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Re: starship-design: Ice Impact Terraforming

From: "Steve VanDevender" <stevev@efn.org>

> And speaking of the wildly implausible . . .
> Let's put it this way.  Moons and planets aren't billiard balls.  Most
> importantly, billiard balls are far more rigid and internally cohesive
> than any planet.  Think of trying to do this with balls of damp sand; on
> a planetary scale, that's about how cohesive a planet is.  You might be
> able to impart some angular momentum to the Moon by smacking it with
> another Moon-sized object, but you're going to end up with a spinning
> collection of very small rocks, not a neat glancing collision that
> leaves both bodies whole and one of them spinning nicely.
> Trying to spin up the Moon with tangential comet impacts at its equator
> is also wildly implausible.  The comets aren't going to impart much
> angular momentum, for one thing; they'll just chip off bits of Moon that
> will spray into space.  Even if you have all the comets you want, after
> a while you'll only end up with a Moon shaped vaguely like an apple core
> that won't be spinning much faster.

Hmm - the direct dynamic solution would be my preference, as a way to spin
up a "solid" body. That's because it is quickest, if the task could be done
that way. There is a solution by gravitational interaction, involving a
large gravitational body in elliptical orbit around the Moon, which would
take longer, but would unquestionably spin up rotation. Farther afield, we
could find electromagnetic solutions, though most such would need large
scale machinery construction.

But back to hitting it with rocks: the impactors are very small compared to
the Moon, order of 1-10 km diameter. The impact trajectory is neither
precisely tangential, nor precisely equatorial. Impacts all have a vector
component of force which can be translated into a contribution to the Moon's
angular momentum. The most effective momentum transfer isn't when the
impactor skips off the surface and escapes, but when its mass is absorbed.

Ejecta are not required, for momentum transfer of the impacting body, into
angular momentum of the target body. Ejecta escaping make litter. Your
examples of eroding the Moon away, relate to impact velocities too high to
do the job. If anything chips off the Moon, and escapes in the direction the
impactor was going, its momentum is wasted because it did not contribute to
spinning up the Moon. For impacts of useful energy, a mountain range, cliff
or crater wall should make a good backstop for low-angle impacts. Whether
the impacting bodies are ice, metal, or rock isn't relevant to momentum
transfer, as long as nothing escapes forward.

Every impactor which hits and is completely absorbed, at any angle other
than vertical, exerts a moment of torque on the Moon. Horizontal absorbtion
at the Equator is the extreme limiting case, of the most efficient momentum
transfer of the impacting body, to angular momentum of the Moon as target
body. No actual strike will be exactly horizontal, but will be angled down
to such a degree as to ensure that ejecta do not escape.

If impact speeds are effectively moderated that no ejecta escape, the
physical properties of the target body are not relevant to its inertial
relationships. If torque is applied to a body made of powder, it will spin.

Johnny Thunderbird