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At hypersonic speeds, Discovery shuttle landing is innately risky
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  • CAPE CANAVERAL, Florida (AFP) Aug 06, 2005
    Slashing speed from nearly 29,000 kilometers per hour (18,000 mph) at up to 1,650 degrees Celsius, landing the US space shuttle permits little error in the best case, NASA officials say.

    The landing Monday of the Discovery shuttle will carry an even greater sense of risk, though, coming thirty months after the Columbia shuttle disintegrated upon reentry.

    Seven astronauts died in February 2003 when the Columbia broke up while descending to Earth after superheated gases entered the spacecraft through holes in its heat shield.

    The tragedy stalled the US space agency's shuttle program so that NASA engineers could reduce the spacecraft's vulnerabilities.

    But the risks are still innate and uncommonly high, officials of the National Aeronautics and Space Administration say.

    "Flying a re-entry is not what normal sane people would normally call safe," said Wayne Hale, shuttle deputy program manager.

    "It is not 100 percent safe," he stressed.

    With little room for error, the shuttle must turn around in space an hour before the scheduled landing and fire its engines to decelerate from 29,000 kmh.

    The shuttle pushes through the outer layers of the atmosphere about 25 minutes later, at an altitude of 129 kilometers (80 miles).

    It is still some 8,000 kilometers (4,970 miles) away from the landing site at Cape Canaveral, Florida.

    As it drops through the atmosphere at a rate of 9 kilometers (5.6 miles) a minute, the shuttle effectively changes from being a space orbiter into an unpowered sailplane.

    "You only get one shot at it, since we are only a big glider," said Paul Hill, the lead shuttle flight director.

    The shuttle continues its descent with its heavily insulated belly facing the earth's surface, angled at 40 degrees.

    The angle of entry is crucial in breaking through the densest layers of the atmosphere.

    A more shallow angle would see the shuttle ricochet back into space, like a rock skimming over the water's surface. A steeper angle would make it overheat. The least error could be fatal.

    As the air pressure builds in the atmosphere half an hour before landing, 85 kilometers (53 miles) above earth, the shuttle's wing flaps and rudder become useable.

    The shuttle makes a series of four steep, 'S'-shaped turns, rolling to the side as much as 80 degrees, in order to slow down.

    Nevertheless, the Discovery will still be moving at supersonic speeds, and the friction of the air will push the temperature of its exterior ceramic tiles to 1,650 degrees Celsius (3,002 Fahrenheit).

    It was during this phase of the landing descent, about 16 minutes before landing, that the Columbia broke up in mid-air over the southern United States in 2003.

    Its heat shield had been damaged by pieces of foam which broke off the shuttle's fuel tank when it was launched.

    Just ten minutes before touchdown, 43 kilometers (27 miles) above the Earth, the shuttle is still moving at nearly 10,000 kilometers per hourmph).

    Two minutes later, the shuttle finally becomes controllable by the astronauts inside.

    Flying through the inner atmosphere over Florida four minutes before landing, the shuttle emits two massive sonic booms as it drops beneath the speed of sound.

    The astronauts begin manually steering the aircraft into its final approach trajectory, about 40 kilometers (25 miles) from the runway.

    Ninety seconds from landing, as the astronauts line up with the runway ahead of them, the Discovery will still be at an altitude of 4,500 meters (14,763 feet).

    Still moving at 518 kilometers per hour (321 mph), the shuttle has to descend at a much more acute angle than a commerical airline, losing altitude about twenty times faster.

    Finally, at 610 meters (2,000 feet) above the ground, 33 seconds before touchdown, the pilot will pull up Discovery's nose to slow it aerodynamically, and then lowers the landing gear.

    The rear wheels hit the tarmac first at 354 kilometers per hour (220 mph), speed breaks are deployed to the maximum, and a parachute is released from the shuttle's rear to help slow it down.

    The pilot gently brings the nose down, applies the breaks, and lets the now-useless parachute fall away from the aircraft. If all has gone perfectly, the shuttle will roll calmly to a stop.




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