QUESTION: 
What ideas do you have for landing a crew of people on Mars?

ANSWER from Andrew Petro on December 3, 1997:
Mars has  a thin  atmosphere so  parachutes can  be used  to slow down
quite a bit as  a spacecraft descends to  a landing.  But  because the
atmosphere is a lot thinner than Earth, parachutes won't work quite as
well on Mars.  So, a spacecraft carrying people would probably descend
on parachutes until it got close  to the surface and then fire  rocket
engines to make a nice soft landing.   (We might use air bags just  to
absorb a little of the impact - but not to bounce like the  Pathfinder
spacecraft did.)  Another reason to use rockets to control the landing
is that  on exploration  missions we  may need  to land  very close to
other spacecraft  that we  have already  landed on  Mars.  Just coming
down on parachutes may not be  accurate enough.  Rockets are a  rather
expensive  way  to  land  but  there  does  not  seem to be any better
alternatives right  now, especially  if you  need to  make a  pinpoint
landing.  Maybe you can think of some other ways to land.

Andrew Petro
Johnson Space Center