Re-entry, the final most dangerous hour

A space shuttle is both lifeboat and death trap. Every launch is a potential explosion, every orbit a flight through a shooting gallery. But the real hazard has always been on the way down.

A space shuttle is both lifeboat and death trap. Every launch is a potential explosion, every orbit a flight through a shooting gallery. But the real hazard has always been on the way down.

The shuttle must hit the upper atmosphere at exactly the right angle, and it gets only one chance. The pilot has notional control but most of the decisions are taken by onboard computers. An hour before landing, and practically one fifth of the way round the globe, the shuttle must fire its rocket engines to slow itself down and drop towards Earth.

Around 3,000 miles west of the runway, it should enter the atmosphere at 400,000 feet, and 22 times the speed of sound, tilting its nose by 40 degrees, so that the thermal tiles on its underbelly will serve as a heat shield, and the hull of the spacecraft will act as an air brake. In effect, the vehicle is being hit by a 1,700mph wind. The orbiter and its crew must rely on the strength of a decades-old engineering design and a set of replaceable foam tiles to take the heat. At this point, all communication is cut off. The temperature of more than 1500C ionises the air around the spacecraft, it strips electrons from the atoms in the surrounding atmosphere.

This creates a zone of electromagnetic disturbance that no radio waves can penetrate. For 17 or more minutes, the shuttle is out of contact. It loses three feet in height for every 15 feet it covers. This is a descent far steeper than made by any powered aircraft.

At around 170,000 feet - about 30 miles - the craft's navigational equipment should pick up beacon signals from its landing strip, and begin to slalom in a series of wide swings to slow its speed further. At 40,000 feet, while still travelling at one and a half times the speed of sound, it executes a 180 degree turn and takes a bearing on its runway. At 13,000 feet, its speed begins to drop to 400 mph; its landing gear drops 11 seconds before touchdown and it hits the runway at more than 200 mph. All this time, it is at the mercy of sudden gusts of wind. There is no margin for error, no second chance and no hope of baling out.


Tim Radford, science editor

The GuardianTramp

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