Colliding/orbiting planets chatter (split from "The real PTM

[Tunnelcat]

(mod note: this thread is split from this point in the "The real PTMC" thread. ie: OP was not intended by the OP as being an OP. (...what?))

[roid]

what is that? an artists' depiction of some kindof plasma engine used to ship asteroids around?

[Lobber]

Thanks for splitting the threads. (And earlier merging of my PTMC thread with the existing PTMC thread)

[Tunnelcat]

what is that? an artists' depiction of some kindof plasma engine used to ship asteroids around?

No, it's supposed to be an artist's conception of 2 planets colliding in slow motion. Apparently his idea is that as the 2 planets get close to each other, the atmospheres begin to react, exchange materials and in the process, create enormous electrical storms between them.



Don't know if that's what really happens, but it reminded me of an orange version of the shield pickup in Descent and I thought it looked cool.

[roid]

the 2 planets that close together would probably tear eachother apart from tidal forces, their surfaces would become molten pretty-quick* from the friction from the tides alone.



lets do it


* ok i have no idea how long it would take, unreferenced relative timescales FTW!
I just know that Europa has liquid under-surface oceans merely from tidal interactions with Jupiter, it should otherwise be a solid iceball afaik. Tidal forces are surprisingly powerful!

[Sirius]

Tides are caused by a body rotating while it's under gravitational pull; situation here is a little different with the entire body moving at the same rate.

Roughly. I expect at extreme proximity there's going to be significant differences in gravitational force between the near and far side of the planet, which could stretch it out enough to cause surface damage. Anything whose shape is determined by hydrostatic equilibrium is probably not going to be particularly resistant to this.

[Lobber]

Tides are caused by a body rotating while it's under gravitational pull; situation here is a little different with the entire body moving at the same rate.

Roughly. I expect at extreme proximity there's going to be significant differences in gravitational force between the near and far side of the planet, which could stretch it out enough to cause surface damage. Anything whose shape is determined by hydrostatic equilibrium is probably not going to be particularly resistant to this.

Actually, tides are caused by differences in orbits of the same object as it orbits another object. The close end of the object to its orbital partner wants to move faster and downward, while the far end wants to move slower and upward. That is why high tide comes not once (when the moon or sun is directly overhead of a point on the earth the tide is high at) a day, but twice a day (when the moon or sun is directly beneath the point of high tide on the earth).

If the object, in this case, the earth, were separate objects or a collection of many pieces, each piece would follow its own orbital path relative to the moon and the sun. However, since the earth is a single object, then tidal forces upon it cause movable parts of the earth to do some moving while still under the powerful gravitational field of said collection of objects, ie, the earth. This is why water reflects the tides so well. The atmosphere also reflects tides but its is far less visible to the observer. The lithosphere is also affected by tides, and in fact also affected by centripedal forces as well, so it is harder to measure, but it has been shown that the earth is pear shaped mostly due to concentrations of mass in the northern hemisphere and the southern antarctic continent. This has little if anything to do with tides, but can be affected by them none the less.

Rotation of the earth therefore only causes centripedal effects and is not related to tides.

[Sirius]

Kinda. If the earth were tidally locked, then if it still had tides they wouldn't change.

[Tunnelcat]

Kinda. If the earth were tidally locked, then if it still had tides they wouldn't change.

That would be weird, and have an almost have infinitesimally small chance of happening. All the planetary bodies would have to have just the right orbital and rotation rates to stay fixed in the way they face each other.

I wonder what a slow motion planetary collision would look like, or if it would even be possible? I would think that the gravitational forces would accelerate the closing speed as they got nearer and nearer to each other, so they would always collide at a high speed. OR, would they attract each other and orbit around themselves as a binary system? I guess it would depend on whether their masses were somewhat equal or not. Just musing here.

[sdfgeoff]

I wrote a newtons gravitational law simulator recently, and you'll be pleased to know that planets could collide slowly. Or in my case, a planets moons together.
I have yet to get two moons to orbit each other and the planet, but it should be possible.

[Sirius]

Tidal lock isn't as unlikely as one might think; a lot of moons (including our own) are in that state. Actually, in a system where nothing changes (e.g. sun going boom) it's ultimately inevitable.

