What would happen to the moon?
January 25, 2006 2:19 PM   Subscribe

If there's anyone here who groks physics and math and astronomy and astrophysics here's a great big what-if for you.

Pretend the planet Earth was destroyed. Smashed into chunks ranging in size from grains of sand up through mountains. Presumably the atmosphere would be lost to space, much of the former planet would fall slowly into the sun or flee outward. What was left would form a new astroid belt, depending on how the actual destruction was caused it would probably become a planet again, at some point. Assuming that nothing directly impacted the moon, what would happen to it? Most of the earth's mass would still be around, just spread out a bit more. Assuming the moon's orbit didn't intersect the debris, would it continue to orbit? Would it fall towards the sun? Fly away from the sun? What?
posted by Grod to Science & Nature (14 answers total)
It depends a lot on how violent the destruction is. If the planet somehow just "breaks up" into a lot of little pieces1, without any additional velocity being imparted to the pieces, then the collection of pieces--and even the atmosphere--would still stick together by virtue of its own gravity. One chunk of the earth wouldn't fly off on its own any more than a rock flies off the surface of the earth under our current situation.

If the destruction is violent enough that you have all these chunks of earth flying away from each other at high speed, then it depends on how quickly they're flying away from each other. If it's fast enough, the effect of their gravity on each other will be negligible, each chunk would react as a little asteroid on its own, with action determined by the velocity imparted by the destruction--some would go towards the sun, into more elliptical orbits; some might go directly into the sun; some might go away from the sun, also into more elliptical orbits; some, if they picked up enough velocity away from the sun due to the destruction, might leave the solar system entirely.

1 Even this is based on a false assumption, since much of the interior of the earth is liquid, it wouldn't just break up into chunks.
posted by DevilsAdvocate at 2:39 PM on January 25, 2006

You've got a very big "depends" in there -- the "depends on how the actual destruction was caused" bit.

This question hugely depends on what you're assuming "smashed" means, i.e., was a large amount of new energy added to the system? Has the earth's rotational kinetic energy changed? This is very underspecified...
posted by dmd at 2:40 PM on January 25, 2006

The moon is already orbiting the Sun. It and the Earth are also orbiting the center of the Earth-Moon system (which is close to the center of the Earth but not exactly)

If the Earth broke up, the Moon would continue in a slightly shifted orbit, depending on the mechanics of the breakup. If the Earth just imploded or exploded in a way that conserved angular momentum, then probably the Moon would continue.

However, if the Earth was destroyed because some huge planet whizzed by and shoved the Earth into the Sun, then the Moon would probably be thrown right in too.

(on Preview: the question is ambiguous on the use of the word "it" but I m going by the title which is "What would happen to the moon?")
posted by vacapinta at 2:43 PM on January 25, 2006


Given a collision which matches those criteria, I would think that the Moon would continue to orbit the cloud which was once the Earth.

It would be pretty tricky for such a collision to actually happen though.

Although IANAAP (I Am Not An AstroPhysicist)
posted by public at 2:45 PM on January 25, 2006

Well, I for one wouldn't assume the mass of the Earth would go anywhere but simply create a belt around the orbit that the Earth used to have. Additionally, I don't know/think this mass would ever form another planet again even if it was to collect into one large mass.

As for the moon. If the mass of the Earth spread out, either in a belt or by flying off in some other direction, to such a degree that the gravitational pull on the moon was lessened, the moon would fly off in whatever direction it happened to be going at that moment. The only think keeping the moon in orbit is that it is constantly falling toward the Earth due to gravity. An object in motion, tends to stay in motion, and without the gravity keeping it from going in a straight line, it would just do that: go in a straight line. As for where it would go, well that would depend on the time of the month and the time of the year this event happened...
posted by pwb503 at 2:47 PM on January 25, 2006

I think it depends on the exact cause of Earth's destruction and how it happens. Notice that the moon orbits the Sun on its own accord, besides orbiting Earth, so if Earth simply disappeared there would be a good chance of the moon keeping a similar solar orbit.

An explosion with sufficient magnitude to cause what you describe will generate a debris impact wave that will certainly intersect the moon's orbit and send it travelling to some direction. At this point Earth's gravity has ceased to have any significant effect. From then, it may be captured by another planet gravity field (Mars or Venus), enter a stable orbit on its own or collapse into the Sun.

