Mars retrorockets in atmosphere

Some additional remarks are appropriate about descent in the Martian atmosphere. Terminal velocity in the thin Martian atmosphere is typically too large to permit soft landing only using a parachute. Even the semi-hard landing of the Mars Pathfinder vehicle used a retro-rocket to null the descent velocity just prior to impact. A scientific concern is that the plume from the descent engines should not deposit fuel contaminants at the landing site, or significantly erode the surface material there. Nozzle design may need to take this into account (e.g. Mars Polar Lander had multiple-nozzle motors). Additionally, the retro system may need to be cut off some metres above the surface, to free-fall.

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