--- In sfconsim-l@yahoogroups.com, Henry Cobb <henry_cobb@...> wrote:

> > From: Isaac Kuo <mechdan@...>
> > Where is this small comet, and how much delta-v are we
> > talking about? How do we move it?

> Moving just takes a mirror in orbit around it and selective
> heating to vent reaction mass.

This is going to be problematic for many reasons. For one thing, the
exhaust velocity will be very poor, but that's just for starters.

If you've got a decent amount of sunlight to work with, then the comet
will be outgassing chaotically anyway due to direct sunlight heating.
The halo of dust/gas will complicate your efforts to concentrate
sunlight onto the surface.

The surface will likely not be snow/ice since that may have already
ablated off. Instead, you may be heating up a dark surface material,
which then loses heat to snow/ice that outgasses in arbitrary
directions at low velocities.

If the comet is small enough to be nudged around by these weak
outgassing forces, then chaotic effects from natural sunlight caused
outgassing will be constantly messing with your aim. For your mirror
to pump more energy into the comet than direct sunlight, then the area
needs to be much greater than the comet's...so...

> You catch a tiny comet just as it goes short period and
> nudge it past a few planets to bring it to Earth.

Only possible if the comet is flying along the ecliptic and already on
a near impact course with another planet.

> Then just baggie it and harvest the water vapor as it melts.

...My gut feeling is that the only way you're going to get any sort of
directed thrust is if you bag it to begin with. That way, all
outgassing in all directions produces gas which ultimately gets
released only in your desired direction. Your bag has one or more
aimable rocket thrusters which you "activate" by releasing a valve.
The exhaust velocity is still pathetic, of course.

> Grab one a decade and you're golden.

How many decades is this mission supposed to take?

Isaac Kuo