If some alien kid just slices the Earth gently with her dimensional scissors, then I don't know what happens. Maybe the moon joins the new belt, since it is already orbiting the sun now, but ceases to orbit what was the Earth.
posted by nkyad at 2:50 PM on January 25, 2006

On a more realistic note, this sort of thing does actually happen, but not between a planet and its moon.

A runaway star can come about when a partner star in a binary system goes supernova. The runaway keeps going at its orbital speed, but due to the sudden drop in mass of the parent within the orbital radius the star just heads off out of the orbit.

Basically only the mass inside the orbital radius will keep the object orbiting. If the mass drops too much the partner will disappear off. You can calculate how much mass must be blown off for this to happen relatively simply.
posted by edd at 2:58 PM on January 25, 2006

edd, relatively simply how?
posted by Grod at 3:02 PM on January 25, 2006

Well, it's a bit late here so I won't figure out all the details from scratch, and it depends somewhat on how you set up your initial orbit... but basically:

You have some initial orbit around a star of mass M. Post-supernova the star has mass deltaM outside the orbital radius, and assuming that ejected mass is in a spherical shell then that mass exerts no gravitational influence and the orbiting object is now orbiting about an object of mass (M - deltaM). You calculate the kinetic energy of the orbiting object and see if it is now greater than the gravitational potential energy that remains.

If you started with a circular orbit and the entire star just went into the deltaM then it'd move off in a straight line. If not enough mass is ejected you'll end up with an elliptical orbit at the end rather than circular, and if enough mass is ejected you'll end up with a hyperbolic trajectory I guess.
posted by edd at 3:09 PM on January 25, 2006

Something like this already happened once in the Solar System's past, it is thought by planetary astrophysicists.

It's how the Earth-Moon system got here. The leading hypothesis is that the Moon (which is very much made of the same stuff as the Earth's crust) was formed when a then-smaller Earth was impacted by a smaller planetesimal some 4 billion+ years ago. The collision completely destroyed the planetesimal, and ripped off huge portions of the Earth's crust (which at the time was mostly liquid). A huge could of molten rock and such ringed the Earth for awhile, which eventually coalesced into what we call the Moon. The Earth picked up some extra mass from the planetesimal. The majority of the mass involved in the impact ended up staying a part of the Earth-Moon system.

So, assuming the Earth was not destroyed by some really outlandish means, something like that would just happen again. The Moon would not go careening of out of the Solar System, nor would it fall into the Sun. It would stay (mostly) where it was, and either absorb the newly liberated Earth mass, or end up in a new system with a new planet formed from said mass.

The only way to avoid that would be truly bizarre assumptions about how the Earth was destroyed (as if that assumption wasn't bizarre enough).

Of course, the way to answer the question concretely is to create a simulation of the system, which could certainly be done -- if you were an astrophysicist. To do that, you'd have to settle on some initial conditions of Earth destruction.
posted by teece at 4:12 PM on January 25, 2006

The Moon orbits the earth in the same direction as the Earth orbits the Sun; therefore while it is inside Earth's orbit it is orbiting the Sun both closer to and slower than Earth. But closer orbits have greater orbital speeds. So I think that if the Earth's mass were dispersed outside the Moon's orbit while the Moon was waning crescent or new, it would fall into the Sun. Conversely, if the Moon had been waxing gibbous or full it would be moving faster than orbital speed and would fall away from the sun. Here, this might help.

If the Moon were waxing crescent or waning gibbous, though, it might have enough momentum to be thrown across Earth's old orbit into (or past) an orbit appropriate to its speed.
posted by nicwolff at 4:22 PM on January 25, 2006

Level with us -- This is actually a question about the Death Star, isn't it?
posted by JekPorkins at 4:51 PM on January 25, 2006

JekPorkins lol that's exactly what I was thinking.
posted by Baby_Balrog at 5:57 PM on January 25, 2006

teece, that is why I was hoping an astrophysicist would drop by. To create a simulation, or explain how such a thing is done.
posted by Grod at 9:59 AM on January 26, 2006

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