Space Shuttle Re-entry
    The loss of the space shuttle Columbia on February 1, 2003, drove home a simple truth: space travel is never routine, no matter how many shuttles launch and land. Success depends on near-perfection--especially during re-entry, where there is almost no room for error. Here's how it's supposed to work.
A Speeding Bullet
    First, the shuttle has to slow down. While in orbit, it's traveling at more than 17,000 miles per hour (27,000 km/h). That really is faster than a speeding bullet. Imagine driving down a highway for five minutes, passing landmark after landmark as you go. The orbiting shuttle would speed by it all in a second.
    To shed some speed, shuttle pilots fire maneuvering thrusters on the shuttle's nose and tail, turning the craft so that its tail faces forward. Then they fire the main thrusters to slow down. Another thruster burn reorients the shuttle to its proper re-entry attitude, with the nose facing forward and raised about 30 degrees. These maneuvers are crucial. Too much speed, and the shuttle will slingshot back into space. Too little, and it will burn in an uncontrolled plunge. And only the proper, nose-up attitude puts the bottom of the shuttle in position to bear re-entry's brunt.
A Flying Brick
    At an altitude of 400,000 feet (about 75 miles up), the shuttle begins to catch a little air. Aerodynamic forces take effect. All thruster maneuvering  ceases, and the shuttle becomes a 120-foot glider. Onboard computers  maintain proper descent attitude and speed by adjusting body flaps, elevons on the wings, and a rudder. Yet without power, and traveling at hypersonic speeds through the thin atmosphere, pilots say flying the shuttle is like flying "a brick with wings."
    Plowing through air, however thin, causes drag, which slows the shuttle down--and creates gravitational forces that stress the ship and its crew. Apollo astronauts regularly endured more than 6 Gs during re-entry, while Mercury astronauts suffered up to 11 Gs, near the maximum  sustained force humans can stand. Shuttle crews experience only about 3 Gs, significantly less than during liftoff, because of the shuttle's longer descent time. But the mechanical stresses on the shuttle are intense, with shear forces exceeding 7,000 pounds per square inch.
A Blunt Bottom
    Still, the most difficult challenge of re-entry comes from another source: heat. Any object moving through the atmosphere encounters friction caused by air molecules hitting its surface. At relatively low speeds, like those of conventional aircraft, the heat caused by this friction is negligible. But as speeds increase, so does friction. For some objects, the extreme speed of re-entry can produce surface temperatures greater than 5,000 degrees Fahrenheit (2,700 degrees Celsius).
    Two factors are crucial in foiling this heat: the shape of the re-entry vehicle and the materials used to make its surface. Strangely enough, the best shape for an object during re-entry is not slender and streamlined, but blunt-faced. A blunt shape creates more drag, which allows the re-entry vehicle to decelerate more in the thin upper atmosphere, so that speeds in the thicker lower atmosphere remain as low as possible. The blunt shape also produces shockwaves away from the object's surface, which deflect more heat away.
Covered in Tile
    Blunt shapes have remained constant throughout the history of space travel. But heat-resistant materials have not. At first, NASA relied on a process called ablation to transfer heat during re-entry. The protective heat shield on those early NASA capsules burned and fell away, taking large amounts of heat with it.
    The shuttle requires reusable materials. So the nose cone and leading edges of the wings, which reach the highest temperatures during re-entry, are covered with reinforced carbon that can withstand temperatures up to 3,000 degrees Fahrenheit (1,650 degrees Celsius). The rest of the underside, as well as the forward fuselage, is covered with more than 20,000 silica fiber tiles that can withstand 2,300 degrees Fahrenheit (1,250 degrees Celsius). "Blankets" of silica fibers cover the parts that face the lowest re-entry temperatures. Underneath the brittle tiles is a layer of material that cushions them from the vibration, expansion, and contraction of the shuttle's aluminum frame and that insulates the frame from the radiating heat.
    Briefly Alone
    The tiles protect the shuttle from the heat, but they can't protect the radio. The intense heat of re-entry strips electrons right off oxygen and nitrogen molecules in the air around the shuttle, forming a highly conductive layer of ionized particles (called plasma) around the vehicle. Because radio waves cannot penetrate this layer, the shuttle can lose
communication with ground control. This "ionization blackout" can occur from 25 minutes to about 12 minutes before landing. During this time, shuttle pilots must often operate without assistance, increasing the difficulty of an already challenging process.
    At an altitude of roughly 83,000 feet (about 16 miles up), re-entry is over. On a good day, all that remains are five minutes of relatively cool, gentle, controlled gliding to the landing site. Yet the tremendous aerodynamic and thermal forces of re-entry always contain the potential for catastrophe. It is a danger the scientists, engineers, and astronauts of the shuttle program know all too well.

              Christopher Call
              August 8, 2005

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