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On the Earth, with our atmosphere, even small objects entering the atmosphere such as Tunguska or Chelyabinsk can do devastating damage due to the shock-wave travelling through the air. People could be injured by the shockwave itself, which can knock down trees if strong enough, or be bombarded with flying debris (usually glass from broken windows). Most asteroids themselves won't even touch the surface before burning up from air resistance.

What would happen on the moon? Assuming this Wikipedia article about the properties of lunar craters is accurate, what would an astronaut in a pressurized spacesuit experience if standing at varying distances from the impact of a small meteroid? (I suppose "small" is a relative term here, so I'm going to say that the crater generated would be about a 20m radius). I'm using this video as a reference for the behavior of Moon dust after being hit by a crater.

What would happen if 3 very unlucky Moon colonists were standing:

  1. A meter away from the impact point? (NOT being hit by the object, but very close)

    • In my head, I imagine they'd be tossed upwards with all the moon dust and probably die from whiplash or having their suit torn open by flying debris. Does that sound about right, or could the ground be blasted away from under their feet and leave them in midair?

  2. At the edge of the future crater? (15-25 meters away)

    • According to the wiki article, the rim of the crater is where much of the ejected material builds up after gravity pulls it back down. Would astronaut #2 be hit by debris launched sideways from the impact, or be buried by falling dust shortly thereafter?

  3. Dozens of meters away, close enough to see the impact?

    • In Earth's atmosphere, they'd be hit by the shockwave in air. On the moon, would there be an earthquake-style tremor or anything of the sort?
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  • $\begingroup$ Hmm, this question is more complex than it seems. No pressure wave, but debris will go farther due to low gravity, and will not be slowed down by air resistance (which means that dust won't "settle" like it does on Earth - every tiny particle becomes a high-speed, deadly projectile). Also, debris is more deadly when a punctured space suit can kill you. Dozens of meters away will almost certainly be deadly, but asking how far away one must be before it is survivable will require a lot of math. $\endgroup$
    – IndigoFenix
    Oct 22, 2019 at 6:19

2 Answers 2

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Dead, dead, and – you guessed it – dead.

For all three of your scenarios, the astronaut would be utterly destroyed.

  1. The inside radius of the future crater is instantly vaporized. It temporarily becomes liquid-like and ripples due to the high energy impact. Nobody would survive within that radius.
  2. At the edge of the crater, the resulting shock wave and the amount of debris (called ejecta) would kill anybody close to the crater.
  3. You are unlikely to survive if you are even remotely close to the crater. Perhaps a few kilometers away you'd be safe, depending upon the size of the resultant crater. Either way, if the size is being measured in meters like the question describes, you are definitely dead.
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    $\begingroup$ On earth we can treat great wounds and survive. On the moon, a puncture on the suit would prove fatal. Only a piece of regolith craking your visor and you asphyxiate. $\endgroup$
    – Gustavo
    Oct 21, 2019 at 21:09
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    $\begingroup$ What shockwave? A shockwave is normally formed when a pressure wave propagates at a supersonic velocity, hence the shock. Without an atmosphere, there is no shockwave. On the other hand, there is most definitely a pressure wave, and ejecta. $\endgroup$
    – Aron
    Oct 22, 2019 at 2:24
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    $\begingroup$ Based on the below, I'd assume there would be virtually no shockwave / pressure wave. In a vacuum, there's basically nothing to pressurize. sciencefocus.com/space/can-you-have-a-shock-wave-in-space $\endgroup$
    – Itinerati
    Oct 22, 2019 at 4:10
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    $\begingroup$ @Aron there's still a shock wave propagating through the lunar regolith. There's a non-zero possibility that the astronaut would not only be dead, but also launched into lunar orbit. $\endgroup$
    – Morris The Cat
    Oct 22, 2019 at 14:08
  • $\begingroup$ @Itinerati That's the thing. We are not talking about a vacuum. We are talking about on the Moon. It has a shock wave moving through the moon rock just like MorrisTheCat described. That, added to the amount of ejecta (depending on the size and speed of the ejecta, each one would have its own blast radius and shock wave), there would be pretty much no survival. $\endgroup$
    – overlord
    Oct 22, 2019 at 14:22
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This 550 pound WWII bomb produced a 5 meter radius crater in soft farm soil. To make a 20 meter radius crater in moon rock would require at least the equivalent of several tons of explosive. The insta-kill radius is going to be at least 10's of meters out from the edge of the crater. Quite likely everybody within the distances you described is automatically dead. Quite possibly at distances 1 and 2 they are dispersed to an extent it is difficult to be sure that anybody was there. At distance 3 it might be tough to be sure how many people were there.

Note added to respond to comments: The atmosphere is not very important in such explosions. The blast effects are not created by the atmosphere, but by the rapid expansion of the material that is being drastically rapidly heated. This is why the crater forms in the first place. The expansion of the material involved in the explosion is negligibly affected by the air. In the cited example there were about 550 pounds of explosives, which dug a crater roughly 3 meters deep and 5 meters in radius. Assuming that the soil is typical density for dry soil of about 1.33 g/cc, that as much as 300 tons of soil moved. The amount of air in the hemisphere above the soil is something like 700 pounds. The air has not got energy to do anything with the soil.

The primary reason the left-over WWII shell had this much effect was that it was at least partially buried. An impact crater is formed because the impacting object begins by at least partially burying itself. This is why, when using explosives, they are nearly always implanted in the object to be blasted.

A crater in rock that was 4 times the radius would move 16 times the volume of rock. (Assuming the depth stayed the same. If the depth was 4 times as much, it would be 64 times the volume of rock.) And rock is typically about double the density of soil. So a 20 meter crater involves explosively moving possibly as much as 2000 tons of rock. (Or 8000 tons if the depth increased.) A little air will make no difference.

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    $\begingroup$ Your answer gives the impression you are basing kill radius on data gathered about weapons being used on Earth where the damage comes from pressure waves in the atmosphere. Can you add information about what or how you think the damage would be caused in a very thin atmosphere? $\endgroup$
    – Muuski
    Oct 21, 2019 at 22:46
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    $\begingroup$ If a crater is formed, an enormous amount of debris will be thrown in all directions, and you will be perforated by shrapnel and then squashed under a pile of regolith and stone. $\endgroup$
    – Richard Smith
    Oct 21, 2019 at 23:06

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