QUESTION:
I don't quite understand the aerobraking that MGS will do.
Since dipping into the atmosphere slows down the spacecraft
to get it into a circular orbit, what prevents the continued drag from
the atmosphere from eventually causing the spacecraft from crashing?

ANSWER from Charles Whetsel on March 21, 1997:
That's a very good question.  While using the aerobraking technique saves
us a lot of fuel on MGS, we stiil have to keep enough fuel in our tanks to
get out of the atmosphere when we get close to out mapping mission orbit.
On each aerobarking orbit, we spend about 30 minutes or so below the top of
the Martian atmosphere.  Initially, the orbit period is around 48 hours, so
we are spending 30 minutes out of every 48 hours in the atmosphere.  At the
end of aerobraking, the orbit period has shrunk to 2 hours, so we are
spending about 30 minutes out of every 2 hours in the atmosphere.  On the
last orbit of aerobraking, after we are close enough to the desired mapping
orbit, we will go through the atmospheric drag pass and then 45 minutes
later, perform a propulsive maneuver at the highest part of the orbit
(called the "apoapse") so that at the next low point ("periapse"), our
trajectory will carry us over the top of the Martian atmosphere, not
through it.  After this, we will need to do a few more small touch-up
maneuvers, but this is more or less how we will end aerobraking.  You are
correct in assuming that if we did not perform this aerobraking "exit"
manevuer (We refer to it as the "ABX" maneuver), we would crash into Mars
very soon (within about 24-48 hours!).  We are all expecting the last two
weeks of aerobraking to be VERY exciting.