Moon Collision

Factual Questions
1

Greetings, everyone. I am, at last check, your newest member.

I have a question (by the way, this text is taken directly from an email message I wrote several days ago, so it may seem -slightly- out of context):

I am writing a story right now and require some assistance.

Here’s the premise for my story: Somewhere far, far in the past (in the millions of years range) Earth has no moon. However, there is a human civilization living on Earth which decides to build a moon. They’ve done space stations, planetary colonies, extrasolar expeditions, etc. and now want something a little closer to home. Anyway, they build this moon orbiting Earth, and it has everything: it’s a resort, laboratory, missions control, etc. However, something goes wrong and its orbit decays, sending it plummeting toward Earth. They are caught off their guard and the entirety of human civilization is wiped out, all except for about 200 on an expedition to Alpha Centauri. The collision of the moon also knocks a large chunk off of the Earth which coalesces to form the Moon as we know it today. I know the plot is not all that believable, but who ever said sci-fi had to be believable?

Anyway, I have some questions about my story. I would like to know, first of all, what the effects on Earth would be if the Moon (our Moon and the moon in the story are almost the same size) were to, over a period of about a month, spiral in towards Earth and eventually collide with it. I would like to know what would happen as the moon got ever closer (i.e. after 1 week Earth would be like this, after 2 weeks like this, after 3… etc.). I would also like to know what the actual collision would be like. And last, I would like to know just what sort of accident could so drastically affect the orbit of the moon.

(Note to Dr. Phil Plait: Hi, if you are reading this! Took your advice, as you can plainly see. Thanks!)

2

Must.Fight.Urge.To.Smack.

3

I really wanted to reply to your question, but if you don’t have any respect for it, why should I?

4

Well! Let me guess: You two are from the crowd who stalk off in an indignant huff everytime someone says “sci-fi”, all the while muttering under your breath, “It’s ‘SF’, for crying out loud!” Well let me tell you: it is you so-called “fans” who have no respect for the genre. When you minimize it down so far that you won’t listen to someone simply because he points out an already accepted truth about writing, you are mocking Ray Bradbury and spitting in the face of H.G. Wells.

Do you honestly think these two, or any of the other greats cared if their stories were believable? No. Because they weren’t. It may come as a shock to you, but you can’t just go and pick up some brass and crystal and build a time machine. Wells was no fool. He knew it wasn’t as simple as that. He knew it wasn’t about reality. In fact, by its very definition, the genre we know as sci-fi is a harnessing of the power of unreality to tell a story, represent a theme.

Also in case you missed it, sci-fi isn’t even about reality! Wells didn’t make The Time Machine because he wanted to write an interesting story (certainly not because he wanted a realistic one!), he wrote it to show danger of the growing gap between the upper- and lower-classes. He didn’t write War of the Worlds to give future generations something to read and say “neat story”. He wrote it to criticize British Imperialism.

More to the point: have you honestly ever read a good sci-fi story that was also believable? Honestly! I know I haven’t! As a matter of fact, I’d wager that I’ve never read any believable sci-fi story, or for that matter, seen a believable sci-fi movie. Yes! It may come as a shock to you that warp drives are, at best, implausible. It may shake the very core of your beliefs that that one little nuke isn’t going to destroy an asteroid the size of Texas. It may even surprise you to learn that the acceleration involved with “jumping to hyperspace” would smash any person to a reddish liquid.

If you could name me one -one- sci-fi story that is realistic, was written with the intentions of being realistic, yet still has a good plot and successfully delivers a meaningful theme, I would honestly be as schocked as I’ve ever been. Perhaps you are afraid to admit that your favorite genre is not believable. Perhaps you have an inferiority complex. I don’t know; I’m no psychiatrist. But whatever the cause, both of those replies were highly irrational, illogical, and quite frankly uncalled for.

5

Calm down, son. Yes, they were a rude, but you’re only fanning the flames.

I have read believable science fiction stories. They tend to be the dryer ones, it’s true. But they’re there. Try some of Niven’s short stories. Inconstant Moon, maybe.

As for your question (and anyone recommended to the SD by The Bad Astronomer gets some extra leeway from me) I’m not sure it’s answerable as posed.

  1. Yes, I could see something causing the moon to impact the earth. Some large impact on the far side might be able to do it. But it would have to be enormous. I wouldn’t even want to calculate it. Note: The big explosion from Space:1999 wouldn’t come within a billionth of a billionth of doing it.

  2. As to what would happen…that’s tough to say. The earth and moon orbit a common point. Off the top of my head I’d say possibly as the moon got closer to that point it might speed up the orbit. And when it impacted…BLOWIE. But it’s been done a million times with asteroid and cometary impacts.

Anyway, let me do this for you…

Dudicon …

Welcome to the Straight Dope Message Board. I hope you enjoy your time here. May it be long and merry.

6

Just to throw one more monkey wrench into your story, let me point out that your statement

isn’t even close to right. The latest thinking is that the collision with a Mars-sized object that threw out ejecta that became the moon happened within 100 million years of the Earth’s creation (or 4.26 billion years ago), when it was still a molten ball in the midst of constant barbardment by the same huge planetesimals that made many of the craters we see on the moon today.

