QUESTION:
What happens if there are strong winds in the landing region when the craft
is surrounded by the airbags? Could the craft just start rolling and
continue to do so much longer than the estimated time? If this happens could
it roll until damage is done to the protective bags? Or do you just sit back
and relax until the wind goes down before final deployment? Maybe the
density of the atmosphere is low enough so this will not be a problem.

ANSWER from Rob Manning:
Good question! As I mentioned in another FAQ, due to the very low density of
the Martian atmosphere, the force imparted by a wind on Mars is much less
than wind at the same speed on Earth. For example, a very high (and rare)
wind speed of 100 mph on Mars would feel the same as a moderate Earth breeze
of only 10 mph. Despite the large surface area that the airbags present to
the wind, that force would not be enough to keep the airbags from rolling
for very long. Even if it did continue to roll and was helped by a slight
terrain slope, after 11 minutes on the surface, the flight software will
slowly begin to deflate the airbags by activating the four airbag retraction
actuators (one for each of the four bags). These motors reel in multiple
retraction cords mounted inside the bags. One of these cords (one on each
bag) will pull open vents that allow the airbag gases to leak out. As soon
as the bags begin deflating, the kinetic energy of any residual rolling
motion will quickly be dissipated much like the drag of a partially flat
tire. In the "airbag roll down the hill tests" which we performed in the JPL
parking lot (one of our more entertaining tests), we confirmed that it would
be very hard to sustain a rolling motion for very long. In those tests we
measured and calculated the rate of kinetic energy loss (i.e. slow-down) on
a flat surface for a given initial velocity. Rocks, sand and gravel only
tend to slow down the lander further.

--Rob Manning