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I've heard that eventually all the space junk in orbit around Earth will form a disc much like Saturn's rings. If this is true, why? I would expect that that space junk would simply form a spherical shell in orbit over Earth due to the mutual attraction of gravity between the individual pieces of space junk causing spreading it out.

Any help is much appreciated.

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    $\begingroup$ This is essentially a duplicate of Why are our planets in the solar system all on the same disc/plane/layer?. As Warrick's answer to that question says, collisions between orbiting objects result in everything gradually collapsing into a single plane. Collisionless material, e.g. dark matter, doesn't do this so it remains as a diffuse sphere. $\endgroup$ – John Rennie Oct 14 '16 at 15:47
  • $\begingroup$ The earth and other non-giant planets do not have rings, so there must be some reason they do not form or do not last long. $\endgroup$ – Keith McClary Oct 14 '16 at 17:24
  • $\begingroup$ @KeithMcClary: the optimum distance for a disk to form in a rocky planet is probably too low and well inside traces of the atmosphere. So in the few thousands/million years the debris would form a disk, its orbit will decay and fall into the planet. $\endgroup$ – rodrigo Oct 14 '16 at 20:34
  • $\begingroup$ Physics of Planetary Rings, p.95 discusses ring formation and size. $\endgroup$ – Keith McClary Oct 15 '16 at 0:30
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Two things.

Firstly, I believe the space junk has a net angular momentum around the Earth's rotation axis. That is because it is launched with angular momentum from the Earth's surface and usually in the direction of spin.

Second, the individual bits of space junk are capable of inelastic collisions that result in kinetic energy, but not angular momentum, being lost.

If these things are true then eventually the system will settle into some configuration with minimum kinetic energy for a given angular momentum, which will be a disc-like structure. However, most space junk is in low-Earth orbit already. If it loses kinetic energy then it will re-enter the Earth's atmosphere and burn up.

I wouldn't know what timescales this might happen on - collisions between bits of junk are reasonably rare - but one danger is a few big collisions might trigger a collisional cascade; much the same process (among planetesimals) is thought to produce debris disks of dust around some stars. As @rob points out, if the timescales for inelastic collisions are too long, then interaction with Earth's extended atmosphere will drag the space junk down before it can form a disc.

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  • $\begingroup$ When considering timescales, remember that most low-Earth orbits have a non-negligible interaction with the atmosphere over years to decades. Whether there'd be enough inelastic interactions to form a disk before the low-altitude stuff re-enters the atmosphere is a numerical question. $\endgroup$ – rob Oct 14 '16 at 21:45
  • $\begingroup$ @rob Indeed. The statement that "all the space junk... will form a disc" is certainly not true. $\endgroup$ – Rob Jeffries Oct 14 '16 at 21:57
  • $\begingroup$ If applied to a protostar, will the protoplanetary disc form around the equator of the star because it has greatest angular velocity and angular momentum(because of differential rotation) around the equator? $\endgroup$ – K Ferreira Jan 24 '17 at 14:04
  • $\begingroup$ @KFerreira The disc form equatorially because that is the plane perpendicular to the net angular momentum vector. It has nothing to do with differential rotation. $\endgroup$ – Rob Jeffries Jan 24 '17 at 17:57

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