Since before President Kennedy pointed to the moon and implored America to find a way to go there, the engineers at NASA have been workers of the previously impossible.
Whether it is choreographing the movements of far-away satellites with balletic beauty or keeping a space station aloft on spare parts and pure ingenuity, NASA has repeatedly found ways to overcome the limits that physics and doubt would put upon its endeavors.
Now, with the realization that 2-1/2 years and $1.4 billion of research and repair did little to solve the foam problem that destroyed the shuttle Columbia, the agency is again being forced to face the challenges presented by the most complex vehicle ever built - and a public increasingly skeptical that it can master the machine.
NASA's greatest asset - its legendary moxie - is under question, and how it responds in the days and months to come will determine not only the future of the shuttle program, but whether NASA will be trusted to set out to the moon and Mars next decade.
"The NASA shuttle team still has the talent and drive it did in the 'old days,' but not as much experience with developing and troubleshooting new systems," said James Oberg, a former member of NASA mission control, by e-mail Thursday. "The challenge here is looking at a problem they have already looked at very hard, in waves, for decades."
The first test will come as early as this weekend. Saturday, Discovery will open its cargo bay and spacewalking astronauts will clamber inside, where they will use a mockup panel of shuttle tiles to test new patches designed to protect cracks and holes in the shuttle's thermal skin.
On this occasion, they won't leave the cargo bay. Though a piece of foam roughly the size of the one that damaged Columbia ripped off Discovery's fuel tank during liftoff, it did not appear to hit the shuttle, and images of the shuttles tiles so far show no major damage. The shuttle should be fine to land on schedule, mission managers say.
Yet the test run is in many ways another indictor of NASA's ingenuity. The patch for a potential hole in the leading edge of the wing looks vaguely like a piece of tar paper pierced by a butterfly bolt and a washer. The "goo" to fill tile divots and cracks comes from a glorified glue gun. It's "This Old House" in moonsuits.
The astronauts themselves, however, are not eager to start hanging out under the shuttle playing Bob Vila. Admittedly, these are just the first prototypes, but the crew suggested that they were not willing to stake their safety on the technology.
And that, some say, speaks to the broader legacy of the shuttle. It is, without question, a magnificent piece of machinery. It has provided the testing ground for a host of pioneering technologies and still is the only reusable spacecraft ever built.
But the leaps in engineering that scientists had to make to accomplish this in the 1970s created a host of unique problems. Foam insulation "has been a challenge for many rockets and many countries," says Dr. Oberg. But because of the shuttle's peculiar configuration - with the vehicle riding astride the fuel tank, not atop it - "only with the shuttle does it become a 'loss of crew' concern."
Now that mission managers have officially grounded the shuttle fleet, they will again apply themselves to these problems. Whether they can fix them will have a profound impact on the course of the space agency - most importantly, returning to the moon. If NASA is defeated by the complexities of its own machine - or if it cannot make quick progress without millions more from Congress - faith in the agency could flag.
"I could see senators asking, 'If you can't do this, how are you going to go to the moon and Mars?' " asks Keith Cowing of Nasawatch.com.
To some, it would be an unfair comparison. Today's engineers are not less worthy than those of the past, rather they have been saddled with old mistakes.
"The original space shuttle was an extraordinary series of unfortunate compromises," says Jim Campbell, editor of the Aero-News Network. Had they planned and designed the shuttle better, he adds, "we would have paid a lot less in the long run."
Those costs are beginning to add up. Though the program has its share of "shuttle huggers" at NASA, who see the vehicle as an engineering marvel and America's only link to human spaceflight, the rationale for the shuttle has long been as much political as scientific. It is an international venture and a provider of 10,000 jobs.
In the face of more delays and perhaps the need for more money from Congress, however, the impact of one 8-by-33 inch piece of foam could be transformational. Says Mr. Cowing: "People are already asking if this is the last flight of the shuttle."
• Adam Karlin contributed to this report.