Engineering constraints on potential 2020 landing sites are based on those derived for the MSL “sky crane” landing system [Golombek et al., 2012], with some important exceptions.
Below -0.5 km MOLA elevation, with respect to the MOLA geoid.
Within ±30° of the equator./
Like MSL, the 2020 mission has a nominal landing ellipse of about 25 km by 20 km, oriented roughly east-west. A potential improvement under investigation, called range trigger, would allow landing within a 18 km long by 14 km wide ellipse. It may be possible in the future that the range trigger ellipse could become as small as 13 km by 7 km.
Terrain Relief and Slopes:
Less than ~100 m of relief at baseline lengths of 1-1,000 m to ensure proper control authority and fuel consumption during powered descent.
Less than 25°-30° slopes at length scales of 2-5 m to ensure stability and trafficability of the rover during and after landing.
The probability that a rock taller than 0.55 m high occurs in a random sampled area of 4 m2 (the area of the belly pan and area out to the inside of the wheels) should be less than 0.5% for the proposed sites. This corresponds broadly to 7% rock abundance, which is near the mode in the rock abundance for Mars as estimated from thermal differencing techniques. Subsequent analysis indicates the most critical area is just the belly pan of the rover, which covers ~2.7 m2 and can tolerate 0.6 m high rocks, which corresponds to about 12% rock abundance. Because rocks will eventually be counted in HiRISE images, rock abundance could locally be up to 20% provided that the overall risk for the ellipse does not exceed the 0.5% probability level.
The Ka band radar backscatter cross-section must be > -20 dB and < +15 dB at Ka band to ensure proper measurement of altitude and velocity by the radar velocimeter/altimeter of the descent vehicle.
Surfaces with thermal inertias greater than 100 J m-2 s-0.5 K-1 and albedo lower than 0.25 and radar reflectivities >0.01 to avoid surfaces dominated by dust that may have extremely low bulk density and may not be load bearing. Surfaces with thermal inertias less than ~150 J m-2 s-0.5 K-1 with high albedo may also be dusty and so should flagged for further investigation. Areas that meet the engineering constraints for 2020 Mars Rover landing sites are shown in Figure 1.
Enhanced EDL Capabilities-Hazard Tolerance:
Under investigation is the possible inclusion of enhancements to MSL EDL capabilities and the possible inclusion of Terrain Relative Navigation technology that would allow the consideration of landing sites with some areas that exceed the relief and rock constraints. A conceptual Terrain Relative Navigation system would allow areas of <150 m radius that violate the slope and rock constraints within the ellipse so long as they are surrounded by areas >100 m radius that appear safe for the landing system (i.e., that meet the engineering constraints).
These constraints are preliminary and subject to change. In some cases they could be broader than what the eventual spacecraft can accommodate; the intent is to be biased toward identifying all scientifically interesting landing sites. It is possible that these capabilities may not ultimately be fully or partially implemented, rendering some initial sites non-viable in the future.