Astronaut Mark Kelly and his crew traveled to the International Space Station in the space shuttle. The shuttle burned off all remaining fuel before entering Earth's atmosphere. When the shuttle returned to Earth it landed at a speed of approximately 354 km/hr or 220 mi/hr, about 20 times the landing speed of a commercial aircraft. What accounts for the shuttle's speed upon landing?

A) Earth's atmosphere

B) initial thrust of take-off

C) angle of approach upon landing

D) gravitational potential energy

It would be D. Gravitational potential energy As the shuttle prepares to return to Earth it reduces its speed so that gravity pulls it out of its orbit and toward Earth. As it approaches the earth the gravitational potential energy is converted to kinetic energy. The drag of Earth's atmosphere slows the shuttle's descent and causes some kinetic energy to convert to heat energy.