What could a space ship do if it stopped because it ran out of fuel?

Category: Space
Published: February 16, 2013

By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University

Space ships do not stop when they run out of fuel. While outer space does contain gas, dust, light, fields, and microscopic particles, they are in too low of a concentration to have much effect on spaceships. As a result, there is essentially zero friction in space to slow down moving objects. Unlike ships in water, a ship in space does not need constant thrust to keep moving forward. A space ship's momentum will continue to carry it forward indefinitely at a constant speed after the engines are turned off. Because fuel is very costly to put up into space (it's heavy, and the fuel-weight factor is self-compounding), space probes in real life use as little fuel as possible. For most spacecraft, this means turning on the engines briefly at the beginning to get off earth and up to speed, and then only turning them on again at the end to decelerate. For most of the voyage, a spacecraft's engines are off and it is coasting under its own momentum. Movies that show spaceships with their engines on all the time are unrealistic. Similarly, when a ship in a movie turns off its engines and slows to a stop, the producers are portraying an unrealistic scenario.

Most spacecraft, such as the Voyager shown here, only spend a very small fraction of their voyage with their engines on and instead cruise under their own momentum. Public Domain Image, source: NASA.

Topics: fuel, inertia, out of fuel, space movie, space ship, space travel