Mini-Throughput: Space Junk Edition

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Michael Siegel

Michael Siegel is an astronomer living in Pennsylvania. He is on Twitter, blogs at his own site, and has written a novel.

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  1. Avatar gabriel conroy
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    says:

    Imagine throwing a baseball harder and harder, seeing it land further and further away until, finally, you threw it so hard that by the time it came back to Earth, it had gone over the horizon and the Earth had curved away.

    Maybe that’s why I wasn’t any good at little league.Report

  2. Avatar Jaybird
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    says:

    Dumb question time: How difficult would it be to de-orbit a satellite that is officially defunct?Report

    • Avatar Michael Siegel
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      says:

      Not a dumb question. It’s hard. The eliptical orbit is the simplest because it puts a much shorter lifespan on the satellite. Every time it dips into Earth’s atmosphere, it loses momentum. You can also attach small thrusters to push the orbit down at the end its useful life. NASA has even discussed min-rockets that could rendezvous and attach to larger satellite to push them down.Report

      • Avatar Jaybird
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        says:

        Hrm. So the mini-rockets would have to rendezvous with the satellite which means getting the EXACT speed within a few feet per second, latch on, then push down for a meaningful duration (turn 150 years into a month, say).

        And, I assume, we’re confident that the satellite will break into harmless pieces in atmosphere instead of turning into an unaimed Rod From God?Report

        • Avatar Oscar Gordon
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          says:

          Yes, mini rockets would burn up. Rods from God work because they are large, solid, rather aerodynamic hunks of dense metal with an ablative heat shield.

          Mini rockets would be lightweight, mostly empty, and not have a heat shield.Report

          • Avatar Jaybird
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            says:

            I’m less worried about the mini-rockets (for some reason) than the Iridium stuff.Report

            • Avatar Oscar Gordon
              Ignored
              says:

              Starlink satellites are ‘small’, and not aerodynamic. De-orbit one and it will burn brightly for a few seconds then eventually fall to Earth as ash.Report

              • Avatar Jaybird
                Ignored
                says:

                My brain is telling me that that doesn’t mean anything for radioactive stuff, though.

                Does it disperse it to the point where it’s more or less meaningless (the equivalent of driving to Denver from Colorado Springs)?Report

              • Avatar Oscar Gordon
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                says:

                Short answer, yes.

                Remember oh so many years ago when NASA wanted to send up long distance probes with radioactive power cells, and Greenpeace, etc. went on the war path because if the rockets exploded on take off, or the probe failed to leave orbit and burned up, all that radioactive material would poison the world!?

                Same thing. We are talking about very small amounts of radioactive material, that would be widely dispersed across the globe. Our bodies are not delicate little flowers that wilt at the first sign of an alpha particle, we can handle quite a bit of radiation, especially if you have a healthy thyroid, and we do, on a daily basis, because radioactive materials exist everywhere (see: Bananas).Report

              • Avatar Jaybird
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                says:

                Cool. I’ll try to freak out less.Report

              • Avatar Oscar Gordon
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                says:

                When to panic? If a chunk of satellite somehow manages to make it through the upper atmosphere (it happens) and it lands in your front yard…

                Don’t panic. Don’t touch it. Call the fire department (not the police!) and let them know you have a potential haz-mat situation. They will send the haz-mat team to asses and deal with it. After that, just keep the neighborhood kids and dogs from trying to lick it until the FD shows up.Report

  3. Avatar Oscar Gordon
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    says:

    I’ve talked a lot about de-orbit methods in previous posts. I personally like the idea of nets, or attaching drogues to LEO objects, rather than trying to push with a rocket (just that much extra reaction mass one needs to bring with).

    De-orbit rockets are better for higher orbits.Report

  4. Avatar North
    Ignored
    says:

    This really reads like an ounce of prevention is worth a pound of cure problem. If everything that goes up from now on has a plan to de-orbit it at the end of its lifecycle is there still enough crap up there to potentially lock us in?Report

    • Avatar Oscar Gordon
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      says:

      It’s more than an ounce, usually a couple of kilograms except for the smallest of satellites.

      We do need more of a plan for all the crap currently up there besides, “Wait for the orbit to decay”. As the junk is, it’s avoidable, but every time we have a collision like we might tonight, that whole dynamic changes.

