Asteroid Mining

"dsw_s" wrote:
> Once you get out of earth orbit and into a trajectory that will take
> you to the other side of the solar system, it costs no additional
> reaction mass to get there.

But if you want to *stay* there, you will have to either slow down or
speed up. Otherwise you will come back to Earth in an elliptical
orbit, or just keep on going.

Sometimes this can be done by using the slingshot effect (the Galileo
and Cassini missions), or aerobraking (various probes sent to Mars).
But neither of these could be used with asteroids. So it *would* cost
reaction mass.

> Once you get almost out of earth orbit en route to the moon, you
> have to spend a whole bunch more reaction mass fighting the moon's
> gravity if you don't want to crash.

Not all that much more: the Moon's gravity well is only about 1/30
that of Earth. Think of the Mercury boosters, which had to lift off
of Earth, versus those of the lunar modules.