Phobos-sized moon hits our moon: survival on earth?

I’m thinking of cosmology for a novel I’m working on, and I wondered if anyone had done the math/calculations to show if a Phobos-sized captured asteroid impacted with the moon, whether a pre-modern civilization on the earth below would be able to survive that event, in such good condition that later peoples would have myths or deity-tales about the event.

Any takers?

Sorry to say, :slight_smile: but humanity will survive. It may even not notice if the impact happens in the other side of the moon.

Phobos is less than 10 miles in diameter, now we are talking about a chihuahua encountering the sole of the foot of the NFL lineman.

I could refer you to NIvenPournelle’s Lucifer’s Hammer for an existing fictional account that plumbs the depths of such an impact, with considerable scientific basis for most of the story points. Not really a good GQ answer, but it’s there.

It’s been theorized that impacts on Mars resulted in debris ejected into space and eventually landing on Earth. If a large enough body hit the moon, it seems even more likely that some objects, perhaps of significant size, would end up on Earth, too. Probably not the end of civilization, but a very large rock splashing into the Pacific would be a bad news day for many creatures and a few major cities.

Mmm, the OP mentions Phobos colliding with the moon, and then implies that the remains of that collision will cause havoc on the earth later, as pointed, it is not likely to affect the earth. Whatever pieces that are ejected will be smaller and just show up on earth as small meteors.

IIRC Lucifer’s Hammer has a comet impacting earth, not the moon.


However, you should admit the truth, you are really Thundarr the Barbarian huh? :p;):slight_smile:

Better for the OP than Lucifer’s Hammer, in which Earth is the “target”, is Jack McDevitt’s Moonfall which is about a large asteroid that impacts and shatters the moon.

True, but there are many similarities (the comet in LH hits as fragments all over the Northern Hemisphere - similarity to attack by lunar blobs, no?)

No, I am the unabashed admirer of a incredible historical figure who occasionally described himself (not inaccurately) as an amateur barbarian. I like to think I am much like him in that respect - intellectually, if not sociologically.

Yes if it is a huge one, no if it is the size of Phobos.

Fritz Lieber’s The Wanderer covers this ground well enough to win a Hugo. He assumed a much larger planet impacting the moon. There’s a lot of destruction on Earth, but civilization survives.

And the already referenced Thundarr the Barbarian from the 80’s has the moon destroyed by a runaway planet and the Earth lay to waste.

(text from the audio, the maker of the Youtube entry calls the planet a comet.)

So, yeah, Ruby-Spears Productions not only got to steal from Star Wars, but also from classic science fiction.

To be really nitpicky, it was a planet-sized alien spaceship that ate the moon for fuel. I only mention this because an impact would have left lots of debris floating around, which Leiber’s scenario avoided.

But Leiber’s analysis of the effects on tides was what really made the book. (Well, of course, that and the characters and plot!)

Another one is Thunderstrike, where a large object that’s on a collision course for Earth is deliberately diverted to hit the Moon* because it’s too massive to force into a miss in to time they have. It’s still a major disaster though; the Moon has to be evacuated and there’s an awful lot of debris.
*Specifically, antimatter charges are used to slow it just enough that the Moon will pass between it and the Earth.

So what I’m hearing is that I can probably up the size of my “Phobos” moon to at least Ceres size, and still be on ok territory as far as a civilization making it through the event. Awesome. That’s what I wanted to happen.

Further expansion of question:

If this were a slow process over many years (orbits decaying into each other, essentially) how would I go about figuring out what amount of debris would be falling to earth, vs falling onto the new combined moon surface, and how varied would the debris sizes be?

In my total ignorance, I’m imagining something like the Perseids essentially every night, as bits continue to fall and flame out, and a decent amount of surviving chunks from golf-ball sized to bus-sized, but is there any way to plot that out or make a more educated guess as to what size bits would predominate, and how many of them there would be, and over what sort of time-period before most of the junk is fallen to either earth or remaining moon?

I suppose I could just pull shit out of my ass, but I find it really interesting when fictional plot events are revealed to be based on attempts to be realistic, so I’d like to do the same in my own work.

Thank you all for the info so far!

if its a slow process the problem you’ll have it that the Phobos or Ceres sized object will never hit the moon, instead it will be torn apart into a ring when it approaches within the Roche limit of the moon.

I think for what your Ceres sized planetoid needs to be a fast moving object coming in from outside the solar system and happen to strike the moon.

Well, I’m just going to wait until you guys figure out if it’s worth the bother or not. And you’ll have to decide the what velocity you want to hit the moon with. You want a brush, a bump, a medium to heavy bump or a full on collision. And what angle (with respect to the moon’s orbit around the Earth) do you want me to go for?

I’m not going to waste any time programming command #1 into the auto-pilot until you guys work that shit out. Maybe I’ll see how much reaction mass is on hand now, but that’s it.

I think the “slow process” he means is after Ceres hits the Moon, with the resulting debris raining down on Earth for centuries or millennia.

But I’m not certain.

You’re both right -

I was figuring the collision process itself would be slow - years or decades or centuries of them being all close and wreaking havoc on each other - I didn’t know that the smaller one would tear itself apart before it actually hit, but that would work also, as long as most of the **mass **of the smaller moon ended up as part of the larger moon eventually.

I also thought that there would be centuries or millennia of debris falling onto the earth from that process.

But I don’t know enough to know either of those for sure.