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The Question

(Submitted February 04, 1998)

I would be grateful if you could answer this question for me.

I have heard about three methods for propelling spacecraft, these are conventional rocket, ion propulsion and anti matter/matter collision. Are there any other methods for propelling spacecraft both theoretical and actual?

If so, could you please outline them.

To tell you a bit about myself, I'm 18 and go to college in Southern England. I study physics at advanced level. Physics is a hobby for me, I will join the Merchant Navy in September as a deck officer, in my spare time on the tankers I hope to study an Open University course in astrophysics or cosmology.

The Answer

There are certainly a number of different methods for propelling spacecraft beyond conventional rockets, ion propulsion, and anti-matter/matter collisions (though I've never heard of the last ever discussed in a practical system).

Among those already in use are aerobraking and gravitational assists. Aerobraking consists of using a planet's atmosphere to slow down a spacecraft when it arrives at its destination. The currently operational Mars Global Surveyor has been using this scheme to lower its original orbit around Mars into a lower one suitable for its mapping experiments. I believe MGS is the first space mission to use this technique. MGS was sent to Mars by conventional means, but use of aerobraking instead of standard retrorocket systems saved lots of fuel and launch costs.

Gravitational assists have been used by a great number of different missions. In this scenario, a space probe approaches a planet from a carefully planned orbit such that the planet's gravity transfers some of the kinetic energy of its orbit to the probe. A probe like Voyager would come in behind say Jupiter, and get an extra kick of energy while slowing Jupiter's orbit around the Sun. Since Voyager is miniscule compared to Jupiter, the difference in Jupiter's orbit afterwards is very very very very small. But the extra velocity the probe gets can be very significant, trimming many years off the voyage time to more distant places. Both the Galileo and Cassini missions were planned so as to swing those probes around planets in the inner solar system (Earth and Venus in both these missions) to work up enough velocity to orbit out to much more distant planets.

On the drawing board side, one idea often discussed is solar sailing. Light has momentum, so if you put up a giant mirror, sunlight being reflected off it would impart a slight force. So you could put up giant mirrors to generate small amounts of thrust. You don't get much in the way of velocity change this way... unless you keep those sails up for a long time! But you could get a probe to pick up a lot of velocity if it spread out sails and kept them up. If the probe was heading towards the outer solar system, however, the amount of sunlight drops off in inversely proportional to the square of the distance to the Sun, so the same sail would receive only 4% as much sunlight at Jupiter as near the Earth, so solar sailing would largely only be useful for exploration within the inner solar system.

Another drawing board idea is something called a mass driver. If say for instance you were using an asteroid as a space vehicle, you could set up a launch rail to throw pieces of the asteroid off in one direction. There are already ways to set up such electromagnetic sleds to throw things off in one direction. So if you've got a lot of spare material you can give the old high-speed heave-ho, you can get some thrust out of the deal. I don't think this one is off the science fiction book rounds, but it is actually a physically plausible way to go about it.

You might also find some useful information on the longer-term planning and ideas sections of the Office of Aeronautics and Space Transportation Technology and also the Office of Space Flight, both at NASA Headquarters. Their WWW addresses are and

Hope this provides the insight you were looking for!

Jesse Allen
for Ask an Astrophysicist

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