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April 11 2013, 08:00 PM   #15
Crazy Eddie

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Re: How does one "lasso" an asteroid?

gturner wrote:
 Crazy Eddie wrote: Nope. Just figure out the asteroid's center of mass and have the tug attach a cable so that it is providing thrust through that point. It then becomes possible to disconnect the cable, reposition to a different vector and then reattach the cable, again thrusting through the center of mass.
Think through that slowly and you can see that mission planners will have a handful.
I have, in fact I'm in the process of writing a book about that very subject. Of course, I had envisioned this as a manned mission -- which would simplify matters hugely -- but doing it with an automated mission is just a matter of better planning and specialized techniques and equipment; suffice to say, even with the manned mission, just attaching the cable to the asteroid probably takes up an entire chapter at least.

 You can't drill into the asteroid without somehow being attached to it firmly enough to apply significant force to the drill bit, which implies that you've already driven some anchors to hold your satellite against the asteroid, which required drilling...
You need pitons with anchor cables to get the initial bracing force, the placement of which will have to be chosen carefully so the drill can be placed near the center of mass. Once the drill is in, it supplements the anchor cables/pitons by distributing the mass more easily. If you can figure out exactly how much force the anchors can take -- based on how deep they're buried and the structure of the asteroid near the surface -- then you have a figure for how much force you can put on when you decide to move the thing. For a carbonaceous chondrite, for example, it would be anywhere between 500 and 3000 Newtons before the anchors are in danger of breaking free. Adding more cables would allow a little bit more force, but would make it slightly harder to reposition the tug for midcourse maneuvers (although lengthening/shortening some of the cables could also accomplish that).

 So you'll probably have to wrap the cable around the asteroid, but then you'll need more than one cable because you can balance a basketball in a loop of cord; it will always fall out one side or the other.
Well, the industry proposals all involve wrapping the asteroid in a large kevlar or vectran net like a giant ziplock bag, for precisely this reason. That to me seems like a good concept if you know exactly what you're going out to capture and fully understand its shape and structure, but it could be problematic if the asteroid has a funny shape or is larger than you expected. It seems to me that the "bag if and haul it" approach works best if you also bring three tons of TNT with the intention of blasting it to pieces before you start hauling it.

 Using a net or bag instead of a single cable might work, but you'll need something to hold the net open so you can get the asteroid inside
Elastic ring around/within the aperture is inflated with air, expands to a larger ring.

 But then you've got the problem that the asteroid is probably spinning very slowly, which is going to reel in the satellite unless the satellite orbits to match the asteroid's rotation
Which is another reason why I favor the cable/anchor method. If the asteroid is slowly tumbling, you can attach a cable on one side and use some controlled thrusts to stop its rotation relative to its intended orbit.

 And when you periodically tug on a cord you're introducing dynamics where the cord's elasticity pulls the satellite back toward the asteroid after each tug, slackening the cable and tending to crash the satellite into the asteroid while getting the loose cable tangled in the satellite's antennas and solar arrays.
That's a matter of timing and throttling the thrusters in the right way in order to avoid sudden snaps of the cable from slack to taught. Gentle upthrust starts each maneuver, followed by a larger thrust once the cables are fully taught. Strictly speaking, even if we were talking about strapping a Merlin engine and 30 tons of propellant to that tug, a 500 ton asteroid isn't going to pull much faster than about 1/8th of a G and the tension on the cables won't be nearly as much as you'd expect, slackness or not. It's really the interface with the asteroid you have to worry about.

 In contrast, a big bucket with a lid and a thruster in back is easy peasy
And also very heavy and very inefficient, especially if you have to have something solid on the open end of the bucket to keep the asteroid from tumbling out of it when you cut thrust. That, plus the fact that keeping the asteroid from rattling around inside the "bucket" would require a bit of engineering workup almost as difficult as attaching the tethers.

Significantly, it doesn't have to be a solid "bukcet" to encompass the asteroid. A collapsible cage that expands to the size of the asteroid and then contracts again to the smallest size it can would solve that problem and give the tug something nice and solid to attach its cables too. In that case, figuring out how to keep the cage intact becomes a different engineering problem than the otherwise GEOLOGICAL problem of finding a piece of the asteroid solid enough to stick an anchor. The nice thing about the anchor method, however, is that you still have options to redeploy to a different spot if one of the anchors breaks loose.
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