Orbiting, Weightlessness, and Re-Entry



What keeps the space shuttle and other satellites in circular (or near circular) orbit?  For the specified altitude (and radius from the center of the earth), the shuttle is traveling at a velocity that creates a centripetal force that is equal to the weight of the space craft at that altitude.  For an altitude of 400 km above the earth's surface, the orbiting velocity is 7,700 m/s.  Because of the near frictionless environment at that altitude, there is little friction to slow the spacecraft, and therefore is able to maintain orbit without firing the engines.  This ability of the spacecraft or satellite to remain in orbit for long periods of time without using an engine to apply thrust is an example of which law?


At the altitude of 400 km above the earth's surface, the acceleration of gravity is about 8.8 m/s2.   With such an acceleration of gravity pulling a person down, how can the person feel "weightless?"  The direction of the velocity (tangent to the radius) is attempting to move the spacecraft out away from the earth (higher altitude), but gravity is pulling the spacecraft down toward the earth.  At the right velocity (orbital velocity - see above), the gain in altitude created by the tangential velocity of the spacecraft is equal to the downward loss in altitude due to gravity.  The spacecraft stays at the same altitude and remains in a circular (or near circular) orbit.  You appear to be weightless because you are constantly falling without losing altitude.

To re-enter and land, the spacecraft (Space Shuttle or Mercury/Gemini/Apollo capsule) must fire an engine to slow the forward velocity of the spacecraft.  As the spacecraft begins to slow, the pull of gravity becomes greater than the centripetal force and the spacecraft loses altitude.  When in orbit the spacecraft has a lot of stored energy as both potential energy and kinetic energy.  For the Space Shuttle, this is 3 trillion Joules of energy, and for an Apollo capsule that is 400 billion Joules of energy.  These are a very large numbers.  When the spacecraft lands, all of this stored energy must be given up.  What happens to this energy?  Most of the energy is converted into heat created by friction in the atmosphere.  Enough heat to destroy the spacecraft and kill the astronauts.  On John Glenn's Mercury flight and the flight of Apollo 13, there were concerns that the heat shields may fail, but they did not.  On February 1, 2003, Space Shuttle Columbia disintegrated during re-entry over Texas, on its 28th mission, killing all seven crew members.  This was the result of missing heat shield.  How is the heat given off without destroying the spacecraft?

Express 3 trillion Joules of energy using metric prefixes:


Express 3 trillion Joules of energy in terms of billions of Joules:


Express 400 billion Joules of energy using metric prefixes:


3 trillion Joules is also equivalent to:


  On the Apollo space capsules, the forward facing side is coated with insulating layers that slowly burns away (taking the heat with it) as the module re-enters the atmosphere.
  On the Space Shuttle, special heat insulating tiles are designed to radiate the created heat out away from the Space Shuttle, while at the same time providing a layer of insulation which prevents most of the heat from transferring through through the tile and into the Space Shuttle.  The heat insulating tiles on the Space Shuttle are capable radiating away 24 million Joules of heat per second.  

PHY280 General Physical Science   /   Webber International University