QUESTION:
If the Mars atmosphere is less than 1% than that of Earth, how can a
parachute of the size you are using be sufficient? It would seem to me that
you would need a parachute close to 1000' wide to achieve the same effect.
Would you please explain the dynamics of placing a lander on Mars, and why a
small parachute would work on Mars as it does on Earth?

ANSWER from Rob Manning:
You ask a very insightful question. The bottom line is you're right,
parachutes this small aren't sufficient on Mars! On Mars Pathfinder, as on
Viking, we use a "small" 40.5 ft (12.5 m) chute. It was scaled so that, with
our lighter lander, it does about as much for the our descent speed as does
Viking's. Our terminal velocity seconds before getting to the ground (where
the atmosphere is "thickest") is still about 65 m/s (146 mph)!!

You are correct, it would indeed take a larger chute to get slower "normal"
Earth-like terminal velocities. Our chute on Mars is about the equivalent of
a chute 38 times smaller in area on Earth (6.5 ft across!), and this
includes the effect of Mars' lower gravity! A chute that could lower our
lander to the Martian ground at a gentle 10 m/s (22 mph) would have to have
an area about 42 times larger than our "little" chute (or a diameter of 263
ft)! That's 42 times the mass (and volume) of our 10 kg chute, or 420 kg,
more than the mass of our entire lander! It wouldn't fit! We would need to
have a "gossamer" (ultra-light weight material) parachute and then figure
out how to get it open at high speeds!

This is why we turned to solid rockets to stop our lander just before we hit
the ground. Viking, too, used liquid rockets to slow the terminal decent.
Also Pathfinder's airbags protect the lander from the local terrain
variations (bumps, craters, rocks, hills, etc.) after the rockets do their
thing.

So why do we do we use a chute at all? Well, parachutes might not be all
that good at laying a lander gently down on the Martian surface, but they do
a spectacular job of braking something moving very fast. Remember, the drag
FORCE a chute generates (therefore its deceleration), is proportional to the
square of the velocity and only linearly proportional to the atmospheric
density; so even a thin atmosphere and a "small" chute will do much to slow
our entry vehicle down once the heatshield's aerobraking has been mostly
achieved.

This is also true of heatshield, our entry vehicle (like Viking's) enters
the upper atmosphere at 7 km/s (or more than 15,000 mph!). Most of this is
reduced by the friction with the heatshield. But even 2 minutes later, our
vehicle is still screaming in at nearly 400 m/s (900 mph) when the parachute
opens before slowing down to 65 m/s near the ground. I'd say that reducing
our velocity by a factor of 6 (a factor of 36 in kinetic energy), isn't all
that bad for only 10 kg of extra payload mass, wouldn't you?

So, the short answer is, you're right, parachutes don't work on Mars like
they do on Earth (neither do airbags, but that is another story), but they
do a great job when you need to slow down something that is whipping through
the Martian atmosphere FAST!

--Rob Manning