posted 04-26-2010 08:03 AM               

The Physics of Space Shuttle Re-Entry

The phase of a spaceflight during which the craft leaves earth orbit and descends through the upper atmosphere is generally known as 're-entry'. In order to be in stable earth orbit in the first place, the craft must have attained and maintained a critical velocity. This orbital velocity is nearly thirty times the speed of sound - around 13 kilometres per second. If the craft moves any more slowly than this, it will descend to a lower orbit under the influence of gravity. Because the craft will now encounter atmospheric resistance, it will lose energy and fall to earth.

In order to make a safe landing, a returning spacecraft has to lose nearly all of that orbital speed. The operation is basically a reversal of the launch phase, and this means that the returning craft must sink as much kinetic energy1 as the propulsion systems generated between lift-off and orbit. Theoretically speaking, there are four fundamentally different methods of doing this:

• Powered Deceleration
  
• Energy Exchange
  
• Mass Shedding
  
• Energy Dissipation

An explanation of the shuttle's methods will be helped by a brief consideration of all four.