Mr.Tucker wrote:Absalom wrote:Arioch wrote:I think cleanup of space junk is going to be a necessity. I recall someone saying that having services like those in PLANETES where they go out and grab each piece individually would not be feasible, but I don't really see an alternative. Maybe you could have robots that go around and knock things out of orbit, but I can imagine that creating as much new debris as it removes.
1) Lasers have been suggested, which for some stuff (high surface area to mass ratio, in an orbit too high to be grabbed by the atmosphere already) I would assume will happen to some level.
2) A possibility that I once read in some sci-fi book was to have a very thin sheet of gold that you just ran into things. At orbital speed differentials the gold that was physically hit would turn into plasma, thereby exerting a small rocket-effect in opposition to the vector of the impacting object. Fortunately, this should be doable with materials other than gold, as it's basically a variant of the Whipple shield.
3) Then there's "space drones" equipped with "proper" Whipple shields designed to stop the object the first time, rather than just slow it down a little. This is useful for a smaller variety of objects than possibility 2, but also does more of the job at once.
4) Finally, there's the space-tug method. Just match the orbit, grab on, and move it somewhere (presumably one of the disposal orbits, unless you're really close to LEO already).
Of those four, 2 - 4 require propulsion and maneuvering, but hopefully that could be accomplished with a combination of flywheels (for quick rotations) and several magnetic tethers (several so that you can switch them on and off to control and compensate for rotation, I'd say that 3 is a safe "absolute minimum, but really, add a few more" value; magnetic because I'd be dubious about using an electric tether beyond LEO).
Indeed, both "laser brooms" and particle beam projectors have been proposed to clear orbital space of debris. I'm inclined to believe particle beams would be more efficient at this, since they can impart more momentum into a target (particles having mass, unlike photons). They would sweep orbital space and nudge the debris into a decaying orbit, over several passes if need be (interestingly, while doing my little foray into Magbeams, I found that this was another potential application).
I was thinking lasers since you can have just a few installations (one, two, three) with huge mirrors and/or lenses to cover roughly the entire Earth-Moon system. I'm not certain that we could reliably aim a particle beam at those ranges.
Mr.Tucker wrote:However, arranging a rendezvous between a piece of debris and a collecting drone seems both time consuming and inefficient, unless said debris was of considerable size.
Yes, well, I figure that a bolt would probably get hit by a laser, as would e.g. paint chips. I haven't looked up any surveys of the debris, but for the physical methods I was thinking "high-mass high-vaporization-temperature bolt in high orbit" to "large satellite".
Mr.Tucker wrote:Sending a drone to crash into orbiting particles may work, but would also take considerable time (new debris is produced each day), and it's hard to imagine sending a 50kg satellite simply to collect a debris the size of a bolt.
Bear in mind that I have NEVER looked for the generation rates of debris, I was just figuring that anything you would deal with via physical technique would be appropriately high-mass, or otherwise difficult to deal with. I was thinking that as you move into the higher numbered options, you would also be moving to ever larger objects. 4 would presumably be the choice used for full-size satellites. 2 would presumably always be used for something that was small enough to be vaporized after one or two hits but too large to get in a reasonable time with lasers (extra points if the sheet is moving against the rotation of the Earth). Choosing between 2 & 3 would be a bit of an issue, but I assume that there might be something that 3 would be the best choice for.
Mr.Tucker wrote:Magnetic tethers require good timing (and trying not to get hit by other things while maneuvering the large structure to collect the primary target), and debris generally have erratic orbits, so it would be, again, difficult and time consuming (they are still marvelous for orbital maneuvering of ships

).
I suggested magnetic tethers so that the most likely sources of future small debris could be dealt with over time. Ion engines would also be an option, but I think that for this use-case a powered tether system is best, since you never have to refuel

. A fairly large MHD or "Lifter" drive might be of interest for LEO maneuvering, too, as would an electric tether system (which, of course, could be integrated into the magnetic tethers). Lots of interesting ideas with this subject

.