NASA is an American space agency that has been responsible for many of the most important space exploration missions in history. A ROCKET is a spacecraft propelled by engines that generate thrust by the expulsion of a gas or liquid. The Lunar Excursion Module (LEM) was a spacecraft used by NASA during the Apollo program to land on the moon. A lander is a spacecraft designed to land on the surface of a celestial body. An orbiter is a spacecraft designed to remain in orbit around a planet, moon, or other celestial body. All of these vehicles have been used by NASA in their space exploration missions.