"What would happen to the moon if the Earth exploded?" and other orbit questions.

Factual Questions
1

Assume that no chunks of Earth hit the moon and destroyed it or damaged it.

Would the moon move in towards the sun and find an orbit around sun? Or would it crash into the sun?

Would it shoot outwards towards the outer rim of the solar system and assume an orbit out there? Or keep moving past the edge of the solar system?

Could it move outwards or inwards and assume an orbit around another planet? Could it crash into another planet like a giant meteor or asteroid?

Asteroid vs. Meteor vs. Comet, whats the difference?

If the Earth were to split in 2 pieces would they maintain the same orbit around the sun? Would they be gravitationally attracted to each other and just “stick together”? Or would they each shoot off and form new orbits/crash into sun or other planet?

If you were to stop the orbital movement of the solar system for a split second, would all of the planets, asteroids, moons, and space ships crash into the sun and melt? If so would they all move towards the sun at the same velocity? Kinda like the feather and bowling ball dropped off a tower experiment.

If they all crashed into the sun would the sun burn out from all the debris it absorbed?

Last, but not least, what initially set the planets in motion? I understand why they stay in motion, but are there any theories as to how they were set in motion to begin with? Would the solar system work the same way if everything orbitted in the opposite direction?

Lot’s of curiousities here. I know most of this couldn’t really happen, but open your minds for a bit.

2

It depends on what the trajectory of the moon was at the time the earth exploded, if it were going ~ rightangles to the sun, my guess would be that it would form a stable orbit and eventually crash into some planet a couple of thousand years later. If it had enough velocity, it might escape the sun and head off into deep space.

An Asteroid is a chunk of rock thaat sits between Mars & Jupiter, a comet is a chunk of rock that isnt an asteroid and a meteor (metorite?) is a chunk of asteroid/comet thats hit the earth.

It depends on how the earth splits into two. If they just suddenly split 5cm apart, then yes, the gravitational attraction would just pull them together again.

If all the planets just stopped dead in their tracks, (they would probably first heat up a lot due to conservation of energy but we’ll ignore that). they would all “fall” towards the sun but they would accelerate at different rates. Gravitational force is the inverse square of distance so the inner planets would start falling faster sooner.

The sun would likely hardly even blink if all the planets fell into them. IIRC, the sun has enough room to fit a MILLION earths (side by side or in total? I forget) but the mass of the planets is insignificant compared to the sun.

The last one is a lot trickier and, IIRC, is one of the major conundrums in planetary evolution. We dont know quite exactly how our solar system got so much momentum but it might just be something we have overlooked.

Everything would be the same if the planets revolved the other way I think. Theres nothing stopping them from doing so.

3

The moon is in orbit around the sun and that orbit has some wobbles in it caused by the presence of the earth. If the earth were suddenly removed what happens next would, as another poster said, depend on where the moon was in its wobbling when the event happened. A computer simulation for a two-body problem with various initial moon velocities shouldn’t be too complex. Anybody go one?

4

Wouldn’t the normal acceleration due to the spin of the earth send each piece flying out ward as soon as the split occured? No matter how small the crack?

5

No; not any more than the spin of the earth causes rocks to fly off the beach and away into space.

6

The earth is held together by gravity. The tensile strength of the material making up the earth is miniscule compared to the force of gravity holding it together. Fortunatly, so is the force generated by the earth’s rotation. If you sliced the earth in half, but didn’t actually move the halves away from each other, nobody would notice as gravity would keep right on holding the earth together. To actually get two seperate pieces you would have to push the two earth-halves away from each other, which would take a hell of a lot of energy. And as you did so, each earth-half would collapse on itself to form a sphere, so you’d end up with two new planets would half the mass of the earth each.

7

It would stay in the same spot orbitting the sun like it does now, more than likely. Depends on how you blew up the Earth which is a task and a half.

Quick answer: Asteroid: smaller than a planet but not orbitting a planet (thus, not a moon). Rather rocky, no tails. Comet: Icy planetessimal, gets a tail when they head toward the sun. Also doesn’t orbit planets. Meteor: debris that collides with the Earth’s atmosphere generally distintegrate upon entry. Sometimes parts of them hit the Earth and become a meteorite.

