ablation- The process by which heat is removed from a reentry body through the melting, vaporizing and scouring off of special materials as the body is heated by friction with the atmosphere.

apogee – In an orbit, the point farthest away from the Earth.

gimbal – A pivot device that allows the entire nozzle and the flow of exhaust gases to be swiveled in any desired direction, thus directing the rocket’s course in the opposite direction.

parking orbit – The initial orbit achieved for a vehicle that is to be sent into space as a probe, for vehicles to be joined in rendezvous, or for reentry vehicles to be returned to Earth.

payload – The load carried by a spacecraft consisting of things (such as instruments or satellites) necessary to the purpose of flight.

perigee – In an orbit, the point closest to the Earth.

propellant – The solid or liquid fuel of a rocket that, when burned in combination with an oxidizer, provides thrust.

satellite – Any body held in an orbit by the gravitational field of another body.

stage – One unit in a multistage rocket. The stage has its own engines and fuel supply, and may have its own guidance system. The booster is the first stage of a multistage rocket.

thrust – The amount of push generated by a rocket engine measured in pounds.