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Mass drivers have been envisioned in numerous Sci Fi tales. These include Earthlight, by Arthur C. Clarke, Robert A. Heinlein's The Moon is a Harsh Mistress, and Heart of the Comet by Benford & Brin, as well as Buc kRogers, Babylon 5 and Halo. Two researchers propose that a space-capable mass driver may be feasible. Startram would act as an electromagnetic catapult,using maglev technology, to accelerate and launch spacecraft into orbit,without using rockets or propellant. Dr. James Powell and Dr. George Maise takea very optimistic view, claiming that a system capable of launching payload into orbit for less than $40/kg could be built using existing technology—if we were to gather international support. The notion of gun-propelled launch goes back to Jules Verne. Sloping a launcher along the western face of Mt.Chimborazo in Ecuador or Mt. Kenya would allow a very profitable/cheap launch system for cargo. But see the concept for a 1200 km long version (to spread out the g-load) for passengers! And yes, we studied stuff like this long ago, at Calspace. See their website at: http://www.startram.com/home
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The answer to Kessler Syndrome seems rather obvious to me - especially since the cost of putting stuff in orbit is rather high right now. All of that orbiting debris should be collected, and stored away until it can be recycled. (I kind of always figured that was one of the side things that went on with the Tank Fam in Tank Farm Dynamo.)Apr 13, 2012
+Tara Li Sure, but collecting all those objects is bound to be a bit of a chore, isn't it? How can it be done inexpensively with current tech?Apr 13, 2012
I guess it can't be done inexpensively, yet - although it may become less expensive than losing satellite function due to collisions.
I wonder whether it is possible to charge smaller fragments with an electron beam or similar and then use electrostatic attraction to alter its course. Basically some sort of floculation for the small particles in orbit, perhaps with some frilly sail with the opposite charge to mop the parts up. Of course, the capturing vessel and the chased fragment ought to be on similar vectors.Apr 13, 2012
+Luke Parrish I wish I had a detailed analysis to give you, but I don't have that kind of time. But then, the proposal is basically a PowerPoint expanded into a wiki. For example, "Active collision warning and avoidance" is a nice bullet point but quite complex computationally and physically; just developing that one technology would be a great leap forward. Satellites avoid collisions now because of ground control, and ground control has the benefit of scanning the entire sky. Moving this to the satellites and making it autonomous without having the satellite spend all its power scanning its neighborhood would be a significant advance!
Unfortunately, Mr. Lofstrom doesn't elaborate on his "aggressive program to remove obsolete thinsats", so it's hard not to assume that it would be an undertaking similar to the effort that put the sats there in the first place, times two: to dispose of a satellite, you have to first get something up there, a given amount of momentum and energy, dock, and then expend almost as much momentum and energy countering that, to make it fall from orbit and burn up. It would be best if the thinsats could mostly deorbit themselves, minimizing how many deorbit missions need to be had.
As for the phased array communications issue... well, that's part of what I do for a living. I work with the guys who implement phased-array beam forming algorithms on server hardware. It's a huge amount of computation and bandwidth; this is the reason I assumed he was not doing this on the satellite side. Let me give you an idea of it: Fundamentally, you need to phase-lock the thinsats to each other. That means GPS-grade or better timing synchronization, and higher frequencies (e.g. GHz) require higher precision. Someone else will have to fill me in on whether GPS timing can be had in the m288 orbit. For now, I'll assume it's doable, and that it's not feasible to include atomic clocks on thinsats (which could be an alternative). Don't forget relativity corrections.
The usual rule of thumb is that you need antenna elements (each a separate radio) spaced half a wavelength apart. That's about 6 inches at 1 GHz. This just gets you the aforementioned 6-degree beam from one thinsat; bear with me. Now, to get other thinsats in on the action, you not only need the timing sync mentioned above, you also need to know exactly, to about 1/30 wavelength, where all the antennas in the array are. This is easy to do on the ground or a single satellite, but harder for an array of satellites. Let's assume you've got that solved, too, even though it is also a significant technology all by itself.
If you don't have antennas every half wavelength, you get a sparse array. This can be quite useful, but it can create "grading lobes", i.e. unintended directions with high gain. So long as these grading lobes stay out of an individual sat's 6 degree beam, you're okay, but I think you have to have the sats no more than a few meters or tens of meters apart to get away with this.
Finally, the data you're actually intending to send the user needs to be shared to all the sats that are forming the beam. This requires a single broadcast channel to all of them, which limits your bandwidth (either only one sat can transmit, or you use something like WCDMA/TDMA to divide the channel). Omnidirectional antennas are also low-gain, cutting bandwidth more. Then the sats steer the beam and transmit. All this is per-user, by the way; multiply by N users.
The real problem, of course, is receiving at the sats, because the sats don't necessarily know where a signal is coming from, and need to compute all possible beams (or a lot of them anyway) to search for handshake signals. Oh, and they need to filter out their own transmissions (sat-to-ground and sat-to-sat).
So please understand that when I see phrases like "no practical limits on download bandwidth to billions of customers on earth" I think "not really".
Anyway, I wish you guys the best of luck. I've spent a lot more time on this than I planned. If you want better critiques than something like this, you should write it up as a set of papers and push it on arXiv or similar.Apr 13, 2012- +Luke Parrish I was thinking, actually, large sections of aerogel, put out, allowed to collect the tiny and small stuff, and then brought in, melted down, and newly generated aerogel put back out. If the ETs had been put into orbit, larger pieces could have been captured and stored in there (though yes, that would be much more energy intensive). I am wondering, though, how much more effective solar sails could be in cis-lunar space with large mirrors to concentrate sunlight on them.
I've wanted to ask Dr. Brin some time about the use of cooling lasers in Sundiver - I've also seen the same trick suggested in Arthur Herzog's eco-crash novel "Heat". Quite a few sites suggest it can't work, but I don't see anything in particular that rules it out.Apr 13, 2012 - Keith Lofstrom has written some specific answers to questions raised here: http://server-sky.com/Brin2012Mar17Apr 15, 2012
