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I've been watching the recent news about the close pass of the interstellar object 'Oumuamua'. It apparently passed as close to us as 60 times the distance between the Earth and the Moon at it's closest point.

That's really not far away at all, relatively speaking, so there is the obvious question of what would happen if it had hit us. That's been asked a lot, and the consensus seems to be that it would be very very bad, although not quite as bad as the one that hit the dinos.

But what hasn't been asked anywhere (as far as I can tell) is what would happen if it were to hit the Moon instead?

What effect would it have on us here on Earth? Would we get showered with debris? Would the Moon's orbit be affected? Would there be any political fallout? A space race to claim rights to the crater for scientific or mineral rights? Or any other effects that I haven't thought of?

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    $\begingroup$ Space treaty forbids using space for industrial activity. After an impact, space remains space... and science has managed to be cross national... $\endgroup$
    – L.Dutch ♦
    Nov 23, 2017 at 13:24
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    $\begingroup$ I'm not aware of such a treaty. What are you referring to? $\endgroup$
    – Elukka
    Nov 23, 2017 at 15:53
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    $\begingroup$ Maybe.. just maybe, a part of the object will get embedded into the Moon's surface, only to be discovered millions of years later by a lucky space-faring species whose evolutionary future is about to get very interesting. ;) $\endgroup$
    – omijn
    Nov 23, 2017 at 16:41
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    $\begingroup$ "60 times the distance between the Earth and the Moon" - which is about 60+ light seconds, and if we recall the sun is about 500 light seconds away, I would not say it was close even on the case of solar system. $\endgroup$
    – MolbOrg
    Nov 23, 2017 at 21:12
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    $\begingroup$ @Polygnome Which says that we can't put weapons in space and nations can't claim sovereignty over places in space. Says nothing about commercial exploitation that I can see. $\endgroup$
    – David Richerby
    Nov 23, 2017 at 22:53

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A lot of people have already mentioned that Oumuamua impacting the moon would not have a serious impact for us here on earth. We have some reason to believe that humans have witnessed massive lunar impacts before. There's a great story from 1178, about a few monks who believed the world was ending when they saw an explosion "split the moon in two" and caused it to sputter out "fire and hot coals." Some people believe what they really saw was the impact that left the Giordano Bruno crater!

So if you want to know what would happen if an object with a lot of momentum hit the moon, the answer is probably, "It would look really cool!"

Besides all that, I want to set the record straight on a few things, since you mentioned Oumuamua specifically. Oumuamua didn't really get all that close to the Earth. In fact, it maintained a healthy distance of at least 24,180,000 km, which is more than 60 lunar distances away. The reason there was a lot of speculation of, "Oh no, what if it had hit us!?" isn't that it got too close, it's because it was going too fast.

Part of what made Oumuamua such an extraordinary discovery is how fast it was going when it passed between the sun and Earth. On September 9th, scientists clocked its maximum velocity at 87.7 km/s. Based on that, they were able to calculate that it was moving at about 26.5 km/s when it entered our solar system.

When it entered our solar system.

No other object so close Earth has ever been confirmed to have actually come from outside our solar system. 26.5 km/s may not sound like a lot, but it's still fast enough for Oumuamua to break the sun's escape velocity. So, since it didn't crash into the moon (or anything else, yet) as it passed through our space, it should now be on its way back out of the solar system!

That's a bit of a tangent, but I think it's worth bringing up. Anyways, here's my back of the envelope guess for what would happen if Oumuamua did strike the moon, right at the perihelion of its orbit. The energy of the impact would be:

  • volume = 180 × 30 × 30 m^3 (observed)
  • density = 2000 kg / m^3 (blind guess)
  • mass = volume × density = 3.24e8 kg
  • velocity = 87.7 km/s
  • kinetic energy = 1/2 × mass × velocity^2 = 1.25e18 Joules

EDIT: A previous version of this answer figured the impact would have an equivalent of 23 tons if TNT. This was way off, because I was misreading "km/s" as "km/h" when I gathered my information. The correct kinetic energy is orders of magnitude higher: 2.98 × 10^8 tons of TNT, (about 10 Tsar Bombas, and a bit less than 1 if you slow it back down to 26 km/s). Apologies for getting that very wrong!

So I'll revise again what I said earlier. It would be really cool if an Oumuamua-like object hit the moon at max speed, and probably would be the sort if thing you could see with the naked eye — just like Gervase of Canterbury allegedly did back in the 12th century.

