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I'm landing an unmanned spacecraft on Mars (or substitute your astronomical body of choice here.) I would like to avoid rock outcrops and land on relatively soft soil. What kind of sensor could distinguish between rock and soil from an incoming lander?

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For a modern autonomous lander, a mix of sensor types will likely be used, but for rock-avoidance, I think LIDAR and optical scanning is probably the most important. Modern computing techniques can derive a 3-D model from stereo camera imaging in real time.

Strictly speaking, that doesn't distinguish between hard rock and soft soil, but between flat and non-flat terrain, which may be the more important distinction to your lander. It might be possible to characterize the consistency of the surface with a mix of visible and IR optical and radar sensing. Preferring a consistently colored flat surface to a varied one could be a valuable secondary guidance criterion.

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  • $\begingroup$ The first paragraph basically describes ALHAT alhat.jpl.nasa.gov (which I assume is just one example of such a system) $\endgroup$ – Erin Anne Jul 7 '18 at 0:13
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In addition to LiDAR and stereo cameras (both work well, cameras are cheaper), you can consider surveying from orbit first and picking a location that is already mostly free of rocks. That's how it's currently done for Mars landers.

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  • $\begingroup$ Of course, if you do this you have to land in an area that is mostly free of rocks the size of your landing ellipse, which while they are getting smaller, are still quite large. $\endgroup$ – PearsonArtPhoto Jul 6 '18 at 2:23
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    $\begingroup$ They're getting quite a bit smaller. MPF was 300 km. MER was 80 km. MSL was 25 km. 2020 will be 10 km. Using the same sensors that 2020 will prove out and adding more propellant for final divert maneuvers, I think it can get down to 1 to 2 km for the subsequent lander. $\endgroup$ – Mark Adler Jul 6 '18 at 2:44
  • $\begingroup$ Didn't realize that 2020 was down to such a small landing ellipse. Neat! $\endgroup$ – PearsonArtPhoto Jul 6 '18 at 12:58

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