Shuttle Discovery is making its descent to Earth for a touch down in California after bad weather in Florida forced a change to the intended landing site.
Nasa hoped to land the shuttle at Kennedy Space Center, Florida, but it will now touch down at Edwards Air Force Base at 1312 BST (1212 GMT).
Earlier, Discovery conducted an engine burn to slow its speed enough to freefall back to Earth.
The shuttle will make a night landing at the runway north of Los Angeles.
At 1206 BST (0706 EST; 1106 GMT), the shuttle fired its two Orbital Manoeuvring System engines for about two minutes 42 seconds, at an altitude of around 329km (205 miles).
SHUTTLE LANDING SITES
Kennedy Space Center - the shuttle's "home" and preferred site for landing
Edwards Air Force Base - the shuttle has landed a total of 49 times at the base
White Sands - back-up known as Northrup Strip lies 45 miles north of US Army missile range
The shuttle started to experience the burning effects of the atmosphere at about 1240 BST (0740 EST; 1140 GMT), pitching down and then up so the protected underside is exposed to the most intense heating.
It hit the atmosphere an altitude of 120km (75 miles) and a speed of about 27,360km/h (17,000mph). Under certain circumstances, the shuttle's belly can be subjected to temperatures of 1,600C (3,000F) - hot enough to melt steel.
Discovery has begun to bank from side to side four times in order to dissipate speed, after which Commander Collins will take manual control to pilot the vehicle to touchdown at about 322km/h (200mph) - roughly 20 times the speed at which a commercial plane approaches the ground.
Discovery will land on runway 22 at Edwards Air Force Base about 54 minutes before dawn. Its steep trajectory will take it over the Pacific Ocean and just north of Los Angeles. Commander Eileen Collins will have to turn the shuttle a sharp 96 degrees to align it with runway 22.
Nasa will spend about $1m on returning the shuttle from California to Florida where its launches take place.
But rainstorms and lightning within 30 nautical miles (55.6km) of the Kennedy Space Center forced Nasa to scrub the two available landing opportunities in Florida on Tuesday.
The crew have several landing windows on Tuesday
And the situation onboard the shuttle meant Nasa were keen to bring Discovery down today.
A Nasa spokesman told the BBC News website that the crew only had enough consumables, such as food and water, to take them through Wednesday only. The shuttle's ability to remove carbon dioxide gas from the crew decks were also due to run down after Wednesday.
The spokesman added: "We don't want to push it that far, we'd prefer to have that day in the bank."
Nasa has been extremely careful for the shuttle's return to Earth, looking for ideal weather conditions to make the shuttle's glide down through the atmosphere as trouble-free as possible.
This is the first flight for the US space vehicle since the Columbia disaster in February 2003; and Nasa is not taking any chances on this outing.
The shuttle craft is vulnerable as it descends; Columbia disintegrated on re-entry. That disaster was caused by damage sustained when debris broke free during lift-off.
Space shuttle Columbia's external tank shed a suitcase-sized piece of insulating foam during launch that punched a hole in the ship's wing, allowing super-heated gases to pierce the airframe during re-entry and pull it apart.
Discovery was due to land on Monday in Florida but bad visibility due to a deck of low-lying cloud forced a delay for 24 hours.
Although Kennedy Space Center, at Cape Canaveral, is the shuttle's "home", Nasa has back-up landing strips at Edwards and the Northrup Strip at White Sands in New Mexico.
Foam debris broke free when Discovery was launched, but the crew made repairs in space and space agency officials say they are confident of a safe return this time.
Nasa spent months changing the way the insulation was applied to the tank. It was thought only small pieces of debris would come off during Discovery's launch on 26 July.
But onboard cameras captured multiple pieces of foam being shed from the tank - one of which was only slightly smaller than the one that doomed Columbia.
Nasa has grounded the fleet until it has made changes, so Discovery's visit to the International Space Station (ISS) may be the last shuttle mission for some time. On Saturday, Discovery undocked from the station before flying around the orbiting platform to check it for wear and tear.