Journey Home Is a Hazardous One-Shot Deal

The return to Earth is a hazardous facet of spaceflight, but NASA has never suffered a reentry failure in its 42-year-history of manned missions -- until Saturday’s loss of the shuttle orbiter 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 designed to keep both heating and the structural load within bounds that protect the orbiter structure and its occupants.

Ceramic tiles dissipate the heat as the orbiter reenters the lower atmosphere. If some of the tiles are critically damaged on liftoff, there is nothing the astronauts can do but hope they have enough protection from the undamaged tiles to get them home safely.

Reentry is a one-shot process that leaves no room for error. The onboard tanks on the orbiter -- the part of the shuttle system that orbits Earth and returns to the ground -- carry just enough fuel to fire its rockets once to initiate reentry. Once it is in the lower atmosphere, the craft becomes a very heavy glider because it has no engine that would allow it to circle and try again if everything isn’t correct on the first landing attempt.

An orbiter such as Columbia circles Earth at an altitude of about 200 miles and at a speed of more than 25,000 feet per second. Landing it involves gradually bleeding off that altitude and speed.

The process begins halfway around the world from the orbiter’s normal Florida landing site. Orbiters normally fly upside down, nose forward. When Mission Control in Houston tells the astronauts it is time to come home, they 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 Earth’s lower atmosphere.

At this point, the crew activates the craft’s auxiliary power units, which provide the energy necessary to operate the craft’s flight controls once it reenters the atmosphere. The auxiliary power units are hydrogen-powered fuel cells, which always have 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, 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 entry begins over the South Pacific between Japan and Hawaii, 5,063 miles from the Florida runway, with the orbiter flying 557,000 feet above Earth at a speed of 25,400 feet per second. A few minutes later, between Hawaii and the U.S. mainland, the orbiter actually encounters the upper fringes of the atmosphere at an altitude of 400,000 feet and a speed of 25,000 feet per second. At this point, it is about 40 minutes from touchdown.

The entry angle is very shallow, only 1 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 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 265,000 feet, the outside temperature has climbed to about 3,000 degrees and the air around the orbiter has become ionized, preventing transmission and reception of radio signals. This blackout continues for 12 minutes.

(The Columbia broke up at an altitude of 207,000 feet, according to NASA.)

By the time the orbiter has descended to an altitude of 83,000 feet, its speed has slowed to 19,000 feet per second, or Mach 2.5 (2.5 times the speed of sound), and it is 59 miles 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 49,000 feet about five minutes before landing.

This is the only point during reentry at which the crew could theoretically escape from the orbiter. At all other points during the descent, the craft is too high and flying too fast. There are no escape pods or ejection seats built into the shuttle, only parachutes.

As the craft nears the runway, the pilot guides it through a circle a little more than a mile in diameter to line up with the runway and make the final approach. The craft is now descending at an angle of about 20 degrees, nearly seven times the 3-degree angle used by a commercial aircraft.

At 2,000 feet, the pilot points the nose up at an even steeper angle to slow the rate of descent more and the landing gear is deployed. Once touchdown has occurred at about 215 mph, the pilot brakes the craft, releases a drogue parachute and deploys speed brakes on the tail to bring it to a halt.