3 ms·

There was an interesting paper (poster linked below[1]) about the unique risks that are posed by high velocity lunar regolith. The amount of exhaust mass you ne

There was an interesting paper (poster linked below[1]) about the unique risks that are posed by high velocity lunar regolith. The amount of exhaust mass you need to decelerate a lander is proportional to that lander's mass. Since you can either eject more mass more slowly, or less mass more quickly (momentum being simply mass times velocity), to save mass for the payload you want to get your exhaust velocity high. However, when that exhaust collides with the lunar regolith it transfers its momentum there. Further, given the low gravity of the Moon, you run the risk of accelerating dust and rocks so much that they actually go into an elliptical orbit with the lander at the perigee of that orbit. If that happens, you touch down and anywhere from 20 to 35 minutes later a rock you kicked up comes right back at you from the other direction! Perhaps worse, that rock hits a satellite or support module that is currently in orbit. In any case its not a good thing and something that was avoided in the Apollo program because the landers had relatively low mass (< 10 tons). [1] https://ntrs.nasa.gov/api/citations/20205007525/downloads/High-Velocity%20Dust%20Mitigation.pdf https://ntrs.nasa.gov/api/citations/20205007525/downloads/Hi...

deleted 5y ago
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lmilcin 5y ago
> If that happens, you touch down and anywhere from 20 to 35 minutes later a rock you kicked up comes right back at you from the other direction! It is not simple as that, it is actually more interesting:) On a spherical ideal body it is impossible for the rock to do this. That is because the only orbit that does not intersect the ground must be exactly parallel to the ground at the landing site. So not only it can't hit the landing pad, it can't even hit the rocket sticking way above the ground. Now, there is couple of interesting reasons I can figure out why this is at all possible on Moon, but I am too lazy or in some cases incapable to estimate which ones are significant. 1. Moon rotates. If the landing happened anywhere but the poles, by the time the projectile completes orbit the surface has shifted (but just a tiny bit since Moon rotates so slowly). If the projectile was shot parallel to the ground in the right direction, it does not need to complete entire orbit to hit the landing pad or the rocket. 2. Moon has lumpy gravity. It is not an ideal spherical body and it is known to have huge gravitational anomalies. The anomaly is so large that satellites around Moon need regular boosts because otherwise their orbit would degrade. 3. Moon has atmosphere. It is as good as vacuum in most circumstances but it might possibly be significant for particles of dust traveling at thousands of meters per second. For those interested, the entire atmosphere of Moon is estimated to weigh around 10t. So don't count on using it Watney-style for anything practical. 4. Solar wind. Very small particles may be significantly shifted in their orbit due to interaction with energetic particles traveling in particular direction (like solar wind). 5. Particles skipping off of Lunar surface? I think it could maybe be possible for a particle being close to complete its orbit to hit something at really extremely small cross section that rather than vaporize it would just deflect it by couple arc seconds and hit the rocket. Of course this is just thought experiment. Dust flooding other structures on the Moon, satellites, orbits around Moon and Earth is reason enough to try to find a solution.
- ChuckMcM 5y ago
  Agreed, it is never simple! What was most interesting to me was the non-intuitiveness of being able to throw a rock into orbit (I know tumang[1] prejudice), with the right conditions the NASA paper suggests some could escape the Moon's gravity completely! [1] Belter word for someone who lives on Earth.

deleted 5y ago

[deleted]

lmilcin 5y ago

> If that happens, you touch down and anywhere from 20 to 35 minutes later a rock you kicked up comes right back at you from the other direction! It is not simple as that, it is actually more interesting:) On a spherical ideal body it is impossible for the rock to do this. That is because the only orbit that does not intersect the ground must be exactly parallel to the ground at the landing site. So not only it can't hit the landing pad, it can't even hit the rocket sticking way above the ground. Now, there is couple of interesting reasons I can figure out why this is at all possible on Moon, but I am too lazy or in some cases incapable to estimate which ones are significant. 1. Moon rotates. If the landing happened anywhere but the poles, by the time the projectile completes orbit the surface has shifted (but just a tiny bit since Moon rotates so slowly). If the projectile was shot parallel to the ground in the right direction, it does not need to complete entire orbit to hit the landing pad or the rocket. 2. Moon has lumpy gravity. It is not an ideal spherical body and it is known to have huge gravitational anomalies. The anomaly is so large that satellites around Moon need regular boosts because otherwise their orbit would degrade. 3. Moon has atmosphere. It is as good as vacuum in most circumstances but it might possibly be significant for particles of dust traveling at thousands of meters per second. For those interested, the entire atmosphere of Moon is estimated to weigh around 10t. So don't count on using it Watney-style for anything practical. 4. Solar wind. Very small particles may be significantly shifted in their orbit due to interaction with energetic particles traveling in particular direction (like solar wind). 5. Particles skipping off of Lunar surface? I think it could maybe be possible for a particle being close to complete its orbit to hit something at really extremely small cross section that rather than vaporize it would just deflect it by couple arc seconds and hit the rocket. Of course this is just thought experiment. Dust flooding other structures on the Moon, satellites, orbits around Moon and Earth is reason enough to try to find a solution.

ChuckMcM 5y ago
Agreed, it is never simple! What was most interesting to me was the non-intuitiveness of being able to throw a rock into orbit (I know tumang[1] prejudice), with the right conditions the NASA paper suggests some could escape the Moon's gravity completely! [1] Belter word for someone who lives on Earth.