WASHINGTON (Reuters) - Likened to an old truck, a clipper ship and a box of aging electronics, the shuttle Discovery heads home on Monday, and even its own commander says it should not fly again until problems are fixed.
Discovery's much-heralded mission -- the first since shuttle Columbia disintegrated over Texas on Feb. 1, 2003 -- faced problems that started before liftoff on July 26.
First there was a balky fuel gauge that postponed the first scheduled launch attempt on July 13.
Then a chunk of foam insulation fell from the shuttle's external tank. It did not hit the orbiter, but was troubling since falling foam debris was what doomed Columbia, and NASA had worked to minimize this problem for two and a half years.
Eileen Collins, Discovery's commander, said from orbit she was disappointed to hear about the falling foam.
"I don't think we should fly again unless we do something to prevent this from happening again," Collins said. Shortly after her comments, NASA said it would keep the shuttle fleet grounded until the problem is addressed.
After that, astronauts inspecting Discovery's underside found two strips of special fabric protruding from between the ship's heat-shielding tiles, which NASA engineers feared might cause dangerous overheating on re-entry.
Spacewalking astronaut Steve Robinson plucked the strips out, but there was more: a damaged piece of insulating blanket under a cockpit window prompted concerns that it might break off and damage a critical part of the shuttle.
Wind-tunnel tests allayed these concerns, but still, Wayne Hale, the shuttle deputy program manager, acknowledged Discovery's return was not "zero risk."
All along, Hale and NASA Administrator Michael Griffin have made clear the shuttle is nearing the end of its lifespan. Hale said at a briefing at Kennedy Space Center that it reminded him "of an old truck I own."
Griffin compared it to ocean-going clipper ships that were state-of-the-art transportation in the 19th century. "The clipper ships were the peak of the sailing art and we don't see those either," he told reporters.
When one of the shuttle's 2.5 million parts broke, Griffin mused, "I wonder whether I could find a single electronics box in my house that's 25 years old and still works. I don't think I can. It's the same thing with the orbiter."
Designed in the 1970s and first flown in 1981, the shuttle was envisioned as a workhorse vehicle that would make space travel commonplace. Nearly a quarter-century later, after two deadly accidents that have killed 14 crew, NASA describes the shuttle as a test vessel whose useful days are numbered.
Slated for retirement by 2010, the three-ship shuttle fleet is meant to spend the next years completing construction of the International Space Station, satisfying U.S. commitments to the space agencies of Europe, Russia, Japan and Canada.
Given the shuttles' unsettled flight schedule -- the next launch window opens Sept. 9 but no shuttles are expected to fly until the debris problem is resolved -- the number of construction flights is limited.
Before the Columbia accident, NASA figured 28 shuttle missions would be needed to complete the station. Just before Discovery's launch, Griffin estimated as few as 15 flights.
After the shuttle, a new crew exploration vehicle is supposed to be developed to embark on President George W. Bush's ambitious plan to return Americans to the Moon by 2020 and send them eventually to Mars.
But a U.S. National Research Council report released this week questioned the scientific justification for human and robotic exploration of Mars.
A CBS News poll published on Wednesday found 59 percent of respondents felt the shuttle program is worth continuing, compared with 75 percent who thought so in February 2003.
What could replace the shuttle? One idea offered by aerospace firm ATK Thiokol is a two-vehicle launch system, derived from components that lift the shuttle to orbit. Instead of having a plane-like orbiter riding beside a fuel tank, the proposed system would put humans atop the rocket in a capsule, similar to what carried astronauts to the moon in 1969.
ATK Thiokol is a unit of Alliant Techsystems .