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When a spacecraft lands on the moon, on unprepared ground, it must fling a lot of rocks and dust all around. If you are building a lunar base, the other components that are already there would be pelted with rocks and dust. So there is a dilemma of it being far enough away to be safe, but close enough for efficient access. It seems to me one of the first things they would have to build is a landing pad. I wonder what speeds and distances retro rocket debris could reach. Is anyone aware of studies of this hazard in regards to lunar base plans?

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    $\begingroup$ During the Apollo program, it was found that some of the debris disappeared over the horizon, and it was estimated it could end up halfway across the moon. space.com/4956-lunar-landers-sandblasted-moon.html $\endgroup$ – Hobbes Jan 3 '17 at 8:57
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    $\begingroup$ Well that indicates that safe distancing is not a good solution then is it? $\endgroup$ – Johnny Robinson Jan 3 '17 at 23:56
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Great question. The exhaust velocity from typical landing engines is about 3 km/s. You can imagine good-sized particles being accelerated to a significant fraction of that, say 1 km/s, which is the muzzle velocity of a rifle. There is nothing in the vacuum to slow them down, other than eventually hitting something, or the ground. 1 km/s is already a good fraction of the orbit velocity (about 1.6 km/s), so you basically have bullet-speed, sand-sized ICBM particles being ejected from the landing location, going tremendous distances. I'd think that no matter how far away you have the landing location for your base, you'd need to plan for some protection from particles coming from that direction.

I'm sure that there have been studies of this, but I don't have any handy references. A literature search will likely turn up something.

And in fact TonioElGringo turned up something. The very first paragraph from the relevant section:

The effects of rocket engine blast on both prepared and unprepared pads is probably the single most important and the most complex to analyze of any affecting pad designs. Blast from the lander rocket engine will present a broad range of issues affecting virtually every aspect of lunar base design.

The study got much lower velocities than I was guessing above, but it was also a rough-order-of-magnitude calculation. They didn't have computational fluid dynamics (CFD) back then. Their general conclusion was that your habitat surfaces would sustain relatively little damage if they were 2 km or more away.

Here is a more recent reference that states:

NASA analysis using gas flow codes has indicated that rocket exhaust plumes from the landing stages can induce high injection velocities of the top layer of the lunar surface;

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Analysis further indicates that these particles can achieve ejection velocities between 300 and 2000 meters per second (m/s) with the smaller particles generally traveling faster. Because there is negligible ambient lunar atmosphere outside the plume, the particles continue at that velocity until striking the lunar surface far away. Some particles travel almost all the way around the moon before impact. The smallest, dust-sized particles achieve near-lunar escape velocity, 2.37 km/s, and even exceed it by a significant margin, sending them into solar orbit, according to some plume simulations.

That study also concluded that keeping your landing site 2 km away from what you are trying to protect would be adequate. This is due mainly to the curvature of the Moon's surface, where at that distance the highest-speed horizontally accelerated material would fly mostly over the artifacts.

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    $\begingroup$ Since the projectiles don't decelerate until they impact, I suppose it is as bad to get hit regardless of how far away one is. OT, but it makes me wonder about mitigation techniques: Air bag landing. Landing pads inside smallish craters with high rims. How much would the altitude of an MSL sky crane help without an atmosphere? All the exhaust will hit the ground at same speed anyway, right? $\endgroup$ – LocalFluff Jan 3 '17 at 11:35
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    $\begingroup$ Distance still helps since the exhaust plumes expand greatly in vacuum. $\endgroup$ – Mark Adler Jan 3 '17 at 14:35
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    $\begingroup$ This is a preliminary study done in the 80s for lunar facilities. Discussion of engine blasts starts at page 19. $\endgroup$ – TonioElGringo Jan 3 '17 at 15:18
  • $\begingroup$ @MarkAdler Of course, but IF hit, then you're hit with the same force whether just nearby or 100 km away. $\endgroup$ – LocalFluff Jan 3 '17 at 17:26
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    $\begingroup$ @LocalFluff The distance I was referring to was the distance from the skycrane to the ground, that you were asking about. $\endgroup$ – Mark Adler Jan 3 '17 at 18:02
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Damage to existing equipment from exhaust plumes is indeed an issue for lunar outpost planning. The best information currently available about this is from the Apollo 12 mission that landed close (155m) from the Surveyor 3 Lander that had been on the moon for two and a half years. Astronauts Conrad and Bean removed pieces of the probe that where taken back to Earth for analysis. The Lander was found to be sand blasted by the exhaust plume.

For further information see https://en.wikipedia.org/wiki/Apollo_12#Moon_landing and a 2011 paper from NASA http://dx.doi.org/10.1016/j.icarus.2010.11.013 "Apollo 12 Lunar Module exhaust plume impingement on Lunar Surveyor III" that uses electron microscopy and other methods to analyze the damage.

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I do not believe this would be a problem with a long term base with a properly placed landing area.

The thing is the base needs a radiation shield anyway and the easiest way to do that is simply pile up stuff already laying on the moon. You need two meters (of actual material. Since it won't pack perfectly you'll need a thicker pile) of material, that's going to shrug off landing/takeoff debris without appreciable damage.

Thus all you need to actually protect is exposed instruments--and they can be sheltered with a berm and designate your area for rockets to be on the other side of the berm.

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  • $\begingroup$ I was about to shout "tunnels!" when I saw there was already an answer to this effect. Kills two birds (radiation, impact risk) with one (heap of) stone. $\endgroup$ – flith Jan 4 '17 at 10:13
  • $\begingroup$ Wouldn't there be a risk of debris circling the moon and hitting structures on the other side of the berm? $\endgroup$ – James Jan 4 '17 at 12:22
  • $\begingroup$ @James Debris that circles the moon is in orbit. If it went over the berm the first time around it will next time also. If you have multiple landing sites you need to shield your instruments from every direction debris could arrive. $\endgroup$ – Loren Pechtel Jan 4 '17 at 19:21
  • $\begingroup$ @flith In the long run you probably will want to dig in--save a lot of hassles with temperature control. I don't think the first bases will work that way, though. $\endgroup$ – Loren Pechtel Jan 4 '17 at 19:23
  • $\begingroup$ Loren, surely you'll want radiation protection too? Digging into the ground will be one of your early priorities for a long-term base. Additionally, you can have a tunnel from the landing pad to your main complex. $\endgroup$ – flith Jan 5 '17 at 15:30

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