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Re^16: "Practices and Principles" to death

by tilly (Archbishop)
on Mar 06, 2008 at 08:08 UTC ( #672393=note: print w/replies, xml ) Need Help??

in reply to Re^15: "Practices and Principles" to death
in thread "Practices and Principles" to death

I don't have energy to respond to this in full. However I am sad to tell you that you're mistaken on many counts. For instance contrary to your claim that I've implied that atmospheric drag begins at a low level, I've said that atmospheric drag will bring junk back to Earth in a time frame typically measured in centuries. (I just double-checked it and found I was wrong. While pieces from that may go for centuries, a lot of it will get fixed much faster.) For another example you got the relationship between size and orbital decay exactly backwards - larger pieces of junk experience more drag but have far more inertia and inertia wins. The same scaling principles that makes small things more affected by air down here operate in space. Another simple mistake is your claim that small particles are in streams. Consider the Chinese space satellite collision - that generated an estimated million pieces 1 mm or bigger and I guarantee it is not in a stream! Also even if you start out with a stream of small particles, miniscule velocity differences will, over the course of years, result in them being many km apart. And even if the initial speed was the same, differing effects of atmospheric drag would move particles apart.

Also there are major difficulties that you are minimizing. It is true that I assumed that you need to rendezvous with the junk to deal with it. It is true that you can more easily find collision courses with it. But read Re^7: "Practices and Principles" to death for how difficult it is to work with collisions at that speed, and recall that any shrapnel is new junk. I think my assumption that you want to match speeds holds!

This may be a good point to point out that a satellite in a Molniya orbit as you suggest using is going to be very far from still relative to any piece of debris it encounters that is not itself in a Molniya orbit. That is because while the satellite is fairly still relative to the Earth's surface, the piece of debris is nowhere near still relative to the Earth's surface, so there is a large relative velocity. In fact this is a general principle. If you encounter a piece of debris and at the point of encounter you do not have a large relative velocity, then you and the debris must be on very similar orbits! And conversely if you're on different orbits, any encounter will be at high velocity. The reason is simple, it is because from your position and velocity you can calculate every aspect of your orbit. So if your position is the same and your velocities are close if and only if you're on very similar orbits.

As for reusing existing spacecraft, review the link above about what collisions look like in orbit. Consider well that shrapnel is new junk. And then I think you'll agree with me that this is an approach that is more likely to create problems than solve them.

This is hardly an exhaustive list of issues I can come up with. (For example I didn't want to get into economic issues.) But it is enough to show how hard it is to solve the problem of space junk.

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Re^17: "Practices and Principles" to death
by BrowserUk (Pope) on Mar 06, 2008 at 09:23 UTC

    Your response is too fast, and so ill considered.

    For example: The point about a Molniya orbit is not that it is stationary with any given piece of debris. It is relatively stationary with a fixed point (relative to the earth surface moving at what? 10.7 km/s) above the earth's surface for a protracted period of time. So if a piece of debris was moving in that frame of reference for a small part of that time, a collision could be arranged.

    Now, if you put the deflector into a highy elliptical orbit specially chosen to (after the manner of the Molniya orbit) cause the deflector to be moving at a low relative velocity (relative to the piece of debris) for the short period of time of the collision, then all the problems associated with high absolute velocities disappear.

    Update: You're right about the Molniya orbit. The apparent "apogee dwell" is due to 1) moving more slowly at the apogee. 2) the small arc of the sky covered (as seen from earth, during the long climb and descent to and from apogee. The relative speed to any given point in space is not vastly reduced. Just the apparent speed across the sky. That still leaves the question of how a paper aeroplane thrown from ISS makes it to earth in a few months with only the strngth of an astronaughts arm to change it velocity?

    (I'd already read apl's post and dismissed is because his banks of earth aren't moving. I also mentioned the speck of paint and the shuttle window incident above. Were the two travelling at slow relative speeds? No. Likely as not they were travelling in opposite directions at the point of encounter!)

    You've already agreed that if the two components are in the same orbit with one moving slightly slower than the other (the docking scenario), then their absolute velocities is irrelevant.

    The point about the Molniya orbit is that it demonstrates that the two components, the satellite and the point above the earth can be travelling in entirely different orbits at vastly different speeds, but for some period (including fairly protracted ones), of their cycles, they are travelling co-incident to each and at low, relative speeds.