You just can’t get away with your premise. Well, maybe in the movies. Or tv. Or comic books. But print sf has a higher standard. What you’re suggesting is just plain silliness. That’s why people are jumping down your throat. SF doesn’t have to be true but it absolutely has to have verisimilitude. You can try starting from the premise that “anything goes” but your career won’t go very far. Sorry.

7

Careful painting with that broad brush, Dudicon. I’ll grant that I did a poor job of pointing out that it was your dismissive attitude revealed by that “who ever said” comment which was the point of contention I had with your original question, but I fail to see how you managed to infer all that vitriol from that little issue. And so far off the mark, to boot.

Oh, and for the record (and because I do have a reputation to maintain around here) the answer to your first question is going to be different for the two bodies if they have different masses, so you’re going to need to say whether they are the same mass, not just the same size.

8

Of course, you can avoid a lot of the conflicts if you have it happen on a planet other than Earth.

As the artificial moon (assuming it had a huge amount of mass, and therefore gravity) spiraled in, the tidal forces would be horrific. The climate would change slowly, then the planet would be swept by tornadic winds. The seas would erupt over the land, having tides hundreds of feet high. Towards the end, as this huge body entered the atmosphere, it would tear the atmosphere from the planet, rendering it sterile for billions of years perhaps. When the moon hit the planet, it would go through the crust, fracturing all the way into the mantle.

Seriously, every single living thing would be wiped off the planet. The planet’s crust would be unrecognizable afterwards. It would be like starting over.

This is, of course, only IMO, based on the science and sci-fi I know and love.

9

Quit splashin’ in the GQ pool, you guys. You’re gonna get chlorine in my martini. If you want to have a water fight, go to the BBQ Pit. They have Super SoakersTM over there.

10

According to these people

That’s a pretty large sized “something goes wrong”. I’d expect that a collision with anything the size of the moon would completely melt the earth’s crust.

11

Dudicon, do you watch sci-fi or read sci-fi?

Written sci-fi typically is very plausible. The authors go to great lengths to make it intelligent. The movies are just buffoonery for the most part.

For example, I think you would have a hard time finding a Larry Niven fan that could even sit though the movie, “Mission to Mars”.

The literate sci-fi say it should be.

If you follow good sci-fi, it usually stays quite up to date with the current best understanding of science.

Yes. If you think otherwise, you clearly don’t read sci-fi “greats”.

That is where the indignation comes from.

Try reading:
Larry Niven
Jerry Pournell
Anderson
Asimov
Haldeman
Heinlein
Or any Hugo or Nebula award winner.

Some of the books may not be believable today if you are really current on your science. But, almost all of them were as good as it got when they were written.

12

Wow, there were only the first four posts when I started writing my last… this must have really hit a nerve.

13

Well, I calculated it at about 40 times that, 6.67*10[sup]28[/sup] J, over in the Time Machine thread. Since that page didn’t show any of their calculations, and I was able to catch a few other mistakes in their physics, I’m going to trust my number more than theirs. I suspect that their error was in not conserving momentum: You can’t put energy directly into KE of a single body.

On science fiction, in general: There’s absolutely nothing wrong with implausible literature. If it’s totally physically implausible, though, then it’s not science fiction any more, it’s fantasy.

14

Hey! What errors in the physics? I actually wrote (or co-wrote) that article on alt.destroy.the.earth lo these many years ago, so I’d be interested to see what errors you found (admitting up front that there are quite probably several). If you want to, you can email me your comments & we can spare the board - it’s up to you.

The main problem I have with the OP is the notion of the “slow spiral”. If you assume we’re talking about motion across decades or even centuries instead of hundreds of millenia, then there’s no such thing as a spiral orbit. Small satellites in Low Earth Orbit will spiral back to Earth due to drag effects (even though the atmosphere is waaaaaaay thin at 300 miles up, it still exerts a small force on a vehicle whizzing through it at thousands of miles an hour), but you wouldn’t see something like this happening with a massive object like the Moon, at least not in the timespan of a typical story.

15

A few questions. They build a moon massive enough to knock a continent-sized chunk out of our planet? Out of what? And why would they build something that large just for a resort/research colony? Did they want tides so they could go surfing? Realistically, no civilization would muck around with something like that because the climactic changes and effects due to tidal forces would be unpredictable.

But if you must…I think you have to take it back a bit farther and assume they want to build such a moon (maybe because they NEED the climactic change made possible by a sizeable moon) and that they decide to do this by redirecting an asteroid. Once you’ve accepted this premise (that they can actually move something that large), then you have your disaster scenario – someone miscalculates, or a booster fails at a critical juncture and it’s Earth-is-the-eight-ball time.

But even in this scenario, you probably don’t get the slow photogenic spiral with continents bulging and casts of thousands running around in panic while cities collapse scenario. At best you’d get the asteroid in a wildly erratic orbit eventually terminating in a direct hit.

