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The scary bit is that each collision generates a new debris field, which can in turn generate more new collisions, so the safety of orbit starts decreasing geom

The scary bit is that each collision generates a new debris field, which can in turn generate more new collisions, so the safety of orbit starts decreasing geometrically.

dasil003 14y ago
Yes, but what are the orbits of the debris and how quickly do they fall into the atmosphere or deep space?
- fhars 14y ago
  It depends on the height. At the height of the ISS, is it about a year or less, at the height of the Chinese test is seems to be a few decades or maybe one or two centuries and at 1000km the lifetime of space debris starts to be measured in millennia. (Combining some numbers from http://www.dlr.de/en/desktopdefault.aspx/tabid-5170/8702_read-18916/ http://www.dlr.de/en/desktopdefault.aspx/tabid-5170/8702_rea... , http://www.spaceacademy.net.au/watch/debris/sdfacts.htm http://www.spaceacademy.net.au/watch/debris/sdfacts.htm and http://en.wikipedia.org/wiki/Space_debris http://en.wikipedia.org/wiki/Space_debris . http://www.orbitaldebris.jsc.nasa.gov/faqs.html#12 http://www.orbitaldebris.jsc.nasa.gov/faqs.html#12 gives numbers that are about a factor of ten smaller, and it also depends on the type of debris, a sheet of mylar experiences a higher drag than a steel nut of the same weight.)
  - alberich 14y ago
    > a sheet of mylar experiences a higher drag than a steel nut of the same weight. Drag in space? At what altitude, more precisely, does the objects start to experiment drag or the lack of it therefore?
    - fhars 14y ago
      Yes, of course. That is the only mechanism that can remove debris from orbit (apart from flying up there and shooting lasers at it and other untested fantastic ideas). And debris in higher orbits survives longer exactly because the earth's atmosphere gets thinner and thinner the further you are from the surface. The upper boundary of the atmosphere is somewhere above 100000km (62000 miles) http://en.wikipedia.org/wiki/Exosphere#Upper_boundary http://en.wikipedia.org/wiki/Exosphere#Upper_boundary
      - alberich 14y ago
        Oh, I thought it was gravity alone that did the work :) Thanks for explaining it.
        fhars 14y ago
        Gravity doesn't do anything on its own, in the absence of other forces, the orbit of a steel nut around the earth would be stable (that's also why we haven't yet crashed into the sun even after being in its gravitational field for billions of years). (Well, you would get some instability because neither the earth nor the nut are perfect spheres or perfectly rigid, but those effects would take billions of years to become noticeable.)
        gngeal 14y ago
        "Gravity doesn't do anything on its own, in the absence of other forces, the orbit of a steel nut around the earth would be stable" Actually, it wouldn't, because orbiting a celestial body radiates energy in form of gravitational waves, but in case of Earth's gravitational field and at the speed of orbiting satellites, this mode of losing energy is negligible.
        fhars 14y ago
        Yes, but I still think that the tidal deformation of the earth under the gravitational influence of the steel nut will vastly dominate general relativistic effects like gravitational waves. You usually use neutron stars orbiting black holes to produce the latter.
    - ars 14y ago
      > At what altitude, more precisely, does the objects start to experiment drag or the lack of it therefore? There is no cutoff. It just gets less and less the higher you go, but there is no point where you can say "zero".
  - samstave 14y ago
    This is something I have been wondering for a while; assuming we could detect it, space debris around a body could be a serious indication of life on a planet, could it not? Especially if it could stay around for so long. Though this assumes that the population of that planet is not nearly as trashy as our own. I wish there were a requirement for all space-faring nations to clean up space debris.

The scary bit is that each collision generates a new debris field, which can in turn generate more new collisions, so the safety of orbit starts decreasing geometrically.

dasil003 14y ago

Yes, but what are the orbits of the debris and how quickly do they fall into the atmosphere or deep space?

fhars 14y ago
It depends on the height. At the height of the ISS, is it about a year or less, at the height of the Chinese test is seems to be a few decades or maybe one or two centuries and at 1000km the lifetime of space debris starts to be measured in millennia. (Combining some numbers from http://www.dlr.de/en/desktopdefault.aspx/tabid-5170/8702_read-18916/ http://www.dlr.de/en/desktopdefault.aspx/tabid-5170/8702_rea... , http://www.spaceacademy.net.au/watch/debris/sdfacts.htm http://www.spaceacademy.net.au/watch/debris/sdfacts.htm and http://en.wikipedia.org/wiki/Space_debris http://en.wikipedia.org/wiki/Space_debris . http://www.orbitaldebris.jsc.nasa.gov/faqs.html#12 http://www.orbitaldebris.jsc.nasa.gov/faqs.html#12 gives numbers that are about a factor of ten smaller, and it also depends on the type of debris, a sheet of mylar experiences a higher drag than a steel nut of the same weight.)
- alberich 14y ago
  > a sheet of mylar experiences a higher drag than a steel nut of the same weight. Drag in space? At what altitude, more precisely, does the objects start to experiment drag or the lack of it therefore?
  - fhars 14y ago
    Yes, of course. That is the only mechanism that can remove debris from orbit (apart from flying up there and shooting lasers at it and other untested fantastic ideas). And debris in higher orbits survives longer exactly because the earth's atmosphere gets thinner and thinner the further you are from the surface. The upper boundary of the atmosphere is somewhere above 100000km (62000 miles) http://en.wikipedia.org/wiki/Exosphere#Upper_boundary http://en.wikipedia.org/wiki/Exosphere#Upper_boundary
    - alberich 14y ago
      Oh, I thought it was gravity alone that did the work :) Thanks for explaining it.
      - fhars 14y ago
        Gravity doesn't do anything on its own, in the absence of other forces, the orbit of a steel nut around the earth would be stable (that's also why we haven't yet crashed into the sun even after being in its gravitational field for billions of years). (Well, you would get some instability because neither the earth nor the nut are perfect spheres or perfectly rigid, but those effects would take billions of years to become noticeable.)
        gngeal 14y ago
        "Gravity doesn't do anything on its own, in the absence of other forces, the orbit of a steel nut around the earth would be stable" Actually, it wouldn't, because orbiting a celestial body radiates energy in form of gravitational waves, but in case of Earth's gravitational field and at the speed of orbiting satellites, this mode of losing energy is negligible.
        fhars 14y ago
        Yes, but I still think that the tidal deformation of the earth under the gravitational influence of the steel nut will vastly dominate general relativistic effects like gravitational waves. You usually use neutron stars orbiting black holes to produce the latter.
  - ars 14y ago
    > At what altitude, more precisely, does the objects start to experiment drag or the lack of it therefore? There is no cutoff. It just gets less and less the higher you go, but there is no point where you can say "zero".
- samstave 14y ago
  This is something I have been wondering for a while; assuming we could detect it, space debris around a body could be a serious indication of life on a planet, could it not? Especially if it could stay around for so long. Though this assumes that the population of that planet is not nearly as trashy as our own. I wish there were a requirement for all space-faring nations to clean up space debris.