes.
Fortunately for NASA, the CAIB required only that a "practicable" solution be ready for the first flight to address "the widest possible range of damage." It was a fudge factor recognizing that no solution would be suitable for every eventuality, said Logsdon, who also served on the CAIB.
It's the final CAIB requirement - the "safe haven" issue - that may discourage O'Keefe from approving a manned repair mission to Hubble, as called for this week by the National Research Council panel.
A damaged shuttle bound for the International Space Station could dock and sustain its crew there for up to 45 days until a shuttle could be launched to return them to Earth.
NASA is looking at contingency plans for extending the length of time space station and shuttle crews could survive aboard the station in an emergency. Engineers are also working to reduce to as little as 33 days the time needed to launch a rescue shuttle mission.
The Hubble telescope poses a different problem because it provides no such haven for astronauts on a crippled shuttle. Instead, the National Research Council report said NASA could have a second shuttle ready to fly should a Hubble repair crew get into trouble.
The panel said NASA has shown it can maintain multiple shuttles at the same time on the launch pads at Cape Canaveral. And it has launched shuttles within as little as two weeks of each other - sufficient time to reach a stranded Hubble repair crew before supplies ran out.