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Comment by gehwartzen | Hacker News Reader

Becouse of the orbital lock it’s interesting to think of it like a shield for earth. I imagine a lot of those craters were by asteroids that would have hit earth, esp the perfectly round ones in the center. The oblong ones or ones towards the edges might have come at an angle that wouldn’t have hit earth. Seems like a fascinating way to study earths impact history if we end up exploring the dark side of the moon in more detail.

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I also am fascinated by the idea of Nemesis [1] — an infrequent event that causes mass-impact throughout our solar system.

[1] https://en.wikipedia.org/wiki/Nemesis_(hypothetical_star)

I believe consensus is that the moon will eventually drift away from the Earth. Do such impacts, predominantly on the far side of the moon, apply enough force to change that prediction? Are such projections of whether the moon leaves Earth orbit (or instead collides with Earth) accounting for this?

The moon is drifting away from Earth at a rate of about four centimeters per year [1]. Inbound impacts on the far side won't significantly affect this because of the way orbital mechanics works. To lower an orbital altitude, you need thrust opposing the orbital vector. (This is why spacecraft orient the way they do, with engines pointing in the direction of travel, for retrofire.) Thrusting toward or away from the orbital center of gravity (in this case Earth) can affect the circularity of the orbit, but that's all. In the case of the moon intercepting an impactor that'd otherwise strike Earth, the thrust vector is necessarily toward Earth, so the effect would be to render the moon's orbit infinitesimally more elliptical, with the increase along the major axis balanced by an equally minimal decrease along the minor axis. (To the extent the force vector of the impact is displaced from the axis through the moon's own center of gravity, that will affect the spin of the moon, but not its orbit.)

According to the linked article, it will take about fifteen billion years for the moon to escape Earth orbit entirely. I haven't checked, but I believe that's considerably longer than it is expected to take for the Sun to expand as a red giant and render Earth uninhabitable. Certainly more wondrous than either event would be for anything then living to remember having once had distant ancestors who called themselves "human"...

[1] https://www.google.com/amp/s/public.nrao.edu/ask/what-happen...

It's compelling to me that the Earth and its moon are so tightly linked that their relationship is projected to outlast a life-providing sun. I believe this is moreso than any other moon-planet pairing in the solar system? Given what we know of incoming asteroids and comets, the probability of their impacts hitting with enough force at a trajectory you describe with enough significance to change the moon's orbit is expected to be low . And we're relatively confident of that?

I don't have numbers or references here, but my intuition from a lifetime of space nerding is that, if the moon were struck with enough retrograde force to materially lower its orbit, in the best case that impact would liberate debris of mass and vector such that our first concern would be planetwide disaster response. (In the worst case, it might shatter the moon entirely and distribute a significant fraction of its total mass across Earth's surface; in such a scenario, the survival of eukaryotic life anywhere on Earth qualifies as a million-to-one-shot win.)

But the odds of such a collision I suspect are significantly lower even than of a similarly sized impactor striking Earth. Granted that the moon helps a great deal in sweeping space near Earth of objects that might otherwise enter our atmosphere, but something with the kind of kinetic energy it would take to produce these outcomes isn't going to spend long enough within the gravitational influence of the Earth-Moon system to be significantly affected before impact. It'd be like trying to swerve an ICBM warhead by blowing really hard on it, and I choose that simile with care - if this ever does happen, I think it might constitute a clear indication that someone somewhere in the universe really hates us!

Do we have an idea of what percentage of impacts were prevented this way? What likelihood is there that the moon saved humanity (more than once?) since the time that we descended from the trees?

I imagine once you figure out which craters correspond to ones that came in at an angle which would have hit earth and compare that to all the impacts you have had on earth one could figure it out.

The thing is that on the moon the craters remain visible for a much longer period of time while on earth they get covered up by plate shifts, erosion, ocean impacts, etc. So instead it might be a way to calculate how many asteroids likely have hit earth in general throughout history whether we have found the craters on earth or not. Then you could calculate based on the size of the craters on the moon which ones would have been earth catastrophic events (taking into account any diminishing impact energy due to our atmosphere). Which would tell us how many the moon saved the earth from. In terms of how many since human ancestors were evolving we would have to then date earth catastrophic craters on the moon. I'm not sure if this can be done through photographs or if we would have to get surface samples.

Then comparing that number to the fraction of earth covered/protected by the moon to the total area on earth we would have a decent idea of how many of these events happened one earth even if we haven't discovered the evidence for them on earth.

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