Commander Steve Lindsey and co-pilot Mark Kelly fired up the steering jets for three minutes, getting Discovery out of orbit and into an unpowered glide toward Earth.

The orbiter and its crew of six started their home-bound plunge about one hour before the scheduled 9:14 am (1314 GMT) landing at the Kennedy Space Center on Florida's Atlantic coast.

The final moments of a shuttle mission are among the most dangerous, and it was upon re-entering Earth's atmosphere that Columbia broke apart on February 2003, killing all seven astronauts.

By firing the steering jets, the crew slowed down the orbiter by about 300 kilometers (190 miles) per hour from about 29,000 kilometers (18,000 miles) per hour.

About 30 minutes before landing, the craft should encounter the first layers of Earth's atmosphere.

As it slices through the atmosphere, the shuttle is no longer powered by engines, but glides toward the shuttle landing facility. It touches down at about 350 kilometers (220 miles) per hour, and coasts to a stop with the help of a drag chute.

Should bad weather prevent the scheduled KSC landing, Discovery has another opportunity about 90 minutes later, and has until Wednesday to be back on the ground. If a Florida touchdown is not possible, it could head to an alternate facility in California or another in New Mexico.

But NASA officials hoped a flawless, on-time touchdown at KSC's 4,572-meter (15,000-foot) runway would cap what they have called a very successful mission.

They also hoped Discovery's mission would help ease concerns over the shuttle program that have persisted since the Columbia disaster, and pave the way for a resumption of regular flights in August.

Like last year's first post-Columbia flight, the latest shuttle voyage was largely aimed at improving safety of the shuttle missions that are critical to plans to complete construction of the International Space Station (ISS) by 2010.

NASA says the orbiting space station is a key component in US ambitions to send astronauts back to the moon, and eventually to Mars.

During their 13 days in space, astronauts Mike Fossum and Piers Sellers performed three spacewalks to test shuttle repair techniques and fix equipment needed to continue building the ISS.

The crew delivered critical supplies and removed vast quantities of trash, discarded material and experiment results from the orbiting laboratory.

They also dropped off European Space Agency astronaut Thomas Reiter of Germany, who became the space station's third resident.

The mission is the second since the Columbia disaster and the first since the shuttle fleet was grounded a year ago because the shuttle's external tank had shed a large chunk of foam on take-off.

A similar piece of foam insulation had peeled off on Columbia's doomed flight, piercing the shuttle's heat shield and causing the orbiter to break apart as it returned to Earth.

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