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### Re^17: "Practices and Principles" to death

by BrowserUk (Pope)
 on Mar 06, 2008 at 09:23 UTC ( #672409=note: print w/replies, xml ) Need Help??

in reply to Re^16: "Practices and Principles" to death
in thread "Practices and Principles" to death

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.
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Re^18: "Practices and Principles" to death
by tilly (Archbishop) on Mar 06, 2008 at 16:57 UTC
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.

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.
Where do I start?

I've never said that it is easy to dodge all space debris. What I've repeatedly said is that any individual known piece of space debris is easy to dodge. Which is true. I've also said that dodging an individual piece of debris is easier than removing it. Which is true. I've also said that collecting debris is easier than deorbiting it. Which is true for all but a handful of large pieces. And that handful is, of course, easy to dodge.

Of course this leaves the huge problems of debris you don't know about. It also ignores the scaling issue, which results from the fact that n satellites and m pieces of debris results in nm combinations that you have to keep track of. And you also have the problem that debris can hit debris, creating more debris.

And the Molinya orbit is a red herring if your aim is to arrange for low speed collisions with debris. (Which is what you were proposing it for.) Sure, your orbits intersect, but at the point of intersection your relative velocity is large. If you just want to intersect debris, then the Molinya orbit is not a bad idea. However you have the huge problem that a high speed impact between a solid object and debris is an event that is very, very likely to result in more orbiting debris, not less. If the solid object is a retired space shuttle, then I guarantee that you'll increase debris, not decrease it. While plenty of other objections can be leveled, that one alone is a deal breaker.

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