Posted March 3, 2015

How the force of a sheet of paper can replace a rocket

An interesting part of Dawn's mission is the ion engine. This works very differently from regular rocket motors. There's no combustion and no hot exhaust gases escaping. Here's how an ion engine works. Picture a boy on a cart. To set his cart in motion, he can throw away some balls. Throwing a ball backwards will push the cart forward. Throwing heavy balls away at a low velocity will push the cart about the same amount as throwing light balls at a high velocity. So is there a difference? Yes: throwing light balls at a high velocity is better, because less weight will give you the same result. And being lightweight is everything in space.

Dawns' ion engine throws out very fast, light balls. Using an electric voltage from its solar panels it accelerates charged atoms (ions) of the gas xenon to 40.000 kms per second and spits them out. Regular rocket motors have combustion gases escaping at only about 4 kms per second. So an ion engine can achieve the same velocity gain using way less 'fuel.' Therefore it can be a lot lighter.

Another interesting thing about an ion engine is: it doesn't need to bring its own energy. It can use solar. It needs to bring no petrol, kerosene, hydrogen, gunpowder or anything. All it needs in the luggage is some random 'stuff' to accelerate outward. The only drawback is: ion engines push out so little material, that they need a lot of time to produce the same effect as a rocket. So a rocket is good for takeoff: it produces lots of violence in a short time. An ion engine needs time but given enough of that, it gives you more bang for your buck. And the thing about Dawn is: it had plenty of time. In the video below Marc Rayman, Dawns' mission director, explains that the ion engine produces a force comparable to the weight on Earth of a sheet of paper. But working continiously during many years this force could accelerate the craft by about 10 km per second. As Rayman says, the journey Dawn has taken could not have been done without an ion engine.

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About Herbert Blankesteijn

Herbert Blankesteijn is a technology journalist from the Netherlands who has written for many prominent Dutch newspapers. He presented and directed television and radio programmes and has 10 books to his name. Herbert is interested in nascent fields such as 3D printing, drones, robotics and the private space business.