The CNN article didn't mention the prospect of creating space debris. The experiment did in fact occur are relatively low altitude (I didn't know that at the time) so it would fall to earth in a few months. However, the issue is still very important and should have been discussed, because objects that are in more long-term trajectories sometimes come quite close to earth for a portion of each revolution about the earth and thus, could collide with the tether.
I emailed "CNN" with the following comment (it has been edited slightly for clarity):
Your article about how NASA has tried to guarantee the safety of the crew and the space shuttle should anything go wrong with the tethered satellite-- (see URL: http://www.cnn.com/wires/TECH/02-19/space_tether/index.html) --completely ignored the most important problem. The creation of a 12+ mile long string of space debris is the most important problem!
Is this string in a low enough orbit that it will fall to earth? (That's good if it is.) If not, what are the plans to recover it? HOW CAN YOU LET NASA, THE ITALIANS, AND THE SWISS IGNORE THIS ISSUE??? DO THE MATH, FOR GOSH SAKES!!! Find some computer grad students with nothing better to do with their super-computer skills (which should be easy, this is actually pretty important stuff.) Compare the statistics on strings flinging through space with the statistics on "points" (small objects) flinging through space.
This thing is disgusting. Not that it would or wouldn't work, but that it hasn't been thought out from this important angle, and you guys didn't report on it AT ALL.
For more information, please visit my web page article I wrote last year at: http://www.animatedsoftware.com/spacedeb/spacedeb.htm
You may publish my comments and my web/email addresses if you like. Thank
you for your attention.
Sincerely,
Russell D. Hoffman
The Animated Software Company
However a few days later the experiment was begun, and with 12 miles reeled out--sure enough--the tether broke near the ship. The satellite, tether in tow, reeled away from the space shuttle at 100 miles an hour.
This caused me to realize a second problem with tethered things in space. They will be extremely difficult to capture! You have to avoid, in this example, a 12 mile long flinging, extremely strong thin rope. (Which is probably tangled...)
I'll bet that the severed rope will break at some point (pun intended), so then you have two separate problems. One: A rope attached to a bowling ball, and the other: A rope flinging free. Neither would be easy to solve.
In the first case the free end might be flinging around at a hundred miles an hour sometimes. Indeed, the end of the rope might fling back and bang into the satellite (bowling ball) and smash it into more space junk.
The ends of the other piece/pieces after it breaks the first time won't have the ball attached, but they might well be circling each other at hundreds of miles an hour. How do resonance patterns in 6 mile (average) cables behave in zero-G situations? (Frankly, this might be the most interesting question that might come of this experiment! Maybe we should go back up to look at it just because it might be interesting...)
They are their own libraries. After all, scientific journals normally today are stored at libraries for years and years so you can go back. If electronic documents are to replace these things, then surely the links must stay up! They should NOT be moved to a deeper vault, or (yikes!) taken offline! Organizational methods should be such that the link addresses can stay the same. And, people should not have to link to a home page but should be expected to provide deeper, permanently linked references. That's publishing on the Web! Shame on CNN for ever taking articles offline or moving them!
I wonder why NASA didn't test the tether with a full-weight (err, I mean full-mass) mockup on an earlier flight--or are .5 billion dollar satellites cheap these days?? I know they tested a shorter tether--but didn't that fail too? (I don't remember. I'll try to update this when I find out the History of Tethered Flight in Outer Space.)
Also, the tether apparently strung out in a straight line from the satellite. That's fascinating! I'm not sure which way it pointed so I'm not sure what the significance of that is, but I suspect that the tether itself tended towards that configuration, however slightly, and in outer space that was enough to straighten it out over some time. For example if it was extruded in a straight line and cooled in a straight line it probably would have a tendancy (in outer space) to straighten out if left alone. Which means, of course, that every piece of string left in space that has a tendancy, due to manufacturing considerations, towards straightening out, will indeed do so if left in space as a piece of space junk. Twelve mile razor blades in space ready to slice through anything in their path. How exciting!
Lastly, I have considered the problem of tethers in space and suspect that it was low frequency standing wave patterns that caused the tether to break. These would, on earth, dissipate due to friction. In space there is practically no friction. One method to fix the tether problem might be to create areas along its length that bend the cable with a spring. This would cause the standing waves to break up at the springs, especially if each spring were of a different strength. The springs could be V-shaped or curly-Q's around the tether, just so they bend it out and back a bit. You would need these spread out along the entire tether, perhaps every 1000 feet or so, maybe every 500. Maybe every mile. Anyway, the bend would take up the force of being pulled apart by two crossing waves, and the springs being all different would prevent spring-enhanced waves. The bend in the tether would be very slight, but whatever amount it is, you need a bigger bend or a longer moment on the linkages the fewer tether-benders you have. A straight line is the shortest distance between two points, and twelve miles of absolutely straight cable has absolutely no play in it, and therein lies the problem (to go straight to the root of it.)
Maybe alternating--in a pseudo-random pattern, of course--thicknesses and resiliences of wire along the tether would be a better solution. No bends needed...
NASA should feel free use, with acknowledgement, any of these ideas if they feel it will help them. Don't know why I'm trying to suggest a way to accomplish something that shouldn't be done in the first place, but that's life. Some day they can try this experiment on another planet, just not EARTH. We need our near-earth-orbit area to be clean of space junk! Don't trash the launch pad!
From EXAMPLES IN THE ART OF COMPLAINING by Russell D. Hoffman
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The Animated Software Company
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Last modified March 27th, 1997.
Webwiz: Russell D. Hoffman
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