  Experiments, analyses, and simulations have shown that the engine exhaust
plume of a Mars lander large enough for human spaceflight will create a deep
crater in the martian soil, blowing ejecta to approximately 1 km distance,
damaging the bottom of the lander with high-momentum rock impacts, and possibly
tilting the lander as the excavated hole collapses to become a broad residual
crater upon engine cutoff. Because of this, we deem that we will not have
adequate safety margins to land humans on Mars unless we robotically stabilize
the soil to form in situ landing pads prior to the mission. It will take a
significant amount of time working in a harsh off-planet environment to develop
and certify the new technologies and procedures for in situ landing pad
construction. The only place to reasonably accomplish this is on the Moon.
