As the space shuttle Discovery navigates its dramatic return to Earth tomorrow, it will come freighted with much more than an exhausted crew and 6,300 pounds of worn-out gear and trash from the international space station.

The very future of the U.S. shuttle program will ride on its wings.

The question is not whether the shuttle will land safely. If it does not, the 25-year-old program will almost certainly have no future, experts agree.

The question is what NASA officials will conclude from the plethora of analyses they will be rushing to complete after the uneventful landing the agency expects.

If the problems relating to the external tank's foam insulation appear fixable with modest amounts of money and time -- and if data gleaned from the shuttle's newly enhanced sensory system indicate that the craft survived because of good engineering and not merely good fortune -- then the agency will be on track to achieve its current, albeit scaled-back, goal: keeping the shuttle alive for perhaps 20 more flights to complete enough of the space station to term it "functional," then move on to a new generation of vehicles.

If NASA must make major fixes, however, with delays of more than a few months and with new budgetary stresses, then Congress will face a difficult choice: pumping significantly more money into the shuttle program or retiring it early.

On the one hand, the government wants humans to have some capacity to get into Earth orbit until a new vehicle is ready -- as a matter of pride and to keep U.S. promises to the many nations that have been building space station components.

Although many see the station as little more than a zero-gravity albatross, it has enjoyed fabulous success as a promoter of international goodwill, and NASA regards it as an essential tool for testing the technologies of future space colonization.

On the other hand, there is a powerful desire to stanch the monetary hemorrhage that has long characterized the space station and shuttle programs, which together consume more than a third of NASA's budget.

"This mission was supposed to be the beginning of the end for the shuttle, not the beginning of another round of upgrades," said Howard McCurdy, a NASA expert at American University's School of Public Affairs. "The history of the shuttle has, unfortunately, always been that the upgrades and operational costs have eaten up the money that was supposed to be the seed corn for new expeditions."

The next measure of whether the shuttle is heading for a repeat of that pattern is the launch of Atlantis, scheduled for September. With the fleet grounded until NASA figures out why the improved foam system did not behave as predicted on Discovery, it is no longer clear that NASA can make the September launch window.

Shuttle officials pushed the launch back to Sept. 22, only five days before the window closes, and it could slip to November, December or next year unless engineers quickly find the cause of the foam debris in what NASA Administrator Michael D. Griffin on Friday termed "a eureka moment."

This seems unlikely. Teams of investigators have been trying for nearly two weeks to learn why Discovery's external tank lost four large pieces of foam during launch, and once they do find the reason, they must determine if the shortcomings were unique to Discovery's tank, in which case they can fly Atlantis immediately, or were caused by a systemwide problem, which would require another redesign likely to consume months.

A launch of Atlantis this year could validate Griffin's contention that Discovery's mission "has been a magnificent flight in every way," but others say any plaudits won by the crew's unblemished performance are outweighed by the cussedness of the orbiter's hardware.

"The impact on NASA's credibility is devastating, and I just don't see how they can continue pretending they have control over this thing," said Alex Roland, a Duke University historian who focuses on the space program. "We're playing this kind of Russian roulette, and given the problems we've seen on this flight, it suggests that the payoff just isn't worth the risk anymore."

Roland dismissed Discovery's voyage as a "comic opera," but even its defenders acknowledge they cannot wait to see the shuttle retired: "The mission demonstrated a capability for crew repair," said John Logsdon, director of the Space Policy Institute at George Washington University. "But the main problem that was there prior to launch is still there: 'How many more times do you fly these risky vehicles?' "

Perhaps 20. NASA plans to do what is possible to finish the space station and close the book by 2010, when President Bush's "Vision for Space Exploration" calls for development of a new "crew exploration vehicle" for colonization of the moon by 2020 and eventual travel to Mars.

A Relic of the 1970s

Shuttle retirement in 2010 is not just political rhetoric. The orbiters, most of whose 2.5 million parts were made with 1970s technology, are getting more expensive to service and maintain.

This is not all the shuttle's fault. The idea at first was to carry satellites into orbit and launch them, but this "turned out to be the most expensive way to get things into space ever conceived," said University of Maryland physicist Robert Park, a long-standing critic of the U.S. human spaceflight program.

Then came the international space station, requiring quick-turnaround, heavy-lift cargo flights that tested the shuttle's design limits. Although the station's early rationale as a choice venue for gravity-free manufacturing never panned out, it has succeeded in promoting international cooperation and national pride.

Discovery spacewalker Soichi Noguchi brought a crowd of Japanese reporters to the current mission, while the new Canadian-built sensor boom starred by conducting inspections of the heat shielding so detailed that a television viewer on Earth could read the serial numbers on individual thermal tiles.

Still, the continued success of the station depends on a shuttle that can make frequent trips to it, and the shuttle, by any measure, is damaged goods.

Robbing Peter to Pay Paul

Keeping these aging spaceships going means that any substantial delay in getting the shuttle aloft again, or any prolongation of the shuttle's life beyond 2010, will cost NASA unanticipated hundreds of millions of dollars in the short term.

Currently, NASA has no alternative but to steal the money from other programs, a time-honored approach that has contributed to the quick torpedoing of previous plans to build a next-generation spaceship.

This time, the money squeeze has provoked draconian cuts in NASA's aeronautics division and unease among Earth and space scientists whose favorite projects are under scrutiny. NASA is considering defunding the fabled Voyager 1, which for $4 million per year is sending back the first satellite-gathered data from the very edge of the solar system.

Instead of beggaring other NASA programs, Sen. Kay Bailey Hutchison (R-Tex.), chairman of the science and space subcommittee, advocates "something creative, that frees up money for NASA."

She envisions turning the space station into a national laboratory with private companies and universities paying to conduct research there. She also has a proposal to bring the Defense Department and its deep pockets into partnership with NASA on aeronautics.

The more direct approach would simply be to give NASA more money so it can more quickly develop and build the new crew exploration vehicle -- even as the shuttle and space station continue to bleed the agency.

"The thing that makes funding easier is success," said House Majority Leader Tom DeLay (R-Tex.), who muscled this year's NASA budget through a recalcitrant Congress. "Success means Congress gets excited, and excitement breeds the next budget."

But for every Capitol Hill enthusiast, there is a lawmaker with a longer view. "The shuttle is just a riskier and more expensive proposition than had been calculated in the past," said Rep. Dana Rohrabacher (R-Calif.), who for eight years was chairman of the space and aeronautics subcommittee.

"It may well be more important right now to just close shop right away and use that money to accelerate the development of the next technology."

Details of the new moon-Mars initiative have begun to circulate beyond NASA walls, and call for abandonment of a reusable winged vehicle in favor of a space capsule like the one used in the Apollo program.

Voyages to the moon would be accomplished through an Earth Departure Stage and a Lunar Lander, to be launched atop a rocket NASA insiders have taken to calling "Big Daddy." Those elements would rendezvous with the crew capsule in Earth orbit before heading for the moon.

The beauty of the design is that it depends on tested technologies, many of which do not require major new investments to certify them as "man-rated." Even so, Logsdon said: "The only issue is whether it can be done on the available budget."

Maybe it can, but only if the shuttle flies again soon, flies often and is retired on time so the river of money that it consumes can be redirected toward more ambitious goals. "They get one more chance to make this work," McCurdy predicted.

The launch of the shuttle Atlantis, scheduled for Sept. 22, will depend on how soon NASA resolves problems with Discovery's foam insulation system.

Soichi Noguchi's participation in Discovery's mission has drawn a Japanese following.