Aucbvax.5942 fa.space utcsrgv!utzoo!decvax!ucbvax!space 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 ---------------------------------------------------------------------- Date: 22 Jan 1982 0805-PST From: Tom Wadlow To: space at MIT-MC Date: 21 Jan 1982 1356-PST From: Alan R. Katz 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 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 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 ******************* ----------------------------------------------------------------- gopher://quux.org/ conversion by John Goerzen of http://communication.ucsd.edu/A-News/ This Usenet Oldnews Archive article may be copied and distributed freely, provided: 1. There is no money collected for the text(s) of the articles. 2. The following notice remains appended to each copy: The Usenet Oldnews Archive: Compilation Copyright (C) 1981, 1996 Bruce Jones, Henry Spencer, David Wiseman.