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Re^9: "Practices and Principles" to death

by BrowserUk (Pope)
on Mar 04, 2008 at 05:23 UTC ( #671794=note: print w/ replies, xml ) Need Help??


in reply to Re^8: "Practices and Principles" to death
in thread "Practices and Principles" to death

Here's an idea that popped into my head a few years ago whilst having an on-line discussion with someone who worked for an agency involved tracking space debris by radar. And it won't go away. I don't know enough about orbital mechanics, nor have a sufficient appreciation of the forces and velocities involved, to dimiss it. So, if anyone can shoot it down, please do and I can give my brain a rest.

My assumption is that for any given piece of debris currently in a long term orbit, it would only take a small (earthwards) delta change in its instantaneous trajectory to cause it to fall back to earth of its own accord. And, for the vast majority of those small objects, that would be a safe method of displosal. Also, that a near-but-not-quite parallel collision between a piece of debris and a solid flat surface, is likely to redirect the debris, rather than for it to penetrate. Like a bulllet ricocheting off a wall.

In the next couple of years the shuttles are due to be stood down on safety grounds. It long struck me that the biggest risks are associate with launch and return with people on board. And the biggest limitations on individual missions is the support needs for that wet ware. As a final act, one or more of the shuttles could be sent into orbit, perhaps to dock with the space station unmanned, or with a skeleton crew to be returned by other means.

If it took up extra fuel (in the bay) for manovering; the robot arm; and a large solid deflector. It could use that deflector, extended below the shuttles orbit to change the orbits of small pieces of debris (the vast majority of the 9000 or so they track), just enough to cause them to re-enter and burn up.

'scuse the crudity of the ascii art (its obviously not to scale :):

____ >[____]> / / / \ \ \ ____---- \ ____---- ____---- . . . . . . . ____---- . . . . Earth V this way . V . V

So, shoot it down--but no missiles please :)


Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.


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Re^10: "Practices and Principles" to death
by Gavin (Canon) on Mar 04, 2008 at 11:40 UTC
    Do you think the arm or deflector could be manoeuvred with sufficient accuracy by remote from earth to obtain the desired effect?

      Truthfully, I've no idea, but I wasn't thinking about manoeuvering either (except maybe rotating the deflector to be at right angles to the debris path). Just the entire assemble, shuttle'n'all. And I'm not at all sure that the shuttles are capable of remote operations either, though if they're not, they could be made so--for some amount of money.

      They already track the debris pretty accurately and can predict their future course sufficient that they can avoid them when planning shuttle launches and manoeuvers, though they probably use wide margins of safety. And they have to be able to position the shuttle pretty accurately for docking, so I don't think control is a problem.

      If you imagine the deflector being a 3 cm thick titanium plate that is elliptical, and as large as can fit in the shuttle bay--say 15ft on the minor axis and 30ft on the major, for a weight of about 4 tonnes.

      If presented to the debris at a 30deg angle to its path--you have a 15ft diameter target. Many times the tolorance for docking. With military spec GPS guidance that should be possible. It really comes down how accurately the debris trajectories are known.

      I think the really major part of such an operation would be planning minimal maneouvering of the shuttle to allow it to intercept the maximum number of pieces without wasting fuel. The shuttle wouldn't have to chase after the debris. Just be in the right place as it comes by.

      The bit I really have no feel for is whether, say a camera hitting the plate would cause all the bits to predictably directed in the desired direction, or whether you'd end up with a dozen more pieces of debris on new paths?


      Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
      "Science is about questioning the status quo. Questioning authority".
      In the absence of evidence, opinion is indistinguishable from prejudice.
        Perhaps there's an opening for a Tractor Beam
        or some method of magnetising the deflector shield to attract and collect the space debris as it passes by.
Re^10: "Practices and Principles" to death
by grinder (Bishop) on Mar 04, 2008 at 21:48 UTC

    I think you are seriously underestimating the complexity of this endeavour. I suspect it would be easier to clean the North Pacific Gyre with a tea strainer.

    Try catching a leaf falling of a tree in autumn some time. Thousands of leaves falling, everywhere, all the time. And yet, you are never in the right place at the right time.

    One could probably sail to the middle of the Gyre, toss a bucket over the side and haul it back up, and find nothing but... water. You might see lots of trash float by, lots and lots and lots, but never grab a single piece.

    And it is the same with space. Sure, we are led to believe that near space is full of all sorts of shrapnel ready to drill holes through delicate equipment like space shuttles or, heaven forfend, people. Go up there for a week, and you'll wind up shredded like character in a Tex Avery cartoon.

    I think people forget how utterly vast space is, even low earth orbit. You are simply not going to rendez-vous with so much as a 2 inch bolt.

    I also think you underestimate the mass the reflector would need to be of any use. Unless it was solid metal several centimetres thick, anything colliding with it would just drill right through. If you have sufficient mass to make it useful, you won't have the energy available to move it around. Make it light enough to move around, and you won't be able to do anything, it will just be shredded like a hand going through a rice-paper screen.

