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It is touted by the media that space is becoming more dangerous and risky due to an increase in space debris. Why haven't more craft been hit by the debris? I presume that it is not at a critical level yet. Do the space agencies have avoidance techniques or are the strikes not reported/not critical?

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@gerrit Does that mean that we are just fortunate that a catastrophic failure hasn't occurred? I presume that there is a critical amount of debris after which it becomes too risky to fly. –  Marmstrong Jun 25 at 14:19
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I think so, but I'll leave the answer to someone who actually knows. –  gerrit Jun 25 at 14:22
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Suppose I told you that "Diet Dr Pepper tastes better". You would be quite right to ask me "better than what?" You say that you want to know why more events have not occurred -- more than what? The question would be more clear if you said how many events you expected, what logic you based that expectation on, and then compared that to the number of events observed. With that information we could tell you why your expectations are out of line with reality. –  Eric Lippert Jun 25 at 20:26
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Space debris creates a protective shield against invading aliens! I don't remember who said it, but that's positive thinking. –  LocalFluff Jun 26 at 13:58

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up vote 35 down vote accepted

They are hit all the time. It just hasn't been critical. So far. Looking at the outside portion of the Hubble WFPC2 camera after it was returned, I saw a huge number of large and small pits in the paint and underlying aluminum from debris hits. Below is an image of that surface after all of those sites were cored out for analysis. Note that the cores are much larger than the size of the damaged areas, but it at least gives you an idea of the number of hits. It was on orbit for 15 years.

WFPC2 after coring

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Would they have any idea how many of those were made by man made debris? –  Marmstrong Jun 25 at 22:46
    
I have no idea, but I'm sure that's one of the things they're trying to figure out with those cores. My guess would be most or nearly all of them. –  Mark Adler Jun 25 at 23:33

The size of the various things that can collide are 'small' relative to the amount of 'space' available.

Increase the surface area, increase the risk of collisions. A tether experiment on the shuttle, that was 12 miles long, tore, possibly due to a collision back in 1996.

The solar wings of the space station have recorded minor debris hits I think.

The shuttle windows have shown debris hits over the year, but very small ones that did not break through.

As gerrit comments, space is really big. Satellites, in general are very small in comparison. The odds of two tiny things hitting each other in huge spaces is fairly low but not impossible. Thus the low number of hits.

Alas, increase the number of objects, size of objects, and the probability goes up.

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Space agencies can track the large satellites but how small of objects can they track for their collision avoidance? I here occasionally of the astronauts on the ISS retreating into a safe zone and the ISS's orbit being raised to avoid debris. –  Marmstrong Jun 25 at 14:27
    
@Marmstrong NORAD/US Space command track down to 10cm size or so, last I heard. Below that, too small, and too many to reliably track. Alas, that is big enough to hurt quite a bit. –  geoffc Jun 25 at 14:28
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@geoffc About how many 10+cm-size objects are there in space now? Or do they just keep an eye out for stuff in the vicinity of active satellites? –  JAB Jun 25 at 19:14
    
@JAB More than 21,000 orbital debris larger than 10 cm are known to exist. The estimated population of particles between 1 and 10 cm in diameter is approximately 500,000. The number of particles smaller than 1 cm exceeds 100 million. orbitaldebris.jsc.nasa.gov –  Dieter Van de Walle Jun 25 at 21:49
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Actually that tether cable was melted by the current in the cable. www-istp.gsfc.nasa.gov/Education/wtether.html –  Mark0978 Jun 26 at 1:31

According to this article -- http://www.space.com/5542-satellite-destroyed-space-collision.html -- there have been only 4 collisions with space debris as of 2009, only one resulting in a satellite being destroyed. I suspect there's a question of definitions there: collisions with tiny enough objects travelling in similar orbits (and thus at similar speeds, resulting in very low delta-v) could be barely noticeable.

Which, I guess, leaves the question of whether the low "actual collision rate" is because the danger isn't really all that big, or because people are actively working to avoid collisions.

Reports that the U.S. government is monitoring 18,000 pieces of space debris (a number I just got from that same article) sound at first hearing like there is a huge amount of debris out there. But space is a big place. There are a lot more than 18,000 things you could run into on US highways, between other cars, pedestrians, animals, objects that fall off the back of trucks, etc. These have certainly caused damage and fatalities, but they have not made U.S. highways so dangerous as to be unusable. (And yes, I'm well aware that there are many ways in which "dead skunk in the middle of the road" and "10 cm object flying about in space" are not the same, the analogy is far from perfect, but I think it's valid enough to make my point.)

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There's a lot of empty space on a shooting range too, but the high speed of the projectiles makes that static observation pretty worthless. –  LocalFluff Jun 26 at 13:56
    
We actually don't know how many satellites were destroyed or severely damaged due to collisions with orbital debris. And two out of those article mentions are collisions between non-debris objects, and created large debris fields with different decay rate themselves. Those tracked debris are all > 10 cm² and can be detected by ground radars. Estimated number of all potentially damaging debris is much larger (millions). See e.g. Richard Crowther's talk and documentation here (panel 3): unidir.org/programmes/emerging-security-threats/… –  TildalWave Jun 26 at 14:45

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