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Sun Jan 24 03:43:22 1982
SPACE Digest V2 #86
>From OTA@S1-A Sun Jan 24 03:35:12 1982
SPACE Digest Volume 2 : Issue 86
Today's Topics:
Harry Stine and Science
Collisions with skyhook
Copyrights & retyping
SPACE Digest V2 #85
Administrivia
Skyhook collisions
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Date: 22 Jan 1982 0805-PST
From: Tom Wadlow <TAW AT S1-A>
To: space at MIT-MC
Date: 21 Jan 1982 1356-PST
From: Alan R. Katz <KATZ USC-ISIF AT>
Subject: Harry Stine and Science
Right on! If I had had a subscription to Analog I would have cancelled
it.
However, you may have missed the excellent refutation of Stine's article
that appeared a few months later.
[Sigh. Am I the only person that sees an irony in the above two sentences?
I fail to see how a person can subscribe to a magazine, enjoy dozens, if
not hundreds, of articles, and then cancel because of one bad piece.
Hasn't anybody ever heard of Letters to the Editor?? --Tom]
------------------------------
From: CARLF@MIT-AI
Date: 01/22/82 15:41:41
Subject: Collisions with skyhook
CARLF@MIT-AI 01/22/82 15:41:41 Re: Collisions with skyhook
To: Space-Enthusiasts at MIT-MC, SPACE at MIT-MC
A.exp at Berkeley asked about the problem of satellites running
into a skyhook. It does indeed seem to be a problem. A five meter cable
would be hit by something big enough to smash it about once a month.
This is based upon data from an article called "Formation of a
spacecraft debris belt" (I think) in a book called "Space systems and
their interaction with the environment" (I think) which is #71 in the
series "Progress in aeronautics and astronautics" (I know).
If two spheres of different radius smash into each other at 10
km/sec there are two possible outcomes. If the small one is
sufficiently tiny, it will only produce a crater on the big one. If the
spheres are closer in size, they will both be totally fragmented. The
ratio of projectile mass to target mass above which both will be
destroyed is called the "catastrophic limit". It depends on the
material of the target. It has values which range
from 2600, for soft aluminium, to 120,000 for glass. Basalt has an
intermediate value of 25,000. I adopted this value for the material of
the skyhook. If we assume a five meter diameter cable to be as hard to
break as a five meter diameter sphere, the cable will be broken by a 17
cm object, making the conservative assumption that the object is moving
at 10 km/sec rather than the 7.7 km/sec of most LEO objects.
The most dangerous zone lies between 700 km and 1200 km
altitude. If we assume that we are putting up the skyhook in 2020, and
that 510 satellites and items of debris are launched each year until
then, then the flux of different-sized objects is as follows:
MASS (kg) FLUX (impacts / m^2 / yr)
1000 5 e-7
10 5 e-6
0.1 1 e-5
0.001 2 e-4
0.00001 1
We see that a 500 km section of 5m cable will be hit by a breaker about
12 times a year.
Clearly we have to sweep out LEO. Fortunately, NORAD is
tracking nearly 80% of the objects in orbit, and the rest can be found
if need be. Useful satellites can be tied onto the skyhook, and useless
ones can be shot down. This is easy to do: just take a big chunk of
someting soft up to the appropriate point on the skyhook, and drop it
into the path of the garbage. The fragments which result will be moving
too slow to stay in orbit, and will fall into the air, to be burned up.
-- Carl
------------------------------
From: TK@MIT-AI
Date: 01/22/82 15:45:16
Subject: Copyrights & retyping
TK@MIT-AI 01/22/82 15:45:16 Re: Copyrights & retyping
To: space at MIT-MC
I hate to be the bearer of bad news, but the copyright issue depends
not in the least upon whether you re-type, or even paraphrase a
copyrighted article. The only possible way in which it is not a
copyright infringement to report news stories in this forum is if this
use falls within the doctrine of "fair use" which, roughly, is use for
individual scholarly purposes. That one is hard to call, indeed, but
the question of whether you retype an article has no relevance to the
discussion. Copyrights are just as infringed (or not, as the case may
be) by hand copying a document as by making a Xerox copy. The
technology and the medium are irrelevant.
------------------------------
Date: 22 Jan 1982 14:16:57-PST
From: Cory.kline at Berkeley
To: E@MIT-MC, SPACE@MIT-MC
Subject: SPACE Digest V2 #85
Regarding the newswire controversy, it seems to me that stories re-
transmitted a day (or several days) late have scant claim to be
catagorized as "news". Feature stories are another matter, of course.
Gary D. Kline
------------------------------
Date: 23 Jan 1982 23:59PST
From: The Moderator <OTA AT S1-A>
To: space at MIT-MC
Subject: Administrivia
I hope the messages in this digest will be the last debating the issue of
redistribution of news service stories. Not, of course, because the issue
is unimportant, but because this is not the proper format for such a
discussion. TK's comment about paraphasing stories is, I believe,
correct. Since, news service stories appear from time to time, in all the
digests and many of the mailing lists I know of, I'm not convinced that
SPACE needs to be the first to renounce their use. As long as people
exercise restraint, I think things will be OK, at least for now. Brief
and Concise are the key concepts that should be used in including news
stories (from whatever source).
I will continue to discuss this issue with anyone who is interested, but
privately, not in the SPACE Digest.
The Moderator,
Ted Anderson
------------------------------
Date: 23 Jan 1982 0233-PST
From: Hans Moravec <HPM AT S1-A>
To: space at MIT-MC
Subject: Skyhook collisions
For an earth skyhook to be practical its material must have a tensile
strength on the order of 500,000 kilograms per square centimeter. Thus
a cross section of one cm**2 at ground height could hoist 500 tonnes, and
10 cm**2 (i.e. a radius of less than 2 cm) could transport hefty
5000 tonnes on each run, which should be enough for a whole lot of
purposes. At the point of maximum thickness the skyhook would have
10 to 100 times the cross sectional area, i.e. a radius of 5 to 20 cm.
Because of the exponential nature of shape of the taper function, most
of the skyhook's length is at the smaller rather than the larger radius.
The average radius is thus less than about 10 cm, about 50 times smaller
than the enormous five meters in CARLF's calculation. This cuts the
hit rate, keeping the other assumptions the same, to about one every
four years.
In any case, sweeping out low earth orbit is a good idea, and
not only for the sake of skyhooks.
------------------------------
End of SPACE Digest
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