Q: My question is about your reentry to Earth’s atmosphere from orbit. I have wondered about this since Alan Shepard’s flight. When you do your deorbit burn and start to return, do you go from flying a happy-go-lucky ship floating through space to a long, nightmarish free-fall to Earth? Does it feel like you are falling at a very high speed? Do you feel hot? (Jim Hoover, Dolgeville N.Y.)
Where I used to fly, a couple of hundred miles up, a spaceship’s speed is roughly 17,500 mph, and it’s traveling in a nearly circular orbit. Firing the deorbit engines only slows the ship down by about 200-250 mph, enough to make the low point of the orbit intersect the atmosphere about a quarter of the way around the planet, roughly 20 minutes after the deorbit burn. Before the burn you’re in free fall; after the burn, you’re still in free fall, falling in a slight downward trajectory toward the atmosphere. There’s no detectable change in sensation, but once you strike the atmosphere at Mach 25, the ship will experience drag and start to decelerate. That deceleration will very gradually settle you into your seat for the reentry phase. Good-bye, free fall! Flying the space shuttle, roughly 40 minutes later, we were on the ground.
The heat shield protects the cabin from nearly all reentry heating. Under my orange launch and entry suit, I wore a water-cooled set of long johns that kept me cool. My body heat was transferred from the water-cooled underwear into the cabin air by a briefcase-sized thermoelectric chiller. In turn, the cabin air temperature in the shuttle was maintained at about 70-75 degrees all through reentry by the orbiter’s cooling systems. The cooling system removed excess heat from the air, electronics and the astronauts, and kept the cabin at a comfortable temperature, dumping the heat into the vacuum outside using a water-to-steam evaporator.