How much does a spacecraft weigh when it is in space?
A. An object in space is said to be in a state of weightlessness, although its original mass remains the same. (Mass can be understood as a measurement of inertia, the resistance of an object to be set in motion or stopped from motion.) Objects in space near the Earth, the Moon, or other large bodies retain a small amount of weight due to the tiny amount of planetary gravity that continues to pull on them. However, orbital motion reduces this condition to an extremely low level of gravity known as microgravity (about one-millionth of the normal gravity we feel at the Earth’s surface). When an object is in orbit about a large body like a planet, it is traveling just fast enough to fall in a continuous curved path around the planet, without flying off or falling down to the planet’s surface. This free fall results in microgravity. Thus, when a Shuttle crew wants to land, they fire the Shuttle’s engines directly into its forward path, slowing the Shuttle enough that it drops out of orbit.
