The space shuttle Discovery is coming home.
At 0706 EDT (1106 GMT), two of its engines fired for just under 3 minutes to start the orbiter鈥檚 return into the Earth鈥檚 atmosphere. It is now committed to landing at 0812 EDT (1212 GMT) at Edwards Air Force Base in California. Discovery will be the 50th shuttle-landing at Edwards.
鈥淲e can offer you a go for the de-orbit burn and you will be go to manoeuvre on time,鈥� shuttle communicator Ken Ham told the crew from Johnson Space Center in Houston. 鈥淭he observation at Edwards is clear. The winds are shifting slightly to the southwest.鈥�
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NASA passed over two chances on both Monday and Tuesday to land Discovery in Florida because of rain and the threat of lightning around Kennedy Space Center. Chief astronaut Kent Rominger, who flew practice landings at KSC in the Shuttle Training Aircraft, reported St Elmo鈥檚 fire, or glow discharge, during his flight.
In contrast, the weather at Edwards is fine and the winds light. But it will cost NASA a million dollars to transport the shuttle back to Florida after landing. It will ride piggyback atop a modified 747 jet for the cross-country flight.
The delays have added tension to an already nerve-wracking end to the mission 鈥� Discovery will be the first shuttle to land since Columbia broke up during re-entry in 2003 with the loss of all seven crew.
When Ham asked Collins how she and her six crewmates would feel about landing at Edwards, she replied: 鈥淲e are ready for whatever we need to do.鈥�
The seven astronauts began the final preparations for landing by drinking lots of fluids to prepare for their return to Earth. Astronauts become dehydrated in space, so the extra fluids help ease them back into the ground environment.
West coast option
At the time of the de-orbit burn, the shuttle is travelling at a speed of roughly 26,500 km/h (16,500 mph). About 30 minutes before landing and at about 120 kilometres altitude, Discovery will encounter the uppermost reaches of the atmosphere about 125 kilometres (78 miles) above the Cook Islands in the South Pacific. The heating caused by air friction reaches its maximum of over 1260掳C about 20 minutes before landing.
To help it slow down during its descent, the shuttle will make four S-turns, or banking manoeuvres. During the first turn, the shuttle tips its wings 74掳 relative to the horizon. While still flying supersonic, the orbiter will create two sonic booms as compressed air in front of the nose and the wings produces shock waves.
At 0808 EDT, just five minutes before landing, the orbiter will be flying slower than the speed of sound, and Collins will take over control of the spacecraft. Traditionally, commanders have given their pilots, in this case, Jim Kelly, some time behind the wheel.
The orbiter will cross the coast of California just north of Los Angeles to avoid the densely populated region. As it nears the landing strip, it will make a right-hand U-turn to line up with the runway. As it touches down, it is still travelling about 346 km/h (215 mph).
So which moment is the tensest for the entry flight team? All of it, entry flight director Leroy Cain said before landing: 鈥淚t鈥檚 pretty much about five minutes before we do the burn until wheel-stop.鈥�
Super-heated gases
Columbia had a hole in its left wing caused by insulation foam that fell off the external tank during lift-off. Nine minutes after Columbia dipped into the outer boundary of the atmosphere, ground observers saw debris falling off the orbiter as it flew over California. Super-heated gases were entering the hole and destroying the orbiter.
After the Columbia accident, NASA completely revamped the space shuttle programme, tackling engineering safety problems and how management decisions are made. The external tank was redesigned to limit the amount of foam and ice that could come off the tank and strike the orbiter.
But NASA still spotted four sizable pieces of foam coming off the external tank during Discovery鈥檚 launch. As a result, the agency grounded the remaining shuttles until they could determine the cause of the foam shedding.
The first ever detailed in-orbit inspection of Discovery鈥檚 heat shield did not find any similar damage on this mission. Video cameras and laser scans revealed scuffs and dings, but no holes.
Catch up with every twist and turn of the mission in our , including images, video, an interactive timeline and our exclusive 鈥淚nstant expert鈥� feature.
