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Old April 11 2013, 06:55 PM   #14
Location: Kentucky
Re: How does one "lasso" an asteroid?

Crazy Eddie wrote: View Post
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. How do you attach the cable to the asteroid? If you are going to try to anchor it, what kind of anchor do you use?

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 probably can't glue something to the asteroid because the surface is likely covered with a fine powder, and in fact much of the outer surface might be pretty loose grains, which could throw any drilling to set an anchor out the window, too.

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. If you try to loop the cord around the asteroid, there's the deployment problem of trying to get the free end of the cord to stay where you left it as the satellite makes a circle. This would be simple if you attached the free end to the asteroid, but that brings up the problem of how to attach the end of a string to an asteroid.

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, and somehow you have to keep the cords that run to the net from snagging the asteroid as you try to complete the operation, and that implies arms to hold the strings to the side or some other pretty sophisticated technique.

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. The only way to de-spin the asteroid is to apply a torque, but cords only work in tension, so you'll have to apply little tugs with the cord, ideally while the thrust is aimed along the desired flight vector to avoid wasting fuel. But you also need the cord to pass to the side of the asteroid's center of mass so that each pull will reduce the rotation rate, and given the relative mass of the asteroid and satellite, and that the asteroid's rotation will probably include tumbling, controlling all that will be complicated.

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. Untangling after such an event would probably take days of mission planning to figure out how to roll, pitch, and yaw the satellite so the cable doesn't do any damage, and that would have to be done with the cable still flopping around in real time. They'd want to avoid that situation at all costs.

So cutting thrust will have to be a carefully controlled operation, more of a return to station-keeping based on video of the cable position and range to the asteroid.

In contrast, a big bucket with a lid and a thruster in back is easy peasy, and you won't lose a single grain of material, as long as the asteroid is small enough to fit in the bucket. If after capture you sprayed in some aerogel or other foam, then the asteroid/satellite assembly becomes a single solid body and dynamic control is simple.
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