16

Well, the OP did say all humanity on Earth was wiped out… Of course, by the only remotely plausible scenarios, all multi-cellular life on Earth would be wiped out. Let’s try to steer this towards an intersection between current science, and the premise of the story.

Check me on this:
A fairly advanced society (interplanetary flight as a difficult but possible effort) decides it just has to have a large mass in orbit around their planet, and aren’t too worried about tidal effects. Let’s say Something Goes Wrong, and their selected mass gets inside the Roache limit, maybe even has a grazing impact with the planet. Large mass breaks-up, planet gets wiped, and perhaps some portions of the arriving mass get captured into orbit around Earth.

Plausable so far?

OK, I’m betting that any society that can travel to the stars and move really large semi-planetary masses will have some form of at least semi-perminant space occupancy. Much of this should survive, at least for a while. They will likely be using some form of hydro- or aero-ponics for atmosphere control and food. So, survivors try to pick-up the pieces by steering comets into the planet, grabing hydrocarbons from the gas giants, and seeding microbes back onto the wreck that used to be the homeworld. Eventually, simple plants (algae, molds, fungi, etc) are seeded back, and finally even more complex plants. If the society had kept pets in space, or lab animals (mice, gerbils, some insects?), some might be deposited back to the planet, if they could hold on the century or two necessary to reestablish a stable biosphere. It would have to be herbivores of some kind at first, maybe insects, gerbils, and/or lab mice? Trouble is, the enviroment would be very simple, and you’d have to really damn patient, allowing lots of time for lower forms to be established before releasing higher forms. Even then, if there’s no appropriate prey for higher forms (what do the spiders eat, if they’re all orbweavers, and you’ve got no flying insects?), you may have to just not fill that niche.

In short, you’d have to terraform Terra.

Now… A space-based society that can do all that would likely survive to enjoy the fruits of their labor, and the folks returning from the stars would find themselves simply pitching-in to help restore the Earth.

Alternative:
There is more than one stellar expedition.
The remnants of humanity living in Sol-space try to survive, and begin the terraforming project (say, getting most of the small the debris cleaned out of orbit, landing the comets and gas-giant hydrocarbons), but don’t have the industrial capacity to maintain themselves, and wind-up dying off before the job is half-done. Cue the star travellers, who probably have some idea of what’s going on, but are in no position to help at first. They arrive, and pick-up where the survivors left off. They then die off, having maybe seeded some higher life onto the planet. Then the next batch of start-travellers return, and continue, and die, and then the next…

Assuming sufficient numbers of expeditions, returning at intervals of 50 - 100 years, after four or five cycles, you might have a bleak, but habitable planet again. At that point, it becomes a surface survival story.

Does this work? Even a little?

17

Somewhere on the net I read a list of classic things that beginning sci-fi writers do wrong. One is that things in orbit “decay” (a weekly problem on shows like star trek, etc). The article points out that once you put an object in orbit it… (drum roll) stays in orbit. The “something goes wrong” part has to be something big enough to move that huge freaking mass towards the earth. Just having it “decay” is a bad idea.

A civilization advanced enough to build a moon also would be advanced enough to do some major sort of evacuation if they had a month’s warning. To think that they would all stand on the surface, looking up towards the sky with a worried expression on their faces for an entire month seems kind of silly. You’d have some survivors somewhere, maybe in space stations, maybe on mars, who knows.

Sci-fi doesn’t have to be one hundred percent believable, but for me at least, the better stories are the ones that require the fewest bits of tinkering with the laws of physics. However, keep in mind that science fiction is two things - science, and fiction. Getting the science right is important, but so is telling a good story.

Good luck with it!

Oh, and to keep it all in perspective, there’s at least one story that Stephen King wrote that even he admitted the science in it was “wonky” (if I recall, that’s the exact term he used). It didn’t exactly end his career.

18

Found it:

Favorite SF Bloopers (Outer Space)
http://web.mit.edu/mbarker/www/writers/t014186.txt

19

Ah, thanks for all of the advice everyone. By the way, sorry for all the heat I caused with that one little sentence. By the way, I would like to clarify something on that point really quickly. When I said sci-fi didn’t have to be believable, I was treading the fine line between “believable” and “realistic”. Of course a sci-fi story should be such that when you read it you don’t say, “Geez, what was this guy thinking?” However, it is fiction, and I would say that grants you some leeway, especially when the leeway is only in the area of “improbable” instead of “impossibility”. My story is improbable, perhaps highly so, but not impossible. I came to this website to clear up the factual bits (i.e. what would actually happen, physically speaking) so that my story wouldn’t slip over into that realm of impossibility. Anywho…

Another thing I should clarify: When I said they built the moon, I wasn’t being entirely detailed enough. What they did was push it out of orbit from Jupiter, and bring it back to Earth. I suppose that I just slipped into saying they “built” it when I didn’t mean to.

Yes, there are survivors, of course, and as of yet I’m not entirely sure how to work them into the story. They escape off of Earth and head to… somewhere else. Incomplete? Yes. But whoever said sci-fi had to be complete? (A joke! That was a joke!)

20

By the way, just so you know: The moon in the story is almost exactly the same mass and size as Triton.