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. Last Updated: 07/27/2016

No Room for Error on Re-Entry

Re-entry is a hazardous facet of space flight, but NASA has not had an accident during the return to Earth in the entire 42-year-history of manned spaceflight -- until Saturday's loss of the Columbia.

Most attention to the risks of spaceflight has focused on liftoff because of the obvious dangers from the powerful rocket engines and the huge amounts of volatile fuel involved.

But bringing a spacecraft back to the ground is fraught with its own problems. It is a tightly controlled process that leaves no room for error. The orbiter's onboard tanks carry just enough fuel to fire its rockets once to initiate re-entry. Once it is in the atmosphere, it has no means that would enable it to abort and try again if the first attempt is flawed.

An orbiter like Columbia circles the Earth at an altitude of about 300 kilometers at a speed of more than 7,500 meters per second -- more than 10 times the velocity of a bullet. Landing it involves bleeding off that altitude and speed.

The process begins halfway around the world from the Florida landing site. Orbiters normally fly upside down, nose forward, cargo hold open. When Mission Control in Houston tells the astronauts it is time to come home, they close the cargo door and use the orbiter's thrusters to turn the ship so it is flying tail-first.

Firing the rockets lowers the craft's speed just enough so that the lowest point of its orbit -- the perigee -- is in the Earth's atmosphere.

At this point, they activate the craft's auxiliary power units, which provide the energy necessary to operate the craft's flight controls once it re-enters the atmosphere. The APUs are hydrogen-powered fuel cells, which have always presented a small risk of exploding.

It takes about 25 minutes for the orbit to degrade enough to begin the actual entry. During this period, the thrusters are used again to flip the orbiter upright and point the craft nose forward, with the nose at an upward angle of about 40 degrees. At that angle, the ceramic tiles on the belly of the craft bear the brunt of friction with the air.

The actual entry begins over the South Pacific between Japan and Hawaii, 8,100 kilometers from the runway, with the orbiter flying 167,000 meters above the Earth at a velocity of 7,620 meters per second. A few minutes later, the orbiter encounters the upper fringes of the atmosphere at an altitude of 120,000 meters and a velocity of 7,500 meters per second. At this point, it is about 40 minutes from touchdown.

The entry angle is very shallow, only one degree. Any shallower, and the craft would bounce back out into space. Any deeper, and it would heat up too rapidly.

With the nose tilted upward, the delta-shaped body would generate significant lift, so the craft is banked back and forth in gentle S-curves to bleed off the lift and allow it to continue its descent. By the time the craft has reached an altitude of 79,500 meters, the outside temperature has climbed to about 1,648 degrees Celsius and the air around the orbiter becomes ionized, preventing transmission and reception of radio signals. This blackout lasts for 12 minutes.

Speed control and shielding are crucial during that interval. When a satellite or other unprotected spacecraft reaches 72,000 meters, it begins to break up from thermal and mechanical stresses.

By the time the orbiter has descended to an altitude of 25,000 meters, its speed has slowed to 5,700 meters per second, or Mach 2.5 (2.5 times the speed of sound), and it is 95 kilometers from the runway. The pilot begins a series of sweeping S-turns, called roll reversals, to slow the craft to Mach 1, which is reached at an altitude of 14,700 meters about five minutes from landing.

This is the only point during the entire re-entry procedure where the crew could theoretically escape from the orbiter. At all other points during their descent, the craft is too high and flying too fast for them to bail out. There are no escape pod or ejection seats built into the shuttle.