You're probably aware that you can extract energy from tidal movement (how much and how efficiently is another question, but still). This energy doesn't come from nowhere though; it will last for a very long time, but ultimately it comes from the rotational kinetic energy of the Earth. Put simply, the action of the tides is (very) gradually slowing down the Earth's rotation through friction. Give it a few billion years and it'd eventually reach the state where the Earth spins at the same rate it orbits, i.e. tidally locked. That's the lowest-energy state it can be in.

There's still the issue of the earth and moon having different orbital cycles, though, so I'm not quite sure how that'd figure into things; the second law of thermodynamics has to apply somehow, but the mathematics are a little complicated It might be why the moon's orbit isn't perfectly stable though.

P.S. Yes, gravitational force is inversely proportional to distance, so the nearer the two bodies get, the faster they'll accelerate into each other. A collapsing orbit would be the most likely scenario I could think of to slow down the impact as well. It would only do so much unless the orbits were so fast they'd rip the planets apart of course!

[Krom]

The moon wasn't always in tidal lock with the earth, it used to rotate so that both sides of it could be seen from the earth. Similarly, days on earth used to be shorter, but have become longer because just as the earth slowed down the spin of the moon, the moon is slowing down the spin of the earth. The gravitational pull of the moon applies an enormous amount of torque on the earth, which is having two primary effects: First it slows down the rotational speed of the earth, making days longer by a tiny amount each day. Second that energy gets put into the moon and actually accelerates it in its orbit, which is causing the moon to drift further away from the earth.

The moon used to be a lot closer and the tides here on earth were much more extreme (more like unbelievably powerful, look up the theories and geological evidence on them), also the days on earth used to be as little as 6 hours long.

Eventually either the earth will tidal lock with the moon, so the moon will only be visible from one side of the earth. Or the earth will literally throw the moon off into outer space...

[Top Gun]

I think from the numbers I've seen on that, the Earth wouldn't become tidally-locked until after the Sun goes red giant anyway, so we'll presumably have other things to worry about.

[Tunnelcat]

I wrote a newtons gravitational law simulator recently, and you'll be pleased to know that planets could collide slowly. Or in my case, a planets moons together.
I have yet to get two moons to orbit each other and the planet, but it should be possible.

CooI! wonder what that would look like? I mean, what would the land masses and any water or atmosphere do as the planets got close? The tidal forces might do some spectacular things to all the stuff that's loosely held to the planet's surface by gravity. The earthquakes and tsunamis that started up might be pretty wild. Not something I would want to be living on when it happened either.

Anybody read the Star Wars Vector Prime series? That very hypothetical scenario, the intentional pulling of a moon into a planet, was part of one of the major battles that ultimately led to the death of Chewbacca.

Sernpidal

[roid]

Tides are caused by a body rotating while it's under gravitational pull; situation here is a little different with the entire body moving at the same rate.

Roughly. I expect at extreme proximity there's going to be significant differences in gravitational force between the near and far side of the planet, which could stretch it out enough to cause surface damage. Anything whose shape is determined by hydrostatic equilibrium is probably not going to be particularly resistant to this.

It's not so much surface DAMAGE i'm talking about, but heating, melting. i'm suggesting that the tidal flexing forces will make both planets into molten balls of magma, from the friction of that tidal flexing. I suppose it doesn't matter much, since well even our planet is technically a molten ball with just a thiiiiin layer of solid on the surface - physically it probably behaves just as a molten planet would. So i guess a molten planet wouldn't really be that weird afterall.
An incredibly hot and active planet might have an unpredictable atmosphere, i guess i'm not sure if lightning would be a thing... and anyway it'd be the least of your worries.

[Tunnelcat]

Yeah, the molten rock and boiling water and steam would get you first. It would still look cool from space.

[flip]

Could a big enough asteroid interact with the moon gravitationally enough to throw the moon's orbit off and cause a dead pool of the oceans?

http://www.nasa.gov/multimedia/videogallery/index.html

I thought this was interesting to see how an earthquake and tsunami actually disturbed the upper atmosphere. It's all connected I guess and rubbing against each other.

