From: johncobb@ut-emx.cc.utexas.edu (John W. Cobb)
Newsgroups: sci.physics
Subject: Re: Space Poop
Date: 9 Sep 92 19:09:36 GMT
Reply-To: johncobb@ut-emx.cc.utexas.edu (John W. Cobb)
Distribution: na
Organization: The University of Texas at Austin

In article <26070@dog.ee.lbl.gov>, sichase@csa3.lbl.gov (SCOTT I CHASE) writes:
|>In article <1865afINN3gt@usenet.INS.CWRU.Edu>,
bj368@cleveland.Freenet.Edu (Mike E. Romano) writes...
|>> 
|>>Interactive collisions in earth orbits will eventually
|>>generate sufficient debris to create a dense Debris Belt
|>>and prevent further launchings.
|>
|>This is hard to believe.  Space is a very big place, even the measly space
|>which defines reasonable satellite orbits.  NASA has studied the
problem, and I 
|>am not under the impression that they are overly concerned.  Do you have any
|>real information about exactly when, extrapolating future expected launch
|>capacity, this will really become a major problem? 

Unsubstantiated Anecdote: I remember hearing some talk a few years back
about a shuttle flight that returned to earth with an impact crater in
its viewing window that had penetrated about 1/2 of the way through. If it
had gone completely through, it would have had catastrophic consequences.
When it was analysed it was found that the pit contained traces of titanium.
So the working hypothesis was that the shuttle had been struct by a hyper-
velocity paint flake.

I think that perhaps some of the ELDEF data might answer the severity of
this problem. So what's the final word there? I seem to remember two things
from the original press when it was brought back 1) It was really bunged up
a surprising amount, even accounting for the fact that it stayed up soo long
and 2) NASA couldn't fond too many takers for the tomato seeds because they
couldn't guarentee they were mutant free, so they recommended schools go
ahead and grow the plants, but not eat the fruit. :>

While I'm engaged in fuzzy memory mode, I'll also give an oblique reference.
I believe <Physics and Society>, the periodic newsletter of the American
Phyiscal Society's forum on Science and Society, ran an article on this
subject 2-5 years ago. The thing I remember most from this analysis was that
the debris rate accelerates rapidly such that even with 0 launches, high
orbits will be trashed in a finite amount of time. It seemed that the exact
value of the finite time had rather large error bars, but that there was
a pronounced threshold effect. The idea was that when there are hypervelocity
collisions between these objects that the result is to create several copies
of themselves. Since collisions are a "N-squared" effect, initially collisions
will be negligible, occuring infrequently, but that they will provide a
steady source of objects. Then as the numbers of objects increase, the
collision rate will increase in a self-feedback fashion and very rapidly
(on the order of a few years) the number of objects blossoms into unacceptably
high levels. The key point being that just as in nuclear chain reactions,
or electron ionization cascades, the orbit debris problem has a pernicious
self-feedback mechanism. So Scott, it might actually be something to worry
about and it may be easy to underestimate the risk until it is too late.

john w. cobb
jwc@fusion.ph.utexas.edu