Theoretical Question

[SteevyT]

Just a little question that sometimes pops into my head, but I can\'t seem to figure out a definitive answer for. Ignoring how it would ever be created with this rotation speed, would it be possible for a planet to have such a fast rotation to have an apparent gravity of zero (or even negative)? At first, I\'m thinking no, because the mass that is spinning too fast to be held down by gravity would just be flung off into space. However, rock is fairly solid from my experience, so if the planet were created, allowed to harden into a ball of rock, then spun up to a necessary speed to cancel gravity, would it still fly apart?

[Lord Strange]

I believe it would work theoretically. Just don\'t leave anything on the surface.

[Tosh]

Such a planet should be spun for extremely high angular speed after being fully formed. Just imagine the collision that could spin a planet that fast... without breaking it to pieces. I dunno could even rock withstand such an impact.

[semininja]

Perhaps a passing asteroid belt brushed too close and spun it up slowly?

[iamwearingpants]

I\'ve contemplated this. Having an equatorial surface velocity equal to orbital velocity at 0m would NOT destroy the planet. It WOULD cancel gravity also.

I\'ve actually simulated it in orbiter, and it worked.

The one problem is, accretion of such a body is totally absurd, so dream on.

[SteevyT]

The one problem is, accretion of such a body is totally absurd, so dream on.

I know it would never happen, but I was still wondering if aside from the impossibility of the mass to come together with such a ridiculous spin, would it fly apart if you could somehow spin a planet up to those speeds.

[iamwearingpants]

Ahhh.

You would need to exert some insane forces on the planet to do that.

It would have to be done over a loooong period of time because of the masses involved.

And I can\'t think of a method of doing it either.

[UmbralRaptor]

It\'s probably only doable with a relatively small rock, perhaps <600 km in radius. Larger ones would tend to act like liquid drops, strongly deforming, and then flying apart.

[Iskierka]

This is very much possible, in theory, and similar to the Roche limit, which is an alternative that defines how close an orbiting body can get to its parent before its parent\'s gravity is stronger than its own at the surface. At this point, assuming no internal material strength to hold it down, the body will break apart. Same thing for this idea; the body could be spinning this fast, and it would survive up to that speed. Beyond that speed, unless there is material strength to come under tension and hold the surface down, it will start to break apart.

[HarvesteR]

I believe it would work theoretically. Just don\'t leave anything on the surface.

Tested it here. Yep. Don\'t leave things unattached to the ground.

This happened by accident when I was writing the planetary rotation for KSP. Since it\'s a rotating reference frame, all we do is compute and apply the centrifugal and Coriolis accelerations, which means the planet itself isn\'t subject to the same forces.

The effect is exactly what you would imagine. Assuming a theoretical planet that can\'t rip itself apart at those speeds, everything not nailed to it is flung out and away. So in Kerbin\'s case, that meant your ship.

Cheers

[TwoHedWlf]

Looks like, for earth, the planet would have to be spinning about 7905 m/s at the surface giving a roughly 84 minute day. Any faster and you better hold onto something.

[Gojira]

Tested it here. Yep. Don\'t leave things unattached to the ground.

This happened by accident when I was writing the planetary rotation for KSP. Since it\'s a rotating reference frame, all we do is compute and apply the centrifugal and Coriolis accelerations, which means the planet itself isn\'t subject to the same forces.

The effect is exactly what you would imagine. Assuming a theoretical planet that can\'t rip itself apart at those speeds, everything not nailed to it is flung out and away. So in Kerbin\'s case, that meant your ship.

Cheers

Would you make a planet that spins this fast?

[Altair1371]

Would you make a planet that spins this fast?

It would be impossible to land on it, so what\'s the point?

[SteevyT]

It would be impossible to land on it, so what\'s the point?

Not if you thrust toward it. That would actually be a fun problem to try to solve.

[Altair1371]

Even if you went straight forward, without an atmosphere (which I doubt such a planet would have) it would be impossible to match the planet\'s rotation anywhere other than the poles.

