The first space shuttle mission in more than two years was postponed less than three hours before its scheduled launch Wednesday when NASA encountered a problem with one of the external fuel tank's low-level sensors.
No new launch date was set, but deputy shuttle program manager Wayne Hale told a news conference that "we would not in any conceivable way be ready to launch before Saturday." Earlier, NASA Administrator Mike Griffin had told visiting congressional members that the next launch attempt could not take place until Monday at the earliest.
Workers at Kennedy Space Center were draining the external fuel tank, Hale said, and the team would likely have more data about the problem late Wednesday night. He said a decision about what to do next would not come before Thursday.
The current launch opportunity extends until the end of July, after which NASA would have to wait until September. For now, Discovery's crew was staying in Cape Canaveral, officials said.
Engineers had encountered problems with the same sensor system during a launch-pad test in April, and NASA was not able to track down the precise cause at that time. A second tanking test in May was successful, but a different tank was installed on Discovery last month. NASA didn't run the tanking test for a third time, but instead checked out the individual components and cleared them for launch.
"We felt like we had a good system today — we had another anomaly," Hale said.
When one reporter asked who would take responsibility for not conducting another tanking test, Hale said he didn't know whether another test would have turned up the problem. Then Griffin interjected, "Guys, this is an intermittent [problem]. It's so far an unexplained anomaly."
The shuttle crew, led by commander Eileen Collins, was already strapped into the shuttle when the launch was postponed at about 1:30 p.m. ET. The seven astronauts climbed back out and were driven back to their quarters while the launch-pad team secured the shuttle.
The space shuttle Discovery had been scheduled to lift off at 3:51 p.m. ET on a mission to resupply the international space station and test safety procedures that had been developed in the wake of the shuttle Columbia's catastrophic breakup in February 2003. All seven of Columbia's astronauts were killed in the tragedy, which led NASA to ground its shuttle fleet.
Until the sensor problem was discovered, the main worry Wednesday had to do with Florida's changeable weather, not technical problems. During the morning, the skies over Kennedy Space Center crackled with lightning, thunder and rain. But strangely enough, weather conditions were all "green" — acceptable for launch — when the postponement was announced.
How the system works
The sensor system works a bit like a car's gas gauge, signaling when the fuel tank's level sinks to 2 percent of capacity. When the gauge starts to read "E," the shuttle's control system readjusts pressure levels in preparation for shutting down the main engines. A faulty sensor could cause the shutdown to happen too early or late.
There are eight of these low-level sensors, also known as engine-cutoff sensors or ECO sensors. Four measure the liquid oxygen in the fuel tank; four measure the liquid hydrogen. Mission managers said one of the sensors for the hydrogen levels was apparently sending out suspicious readings during a test — basically, continuing to signal that it was covered with liquid hydrogen even when the system was set intentionally to read "E."
The criteria for launching the shuttle require all four sensors to be working, even though the system could handle two failures during launch, Hale said.
Mission managers said the source of the problem might be with the circuitry or wires outside the fuel tank rather than with the sensors themselves. If the sensor inside the tank is found to be at fault, the shuttle might have to be brought back to the Vehicle Assembly Building for the tank repairs — and in that scenario, launch would likely be delayed until September.
Earlier, another launch-day snag was handled more easily: During early-morning preparations for fueling up the shuttle's external fuel tank, engineers encountered a problem with a launch-pad heater. A replacement heater was sent to the pad for a quick change, and after an hour's delay, the fueling process and other preparations went ahead smoothly.
Mission managers said that their response to the day's glitches showed that NASA's safety process was working properly. "We built a very tight safety net to make sure we don't fly until it's safe," Hale said.
Discovery's mission management team has been extraordinarily careful about the launch preparations, following the lessons learned in the wake of Columbia's loss.
NASA said it would launch no shuttles until the issues raised by the Columbia tragedy were identified and corrected. Investigators determined that Columbia's wing was damaged by foam insulation that flew off the external fuel tank just after launch — a mishap that was caught on tape but dismissed as a cause for concern at the time. Sixteen days later, during the shuttle's atmospheric re-entry, hot gases entered through the breach in the wing and destroyed the shuttle from the inside.
The Columbia Accident Investigation Board cited technical flaws as well as lapses in NASA's "safety culture." Since then, NASA says it has changed the agency's culture to be more attentive to problems, and it has reworked dozens of shuttle components and processes to minimize the risk of damage during launch.
More than 100 cameras have been set up to monitor Discovery's liftoff for any sign of flying debris. After the ascent to orbit, astronauts will use a brand-new 50-foot (15-meter) extension boom, equipped with laser rangefinders and a high-resolution camera, to take an up-close look at the shuttle's protective skin.
Is the shuttle safe?
NASA Administrator Griffin told reporters on the eve of the launch that the shuttle was as safe as engineers could make it. "There is nothing that we know of that we have not addressed," he said.
At the same time, he acknowledged that risks still remained. "Can there be something that we don't know about that can bite us? Yeah. This is a very tough business," he said.
Ironically, a launch-pad accident came to light just hours later: Engineers found that a 2-pound (1-kilogram) plastic-and-foam cover had fallen off one of Discovery's cockpit windows and damaged two tiles on an engine pod. Fortunately, the tiles were attached to a panel that could be easily switched out, and the pre-launch schedule was not seriously disrupted.
NASA sources told NBC News that the cover might have been dislodged when an airbag was being retracted from that area of the shuttle.
The mission agenda
Collins and her crew have been training for this flight for more than two years.
The shuttle is to bring 28,000 pounds (12,700 kilograms) of payload to the space station, including food and supplies, a replacement guidance gyroscope and a space storage platform. Three spacewalks have been scheduled:
- Spacewalkers will test tools that have been developed since the Columbia tragedy to fix cracks in the shuttle's protective skin, using sample materials. Those tools include a daub-on applicator, a special type of caulking gun and a series of patches equipped with expansion bolts. Griffin acknowledged that the tools on this mission didn't have "any substantial repair capability" but said the lessons learned would lead to improvements.
- A second spacewalk is aimed at removing a balky gyroscope from the space station and putting in the replacement. Backup gyroscopes have still been keeping the station in the proper position, but NASA wants to have the full complement working.
- The storage platform, which can hold supplies for future spacewalks, will be installed on the station's exterior during the final scheduled outing.
During the preparations for launch, Columbia was never far from the minds of the astronauts and launch workers. Collins has said her crew would present a tribute to Columbia's astronauts during Discovery's mission, but she kept mum about the details.
The families of the Columbia astronauts praised NASA's efforts over the past two years in a statement released Tuesday, and said that "the exploration of space must go on."
"We hope we have learned, and will continue to learn, from each of these accidents, so that we will be as safe as we can be in this high-risk endeavor," the families said. "Godspeed, Discovery."
Griffin said Discovery's mission would honor not only Columbia's astronauts, but other space explorers as well: "Every space launch we do is a tribute to all those who have gone before — the crews who have died as well as the crews who lived."
He specifically cited the loss of three astronauts in the 1967 Apollo 1 fire and seven astronauts in the 1986 Challenger explosion. "Going back even further to 100 years of aviation, the safety systems that we who fly have learned and know are written in other people's blood," Griffin said.