[Top Gun]

A garden-variety asteroid wouldn't do anything significant, but something the size of http://en.wikipedia.org/wiki/Ceres_%28dwarf_planet%29Ceres[/url]? Then you might be talking. It'd depend on a lot of parameters, and I'd probably have to do some calculations that I either barely remember or never learned. It'd also require something to get Ceres moving on a collision course with the Moon, which isn't going to happen on its own.

(Incidentally, the generally-accepted theory for the formation of the Moon involves an Earth-sized object slamming into the Earth during the early formation of the Solar System.)

[flip]

Lol, well as long as I'm wrong about like isotopes being attracted to each other over long distances, we probably won't have to worry about Ceres I'd like to see that math genuinely, just to see the process.

EDIT: Just the formula actually, I'd like to figure it out myself if I could.

[Sirius]

Second that energy gets put into the moon and actually accelerates it in its orbit, which is causing the moon to drift further away from the earth.

It took me a very long time to figure out how this would work mechanically, and even afterward, the earth is such a regular shape I can only imagine this would be very, very slight. Though I guess with its sheer size, even a small deformation such as that caused by tidal forces would add up to a lot of displaced mass to throw off the centre of gravity...

[flip]

http://www.orbitsimulator.com/gravity/a ... ceres.html

I'm gonna need some time to figure this out, but I wonder if either Ceres or Pluto were to be displaced, how it would effect the orbit of the inner planets. Would they draw closer to the sun or be pushed away. They definitely seem to counter-balance. I mean, If everything orbited in the same plane, would seem that the rotational forces would eventually sling everything outward. I'm looking at the Solar System as a whole unit here.

http://universesandbox.com/about/

[Krom]

It took me a very long time to figure out how this would work mechanically, and even afterward, the earth is such a regular shape I can only imagine this would be very, very slight. Though I guess with its sheer size, even a small deformation such as that caused by tidal forces would add up to a lot of displaced mass to throw off the centre of gravity...

The tides are only a convenient visible example of it, the process would still happen even if earth had no water or air.

To grasp it mechanically, you need to remember that the earth and the moon aren't single points of gravity, the whole of the earth pulls on the moon and the whole of the moon pulls on earth. So they experience gravity not just from a center point of each body, but from the entire face. And because the earth spins fairly rapidly under the moon, one side of the earth is always advancing towards the moon, while the other is always receding away from the moon. Because gravity works as an acceleration, the pull on the receding side of the earth is stronger than on the advancing side, so it acts like a rope that tugs the moon along just like a yo-yo on a string.

Another thing to help think about it is to just remember that the moon takes about a month to complete an orbit around the earth, or in other words: the earth spins almost 30 times faster beneath the moon than the moon spins around the earth (27 times faster actually...).

The amount of energy dissipated by this process is enormous, but because the earth and moon are so massive it still takes tens of millions of years to make much of a difference.

[Sirius]

I'm thinking it wouldn't happen if the ground itself weren't slightly distorted though? The sum of the gravitational forces of a perfect sphere would only pull one place - toward the center (unless the movement of a particle actually changes the gravitational force it exerts in which case I guess I need to stop thinking Newtonian mechanics!). However, if it's more egg-shaped, then it depends on the location of the observer - in this configuration the receding face is nearer so exerts a little more, and it is moving in the direction of the orbiting body's orbit, so the net effect is to accelerate it... that would be counteracted when the other vertex of the "egg" rotated nearer except that in this case it is being continually deformed by gravity so the shape relative to the orbiter stays roughly the same.

Or, it could just be that movement thing.

(P.S. For those who haven't done it, this is the sort of stuff that makes calculus kind of fun, if difficult.)

[Top Gun]

http://www.orbitsimulator.com/gravity/a ... ceres.html

I'm gonna need some time to figure this out, but I wonder if either Ceres or Pluto were to be displaced, how it would effect the orbit of the inner planets. Would they draw closer to the sun or be pushed away. They definitely seem to counter-balance. I mean, If everything orbited in the same plane, would seem that the rotational forces would eventually sling everything outward. I'm looking at the Solar System as a whole unit here.

Taking Ceres out of the picture wouldn't noticeably affect the orbits of the inner planets. Gravitational force falls off as the square of the distance between two objects, and Ceres has a (comparatively) small mass to begin with, so it's not tugging on anything too strongly. And Pluto is waaaaaaaaay too far away from everything else to have any effect. Now, obviously this all changes if either of these objects magically happens to buzz through the inner Solar System.

