5 ms·

> Even a fleck of shed paint a tenth of a millimeter across carries as much kinetic energy as a rifle bullet when it's traveling at orbital velocity True, but

by peeters 11y ago

> Even a fleck of shed paint a tenth of a millimeter across carries as much kinetic energy as a rifle bullet when it's traveling at orbital velocity True, but most of the other objects near it are also at or near orbital speed, so what's the kinetic energy when compared to an object it would actually be likely to hit? (The situation in Gravity made no sense to me. The shuttle encounters the debris field every 90 minutes...why was that debris not also orbiting the earth with roughly the same period as the shuttle?) Is the solution a matter of countries agreeing to "lanes" for orbits that differ non-trivially? (e.g. 300-330km is reserved for equatorial orbits, 340-360 km is reserved for polar orbits, 380-400km is reserved for retrograde orbits)

antognini 11y ago
I could imagine that the two orbits were inclined relative to each other. If the shuttle is orbiting around the equator and the debris field is orbiting from pole to pole then you would encounter it on every orbit. (Although then you would encounter it twice on each orbit...)
SideburnsOfDoom 11y ago
> True, but most of the other objects near it are also at or near orbital speed, so what's the kinetic energy when compared to an object it would actually be likely to hit? If they're moving in opposite directions, then you just double it.
- goodcanadian 11y ago
  Actually, I think you quadruple it as kinetic energy goes as v^2.
TorKlingberg 11y ago
If they orbit in opposite directions, the relative speed is twice the orbital speed. Low earth orbits go in all kinds of directions. In geostationary orbit everything is moving in more or less the same direction and speed.
- peeters 11y ago
  I really didn't think there was that much variety in LEO due to the cost of launching in a retrograde orbit. I thought it'd be mainly prograde plus some normal velocity due to inclination.
  - capnrefsmmat 11y ago
    The inclination matters, though. If something heading north at 41 degrees inclination meets something coming south at 41 degrees inclination, it's a serious collision. (And, of course, every satellite has to spend half its orbit going north and half going south)
  - TorKlingberg 11y ago
    Retrograde orbits are uncommon as far as I know, but polar orbits are common for earth observation. A satellite going south could meet one going north. Also unless you launch from the equator inclined orbits are cheaper to reach than equatorial. The ISS for example goes just far enough north to pass the Baikonur Cosmodrome.
- marcosdumay 11y ago
  If they are in opposite directions, they also will fail to cause Kessler Syndrome. If a satellite is hit by a debris with opposite velocity, it'll either fall down, or keep a similar but slightly lower and slightly mode dispersed orbit. It's a 90° angle that is the worst.
dalke 11y ago
The orbital mechanics in Gravity is simply wrong. "340-360 km is reserved for polar orbits" Consider a perfect polar orbit. At time 0, one satellite is above Yamagata heading due north , Japan and another is above Louisville, Kentucky, US also heading due north. These two satellites are on a (possible) collision course directly above the north pole, because all polar orbits go above the poles. "Lanes" wouldn't work because of orbital decay. For example, the HST is ~550 km altitude. That's close enough that natural atmospheric reentry would happen in about 20 years. If that were to occur, it would descend through the lower 'lanes'. Rather, it would only work if every satellite had active station keeping, had an active method to quickly deorbit, never failed, and never blew up (as DMSP-F13 is believed to have exploded due to a battery charging system - http://www.space.com/29996-us-military-satellite-explosion-dmspf13-cause.html http://www.space.com/29996-us-military-satellite-explosion-d... ). But if those assumptions are assumed, then we wouldn't have collisions in the first place.
- peeters 11y ago
  Thanks for the response; great points!

by peeters 11y ago

antognini 11y ago

I could imagine that the two orbits were inclined relative to each other. If the shuttle is orbiting around the equator and the debris field is orbiting from pole to pole then you would encounter it on every orbit. (Although then you would encounter it twice on each orbit...)

SideburnsOfDoom 11y ago

> True, but most of the other objects near it are also at or near orbital speed, so what's the kinetic energy when compared to an object it would actually be likely to hit? If they're moving in opposite directions, then you just double it.

goodcanadian 11y ago
Actually, I think you quadruple it as kinetic energy goes as v^2.

TorKlingberg 11y ago

If they orbit in opposite directions, the relative speed is twice the orbital speed. Low earth orbits go in all kinds of directions. In geostationary orbit everything is moving in more or less the same direction and speed.

peeters 11y ago
I really didn't think there was that much variety in LEO due to the cost of launching in a retrograde orbit. I thought it'd be mainly prograde plus some normal velocity due to inclination.
- capnrefsmmat 11y ago
  The inclination matters, though. If something heading north at 41 degrees inclination meets something coming south at 41 degrees inclination, it's a serious collision. (And, of course, every satellite has to spend half its orbit going north and half going south)
- TorKlingberg 11y ago
  Retrograde orbits are uncommon as far as I know, but polar orbits are common for earth observation. A satellite going south could meet one going north. Also unless you launch from the equator inclined orbits are cheaper to reach than equatorial. The ISS for example goes just far enough north to pass the Baikonur Cosmodrome.
marcosdumay 11y ago
If they are in opposite directions, they also will fail to cause Kessler Syndrome. If a satellite is hit by a debris with opposite velocity, it'll either fall down, or keep a similar but slightly lower and slightly mode dispersed orbit. It's a 90° angle that is the worst.

dalke 11y ago

The orbital mechanics in Gravity is simply wrong. "340-360 km is reserved for polar orbits" Consider a perfect polar orbit. At time 0, one satellite is above Yamagata heading due north , Japan and another is above Louisville, Kentucky, US also heading due north. These two satellites are on a (possible) collision course directly above the north pole, because all polar orbits go above the poles. "Lanes" wouldn't work because of orbital decay. For example, the HST is ~550 km altitude. That's close enough that natural atmospheric reentry would happen in about 20 years. If that were to occur, it would descend through the lower 'lanes'. Rather, it would only work if every satellite had active station keeping, had an active method to quickly deorbit, never failed, and never blew up (as DMSP-F13 is believed to have exploded due to a battery charging system - http://www.space.com/29996-us-military-satellite-explosion-dmspf13-cause.html http://www.space.com/29996-us-military-satellite-explosion-d... ). But if those assumptions are assumed, then we wouldn't have collisions in the first place.

peeters 11y ago
Thanks for the response; great points!