Larry J wrote:
That means [the 2003 MERs] have to handle whatever weather conditions that exist on Mars at the time instead of waiting for better conditions (as Viking did).

Alex write:
Yes, the MERs will be on a ballistic trajectory that enters the Martian atmosphere prograde. However, a small nitpick: the delay with the two Viking landers was due to rugged terrain at the pre-selected landing sites, not weather conditions.

Larry J wrote:
When you're talking about an airbag under a parachute or in freefall, it's going to be moving across the gound pretty fast in a high wind scenario. So long as they're strong enough to handle the higher impact speed, it shouldn't be too much of a problem.

Alex writes:
Well, I can tell you firsthand that the 2003 MER Project is *very* concerned about low altitude winds and wind shear, so much so that an engineering constraint on the landings sites is that both parameters, when integrated, must be less than ~20 m/s. Of course, these are predicts, and not measured in real time. One will note that Mars Pathfinder, which landed at Ares Valles at 3 a.m. local time, when wind shear was predicted to be low or non-existent, still managed to bounce nearly 2 km from the initial impact point. The two MERs will enter during daylight hours when wind velocities may be even higher; for that reason, strengthening the airbages to take more horizontal velocity during impact (as well as accounting for MER's higher mass) is a prime focus of current project engineering, and one of the reasons that the mass margins have been substantially eroded.

Larry J wrote:
This would probably be a lot more difficult to handle in a powered landing like MPL.

Alex writes:
Depending on their magnitude, strong wind gradients can affect soft landers as well. However, landers like MPL have the ability, unlike MPF or MER, to attempt to null out the horizontal velocity by thruster firing during descent, combined with active closed-loop guidance. That said, powered descent has its own hazards such lower tolerance to high local slope angles.

Alex R. Blackwell
University of Hawaii