(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
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.
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
Hope this provides the insight you were looking for!
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