# Phobos-sized moon hits our moon: survival on earth? **URL:** \ **Category:** Factual Questions\ **Created:** [September 14, 2013, 5:09pm UTC](https://boards.straightdope.com/t/phobos-sized-moon-hits-our-moon-survival-on-earth/668851 "2013-09-14T17:09:27Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![Lasciel](https://avatars.discourse-cdn.com/v4/letter/l/e79b87/32.png) [@Lasciel](https://boards.straightdope.com/u/Lasciel)\ **Post date:** [September 14, 2013, 5:09pm UTC](https://boards.straightdope.com/t/phobos-sized-moon-hits-our-moon-survival-on-earth/668851/1 "2013-09-14T17:09:27Z")
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? ---
**Author:** ![GIGObuster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gigobuster/32/421_2.png) [@GIGObuster](https://boards.straightdope.com/u/GIGObuster)\ **Post date:** [September 14, 2013, 5:22pm UTC](https://boards.straightdope.com/t/phobos-sized-moon-hits-our-moon-survival-on-earth/668851/2 "2013-09-14T17:22:36Z")
> [@Lasciel](#): > > 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, 🙂 but humanity will survive. It may even not notice if the impact happens in the other side of the moon. > **[Moons Archives](https://www.popsci.com/category/moons/)** > > See the latest Moons stories from Popular Science. See news, trends, tips, reviews and more at Popular Science. > [@](#): > > “If an asteroid hits the moon, it will just get another crater,” says Gareth Wynn-Williams, an astronomer at the University of Hawaii. It would take a moon-size object to move the moon, says Clark Chapman, a planetary scientist at the Southwest Research Institute, and most likely the moon wouldn’t survive. Hitting it with a much larger, denser object would be like whacking an egg with a golf club. > [@](#): > > OK, how about if the largest known asteroid, Ceres—which at 600 miles across is roughly the size of California and Nevada combined—did manage to slip out of its place in the asteroid belt and set out on a collision course for the moon? > > Hardly a budge, Wynn-Williams says. It’s the equivalent of a four-year-old trying to knock over an NFL lineman. The moon orbits the Earth at some 0.635 miles per second. This orbital momentum is so great that it would overwhelm the impact force of a collision and just continue zinging around the planet. 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. ---
**Author:** ![Amateur\_Barbarian](https://avatars.discourse-cdn.com/v4/letter/a/59ef9b/32.png) [@Amateur\_Barbarian](https://boards.straightdope.com/u/Amateur_Barbarian)\ **Post date:** [September 14, 2013, 5:25pm UTC](https://boards.straightdope.com/t/phobos-sized-moon-hits-our-moon-survival-on-earth/668851/3 "2013-09-14T17:25:11Z")
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. ---
**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\ **Post date:** [September 14, 2013, 5:35pm UTC](https://boards.straightdope.com/t/phobos-sized-moon-hits-our-moon-survival-on-earth/668851/4 "2013-09-14T17:35:28Z")
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. ---
**Author:** ![GIGObuster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gigobuster/32/421_2.png) [@GIGObuster](https://boards.straightdope.com/u/GIGObuster)\ **Post date:** [September 14, 2013, 5:37pm UTC](https://boards.straightdope.com/t/phobos-sized-moon-hits-our-moon-survival-on-earth/668851/5 "2013-09-14T17:37:25Z")
> [@Amateur\_Barbarian](#): > > 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. 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. Aside: However, you should admit the truth, you are really Thundarr the Barbarian huh? :p;)🙂 ---
**Author:** ![OldGuy](https://avatars.discourse-cdn.com/v4/letter/o/3bc359/32.png) [@OldGuy](https://boards.straightdope.com/u/OldGuy)\ **Post date:** [September 14, 2013, 5:38pm UTC](https://boards.straightdope.com/t/phobos-sized-moon-hits-our-moon-survival-on-earth/668851/6 "2013-09-14T17:38:19Z")
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. ---
**Author:** ![Amateur\_Barbarian](https://avatars.discourse-cdn.com/v4/letter/a/59ef9b/32.png) [@Amateur\_Barbarian](https://boards.straightdope.com/u/Amateur_Barbarian)\ **Post date:** [September 14, 2013, 5:44pm UTC](https://boards.straightdope.com/t/phobos-sized-moon-hits-our-moon-survival-on-earth/668851/7 "2013-09-14T17:44:49Z")
> [@GIGObuster](#): > > IIRC Lucifer’s Hammer has a comet impacting earth, not 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?) > [@](#): > > However, you should admit the truth, you are really Thundarr the Barbarian huh? :p;)🙂 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. ---
**Author:** ![GIGObuster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gigobuster/32/421_2.png) [@GIGObuster](https://boards.straightdope.com/u/GIGObuster)\ **Post date:** [September 14, 2013, 5:50pm UTC](https://boards.straightdope.com/t/phobos-sized-moon-hits-our-moon-survival-on-earth/668851/8 "2013-09-14T17:50:30Z")
> [@Amateur\_Barbarian](#): > > 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?) Yes if it is a huge one, no if it is the size of Phobos. ---
