"So they're in for a very interesting set of sensations. No doubt about that."

Hitching a ride aboard shuttle Discovery, U.S. astronaut Bill Shepherd, 51, and two Russian cosmonauts -- Yuri Gidzenko, 38, and Sergei Krikalev, 42 -- are scheduled to return to their home planet at 12:56 a.m. EST (05:56 GMT) Wednesday.

High winds, low clouds and rain at Kennedy Space Center might force NASA mission managers to keep the crew in orbit yet another day, or divert Discovery to a back-up landing site at Edwards Air Force Base in California.

But this much is fact: Shepherd and company face an grueling return to normal gravity and months of physical therapy once they do touch down on terra firma.

The reason is simple: Long stays in weightlessness wreak havoc on the human body.

The heart, lungs and other major organs go through a deconditioning that weakens them. Leg and other muscles atrophy from lack of use and the skeletal system withers away, resulting in a gradual but potentially serious bone loss.

Blood and other body fluids pool in the head and upper torso, giving space travelers bloated faces and bird-like legs.

And when Shepherd and his crewmates first feel the tug of normal gravity 76 miles (122 kilometers) above the planet, that pooled blood will rush back down into their lower torsos, leaving them lightheaded at the very least.

"So in effect, they're going to feel sick when they get off the orbiter," said Terry Taddeo, the NASA flight surgeon that has been looking after the so-called Expedition One crew throughout a four-and-a-half-month stay in space.

"They're going to feel weak, and because of some of the deconditioning that occurs in zero gravity, when they try to stand up, they're going to feel lightheaded. So all of the above is going to make it not a pleasant experience, although, of course, it will be very exciting for them."

Shepherd, for one, is not looking forward to the experience. Gidzenko and Krikalev, both Mir veterans, have been through it before, but their station skipper will be making his rookie return from a long-duration spaceflight.

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"To be honest, I'm not that anxious to see what it's going to be like," Shepherd told reporters during a space-to-ground news conference last week. "Sergei and Yuri have done this before, and they're telling me it's going to be arduous."

"The re-adaptation back to Earth gravity is not easy after long-duration spaceflight," Krikalev added in an interview Tuesday. "I know that for the first several days, it's going to be tough on our bodies, our balance. But in a few days, I hope, we will be back to normal life -- although it takes a little more to come back to [preflight] condition."

The physiological phenomena are so severe that both NASA and its Russian partners go to great lengths to keep space fliers fit before, during and after excursions in weightlessness.

Launched Oct. 31, Shepherd, Krikalev and Gidzenko boarded the station Nov. 2 and up until their departure Sunday, they worked out two hours per day with the most advanced exercise equipment ever delivered to an orbiting outpost.

At their disposal: a high-tech treadmill, a stationary bike and a resistive exercise device.

The latter enables orbiting astronauts and cosmonauts to do squats, leg curls, calf raises, knee lifts and other calisthenics to exercise their legs, hips, trunk, shoulders, arms and wrists.

"We've provided them with exercise hardware better than anything that has flown before," Taddeo said. "So this crew will be better conditioned on return than any other crew."

Still, Shepherd and his crewmates will be strapped into specially designed "recumbent seats," which are chaise lounge-like contraptions that will allow the crew to lie down during Discovery's supersonic dive back through the atmosphere.

Thomas will help the three men get into the couches, which will be set up on the mid-deck of the shuttle's crew cabin, and he'll remain by their sides during the reentry ride.

"I'll just sort of keep an eye on them...to make sure that they're feeling okay. And then the main thing is going to be at wheels-stop, at landing, I think it's going to be important for them not to get up quickly," he said.

"I will help them un-strap and get their helmets off, so that they can just be physically comfortable. But I think that we need to wait until everybody else leaves the shuttle...and then the specialists can come in and help them get out, and they can take their time," Thomas added.

"You need to take your time after a long flight to get your land legs back, so to speak."

Taddeo and two other flight surgeons -- one Russian, one American -- will board Discovery and provide immediate medical attention after the shuttle rolls to a runway stop.

Fire, crash and rescue technicians then will carry the crewmembers off the shuttle either sitting upright or lying supine on a device that doubles as a chair or a gurney. Unless, of course, any of the three want to walk off the ship, in which case the doctors won't stop them.

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The flight surgeons, however, prefer the gurney.

Said Taddeo: "This is to preserve the validity of any medical and scientific data that we will get post-flight."

A battery of medical tests will following during the first six hours after touchdown, and then Shepherd and the cosmonauts will sleep eight hours before specialists help them board a plane for Ellington Field near Houston.

"Keep in mind, of course, that if for any reason we don't think the crew is ready to travel, then by all means we'll just keep them at the landing site," Taddeo said.

Sooner or later, Shepherd, Gidzenko and Krikalev will recline on couches in the Gulfstream 2 aircraft for the two-hour trip back to Texas.

Once there, though, American and Russian medical protocol will put Shepherd and his crewmates on slightly different paths.

"Bill Shepherd will be allowed to return home," Taddeo said, in order to "let him get familiar with his wife again and his normal life."

Gidzenko and Krikalev will spend two to three nights at Johnson Space Center crew quarters before temporarily moving into nearby apartments with their wives and children.

The cosmonauts and their families are scheduled to head back to Moscow 10 days after landing. They'll rehab at the Gagarin Cosmonaut Training Center outside the city.

Shepherd, meanwhile, faces at least two months of intense physical therapy, which will begin with assisted walking and stretching exercises. Within a week, he'll be working out in a swimming pool and riding an exercise bike before light weight training is added to the rehab regime.

For a month and a half, Shepherd's workdays will be limited to six hours, two of which will be spent in physical therapy. Mission debriefings will consume most of the rest of that time.

The intensity of exercise, meanwhile, will gradually ramp up over the course of time, and a special diet will be prescribed to counteract the loss of bone mass.

Rebuilding strong bones will take a few months, but Shepherd, an avid aviator, could be plying the Texas skies before the first of May.

"Theoretically, you could have somebody return [to Earth] and be able to go back to driving their own car -- conceivably, even be allowed to fly their own airplane -- after about 30 days of rehab," Taddeo said.

And while tough days lie ahead, Shepherd likely will look back on rehab as one of the most extraordinary periods of his life.

"It's a very unique part of doing a long duration flight, actually -- to come back and relive gravity again," Thomas said. "It is the most unique experience that anyone can go through."