    God, I sound like a hippy but still, I say we should avail ourselves to cleaning up the oceans. The heavens can wait.

    • another intruder with the mooring in the heart of the Perl

      It would be exceedingly unlikely that any concept thrown together in the 20minutes BrowserUk took to think through his reply (I normally take about 3, at best!) would be workable in its first incarnation. Heck, it's unlikely that any code I write is going to be workable in the first incarnation.

      The key here is that there are solutions and, frankly, it's not that hard to get them going. And, it's not a matter that you're going to rendezvous with a 2" bolt this year. The question as posed 6 replies back was how to effectively make space something that private enterprise can colonize. tilly brought up the tragedy of the commons and cleaning space. I replied with one possible way that private enterprise can solve that specific problem. BrowserUk then replied (several replies later) with one possible way to solve the technical problem of cleaning space. I didn't care what the solution was, only that if there was enough potential revenue, a solution would be found. Just like the problem of clearing minefields post-conflict. Right now, it's expensive. But, imho, that's just because the value of clearing the mines is too low. If several US states were heavily mined, the cost of de-mining them would go down significantly because there would be competition to clear the land because there would be significant value in having that land cleared. Right now, there are mines in places where the land has little value. Increase the value of the land to be cleared and you increase the number of companies willing to invest in mine-clearing technology, thus driving down the cost of removing a single mine.


      My criteria for good software:
      1. Does it work?
      2. Can someone else come in, make a change, and be reasonably certain no bugs were introduced?
      Try catching a leaf falling... toss a bucket over the side and haul it back up...

      Leaves are subject to winds which are chaotic. Sea borne litter is subject to currents and winds and is even more chaotic.

      Space debris moves according to well-known and relatively simple mathematics. They can even account for atmospheric drag. Their paths are very predictible, barring collisions which do occur, but relatively infrequently due as you pointed out, the vastness of the volumes of space involved. And they are predicted, for every space mission. Launches and returns.

      An example of the predictability of space debris trajectories.

      I also think you underestimate the mass the reflector would need to be of any use.... If you have sufficient mass to make it useful, you won't have the energy available to move it around. Make it light enough to move around, and you won't be able to do anything,

      A 1.25" x 15ft x 30ft (3cm x 4.5m x 9m) titanium ellipse: Mass 4 tonnes. You could double the thickness for 8 tonnes and the shuttle could still easily get it up there. And the arm could deploy it. The Columbus science module taken up last month was 15ft x 30ft and 12.75 tonnes. It was manipulated from the cargo bay into position for attachment using a robotic arm. Albeit ISS' arm, not the shuttle's.

      The reason debris is such a risk to ISS modules is because their walls are made of two skins of thin aluminium (3mm or 4mm?). Some of the debris are bits of titanium which is nearly twice as strong and 60% denser. Soft, thin collides with harder denser, and the latter wins.

      If you fire a bullet at brick wall face on, it penetrates. Fire it at an angle and it bounces off. I seem to recall an incident of a gunman firing at the driver of a car through the side window at an angle, when the glass crazed but the bullet bounced off rather than penetrating. But it might have been a movie rather than a real life incident?

      I'm quite expecting for the idea to be shown to be bunkum, but most of your points don't do that. Your point about the vastness of the territory is valid, but remember the idea is send the shuttle up at the end of it's useful life never to return. Remote controlled, with vastly increased manouvering fuel and no rush to get any particular place fast.

      So you aim it at the point you want it to be to intercept one or more pieces of debris in a few days or weeks time and give it a gentle nudge in that direction. In a vacuum, it'll get there eventually. And you concentrate your efforts on clearing specific important orbits, like the frequently used equatorial launch paths used by things like the European ATV and Russian Progress supply vehicles to and from the ISS.


      Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
      "Science is about questioning the status quo. Questioning authority".
      In the absence of evidence, opinion is indistinguishable from prejudice.
Re^10: "Practices and Principles" to death
by tilly (Archbishop) on Mar 05, 2008 at 04:00 UTC
    First of all let's figure out what is required to bring something down to Earth that is orbiting, say, 499 km up.

    The lowest effort way to bring an object in a circular orbit into an orbit where it will intersect the Earth is the rocket firing at apoapsis required in a Hohmann transfer orbit. Thanks to wikipedia I know that the change in velocity required is:

    sqrt(u/r2)(1-sqrt(2r1/(r1+r2)))
    where u is a constant that for the Earth is about 400,000 km**3*s**(-2), r1 is 6470 km (99 km high, just grazing the atmosphere. And r2 is 400 km more than that, or 6870. Plugging those numbers in you need to slow down by about 0.115 km/second, or about 415 km/h.

