WASHINGTON — When the wheels of the space shuttle Discovery at last rolled to a safe stop at the end of a dusty runway before dawn in the California desert, the rest of America's human-spaceflight program began to accelerate into an unknown future.
After more than two years of research and repair, the primary problem that downed Columbia remains. Foam continues to fall from the fuel tank during liftoff, and the shuttle fleet will not fly again until the situation is corrected.
Yet Discovery's two weeks above Earth offered the most thorough examination to date of the changes made to improve shuttle safety. And in almost every respect, managers accounted the mission a success - from the new pictures they snapped during launch and on orbit to the first-of-its-kind spacewalk to tidy up the shuttle tiles.
Indeed, despite the suspension of flights, Discovery has infused the shuttle program with a new momentum and the hope that its companion, Atlantis, could launch in months, not years.
"It was probably one of the most successful missions that [the shuttle progam] has ever flown," says Keith Cowing of the watchdog website Nasawatch.com. "I wouldn't be surprised if NASA tries to fly in the fall."
There is clear skepticism, given the past promises of the National Aeronautics and Space Administration.
Before Discovery launched, shuttle managers had all but promised that no foam chunks the size of the one that smashed a hole in Columbia's left wing would break free of the tank this time. Yet one of the new cameras mounted beneath Columbia clearly showed a large piece falling from an area near the fuel tank's liquid-hydrogen pipe - an area called the protuberance air load (PAL) ramp.
Significantly, this is an area of the fuel tank that NASA did not redesign after Columbia. Engineers felt other areas needed more urgent attention, pushing the PAL ramp off the priority list.
Administrator Michael Griffin has since acknowledged that this was a mistake. But shuttle engineers note that the areas of the tank that were redesigned performed well, with little foam shedding. And they are optimistic that similar changes will resolve the problem with the PAL ramp and allow a quick return to space.
In a press conference Tuesday morning, NASA officials were hesitant to guess when the shuttle would return to flight, but some engineers have insisted that the foam loss during the launch was not as devastating to the program as has been suggested.
"This was always listed as a test flight, and as such, any problems that could arise would lead to actions like this," says a former NASA engineer, who recently left out of frustration over the lack of focus at the space agency. "I don't think people believe the shuttle will stay grounded long."
Moreover, information gleaned from the flight will give a clearer indication of how the redesigned fuel tank performs than do tests and computer simulations.
"Now we have real data to go along with the analysis and conjecture," said Dr. Griffin in Tuesday's press conference.
Aside from tending to the International Space Station, getting this "real data" on how the shuttle redesigns worked was the primary function of this mission.
Now mission managers know they can get highly detailed images of the shuttle. They know that astronauts can tinker under the shuttle without damaging the heat-resistant tiles. And they know how the space station can lend its long arm to help.
To some, the tinkering could become a common occurrence as new images show mission managers more of the shuttle's nicks and dings than they have ever seen before. "From now on, every mission is going to have something like this," says Mr. Cowing. "Now that you can detect it, you worry about it, and you can't say ... it's too dangerous [to repair], because you've done it."
Critics see this as further proof of the shuttle's flaws - both the machine and the mission. While space exploration is worth a degree of risk, they say, the shuttle and space station are more about politics and international prestige than science.
"If they haven't made it safe by now, they're never going to make it safe," says Alex Roland, a NASA historian. The program "is worthwhile in the sense that it creates a feel-good enterprise, but the risk far exceeds the payoff."
If anything, though, Discovery has bound the fates of the shuttle and the space station even more closely together. The shuttle program exists only to complete construction of the unfinished space station. Yet this mission suggests that one of the space station's most valuable functions might be as an orbital garage - something that was in the first plans for the station but was eventually scrapped as costs spiraled.
"It is being pushed in the direction of its original vision," says Howard McCurdy, a NASA historian. "It is an inadvertent turning point arising as a result of the Columbia accident."
Said Griffin: It's going to be really hard to top this mission."