Depends on what you mean by “split”. Do you mean slice down the middle with a knife? Then you get option number 2.

Conservation of angular momentum got you down? The only way to stop the movement would be to apply a deccelerating force in an opposing way which would generally result in a collision and destruction. If you got your collision exactly right (and inelastic) you could cause the oribtting body to acclerate radially with now tangential velocity.

No, Sun big. Really big. The rest of the solar system is just details (with the possible exception of Jupiter who is a whopping 1/1000 the mass of the star at the middle).

This question is like asking how does a duck get out of a bottle? (Answer: It’s out! … Thanks to R. Gott for the koan). There are initial perturbations which are necessarily amplified upon collapse. It’s very difficult to get something that DOESN’T have an initial velocity. This is due to the fact that stuff nearby is lumpy and there’s a lot of pushing and pulling going on. In order to NOT have an initial velocity, you’d have to have your solar system seed in an empty place (not condusive to solar system building).

8

Of course, this should be accelerate radially with no tangential velocity.

9

According to a friend of mine who claims to know what he is talking about (you guessed it - I have no cite), the trajectory of the moon is always pretty close to being at right angles to a line drawn from the moon to the sun.

“But wait,” I hear, “the moon goes around the Earth! Some of the time it must be moving directly toward the sun, and some of the time it must be moving directly away from the sun.”

Nope. Keep in mind that the moon is not a typical satellite. The ratio of the mass of the moon to the mass of the Earth is much greater than most satellite/primary ratios.

The Earth and the moon are both in orbits which are close to being elipses, but each is affected by the other. The Earth-moon gravitational attraction causes both bodies to speed up and slow down, as whichever one is in the lead at any given time pulls the other forward, while whichever one is trailing slows the other down. Likewise, whichever one is farthest from the sun at any given time pulls the other outward, while whichever one is closest to the sun pulls the other inward.

The Earth has the greater mass, so I assume that its orbit is closer to a true elipse than the orbit of the moon, but (according to my friend), even the wobbly orbit of the moon is always convex outward. This is possible because the Earth-moon orbit around the sun is so much bigger than the average distance from the Earth to the moon.

So the moon does not go around the Earth. The Earth and the moon dance around their common center of gravity, moving such a small distance from their common center of gravity (compared to the size of their orbit around the sun) that each of them is always moving at almost exactly a right angle to a line drawn from the object in question to the sun.

Therefore, the moon would not fall into the sun or escape from the sun. If the Earth disappeared into a space warp, the moon would continue on, its orbit now less wobbly and closer to a true elipse. The shape of that elipse would be slightly affected by where the Earth and the moon were in relation to each other at the time the Earth disappeared, but not by much.

In the slightly more likely instance that the Earth actually exploded, there would be even less effect on the moon’s orbit, because much of the mass of the Earth would stick around, more or less in the same orbit it was always in. Depending on how violent the explosion was, the moon might gather up many of the pieces of the Earth and become the more massive body, or most of the pieces of the Earth might be pulled back together again by their mutual attraction.

10

I’m sorry, but I can’t answer your question at the moment; my lludium Q-36 Explosive Space Modulator is in the shop for repairs. (Turns out the problem was Bugs; wouldn’t you know it.)

11

No kidding? I never even thought of that as a possiblity. Where did you get this idea? Not that I don’t believe you, but does anyone else have any input on this phenomenon?

12

It’s pretty simple. On the scale of something the size of a planet, gravitational forces will completely dwarf the tensile strength of any matter. There simply isn’t anything in the universe that can have the mass and density of a planet and not assume a nearly spherical shape. Basically, every part of the planet wants to reach the point of minimum potential energy, by moving as close to the center of the mass as possible. The shape in which every part of the planet is as close as it can get to the center is a sphere.

There was a pretty good discussion on this board a little while back on why planets are spheres and not, say, cubes. If the hamsters are cooperativ you might be able to search for it.