Soyuz undocks from space station

Updated 11:16 PM ET

Closing out a 166-day stay in orbit, two cosmonauts and a NASA astronaut strapped into a Russian Soyuz spacecraft Tuesday, undocked from the International Space Station and fell back to Earth, settling to a jarring rocket-assisted touchdown on the steppe of Kazakhstan.

Suspended below a large red and white-striped parachute, the Soyuz TMA-08M descent module completed the final stages of the flight within easy view of Russian recovery forces and long-range tracking cameras, setting down at 10:58 p.m. EDT (GMT-4; 8:58 a.m. Wednesday local time).

Recovery crews and flight surgeons quickly rushed to the scorched descent module to help commander Pavel Vinogradov, flight engineer Alexander Misurkin and NASA SEAL-turned-astronaut Christopher Cassidy out of the cramped spacecraft.

After initial medical checks and satellite phone calls to family and friends, the station fliers were to be moved into a nearby medical tent for more extensive checks as they begin readjusting to gravity after five-and-a-half months in the weightless environment of space.

Cassidy planned to participate in a new project to help researchers get a better idea of how long-duration spaceflight might affect astronauts on eventual flights to Mars.

"This will be the first opportunity where we ask the crew members post landing to do some exercises," said Mike Suffredini, space station program manager at the Johnson Space Center in Houston.

The idea is to get a better idea of what hurdles a Mars crew might face after a year-long flight without a medical team standing by to help out.

"And the question is, what is their condition, what can we expect them to do?" Suffredini said. "And it will kind of lead our thinking on how the first few days of any exploration mission would take place to make sure the crew doesn't hurt themselves in the process of landing and getting themselves ready to operate on the surface of a foreign planet."

As a former Navy SEAL and veteran of a 2009 shuttle flight, Cassidy is more familiar than most with physical fitness. But he said he expects vestibular difficulty -- poor balance and coordination -- to be more of an issue than physical strength.

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Soyuz undocks from space station