"We can't rush to judgment on it because there are a lot of things in this business that look like the smoking gun, but turn out not even to be close," Dittemore said.
In the meantime, the shuttle program has been put on hold as the NASA community will take time during the next few days to remember astronauts Rick Husband, Willie McCool, Kalpana Chawla, Laurel Clark, Mike Anderson, David Brown and Israel's first space flyer, Ilan Ramon.
"It's a tragic day, not only for America, but for the whole world and especially for the families of the astronauts," said U.S. Sen. Bill Nelson, D-Fla., who flew aboard Columbia during a 1986 mission as a U.S. congressman from the district that includes the Kennedy Space Center.
"The American people have started to think that flying in space is like getting into a car for a Sunday drive, but it's anything but that. Spaceflight is still a risky business," Nelson said.
Final minutes
Based on the latest information provided by NASA, here's what happened:
- At 5:35 a.m. EST (1035 GMT) Columbia's clamshell-like payload bay doors were closed and locked. Heat-dispelling radiator panels line the inside of the doors, which must remain open for the majority of each shuttle mission.
- At 6:50 a.m. EST (1150 GMT) commander Husband and pilot McCool donned their orange launch and entry suits and then strapped themselves in to the front left and right seats, respectively.
During the next hour or so the rest of the crew put on their suits and took their seats as well. Kalpana served as flight engineer and sat between the commander and pilot, just behind them. To the right of her was Clark.
The three remaining astronauts sat in the mid-deck underneath the flight deck.
While there was some brief discussion about fog at the landing site, the weather was forecast to be ideal, and Columbia's crew was given a go to fire the braking rockets that would commit the shuttle to its re-entry and touchdown.
- At 8:15 a.m. EST (1315 GMT) Columbia's twin orbital maneuvering engines were fired for about two-and-a-half-minutes, slowing the shuttle by about 176 mph, just enough to knock the spaceship out of orbit.
- At about 8:44 a.m. EST (1344 GMT) Columbia reached the 400,000-foot altitude that NASA considers the beginning of Earth's atmosphere. The shuttle's outer temperature began climbing but it would not reach its peak for another 15 minutes.
- At 8:53 a.m. EST (1353 GMT) Mission Control detected the failure of a pair of temperature sensors on the hydraulic system that moves the left wing's flaps.
- At 8:56 a.m. EST (1356 GMT) a set of sensors inside the left-hand main landing gear's wheel well saw a temperature increase in a brake line and the tires themselves.
"During this time the vehicle is performing fine. We had no indications of any problem," said Milt Heflin, a NASA mission operations manager.
- At 8:58 a.m. EST (1356 GMT) three more sensors in the left wing, these embedded into the vehicle structure, appeared to fail.
- At 8:59 a.m. EST (1359 GMT) another eight sensors on the left wing, these measuring tire temperatures and pressures, also failed.
One of those sensor failures prompted an alert message to appear on the shuttle's cockpit displays, as well as at Mission Control.
Following a routine procedure, astronaut Charlie Hobaugh radioed up to the crew, "Columbia, Houston. We see your tire pressure message and we did not copy your last."
"Roger. Uh..." Husband replied, but the transmission abruptly cut off. No further word was heard and all data being automatically sent to Houston suddenly stopped.
"As far as I know that was the last transmission from the crew," Heflin said. "Then we lost all vehicle data."
It was 9 a.m. EST (1400 GMT).
Columbia was 207,135 feet over central Texas moving 18.3 times the speed of sound, or more than 12,000 mph. There was no hope of any survivors.
More questions
Why all of this happened remains the big mystery.
Detectives faced with a mystery usually start by trying to determine if anything happened out of the ordinary, while also investigating every possibility.
With that in mind, NASA will spend time looking hard at an incident that took place on launch day when a piece of insulating foam fell from the external tank and apparently struck Columbia's left wing.
It is possible that the large chunk of debris damaged enough of Columbia's black heat protection tiles on the bottom of the wing to trigger Saturday's catastrophic series of events.
The shuttle's thermal protection system tiles are extremely fragile when struck by an object, yet easily withstand the flow of air as the shuttle flies through the atmosphere during launch and re-entry.
The tiles also take a long time to repair or replace when damaged, which is a big reason why NASA won't launch the shuttle through clouds or rain.
The black tiles are designed to withstand the greatest temperatures on re-entry and some 20,000 of them covered Columbia. White, felt-like blankets handle the less extreme temperatures. The pattern of tiles and blankets give the shuttles their unique livery.
If enough of the black tiles were severely damaged by the insulation debris, or a smaller number were completely ripped free from the shuttle's belly, then the amount of resulting heat during re-entry could build enough to begin melting the aluminum structure underneath.
If that turns out to be the case, it does explain the series of events leading up to the loss of contact with Columbia. The various sensor failures and readings could be an indication the wing was failing.
Alternatives
Other possibilities could explain the sensor problems as well, including a structural failure of the wing for some unknown reason unrelated to the tiles, or that the shuttle was slightly off course and momentarily generated more heat than the shuttle was designed to handle.
NASA officials refused to speculate, but did acknowledge the potential link between the launch-day debris and the landing-day disaster.
"As we look at that now in hindsight, we can't discount that there might be a connection," Dittemore said.
Mission managers had reported on Friday that they didn't believe any potential damage to the tiles was cause for alarm.
In fact, there was nothing anyone could do about it.
"There's nothing that we can do about tile damage once we get to orbit," Dittemore said. "Once you get to orbit, you're there and you have your tile insulation and that's all you have for protection on the way home from the extreme thermal heating during re-entry."
Dittemore said there is no way for spacewalking astronauts to fix tiles on the shuttle's belly.
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