    Your other points are just spoilers:

    • Small particles:

      a) I said I think. Not a "claim".

      b) Not all the small particles from any given source of course. Especially not all the particles from an explosion or collision. But are you prepared to deny that any 2 or more small particles will follow similar orbits for a protrated period?

    • Drag effects and size:

      Like I said, the math gets complicated. Have you considered the terminal velocity affect? The idea that a thing falling to earth under gravity will reach a maximum velocity and no more.

    • How does your physics intuition rate the chances of a paper aeroplane being launched by hand from ISS reaching the earth's surface?

      Because at least one scientist believes that it will. Yup! A piece of paper with a launch speed relative to the ISS of whatever an astronaut can generate with his arm. Does that give you any pause for thought?

      Or is that scientist just crazy to think that such a small change in absolute velocity from the ISS' 27,700 kph could result in an orbit that would return that piece of paper to earth in a reasonable time frame ("several months" according to the scientist)?

    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.
      From the amount you struck out, I believe you realized that my response was not so ill considered as all that.

      About the paper airplanes, I'm painfully aware that space is so alien to my experience that any intuition needs to be backed up by calculation. But I note that the ISS is about 60 m long and the airplane is 8 cm long. That is a 750-fold difference in length. Let's leave out differences in materials and shape. That makes the cross-sectional area (and therefore drag) of the space station be 7502 times the airplane. The mass of the space station is 7503 times the airplane. The resulting acceleration due to drag on the space station is 1/750'th what it is for the airplane. So 6 months for the plane to come down is the same as centuries for the space station.

      I can believe that.

      Edit: Missed a factor of 10 on a calculation, fixed. Also added explanation of why the effect of drag on time to hit Earth scales linearly with length. Please note that the linear scaling is for the same shape and materials. The space station has a different shape and materials than the airplane. In particular the space station is hollow. Thus it will probably come down much faster than naive scaling up of the airplane would suggest.

        For the record. So much of what you have stated above is so inaccurate that it makes it pointless to try and argue with you.

        Example: You suggest that it would be easier to collect the debris than to deflect it into a decaying orbit. Do a little research and you will find that the 8000 or so tracked objects represent a total mass of approximately 3000 tonnes. Where you gonna put it? Some pieces are 20 metres in length.

        Example: You suggest that it is "easy to dodge space debris". The 8000 tracked items represent only those pieces > 10cm. They are but the tip of the iceberg. There are estimated to be upward of 100,000 (maybe as high as 500,000) pieces 10cm<debris>1 mm. A "spec of paint" was enough to punch a pretty substantial hole in the "bullet proof" window of the space shuttle. How do you dodge something you have no idea is coming? (Not to mention that "station keeping fuel" is one of the major limitations upon the lifetime of a satallite.

        For example. One of the major roles of the 1.3 billion Euro ATV, is to use it's thrusters to regularly push the ISS higher to compensate for its otherwise rapidly decaying orbit (Remember that discussion? So big objects don't decay due to atmospheric drag huh!)

        I looked to refuting your blanket dismissal of the feasibility of the ideas I expressed above. It turns out that the math underlying Orbital Mechanics is far less onerous than I had thought--once you find the right source of practically, rather than theoretically expressed information. What eventually stopped me was the requirement to be an "approved registered user"(*) in order to obtain the basic data (TLEs) with which to construct a simulation. Well that, and the distinct impression that it wouldn't sway you one iota.

        Al-Qaeda might nip up there and start throwing them at the US?

        There are reasons why using a redundant space shuttle wouldn't be a good idea. None that you broached though. It's mostly to do with its mass. Regularly changing the orbit of so massive a structure, most of which would be redundant for the task, it too costly. However, that doesn't mean that it would make sense to use a to-be-retired-shuttle to put such a project into orbit.

        And the Molinya orbit wasn't such a red-herring after all. For two reasons.

        • Firstly, a highly elliptical orbit is advantageous for this because it is much cheaper (in fuel) to alter the inclination of a satallite when it is further way from the Earth. Further away means slower velocity, mean less fuel to alter the inclination.
        • Secondly, the elliptical orbit takes the deflector across many circular orbits (the ones where most satellites tend to be stationed), and as you transition those circular orbits in the elliptical orbit, the angular velocity of both is the same. Ie. As you transition the path of those pieces of debris in the circular orbits, provided that you are moving in roughly the direction and inclination, the relative velocities will be minimal.

        Some data

        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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