The tail of the space shuttle Discovery, framed by the window of the international space station's Destiny laboratory before undocking over the weekend. Photo: NASA

The Discovery astronauts tested the shuttle's re-entry systems late Monday and packed up for landing early Wednesday to bring the international space station's first crew back to Earth after 141 days in the weightlessness of space.

Re-adapting to gravity will pose a major challenger for Expedition One commander William Shepherd, Yuri Gidzenko and Sergei Krikalev, one requiring extensive physical therapy to regain the strength and coordination needed to cope with a world of ups and downs.

"The adaptation back to Earth's gravity is not easy after long duration flight," Krikalev told CBS News early today. "But that's why during flight we exercise almost every day, trying to be in shape in case of an emergency or even a normal landing.

"I know that for the first several days it's going to be tough on our bodies and our balance," he said. "But in a few days, I hope, we'll be back to a normal life."

Discovery is scheduled to land at the Kennedy Space Center at 0556 GMT (12:56 a.m. EST) Wednesday. But forecasters say low clouds, high crosswinds and possible rain showers associated with a low pressure system threaten to block a Florida landing.

Conditions are expected to improve somewhat by Thursday, but the outlook remains marginal.

Entry flight director Wayne Hale has activated Edwards Air Force Base, Calif., as a backup landing site for Wednesday. If the Florida forecast holds up, Discovery's crew could be diverted straight away to Edwards.

That's because conditions at Edwards are expected to worsen slightly by Thursday, making a Mojave Desert landing a fairly strong possibility for Wednesday. If the forecasts hold up, that is.

Whenever Discovery returns to Earth, Shepherd and company will face a lengthy recovery process to readapt to the tug of Earth's gravity.

The Expedition One crew answer questions from reporters today from the flight deck of Discovery. Photo: NASA

"It's going to be as if you're encountering gravity all over again for the first time," said flight surgeon Terry Taddeo. "It's like being stuck to a big magnet. Everything they used to take for granted is now going to take a lot of effort and in some cases, a lot of thinking about what you're doing.

"They will experience some neuro-vestibular effects. They're going to feel sick when they get off the orbiter, they're going to feel weak. When they try to stand up, they're going to feel light headed. All of the above is going to make it not a pleasant experience, although of course, it will be very exciting for them."

The Expedition One crew took off aboard a Russian Soyuz rocket on Oct. 31 and docked with the international space station two days later, becoming the complex's first full-time crew.

After overseeing the station's initial assembly and activation, Shepherd formally turned the ship over to Expedition Two commander Yury Usachev during a brief ceremony Sunday night.

An on-time landing Wednesday for Shepherd's crew would mark the end of a voyage spanning 140 days 22 hours three minutes and 19 seconds.

"I have certainly mixed emotions" about returning to Earth, Shepherd said. "Turning over command of the space station to Yury Usachev was the highlight of my professional career. I'm sad to depart. I will be happy to be safely on the ground with family and friends. So it's kind of a day with very high emotions but very different ones as well."

Cosmonaut Sergei Krikalev poses for a photo aboard the international space station as a shuttle approaches for docking in February as seen through a port hole. Photo: NASA

While all three crew members typically exercised one to two hours per day aboard the station, long-term exposure to weightlessness results in bone loss, muscle deconditioning and neuro-vestibular changes requiring significant rehabilitation.

"While we consider this crew to be the best conditioned and the best exercised crew that has flown a long-duration mission, what they have to look forward to is a period of several weeks and months of rest and rehabilitation afterward," Taddeo said.

To ease their return, Shepherd, Gidzenko and Krikalev will make the return to Earth resting on their backs in recumbent seats on Discovery's lower deck.

As soon as Discovery's hatch is opened, two NASA doctors and a Russian flight surgeon will examine the returning station astronauts and help them exit the spacecraft.

"We will be able to walk on the first day after our arrival, although it's not smart to try to do too much in the first several days," Krikalev told an interviewer today. "So we will exercise from the first day and as I said, we'll walk maybe a little the first day and a little more the next day."

The three station fliers will remain at the landing site - Kennedy or Edwards - for a full day before flying to the Johnson Space Center in Houston aboard a NASA training jet.

Astronaut Bill Shepherd in the Zvezda service module during his stay aboard the space station. Photo: NASA

Once back in Texas, "Bill Shepherd will be allowed to return home," Taddeo said. "Once there, we hope to protect his privacy, if not for medical reasons at least to restrict access, let him get familiar with his wife again and his normal life.

"The Russian side has requested that the cosmonauts spend the first two to three nights at JSC crew quarters. While our systems and our thinking about medical pre-flight and post-flight activity is very similar, there are some small differences.

"And one of them is they prefer to keep their crew in a similar quarantine to what they do pre-flight," Taddeo said. "So they'll be in crew quarters for the first day or two, after which they will then be allowed to go over to an apartment that is available to them and their families."

Gidzenko and Krikalev are tentatively scheduled to fly back to Russia about 10 days after landing. But the rehabilitation process will continue indefinitely. And it will begin almost immediately.

"The kind of activities they'll be doing, the first few days it'll be restricted to mostly assisted walking," Taddeo said. "There are some particular types of stretching we can do where they sort of relax and hold and push and you begin to re-develop all the motor-neuron connections, if you will.

Cosmonaut Yuri Gidzenko on the flight deck of Discovery after moving into the shuttle last week. Photo: NASA

"Then we'll start with some gentle activities in the pool, some cycling, nothing that has any kind of impact you could impart to the individual. Then hopefully, by about the first week, you'd actually start them on a very light weight-training regime.

"And then just simply ramp up from there, depending on the results you're getting, the status of the crew member, etc.," he said. "Theoretically, you could have somebody return, be able to go back to driving their own car ... after about 30 days of rehab."

But it will take two months or more to see "significant improvement," Taddeo said, adding that bone loss might not be fully reversed for an additional few months.

"We have a requirement that for the first 45 days after landing that they only work a six-hour day and that two hours of that day are devoted to rehabilitation," he said.