      We also need an enforcement mechanism. Cooperative agreements are nice, until North Korea or Iran decides to launch stuff and ignores the good sense agreements. Do we let them get away with it, or do we summarily de-orbit their satellites for them until they get the hint?Report

  5. Avatar Fish
    Ignored
    says:

    My wife has been dealing with this very subject off and on throughout her career. Getting international agreements is really, really difficult. Getting individual space agencies within the United States to agree on policy is also really, really difficult. Getting other countries to accept responsibility when they accidentally drop something on a neighbor is nigh on impossible, even when the historical data exists to tie a piece of space debris to a particular body launched from a particular site on a particular day at a particular time.Report

  6. Avatar George Turner
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    says:

    Years ago I did some math on a re-usable sub-orbital de-orbit system. Suborbital payloads, especially straight-up, straight-down, are vastly easier than orbital, and a purely vertical leap means the vehicle can return to the launch site.

    A single-stage re-usable rocket like new Shephard or New Glenn, or similar with a small second stage, simply lobs a series of payloads up to intercept, making an apex almost right in the orbital path of the debris, right before the debris arrives. The interceptors then fire large, horizontally-opposed, cold-gas thrusters along the flight path of the debris.

    These apex burns last only a few seconds, during which the interceptors can hover at a nearly fixed position (it barely move at the brief apex burn, anyway), and then make a collision-avoidance burn. They are unmanned, so G’s during the avoidance-burn aren’t a problem. What these do is spray out a stream of relatively slow-moving exhaust along the flight path of the satellite, and the satellite has to fly through that exhaust gas, which creates drag.

    As an example calculation, you’ve got a dead satellite in a circular orbit 300 km up. It weighs 100 kg, has an area of 4 square meters, and a drag coefficient of 2 (it’s not the least bit aerodynamic).

    The suborbital launcher delivers ten small interceptors along the orbital path of the satellite. They separate a few kilometers apart and each fires opposed CO2 cold gas thrusters, which have an ISP (specific impulse) of 71 seconds, a thrust (each direction) of 13,000 lbs (they’re large), and a propellant flow rate of 83 kg/second. The gas exists the large nozzles with a divergence angle of 8 degrees (4 degrees on each side), which creates an expanding, outward traveling gas cloud. The burn time is 2.5 seconds, so each thruster consumes 550 lbs of CO2. The ten interceptors combined expel 5,500 lbs of CO2. During the last half second the burn stops and the interceptors dodge the target satellite. The exhaust velocity of the cold-gas thrusters is 700 m/sec, less than a tenth of orbital velocity, so the expelled CO2 just falls back into atmosphere.

    So now you have 10 intercept points, each with two cones of exhaust aligned along the flight path of the dead satellite. During those three seconds, the cones will expand so they extend for 2 km on each side of the interceptor, with a diameter of about 1,100 feet on the far ends. The cloud isn’t very dense, just 1.8 milligrams per cubic meter at the far end and getting denser toward the apex of the two cones of gas. But that’s enough to create quite a bit of drag. The actual apex area has no gas because the interceptor dodged out of the way, and because a really dense gas could from very near the nozzle would risk breaking up the satellite into even more debris.

    The result is 20 little impulses from 20 little cones, each last 0.27 seconds. The oncoming thrusters have a slightly higher impact velocity than the trailing thrusters, and in this case the leading half of the 10 cones each produce a delta V of -4 m/sec, while the trailing thrusters produce a delta V of -3 m/sec. The applied drag forces varies rom 100 to 1000 pounds, peak. The total delta V for the satellite is -70 m/sec. The result of that is that the satellite goes into an elliptical orbit with an apogee at the original 300 km, but with a perigee at 62 km, which will cause the orbit to decay until it eventually burns up in the atmosphere.

    It seems technically feasible, but the question is who would pay for it, and whether an on-orbit garbage-collecting tug would be a cheaper and more efficient option.

    And that’s it, mission accomplished. The satellite, 100 km up, will fly through the exhaust stream of the two apex motors. Their exhaust will have spread to a diameter of a kilometer or so when the debris’ first encounters it, so the density is extremely low (and of course a function of thrust and their ISPReport

  7. Avatar Michael Cain
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    says:

    From memory, so suspect, but hasn’t almost everyone agreed that geostationary satellites will contain enough fuel to be put in a “harmless” higher orbit near end-of-life? The “big” for geostationary orbital space is relatively small, and when one of those birds dies there’s usually someone waiting to put a replacement in almost exactly the same place.Report

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