Columbia's final moments
8:53 a.m. EST
There is a loss of a temperature sensor in the hydraulics system in the trailing edge of the left wing.
Sensors in the left main gear tire well report a temperature jump.
Three temperature sensors in the left side of the vehicle fail. At this stage Columbia is at an altitude of almost 64 kilometres and its speed is in excess of Mach 18 (20,000 km/h).
Another eight sensors on the left wing, measuring tire temperatures and pressures, fail.
Mission control contacts the orbiter: 'Columbia, Houston. We see your tire pressure messages. We did not copy your last.' There is a short delay before the shuttle crew reply: 'Roger ... Erm ...' The signal is lost.
-*The orbiter reaches temperatures in the region of 1,650 C, its shell of protective heat-resistant tiles is crucial to its safe passage.
A hot topic
The heat tiles were a worry when Columbia first roared into orbit in 1981, a maiden flight postponed several times because the tiles wouldn't stick to the outer skin of the orbiter.
They were even a minor concern in 1984, when Canada's first space traveler, Marc Garneau, took his initial first flight aboard challenger and a small section went missing on a rear bulge after launch.
And now, the more than 24,000 soft, brittle tiles are one of the focuses of the investigation into Columbia's demise.
During the launch, 18 days ago, cameras caught a fragment of insulating foam from the external fuel tank bouncing on Columbia's left wing. Investigators are checking whether the collision damaged the thermal tiles more seriously than anyone realized.
One reason Columbia was two years past its originally scheduled launch date was that the chalk-like insulating tiles had to be repeatedly tested and retested as workers toiled day and night to replace hundreds of tiles which became unglued.
What do the tiles do?
Friction with air during lift-off and re-entry creates surface temperatures above melting point of the aluminum airframe (660 C).
they are made with lightweight silica, the tiles are so heat-resistant a blowtorch could be applied on them until red hot, yet they would cool quickly enough they could be touched with bare hands within seconds.
Each tile is custom made to fit a specific part of the ship, the average surface area of a tile is 30 centimetres square.
Manoeuvring the orbiter for re-entry
In most cases, the orbiter will have been flying nose-first and upside down (the base of the orbiter provides the best buffer against solar heat and radiation).
1. The crew fire the RCS thrusters to turn the orbiter tail first.
2. When the orbiter is tail first, the engines are fired to slow the speed of the orbiter, causing it to fall back to Earth. It will take about 25 minutes for the orbiter to reach the upper atmosphere.
3. During that time, the RCS thrusters are used to pitch the orbiter at about 40 degrees, an angle at which the bottom of the orbiter faces the atmosphere.
During this phase, radio contact is lost due to the tremendous heat caused by friction. Thrusters on the orbiter bank it from left to right, the resulting slalom motion helps to reduce the speed.
Columbia disintegrates during re-entry, less than 16 minutes from its scheduled landing.
4. After reaching the Earth's atmosphere the orbiter would normally descend to a runway in a very steep glide, using its air brakes to slow its rate of descent.
-*Authorities warn wreckage could be contaminated with toxic fuel.
-*Dozens of people admitted to hospitals after handling smouldering metal debris.
SOURCES: NASA / HOW STUFF WORKS / SPACEFLIGHT NOW / BBC / HOW THINGS WORK TODAY / RICHARD PALMER / TU THANH HA / REUTERS / GRAPHIC NEWS / THE GLOBE AND MAIL