Re: keeping those darned planets apart

[William Beaty <billb@ESKIMO.COM>]

On Thu, 26 Aug 1999, Glenn Knapp wrote:

> This looks like a conservation of momentum deal to me. Their momentum in the
> one direction would have to "cancel out" the momentum of the planet

How about an asteroid and an electromagnetic cannon?

That's one proposed method of "asteroid mining": bringing big rocks down
to earth orbit from out past Mars.  The "cannon" propulsion system, if it
was aimed at another asteroid of similar mass, could be used to keep them
from falling together.  The velocity of the "bullets" would of course be
very high.  To guarantee inelastic collison, we could aim them into a deep
well in the asteroid's surface (maybe it would form lava at the bottom).
The whole thing would be powered by solar panels on the surface of the
body which carried the cannon.


>   To make this work, you'd need to fire one heck of a lot of bullets
> really fast.  This means that the mass of your planet is going to be
> decreasing.

As somebody else pointed out earlier, the thrust might need to be
continuously adjusted... or would it?  The attraction is m x m, right?, so
the gravitational force doesn't care if we eventually transport a major
portion of one asteroid over to the other.

> At the same time, the mass of the other planet is going to
> be increasing.  Eventually you'd run out of planet - you made it all
> into bullets that you fired into the other world.
>
> That would be weird.

But don't you think it's also weird that there's no force holding the
asteroids apart?  I simply fail to get my mind around the concept at all.
As far as the propulsion system is concerned, the other asteroid need not
even be there.  Only when we exchange bullets in TWO directions do we
create an interaction identical (in the long run) to a Newtonian force.
This asteroid/cannon thing is like a force which doesn't actually cause
interaction.  If we view it as a force, then it is a "force" which doesn't
obey Newton's 3rd: as we employ bullets to "push upon" the other
asteroid, that other asteroid does not "push upon" the cannon in return.
That "force" which holds the asteroids apart is not a normal one where the
force-pair has two ends!  We push the other asteroid, but cannot "feel" it
out there, resisting us.  Now THAT is weird!


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