Like back-to-back cannon blasts, the loud shock wave shook doors and windows in nearby towns as Discovery arced out over the Atlantic Ocean and then dropped toward a marsh-surrounded runway at NASA's coastal spaceport.

Awaiting the ship's arrival: A convoy of two dozen ground support vehicles and a team of 150 fire, crash and rescue workers as well as shuttle systems specialists.

Also on hand: U.S. and Russian flight surgeons who will carry out a battery of post-flight medical tests on Usachev and his colleagues, the three of whom now face a 45-day physical rehabilitation program.

The chief goal, said NASA flight surgeon Terry Taddeo, is "to get them back in shape, as close as we can to where they were before they flew, allowing them to get back to their normal duties."

Long stays in weightlessness weaken the heart, lungs and other major organs. Muscles atrophy from lack of use and the skeletal system withers away, resulting in a gradual but potentially serious bone loss.

What's more, blood and other body fluids pool in the head and upper torso, leaving long-term space travelers woozy upon their return to Earth. The reason: Pooled blood rushes back down into the lower torso once they reenter the planet's gravitation field.

The physical rehab program will start with a special calcium-enriched diet and relatively easy workouts, gradually increasing to more vigorous exercise over the course of the next two months.

"The first week is going to involve some fairly light workouts for them. They'll be doing some jogging in the pool. They'll be doing some active and passive stretching, and they'll be getting some massage for muscles soreness," Taddeo said.

"Despite the fact they've been doing some exercise in orbit, they're still going to be straining some muscles that they haven't used for a while."

Lap swimming and more energetic exercise will follow the second week along with calisthenics such as sit-ups, push-ups and simple gymnastics, and by their fourth or fifth week back on Earth, the trio likely will begin jogging, lifting weights and doing more strenuous exercises.

"It's just a ramp-up over those 45 days, and we're going to tailor it to the individual," Taddeo said. "Some people may make faster progress; some people may make slower progress."

But within a month or so, the station trio should be able to drive their own cars, or in Voss' case, fly airplanes. And Helms will be able to resume her favorite hobby: Cross-country skiing.

A relative quick recovery is expected despite their five-and-a-half-month stay in weightlessness. That's because the trio spent an hour or two every day working out on the station's treadmill, exercise bike and a resistive device that enabled them to do calisthenics.

The orbital exercise regime kept Usachev, Helms and Voss in relatively good shape, and it is expected to serve them well during their recovery.

"They're very well-conditioned, all things considered," Taddeo said. "The fact that they've been able to get through these exercise routines we've given them -- and the fact that they were doing a lot of good exercise even before they flew -- leads me to think that we won't have a problem with them."

The last leg of the station crew's journey began a little later than planned. A rain shower popped up just two miles (3.2 kilometers) south of NASA's shuttle runway, prompting mission managers to forego a 12:46 p.m. EDT (1646 GMT) landing opportunity.

Also threatening to stall the homecoming: A billowing bank of clouds that even the shuttle crew could see in otherwise sunny Florida skies.

"Florida is all clear except for that one big cloud right there on the approach to (the runway)," shuttle skipper Scott Horowitz said as Discovery flew over Cape Canaveral on the shuttle's last lap around the planet.

"You can tell it is just sitting there right at the Cape," he added. "Maybe it'll go away."

Towering and puffy white, the clouds cleared up just enough -- and just in time -- to give the shuttle crew a green light for a supersonic dive back through Earth's atmosphere.

Soaring high above the Indian Ocean, Horowitz ignited the ship's twin orbital maneuvering engines at 1:15 p.m. EDT (1715 GMT), a three-minute firing that sent Discovery on an hour-long freefall back to Earth.

Discovery zoomed south of Australia before heading out over the Pacific Ocean, where Usachev and his colleagues felt the pull of normal gravity for the first time since March 8.

The shuttle made landfall over Mexico, cruising along a flight path that took the ship north of Tropical Depression Chantal. The ship then skirted along the southern parts of Texas, Louisiana and Alabama before making a beeline over central Florida on its way to KSC.

The high-speed landing brought an end to NASA's 106th shuttle mission, a round-trip taxi mission that involved ferrying U.S. astronaut Frank Culbertson and two Russian cosmonauts to the station for a four-month research tour.

Discovery also delivered 7,400 pounds (3,330 kilograms) of food, clothing, research equipment and scientific experiments for the so-called Expedition Three crew, which includes Vladimir Dezhurov and Mikhail Turin.

Some 3,700 pounds (1,665 kilograms) of carry-on luggage, surplus station gear and garbage was brought back aboard the shuttle along with Usachev, Helms and Voss. Their station replacements will remain in space until Dec. 9.