**Author:** ![RealityChuck](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/realitychuck/32/195_2.png) [@RealityChuck](https://boards.straightdope.com/u/RealityChuck)\ **Post date:** [September 14, 2013, 6:50pm UTC](https://boards.straightdope.com/t/phobos-sized-moon-hits-our-moon-survival-on-earth/668851/9 "2013-09-14T18:50:34Z")
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. ---
**Author:** ![GIGObuster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gigobuster/32/421_2.png) [@GIGObuster](https://boards.straightdope.com/u/GIGObuster)\ **Post date:** [September 14, 2013, 7:11pm UTC](https://boards.straightdope.com/t/phobos-sized-moon-hits-our-moon-survival-on-earth/668851/10 "2013-09-14T19:11:43Z")
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. [![](https://img.youtube.com/vi/LhAobPugvsk/hqdefault.jpg "Thundarr the Barbarian TV cartoon intro (1980)") ](https://www.youtube.com/watch?v=LhAobPugvsk) > [@](#): > > In the year 1994, from out of space a runaway planet hurtles between Earth and the moon. The moon is destroyed, and the Earth loses its ozone layer–causing the entire planet to be laid waste. Centuries later, Earth has become “a world of savagery, super-science, and sorcery,” inhabited by various evildoers, scavengers, and magicians. Thundarr, a warrior who wields a powerful sword, is joined by his friends Ookla the Mok and Princess Ariel as they make their way through this strange world (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. ---
**Author:** ![Trinopus](https://avatars.discourse-cdn.com/v4/letter/t/2bfe46/32.png) [@Trinopus](https://boards.straightdope.com/u/Trinopus)\ **Post date:** [September 14, 2013, 9:51pm UTC](https://boards.straightdope.com/t/phobos-sized-moon-hits-our-moon-survival-on-earth/668851/11 "2013-09-14T21:51:26Z")
> [@RealityChuck](#): > > 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. 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!) ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [September 14, 2013, 10:02pm UTC](https://boards.straightdope.com/t/phobos-sized-moon-hits-our-moon-survival-on-earth/668851/12 "2013-09-14T22:02:14Z")
> [@OldGuy](#): > > 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. Another one is [Thunderstrike](http://www.amazon.com/Thunderstrike-Michael-McCollum/dp/1929381107), 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. ---
**Author:** ![Lasciel](https://avatars.discourse-cdn.com/v4/letter/l/e79b87/32.png) [@Lasciel](https://boards.straightdope.com/u/Lasciel)\ **Post date:** [September 15, 2013, 12:59am UTC](https://boards.straightdope.com/t/phobos-sized-moon-hits-our-moon-survival-on-earth/668851/13 "2013-09-15T00:59:28Z")
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! ---
**Author:** ![coremelt](https://avatars.discourse-cdn.com/v4/letter/c/4491bb/32.png) [@coremelt](https://boards.straightdope.com/u/coremelt)\ **Post date:** [September 15, 2013, 2:35am UTC](https://boards.straightdope.com/t/phobos-sized-moon-hits-our-moon-survival-on-earth/668851/14 "2013-09-15T02:35:55Z")
> [@Lasciel](#): > > 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? > ! 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. > **[Roche limit](https://en.wikipedia.org/wiki/Roche_limit)** > > In celestial mechanics, the Roche limit, also called Roche radius, is the distance from a celestial body within which a second celestial body, held together only by its own force of gravity, will disintegrate because the first body's tidal forces exceed the second body's self-gravitation. Inside the Roche limit, orbiting material disperses and forms rings, whereas outside the limit, material tends to coalesce. The Roche radius depends on the radius of the first body and on the ratio of the bod Th... 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. ---
**Author:** ![Ranger\_Jeff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ranger_jeff/32/2065_2.png) [@Ranger\_Jeff](https://boards.straightdope.com/u/Ranger_Jeff)\ **Post date:** [September 15, 2013, 4:04am UTC](https://boards.straightdope.com/t/phobos-sized-moon-hits-our-moon-survival-on-earth/668851/15 "2013-09-15T04:04:42Z")
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. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [September 15, 2013, 12:38pm UTC](https://boards.straightdope.com/t/phobos-sized-moon-hits-our-moon-survival-on-earth/668851/16 "2013-09-15T12:38:14Z")
> [@coremelt](#): > > 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 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. ---
**Author:** ![Lasciel](https://avatars.discourse-cdn.com/v4/letter/l/e79b87/32.png) [@Lasciel](https://boards.straightdope.com/u/Lasciel)\ **Post date:** [September 15, 2013, 4:23pm UTC](https://boards.straightdope.com/t/phobos-sized-moon-hits-our-moon-survival-on-earth/668851/17 "2013-09-15T16:23:40Z")
> [@ZenBeam](#): > > 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.