Re: starship-design: Ice Impact Terraforming

[Steve VanDevender <stevev@efn.org>]

bugzapper writes:
 > Ah. My bro' reminds me "Space - 1999" was the TV series abaout a runaway
 > moon, actually used as a planetoid type starship. Flying around in a rogue
 > moon has never attracted me. Seems kind of extreme, an admission that the
 > starship problem can't be solved. I think otherwise, obviously.

In "Space: 1999" the moon wasn't _meant_ to be a starship, but it got
blown out of orbit by an explosion in a nuclear waste dump on the far
side (I think they even went so far, in their lack of attention to
physics and astronomy, as to call it the "dark side" of the moon).  This
was, of course, wildly implausible.  It was also wildly implausible that
the inhabitants of Moonbase Alpha managed to visit many distant solar
systems within a relatively short time.  I remember that it was great
fun when I watched it as a ten-year-old, but I was no more seriously
proposing turning the moon into a starship than you were.

 > About the Moon, though. At the pool table, I have seen collisions set the
 > three ball spinning rapidly, standing stock still on the table. I think
 > there probably are collision solutions which will drastically affect the
 > rotation of a large body, without appreciable damage to its orbit. The more
 > conservative approach is to use multiple controlled strikes, tangentially at
 > the equator, to spin up a body. A balance should be struck, between
 > collisions which would increase the orbital velocity, approaching from
 > behind in the orbit, and those which would slow the orbit by coming from
 > ahead. And, no, I haven't calculated how much energy it would take, to spin
 > up the Moon enough to make trees happy. Lots, huh?

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.