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Old November 30th 05, 03:59 PM posted to
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Default Towing Asteroids by Gravity

wrote in message
I note several grammar errors in my Nov 23 post and I think they make
it difficult to understand. I've corrected the errors (that I see) in
the text below. Sorry!

Dave W wrote:
I'm intrigued as to why there are no references to the above in any of

'space' newsgroups.

um, I think you may be under the false impression that doing this would
somehow magically save fuel or something. If I understand you
correctly, you seem to think that if I move a large body (such as an
asteroid), and if that large body's gravity pulls a smaller body (a
towed asteroid), then I am pulling the smaller body for free (without
expending additional fuel). Sorry, but it don't work that way.

The smaller, "towed" body pulls back on the larger body, and you expend
exactly the same amount of fuel as you would expend if you connected
then up via a cable and pulled them both.

Actually, using gravity to tow the body is likely less efficient, because
you have to angle your engine's exhaust off the centerline which goes from
the engine through the center of mass of the body you're trying to tow. To
balance this out, you use engines in pairs. This extra angle will result in
some loss of efficiency when compared to landing on the body and orienting
your engine(s) such that the exhaust is expelled along the centerline which
goes from the engine through the center of mass of the body.

The advantage of towing the body with gravity is that you don't have to
worry about other issues, like designing your spacecraft to not only land
on, but affix itself to a body which has potentially unknown surface
features and material properties.

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