    So if you line everything up correctly, your "gentle" impact has to change the velocity of the junk by several hundred km/h. Basically you're having the junk hit about as fast as a bullet hits, and are hoping that the junk bounces off in one piece and doesn't knock off any shrapnel. (Pieces of shrapnel would, of course, themselves become space junk that would need removal as well...) Also to have the collision happen at less than tens of thousands of km/hour, you have to set up a fairly precise rendezvous between the spacecraft and the junk, which maneuver will be the vast bulk of the overall cost.

    The difficulty and risk of this maneuver doesn't strike me as remotely worthwhile. I believe it would make more sense to rendezvous fairly precisely with each piece of junk, take it into the spacecraft, and rendezvous with the next piece. The weight you save on the spacecraft (lighter = cheaper maneuvering) and lower risks more than make up for the extra bit of maneuvering you have to do.

    EDIT: Clarified the shrapnel issue, and explained further why this makes no sense.

      On the basis of those numbers it looks pretty damning. The high absolute speeds involved make it sound implausible, but you have to consider the relative speeds. When a shuttle docs with ISS, they are both travelling at over 27,000 kph, but their relative closing speed is sub 1 meter per minute.

      I understand the shrapel issue, but again it sounds dramatic because of the high absolute speeds involved. Once you start considering that for the deflector to be in (roughly) the same orbit as the debris, it has to be travelling at approximately the same speed, then the relative speeds involved become much less. (Or much more if they were orbiting in opposite directions. :) It's this aspect, maneouvering to match the direction and altitude that is the likely downfall of the idea. Unless you can pre-compute an efficient sequence of transfers from one debrise encounter to the other.

      However, the Hohmann Transfer Orbit is a one-shot, circular to circular transfer. I suspect, but cannot prove, that rather than attempting to deaccelerate the debris to initiate a direct transfer from a circular LEO orbit into a atmosphere scrapping lower circular orbit in one hit, chanhging it from circular to elliptical may be enough and require less deacceleration.

      Changing it's orbit from circular to elliptical, with a atmosphere scraping pericentre that occurs 180 degrees away from the collision (ie. on the other side of the earth half an orbit later), would (I think) require less deacceleration. And then you get the atmospheric drag and gravity working for you. Ie. Most orbits aren't actually circular but elliptical. If you can arrange the collision on the 'out-bound leg' of the orbit, when gravity is working with you, I think that the effect of the deacceleration you obtain from the collision is hieghtened?

      There is some tantalising stuff in the section "low -thrust transfer" on the page you linked. And a little more in the next section "Therefore, relatively small amounts of thrust at either end of the trip are all that are needed to arrange the transfer.". Mars orbitors tend to enter mars orbit in highly elliptical orbits initially, because it requires less deacceleration, as the planets gravity tends to aid the manoeuver. They then use apocentre burns, when the vehicle is travelling at it's slowest due to having been fighting the planets gravity for half an orbit, to slowly circularise the orbits. Of course they are also usually transfering into polar orbits at the same time, so the maths gets way too complicated for me to understand.

      I'll say it again. I've not enough knowledge to understand how far off base I really am. I kind of wish Mr. NASA (or Mr. ESA or Mr RKA), would pop by and simply say: It won't work. Then I could stop thinking about it. Of course, I'd still like to hear why it wouldn't, but the chances are I wouldn't understand the math :(


      Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
      "Science is about questioning the status quo. Questioning authority".
      In the absence of evidence, opinion is indistinguishable from prejudice.
        Re-read it. The Hohmann Transfer Orbit is a 2 shot circular to circular transfer. First you turn it elliptical, then you turn the elliptical circular. I discussed only doing the first shot, which would turn the circular orbit into an elliptical one. I was therefore discussing the idea of what it takes to turn a circular orbit into an elliptical one that grazes the atmosphere.

        About matching speeds. With work you can match orbits very precisely. But the point I was making is that if you have, say, a 10 cm piece of metal in orbit next to your spacecraft, it is easier to take that piece of metal and store it in your spacecraft for later disposal than to suddenly knock it out of orbit.

        However doing this means taking a spacecraft and maneuvering it to match the junk. Every piece of junk you try to maneuver to takes a lot of energy, energy means fuel, and fuel means cost. This is no big deal if you're going to the ISS or servicing a satellite because that docking maneuver is the whole point of the trip. However you generally don't have resources to do a whole lot of those maneuvers. And it is truly cost prohibitive to dock individually with every dropped screw that is up there.

        About low thrust. First, that is relatively low thrust. But you still need just as much energy in the end. Unless you use the chaotic nature of orbit to wind up getting repeated net gravity boosts from the Moon and the Sun to move. This would potentially let you do your rendezvous much more cheaper per rendezvous, but at a cost of only picking up one piece of junk every year or two. Amortized over several decades, this might be reasonably cost effective. But to make a dent in the existing problem you would need a flotilla of these garbage satellites, and you'd need a long time. There remains the question of who would design and build these satellites, or what their incentive would be.

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