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Touchdown’s terror hour

Houston, Feb. 2 (AFP): The return to Earth by any US space shuttle is a dizzyingly risky operation.

In the 42 years of US manned space flight there had never been a fatal accident on the return from a mission, but that all changed with Columbia yesterday.

James Oberg, a space expert and former Nasa worker who has written many books on the topic, said: “There is a lot of danger when it is coming down.”

The danger starts at one hour before landing.

Touchdown minus one hour: With the green light from ground control, the crew manoeuvres the shuttle to slow down enough to fall out of the Earth’s orbit. Columbia’s orbit was at 280 km.

Touchdown minus 32 minutes 14 seconds: The shuttle is at 122 km altitude, travelling at 27,400 km an hour, but is still 8,500 km from its target, the Kennedy Space Center in Florida.

In the “interface” period, the vessel hits the upper layers of the upper atmosphere and air friction slows the shuttle, which is dropping towards Earth at a rate of descent of 9 km a minute.

The shuttle must then tilt upward to present its flat bottom, covered with heat-resistant ceramic tiles, to the upper atmosphere, with the nose angled upward at 40 degrees, the so-called “angle of attack”. This angle is critical to the shuttle’s re-entry. If it is greater than 40 degrees, the craft could flip over backward from the atmospheric thrust. Less, and it would risk entering the atmosphere too fast, and overheating and possibly melting its aluminum shell.

Touchdown minus 28 minutes, 42 seconds: At a speed of Mach 25, 25 times the speed of sound, and an altitude of 85 km, the hydraulic wing flaps, or elevons, play a key role, helping the shuttle to make a series of wide turns to reduce the speed.

Touchdown minus 26 minutes, 56 seconds: The shuttle uses its elevons to make a series of 60 degree left-to-right banking manoeuvres to slow the craft without having to lift the shuttle nose, while increasing the descent rate.

Touchdown minus 26 minutes, 4 seconds: The shuttle is flying at Mach 19. The heat tiles on the bottom of the craft are at 1,650 degrees celsius.

Touchdown minus 16 minutes: This is the moment when Columbia broke up, at 200,000 feet and traveling at 20,000 km per hour, roughly six to seven times the speed of a rifle bullet. The craft was making a banking manoeuvre at 57 degrees.

Touchdown minus 12 minutes: The angle of attack, at which the nose is raised, decreases to maximise the effect of the speed brake flap.

Touchdown minus 10 minutes, 33 seconds: The shuttle is traveling at Mach 7.3, about 10,000 km per hour at 140,000 feet. Internal navigational systems are shut down in favour of radio-navigation devices used in commercial jets.

Touchdown minus eight minutes, 44 seconds: At Mach 5 and 118,000 feet, the craft becomes more controllable.

Touchdown minus three minutes 57 seconds: At an altitude of 46,000 feet, the shuttle speed drops below the speed of sound. At this point, the pilot takes over manual control of the shuttle from the onboard computer.

Touchdown minus one minute, 31 seconds: At 15,000 feet, the pilot can see the landing strip 10 km away but is moving at 510 km an hour. The shuttle is descending at a precipitous angle of 18-20 degrees, compared with a commercial airliner’s descent of about three degrees.

Touchdown minus 33 seconds: At 2,000 feet, the shuttle brings up its nose to slow the craft from 555 km per hour.

Touchdown minus 20 seconds: At 300 feet, the landing gear drops.

Touchdown minus 10 seconds: The shuttle is over the start of the runway.

Touchdown minus 0: The shuttle touches down. A parachute is deployed with the shuttle moving at 350 km per hour. Brakes are applied and the shuttle comes to a stop within 10,000 feet.

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