Science | It's a triumph of international cooperation and engineering know-how. But its promise of enhancing our knowledge of living and working in space remains unfulfilled as its completion faces yet another delay. And the multibillion-dollar project's continued reliance on the Russians in light of the space shuttle's problems remains an embarrassment for NASA.

NASA's space shuttle woes may have indefinitely delayed future missions, but the turmoil also has retrained a harsh spotlight on the billions spent on the shuttle fleet's de facto mother ship: the half-finished international space station.

The logistical mess posed by one errant piece of foam and two dislodged gap fillers only sharpens the question many researchers have been asking for years: What, exactly, have we gained from the orbiting station? Is it a technological eagle showcasing our ambitions in space, or an ungainly turkey gobbling NASA's budget with little obvious return?

The station has demonstrated international cooperation and a good bit of engineering know-how. But one of its principal promises — that it would increase scientific understanding of the requirements for living and working in space, perhaps a jumping-off point for missions to the moon and Mars — has received another hit with the probability that the station's completion date will be even further delayed.

In all likelihood, Russia will again be called upon to support the station in a role akin to the one it played during the 2 ½-year lag between the Columbia and Discovery missions, when its Soyuz rocket missions at regular six-month intervals were the sole sources of supplies and crew replacements.

Apart from the embarrassment in relying on Russia's less glamorous but more reliable delivery system, NASA now faces the vexing issue of what to do if the shuttle cannot meet its obligation to deliver expensive European- and Japanese-constructed modules to the space station — something no other current delivery system can do.

Within Discovery's cargo bay, the Italian-built Raffaello module ferried two tons of supplies to the station, part of the 15 tons of equipment delivered overall by the shuttle. Discovery will return home with about 13 tons of equipment and trash, which had accumulated over the past 2 ½ years and made for cramped conditions aboard the space station, inhabited by American John Phillips and Russian Sergei Krikalev.

With the possibility of an extended downtime for its shuttle fleet, NASA is beefing up support of the space station in other ways. Over the weekend, the space agency decided to extend the mission by a day to transfer additional supplies, including nitrogen, water and laptop computers.

The extra work force also should come in handy for helping to repair the station's breakdown-prone exercise treadmill.

Monday, spacewalking astronauts cleared up another major headache by replacing a broken gyroscope — a washing machine-sized spinning wheel that helps to orient the station — and repairing a second one.

NASA Administrator Michael Griffin has insisted that another mission may yet fly before the end of the year. With the clock ticking on a fleet scheduled to retire by 2010, the agency faces considerable pressure to meet its international obligations and complete the station.

Laurence Young, the Apollo program professor of astronautics at the Massachusetts Institute of Technology, said in an interview before the Discovery launch that finishing the job is important for other reasons. Medical scientists need to know what impact space radiation has on humans, and what constant weightlessness does to the body during years of travel in space. Otherwise, there's not much point in planning a trip to Mars.

"It's the only place now to do research on extended duration, with multiple human subjects, exploring microgravity. I believe that if we shut down the space station, in a few years we'll have the science and technology community asking to build a new one," Young said.

But even before the Discovery mission, other observers argued that public enthusiasm for completing the space station had waned considerably.

"It's a white elephant," said Alex Roland, a history professor at Duke University who served as a NASA historian for eight years. "It's a gift that is so expensive to maintain that it bankrupts you. And that's what we've got. It's spectacular. It's marvelous. No one else got one. But we can't afford it and we have no use for it."

NASA has estimated the cost of the station over its lifetime at $100 billion.

Donna Shirley, who worked on the space-station project in the early '80s while at NASA's Jet Propulsion Laboratory in Pasadena, Calif., said one of the station's limiting factors is that it lacks a major plan for a centrifuge capable of simulating different gravitational conditions. The moon's gravity is one-sixth that of Earth's, she said, while gravity on Mars is equivalent to 3/8 the gravity on Earth.

"And we have no idea of whether people are going to be functional — you don't know if one-sixth gravity is going to be enough to fix these problems that are apparent in zero gravity," she said.

With further delays in completing the station, much less conducting research, such criticisms are bound to multiply.

Howard McCurdy, a professor of public administration at American University in Washington, D.C., and a longtime NASA observer, described the shuttle and space station as inseparable "Gemini twins" whose expensive maintenance demands may now force delays in NASA's other activities.

To move on toward its primary mission of space exploration, then, McCurdy argues that NASA should fly a limited number of shuttle flights to meet its obligations in completing the space station, then discontinue the shuttle program and find some other means of maintaining the station.

No easy task. But its success may decide how a rather large orbiting bird is best remembered: as an eagle, or a turkey.

Robert Cooke contributed to this story.