Cargo weight is defined as the payload control weight plus
the weight of the attached hardware used to secure the payload to the
orbiter. Allowable cargo weight is determined by altitude and orbital
inclination. For example, on a standard inclination of 28.45 degrees,
maximum cargo weight capability in a circular orbit at an altitude of 100
nautical miles is about 55,000 lb. This capability decreases with altitude
and falls to about 40,000 lb. in a 300-mile circular orbit. At the higher
inclination of 57 degrees (also a standard inclination), cargo weight
capability is 40,000 lb. in a 100-mile circular orbit. This decreases to
slightly over 20,000 lb. in a 320-mile-high orbit. These weights are those
for a nominal ascent for what is described as a "simple, short duration,
satellite deploy mission."
The 20,000 lb. number roughly corresponds to the International Space
Station at its higher elevations. Note: Columbia's numbers for this are
2,000 lbs lower as it is a heavier orbiter.