QUESTION:
Just a quick one. Soft landings on Mars, do you use a decent engine or
parachute. If engine, what is used for propellant. I'm looking for a
little more info than liquid or solid. If either its either could you
tell me the main ingredient.

ANSWER from Charles Whetsel on December 29, 1999:
Descending from space to the surface of Mars requires 3 different types
of deceleration.  First, the spacecraft must surface atmospheric entry
through a very thin atmosphere at very high speeds.  This is done using a
hard heatshield to protect the spacecraft from getting very hot during
entry.  After the heatshield has slowed the spacecraft enough, the most
efficient way to descend through the middle atmosphere is using a
parachute.  This allows the spacecraft to safely drop through much of the
atmosphere, but because the atmosphere is so thin on Mars, it isn't
practical to descend all the way to the surface on a parachute (you would
be going too fast when you got there).  For Mars Pathfinder, when the
spacecraft got close enough to the surface, it cut itself free of the
parachute and inflated airbags to cushion the blow when it hit the surface
(called a "hard landing").  For the Viking Landers and the Mars Polar
Lander, once the spacecraft had released itself from the parachute, liquid
rocket engines were used to decelerate the spacecraft to a safe speed for
the landing legs (called a "soft landing").  For both Viking and Mars Polar
Lander, the rocket engines used a fuel called hydrazine.