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Space debris - a solution?

  1. Sep 3, 2011 #1
    Some of you may have heard of the study just published by the NAS of the prospects of dealing with space junk - follow http://www.scientificamerican.com/article.cfm?id=orbital-debris-space-fence for the story and links.

    Most of the hazardous debris consists of extremely small particles. Further, due to the orbital advantage that arises from launches in an easterly direction, most of the debris is orbiting in this direction, albeit to some extent in inclined orbits.

    The problem with capturing the debris by mechanical means is that because of the high velocity, a collision of the particles with any capturing device is liable to result in disintegration of the particle, thereby increasing the number of particles and making the situation worse.

    I'd like to invite comments on the following idea.

    I propose launching clouds of gas into a retrograde orbit, in the opposite direction to the debris. Debris particles that enter the cloud undergo drag braking. The effect depends on the particle's cross section and its mass, meaning that the braking effect on smaller particles is greater than on larger particles, and the effect on objects as massive as satellites would be negligible. The braking effect causes the debris to descend into a lower orbit and finally into the atmosphere. The effect on satellites could be mitigated further by choosing orbital radius and inclination to avoid satellites.

    The choice of medium to use as a gas would need to be defined. For practical reasons, it could be launched as a vaporizing liquid, or it could be a sublimating solid. It should remain gaseous under orbital conditions, meaning zero pressure and an average 255K temperature. The cloud needs to be tenuous enough that satellites to not undergo excessive heating. The molecular weight should be such that it should not disperse to quickly, but equally it should disperse within a reasonable time scale.
     
  2. jcsd
  3. Sep 3, 2011 #2
    I think you would need a HUGE amount of gas and a massive delivery mecahanism.
     
  4. Sep 3, 2011 #3

    Integral

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    And... Not very selective, suppose there was a satellite in that orbit that you wanted to keep.
     
  5. Sep 4, 2011 #4
    The braking effect works preferentially on smaller and lighter objects. A fleck of paint 2mm across might weigh a few milligrams. An object as large as a satellite weighing maybe 500kg would be unaffected.
    The paint fleck has a mass to area ratio of the order of 1 Kg/m2, whereas the satellite mass to area ratio is of the order of 200 Kg/m2. That means that braking action on the paint fleck is 200 times greater than on the satellite.
    It is estimated that around 95% of space debris consists of objects of 1cm or less. These are objects that cannot be tracked.
    To consider the effectiveness of a gaseous braking system, we need to consider the volume of space swept out by an object with cross-sectional area A moving at velocity V over a period of time. In low earth orbit with a speed of 8km/sec, an object of 1cm2 cross sectional area sweeps a volume of 0.8 m3/sec, or 2.5x10^7 m3 per year. The braking effect then depends on the ratio of the mass of the particle to the mass of the gas encountered in this time. For a particle with a mass of 1 gram, an encounter with 10% of its mass in one year would require a gas cloud density of 0.01 grams per 2.5x10^6 m3, or 4x10^-9 grams/m3. This is evidently an extremely low density. A 1000Kg gas cloud dispersed to this density would occupy 2.5x10^14 m3 or 2.5x10^5 km3.
    Anyone like to check the numbers? This sounds too good to be true.
    At the moment I'm only looking at order of magnitude estimates, ignoring the distinctions between elastic and non-elastic collisions, and the fact that the gas cloud is in retrograde motion.
     
  6. Sep 21, 2011 #5
    The gas would just expand, disperse and be gone in a matter of seconds...

    ( Okay I know wake shields can be used to create an ultra-high vacuum, but we're not talking detectability, but macroscopic drag...)

    IMHO, you'd do better with a coarse net full of aerogel. Hang a solar-powered transponder in it, and let the fly-swatter go on a tether. Incidentally, the recipe for aerogel being vacuum evaporation of a frozen solution to leave the matrix, all conditions convenient in low orbit. Recycle solvent, of course, of course...

    Just call it a Splatnik...
     
    Last edited: Sep 21, 2011
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