As for the earlier topic...well, it'd take a heck of a lot more than a single equation. Right from the get-go, you'd have to deal with orbital mechanics to determine how the gravitational attraction between Ceres and the Moon would affect their trajectories as they closed in on each other; because of the inverse-square property of gravity, that attraction would only increase as the collision became more imminent. But let's say we skip past that part, and move straight to the trajectories and velocities at the moment of impact. The overall momentum of the two objects has to be conserved after the collision (minus the energy losses involved, which when it comes to two planet-sized objects colliding would be pretty huge), so you'd have to determine the nature of the collision...just looking at the relative sizes and masses of the two bodies, I'd estimate that it would be far close to an inelastic collision than an elastic one. Provided we ignore any spin imparted to the Moon, we'd then have its velocity change from the collision; we'd then need to set up a vector situation to determine what component of the imparted velocity was tangential (along the orbital path), and what component was radial (toward/away from Earth). Based on all of that, then you'd have to pick up orbital mechanics again, and see how much the Moon's orbit around the Earth would

Oh, and all of that would give you only the most stripped-down, idealized picture of what would happen, because of all the assumptions involved. To get a true understanding, you'd have to model the system via computer simulation, incorporating all of the relevant physical parameters. And depending on the initial conditions, the result might be pretty extreme...for all I know, if Ceres was moving at some crazy relativistic speed towards the Moon, the impact might just completely shatter it.

[flip]

I appreciate the effort TG, but earlier I realized the same thing. It would take extensive study and time to prove it out myself, so I'll just stick with the conclusions of others based on hard data. WHo knows, maybe I will go back to school because I get a serious hard-on about physics.

I would still have to say that Cere's would affect the orbits of at least Jupiter and Mars, because Cere's was predicted to exist because of the gap between the 2. Now, Take Cere's out of the equation and that gap no longer exists. At that point, Those 2 planets would move, at least slightly, and then cumulatively every other body orbiting the Sun. The 2 common things I see between Cere's and Pluto, besides their unique elipse's around the Sun, is that all the trash gets collected in those spots. So, I imagine if you took one or the other out of the equation it would get pretty nasty quick. Or, If one's orbit changed just enough to move those debris fields into collisions with other planets or asteroids, all bets are off.

EDIT: I was watching H2 earlier and they were talking about the LHC and how the math supports the existence of wormholes, which in turn becomes very paradoxical, so there must be some magic involved

[Top Gun]

Well, I'm not denying you'd see some slight orbital changes, enough to be measurable, but nothing you'd be able to notice without instrumentation. Ceres contains a third of the mass of the entire asteroid belt, but even then, its mass is only 1% that of the Moon's. The asteroid belt as a whole essentially formed because Jupiter's immense mass prevented the formation of a full-fledged planet in that region. Removing Ceres wouldn't automatically cause the rest of the belt to go haywire, though it might affect some asteroids around it.

[Krom]

I'm thinking it wouldn't happen if the ground itself weren't slightly distorted though? The sum of the gravitational forces of a perfect sphere would only pull one place - toward the center (unless the movement of a particle actually changes the gravitational force it exerts in which case I guess I need to stop thinking Newtonian mechanics!). However, if it's more egg-shaped, then it depends on the location of the observer - in this configuration the receding face is nearer so exerts a little more, and it is moving in the direction of the orbiting body's orbit, so the net effect is to accelerate it... that would be counteracted when the other vertex of the "egg" rotated nearer except that in this case it is being continually deformed by gravity so the shape relative to the orbiter stays roughly the same.

Or, it could just be that movement thing.

(P.S. For those who haven't done it, this is the sort of stuff that makes calculus kind of fun, if difficult.)

Think of it this way: if you took yourself to exactly the center of earths gravity, how much would you weigh and why?

[roid]

i think i remember a DBB thread about that a few years back. The general consensus is that a hollow of any size in the center of any spherical mass in space (ie: a planet) - anything anywhere inside that hollow will be completely weightless as if it were floating in free space. (ie: it doesn't even have to be in the center of the hollow, which is weird). The nature of the shape is such that as you move away from the side of the hollow, the nearing mass of the opposite side of the hollow precisely counteracts any change in apparent gravity. Right?
Pretty cool.