[Phoxtane]

I\'m all for it. If it\'s that hard to land on, add an easter egg to make it desirable to land on!

[SteevyT]

I\'m all for it. If it\'s that hard to land on, add an easter egg to make it desirable to land on!

Wouldn\'t being able to say that you landed on it be enough?

[semininja]

Even if you went straight forward, without an atmosphere (which I doubt such a planet would have) it would be impossible to match the planet\'s rotation anywhere other than the poles.

All it means is that the geostationary orbit altitude is zero AGL; there\'s no \'impossible\' there.

[Jesus KerBeard]

An atmosphere could make the fuel requirements pretty intense, but without one mountains are the only thing that might pose an obstacle to lowering a spaceplane\'s orbit to the ground.

[TwoHedWlf]

An atmosphere wouldn\'t be possible, it would all get flung off into space.

What you might need to do is get down into an orbit a few meters up, then fire some harpoons into the planet to effectively hang yourself from.

[SteevyT]

What you might need to do is get down into an orbit a few meters up, then fire some harpoons into the planet to effectively hang yourself from.

And hope the harpoons don\'t weaken the top few meters of stuff enough to be flung off the planet.

[Luckless]

An atmosphere wouldn\'t be possible, it would all get flung off into space.

This depends on if the atmosphere is created before or after the planet is brought up to speed.

If it comes after the planet is spinning, then yes, stuff gets blasted out like a cool space fountain.

If however, the air was there (And suitably dense enough) before acceleration, then you get some really cool weather effects. Namely, insane conditions near the surface. The spinning planet will accelerate the atmosphere/cause drag, creating huge amounts of heat during the energy transfer. Hot, fast moving air is this pushed upward against slower \'cool\' air, where it slows back down again, converts more energy to heat, and then gets pushed back down by atmospheric pressures to be warmed up again.

Your shear forces from up/down drafts is going to be a pure nightmare, and actual temps really depend on the gasses involved, and planet-star relationships. Also, remember your actual zone where the gravity issue comes into play is near the equator, and surface velocities are going to slow as you move north or south toward the pole. Weather patterns would be wicket indeed. (And erosion/sand storms? That would be truly hellish.)

I checked the numbers in a fluid dynamics system that I shoehorned to run a quick test. Looks possible, but my numbers were a bit of a hack job, and the sim was kind of low density, but still.

But as for the planet itself, you are going to have to consider body deformation. Look at the speed of earth\'s rotation, and the amount our planet is deformed from a sphere. Now consider what happens when you speed it up.

I wouldn\'t want to try landing on it.

[khyron42]

yeah. unless it\'s perfectly rigid, you\'ll end up with, um, problems.

from the poles, the full pressure of rock is still pressing down on the core.

from the equator, the entire rock column has greatly reduced pressure. And the more it deforms outwards, the more of it is moving faster than orbital speed and wants to fly outwards, and pulls outwards on the bits it\'s still attached to.

I think what you\'d end up with is cataclysmic plate tectonics, gradual loss of a portion of the planet\'s mass as things loosened and flew off.. a continuing process of that... and eventually a really thick, ovoid disk (like a round, flat pebble from a creek) with the old core sticking up through the middle and the outer edge of the disk just a little beyond orbital speed, being held to the disk by cohesion only.

Is there anyone who can build a deformable physics model of it and record a video of the effects? I think this one would be fun to watch! A demonstration of Roche\'s limit as applied by a body\'s own rotation.

[DonLorenzo]

Approaching it from space, wouldn\'t you still be attracted to the surface as per normal (maybe slightly sideways due to rotational frame dragging) and then just mashed by the insanely fast terrain? Or if you did match speed (don\'t see how you could without an atmosphere, you\'d go in a high orbit again?) it would be like landing on an asteroid maybe (very low/no gravity). Am I missing something here?

[Meatsauce]

Would that matter; I thought gravity is a result of mass, not velocity.