Grieving NASA scientists began painstaking detective work yesterday to pinpoint the cause of the catastrophe that tore apart the space shuttle Columbia as the aging spacecraft plummeted into the atmosphere at more than 12,000 mph.
They have a critical clue in the first sign of trouble during re-entry: the successive failure of temperature sensors embedded in the left wing, the left tire well and the hydraulic system that controls the left wing flaps.
"It's as if someone just cut the wire," said NASA shuttle program manager Ron Dittemore.
The fact that the sensors cut off, sending a warning message to the astronauts' display screen that they appeared to acknowledge in their last radio transmission, might mean that ferocious heat caused by air friction was burning through the protective tiles that wrap the craft.
The failing sensors could also mean trouble with the hydraulic controls, interfering with the precise, computer-controlled choreography of the flaps that steer the shuttle.
The speed of re-entry at 39 miles above Earth would mean that even the tiniest unintended flap movement would send the 90-ton, 122-foot vehicle into a lethal tumble, instantly breaking it into pieces.
NASA officials were also considering the possibility that the left wing was damaged shortly after takeoff Jan. 16, when a chunk of foam insulation peeled off the external fuel tank and struck the leading edge of the wing.
A final verdict on what caused the first fatal accident during re-entry in 42 years of space missions will not be possible for days or weeks at the earliest. "There are a lot of things in this business that look like the smoking gun that turn out to not even be close," Dittemore said.
But NASA officials and spaceflight experts said yesterday that even as the strapped-in astronauts enter the last minutes of their return home, the physical conditions outside the craft during re-entry create a dangerously unforgiving situation.
Critical flight moments
The three most hazardous moments in any shuttle flight are takeoff, the point one to two minutes later when forces on the shuttle reach their maximum, and re-entry, said Mark Lewis, an aerospace engineer at the University of Maryland who is leading an effort to design the next generation of space shuttle.
The Challenger disaster, in which the spacecraft blew apart 73 seconds after takeoff, led to intensive research on the first two critical points, but the re-entry phase has gotten less study, he said.
"I've told my students for years that the next time we lose a shuttle, it will be on re-entry because we're paying so much attention to launch," said Lewis, director of the university's Reusable Launch Vehicle Institute.
As with all shuttles landing at the Kennedy Space Center in Florida, the descent of Columbia began about 200 miles over the Pacific Ocean. The shuttle, traveling 17,000 mph, rotated tail-first and at 8:18 a.m. fired its main engines for about two minutes, slowing the vehicle.
NASA considers this "burn" the most critical moment of re-entry: If it is too short, the shuttle could hit the atmosphere at too shallow an angle and skip off into space. If the burn is too long, the shuttle could come in too steeply and be incinerated.
Once the shuttle wings bit into the thin upper atmosphere, the spacecraft began a series of S-shaped banking maneuvers to slow its descent. At that point, the shuttle was coasting like a glider - though engineers say it more closely resembles a falling brick. Had all gone according to plan, Columbia would have touched down at Kennedy at 9:16 a.m.
At Mission Control, the first signs of trouble came at 8:53 a.m., when hydraulic sensors on the left wing stopped functioning, chief flight director Milton Heflin said. At 8:56, the temperature gauge in the left main landing-gear well stopped signalling.
Those irregularities, however, did not raise undue alarm. It's not unheard-of for sensors to wink out. "The vehicle was performing fine," said Heflin.
Then, at 8:58, three more temperature sensors embedded in the left wing cut off. A minute later, the left inboard and outboard tire pressure sensors began to malfunction.
NASA flight controllers radioed:
"Columbia, Houston, we see your tire pressure messages and we did not copy your last. ..."
Columbia responded: "Roger, uh . ... "
"Then we lost all vehicle data," Heflin said.
At last contact, Columbia was at an altitude of 207,135 feet, traveling at Mach 18.3, or about 12,500 mph.
Scraps of data
The scraps of sensor data radioed back to the ground in the last minutes will be the core of the investigation. "We're getting some hints of where we need to look," Dittemore said.
Officials said it doesn't appear that Columbia's flight computers or hydraulic controls malfunctioned because the spacecraft hewed perfectly to its flight path until the moment it was lost. Nor did the age of the oldest U.S. shuttle - which first flew in 1981 and was completely overhauled two years ago - seem a likely factor in the accident, they said.