So if you made a small hollow in the center of the earth, it would be like a gravity-free bubble, you would float in it as if you were in space. You would neither be drawn to the outside of the hollow, nor the middle of the hollow. If you kept making that hollow bigger and bigger, the effect would remain the same. You could prettymuch hollow out the entire planet, and the space inside would always remain gravity-free.

(disclaimer: you'd still be effected by other bodies other than the planet you are inside though (such as the sun), but then so would the planet you're inside, so you still wouldn't really perceive it.
Also i suppose inconsistencies with the planet's non-quite-perfectly-spherical shape might give slight gravity disturbances to the hollow. And i can't even remember how the planet's spinning effects the whole thought exercise.)

[Top Gun]

Yep, Roid's right. As far as spinning goes, if you were standing or holding onto the "floor" as the spin started, you'd feel the effects of centripetal force and experience weight in a "downward" direction. But if you were already floating when things started spinning, or somehow had someone plop you down in the center afterwards, you'd just keep on floating there.

[roid]

If the hollow is full of atmosphere however, then you may be buffeted by winds upto 1670 kph (as that's how fast the earth is spinning, it's greater than the speed of sound! yikes). Said wind may draw you to the center, or outside, of the sphere (i dunno which. Vortexes... how do they work? ,.-~*´¨¯¨`*·~-.¸ ★■◆●' miracles ,.-~*´¨¯¨`*·~-.¸).

[Top Gun]

Hey, physicists always assume a vacuum.

[Sirius]

The wind is pretty quickly going to start moving at a similar angular velocity to the surface I think.

Still can't quite figure out Krom's thing though.

[Lobber]

This thread really took a left turn after that nifty atmosphere of two planets picture.

Any way to spawn a new thread based upon the current discussion? Or should I start another new thread about the new NEW new PTMC?

[roid]

okie done
here's a back-link to the original thread: "The real PTMC"

[Jeff250]

I'm thinking it wouldn't happen if the ground itself weren't slightly distorted though? The sum of the gravitational forces of a perfect sphere would only pull one place

A point mass is attracted to an object's center of mass, but the earth is a bunch of points. Since the points on the earth closest to the moon experience greater gravitational pull by the moon than the points furthest from the moon, it's harder to rotate the points closest to the moon away from it than it is to move the points furthest from the moon facing it.

[Tunnelcat]

This thread really took a left turn after that nifty atmosphere of two planets picture.

Any way to spawn a new thread based upon the current discussion? Or should I start another new thread about the new NEW new PTMC?

Here's another nifty picture to go along with my earlier 2 planets lightning picture. This is what happens when fine rock particles are moved rapidly and violently against each other within an atmosphere. Viola, lightning!

Now think of 2 planets coming close to each other, gravity ripping loose large pieces of planetary crust during the event and causing volcanism on a massive scale? And imagine both planets contributing their atmospheres and fine pulverized rock between themselves as they get close and you'll probably get something similar to that nice picture I previously posted.

[Top Gun]

I love pictures of volcanic lightning. Some of them seriously look like the very definition of the apocalypse.

[Tunnelcat]

Here's a gallery of really spectacular ones.

http://volcano.oregonstate.edu/lightning_gallery

[Sirius]

A point mass is attracted to an object's center of mass, but the earth is a bunch of points. Since the points on the earth closest to the moon experience greater gravitational pull by the moon than the points furthest from the moon, it's harder to rotate the points closest to the moon away from it than it is to move the points furthest from the moon facing it.

Very true, I'm just not seeing how it affects the moon yet

[Jeff250]

Ah, I see now what your question is: how does all that accelerate the moon's orbit?

I don't recall offhand, but I think it would have to be something like what you suggested, the moon trying to "catch up to" the earth's high tide as it rotates away. It seems like the rise of the high tide would always be slightly ahead, like a dangled carrot in front of a horse.

I think that this is necessary for explaining why the earth's rotation would slow down too, the tide trying to catch up to the moon. I don't think my previous post's explanation is sufficient, since without a tide, the earth would be effectively symmetric.