Originally designed in 1972 and first flown in 1981, the space shuttle relies on aging technology and is far more expensive to operate than anticipated. Internal investigations after the Columbia disaster two years ago exposed a wealth of still-unresolved problems inside NASA, and photographs of Discovery's flight revealed falling debris on liftoff.
As a result, the space shuttle fleet is temporarily grounded again and could be retired earlier than expected. Speaking to reporters on Tuesday after Discovery's landing, NASA Administrator Mike Griffin refused to speculate on when launches might resume. "We're not going to go until we're ready to go," he said.
On the latest flight, which involved restocking the international space station, Discovery's crew had to undergo a risky spacewalk to make unexpected repairs. Astronaut Stephen Robinson removed fabric protruding between thermal tiles that could have caused heat problems when the shuttle entered the Earth's atmosphere.
During the 29 months that the shuttle fleet had been grounded, NASA had taken extraordinary troubleshooting steps: It scanned shuttle panels for cracks with ultrasound and X-rays, redesigned the external tanks, added new sensors, and created a new level of bureaucratic strata focused on mission safety. But problems still arose.
Those glitches raise broader questions about the future of the shuttle fleet: When should it be put out to pasture? Should taxpayers fund a successor space vehicle that would operate under NASA's direction? Or can those functions be performed more cheaply and safely by a booming private sector that barely existed three decades ago?
Politics and symbolism seems to favor the second choice, especially because American spaceflight has been so closely linked to national pride. Even though many routine missions to space can be performed by robots--and there has been talk of redesigning the shuttle to operate under remote control--manned spaceflight continues to capture the public's attention and yield fatter budgets for NASA. (The agency's budget reached its maximum during the Apollo flights.)
One reason to keep the shuttle flying until a replacement becomes available is its ability to deliver supplies and equipment to the international space station and return items or people from orbit, also called "down-mass capability."
"If the shuttle is retired in 2010, that down-mass capability will clearly be unavailable," says a report prepared for the American Academy of Arts and Sciences by George Abbey and Neal Lane of Rice University. "There is no space vehicle other than the shuttle that has significant down-mass capability, nor are there plans for such a vehicle."
Also, if there's a problem and a shuttle were unavailable, the crew would be forced to rely entirely on the Russian Soyuz spacecraft to escape.
NASA had already planned to retire the space shuttle in 2010, to be followed by a new "crew exploration vehicle" in 2014 that could take astronauts to the moon and beyond. And President Bush said last year that the United States should undertake a long-term robotic and manned exploration of the solar system based on the CEV model.
"There are two alternatives," said John Logsdon, director of George Washington University's Space Policy Institute. "One is that NASA discovers that the shuttle is too risky to fly so it's retired and some other means, probably Russian, is used to supply the space station in its current configuration. I think that's the less-likely option but not impossible. The other is a kind of orderly program of shuttle flights to finish some version of the station."
Rep. Ken Calvert, a Republican from California and the chairman of the House of Representatives' science subcommittee, said this week that "I know we must retire the shuttle and replace it with a new vehicle. I am confident that NASA Administrator Mike Griffin will work toward a new crew exploration vehicle that will allow America to continue leading the world in human space flight."
Plans for the new CEV are still preliminary, but NASA seems to be moving toward an Apollo-like capsule design and announced last month that it has awarded contracts to Lockheed Martin and Northrop Grumman to sketch out early designs of a six-person vehicle that would be capable of reaching the moon.
