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November 29, 2008

Shuttle Return Home

The STS-126 crew onboard the Space Shuttle Endeavour is planning to land tomorrow at Kennedy Space Center in Florida. However, a cold front might divert them to Edwards AFB in California instead.

The first landing opportunity is at on orbit 248. The deorbit burn would occur at 11:14 AM Central time for landing at Kennedy at 12:19 PM Central time. The next opportunity is on the next orbit with the burn at 12:50 PM and landing about an hour later. If the weather is not favorable, then the crew may do their burn at 2:20 PM for a landing at Edwards at 3:25 PM. The last opportunity for the day is at 3:57 PM for landing at 5:00 PM at Edwards.

The shuttle does not stay in space because there is no gravity! It stays up there because it is traveling fast enough to balance out the pull of gravity. To return to Earth, it must slow down. To do that, the shuttle points its orbital maneuvering engines into the direction it is traveling. Then like a person on a skateboard holding a fan into the direction they are rolling, the engines are fired to slow the shuttle down. This burn is made on the opposite side of the Earth from the landing site. Each orbit is about 90 minutes, so these opportunities are about 90 minutes apart.

There is not an opportunity to land on every orbit because the Earth rotates "out from under" the shuttle as it goes around, slipping to the west a little each orbit. This is also why the Florida opportunities come before the California opportunities.

The shuttle's flight was already extended an extra day to allow time for troubleshooting the water recycling system. If conditions are not favorable at either landing location, the shuttle has enough supplies on board for another day in space. Landing at Edwards would require the shuttle to be ferried to Florida later, so a Florida landing may be worth the wait.

Returning expedition crewmember Greg Chamitoff will have the benefit of a reclined seat for entry. In freefall, blood accumulates in the upper body because its weight does not help pull it toward the feet. The body adapts by reducing the total volume of blood in the body. The heart does not have to work as hard, and shrinks in size during long stays in space. Chamitoff has been in space for six months.

When an astronaut returns to Earth, the weight of the fluid in the body once again causes it to be pulled to the feet. The weakened heart beats faster to maintain blood flow to the brain, but with less blood to pump, is not able to completely compensate. Thus, if the astronaut returns in a sitting position, they will feel light-headed and may faint.

(Fainting lowers the head relative to the heart, so increases blood flow to the brain, restoring consciousness. Note, this would not work in freefall. To revive someone in freefall, another crewmember would have to swing them around by the feet!)

Once astronauts are no longer in freefall (they "hit" the floor of the shuttle and stop falling while flying, just like on aircraft), the brain triggers the body to make more blood. Blood is mostly water, so the astronauts drink lots of water prior to entry. The heart returns to normal within a few weeks.

Wherever they land, the crew is likely to return to Houston the next day for their traditional greeting at Ellington. They will receive the thanks for a job well done in readying the space station for an increased crew next spring.

To the stars,

Marianne Dyson
Member of the NSS Board of Advisors

Posted by m_dyson at November 29, 2008 08:31 PM



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