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    $\begingroup$ The perihelion was much nearer to the sun (~¼) than the earth and moon, so it would not have the same speed at 1 AU. Could you calculate how fast it was when crossing the earth orbit? $\endgroup$
    – Paŭlo Ebermann
    Nov 23, 2017 at 20:55
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    $\begingroup$ Are your units correct? km/h is stupid slow. Wiki on it shows 26.5 km/s not per hour which makes more sense. Wiki confirms the 87.7 km number as km/s not per hour as well. $\endgroup$
    – Twelfth
    Nov 23, 2017 at 21:06
  • $\begingroup$ @Twelfth: Thanks for correcting me, you're right my units were all wrong. $\endgroup$
    – jen-rose
    Nov 23, 2017 at 22:24
  • $\begingroup$ @PaŭloEbermann It depends a lot on many factors, really. ~25km/s is the velocity at "infinityAU". Remember we are talking realtive to sun an we are also moving relative to it. The major factor are the trajectory and angle of impact but you must also take in account the acceleration due moon-earth gravity well $\endgroup$
    – jean
    Nov 23, 2017 at 23:31
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    $\begingroup$ "That's a bit of a tangent" ... hehe, I see what you did there, :-D $\endgroup$
    – Simba
    Nov 24, 2017 at 9:09
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None

No, we would not be showered by debris, because since the Moon is in orbit around Earth, so would any debris be.

No, the Moon's orbit will not be affected by something as small as ʻOumuamua. At an estimated volume of 0.00016 km3, and the Moon having a volume of 21 900 000 000 km3, an impact at even an absurd a speed like 1 000 km/s, would affect the Moon's orbitial speed by less than one in one hundred million.

No, there will be any political fallout.

No there will not be a space race to claim anything up there — except possibly scientific bragging rights — because it is not allowed to claim the Moon, or any part of it, for any one nation.

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    $\begingroup$ Not true that all debris would remain in orbit. Some would be launched on escape trajectories, some would have orbits that intersect Earth. "Showered" is perhaps an overstatement, but I'd expect there would be lots of meteors for a while. There are, in fact, a number of meteorites that originate from lunar asteroid impacts: meteorites.wustl.edu/lunar/moon_meteorites.htm $\endgroup$
    – jamesqf
    Nov 23, 2017 at 19:26
  • $\begingroup$ The Moon Treaty failed to be ratified by any major country. The Outer Space Treaty of 1967 however is ratified by almost everyone, including all spacefairing nations.and forbids claiming of the moon or any planet by any nation. $\endgroup$
    – Polygnome
    Nov 23, 2017 at 22:54
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    $\begingroup$ Earthlings and their international law, bah. $\endgroup$
    – Carl
    Nov 23, 2017 at 23:18
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Answering by parts.

Would we get showered with debris? It depends on a lot of factors, composition, mass, angle of the impact, etc. But not by something like Oumuamua little guy.

Would the Moon's orbit be affected? Not significant change in orbit or rotation unless it's really massive/fast object impact. Anyway the energy necessary to achieve it is more likely to rip the moon (with very nasty effects for us). Again not by something like Oumuamua little guy.

A space race to claim rights to the crater for scientific or mineral rights? Unlikey, the moon in being hit that way for millions years and any result for such impact is already there. Any mission can take years to be prepared anyway. Also to be sure there's something really worth the effort we can need proof (debris?) in the first place.

Would there be any political fallout? Unlikey, if there's no race for the moon or the impact ground zero local why bother? There's also a lot of hassle here for oil and others resources.

Or any other effects that I haven't thought of? Only if you handwave anything like a gigantic robotic alien species spaceship crash or any other nonsense.

The bottom line is. That's a very common thing and happened a lot in the past and if you stick to reality and science there's nothing breathtaking happening and little chance something interesting come form such event to justify a mission to moon or anything beyond focusing some telescopes.

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If four working retroreflectors which are placed on the Moon during Apollo Program would be destroyed, then unfortunately some on-going projects such as measuring Earth-Moon distance, orbit and the attitude of the Moon and several scientific experiments would also be destroyed.

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  • $\begingroup$ We could probably put new reflectors up there pretty quickly if we wanted to. Just not by sending human astronauts to do it. $\endgroup$
    – user
    Nov 23, 2017 at 16:21

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