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Thread: Space Cowboys: Questions from the Uneducated

  1. #1

    Space Cowboys: Questions from the Uneducated

    Just rented Space Cowboys this weekend and enjoyed it but had two questions, the second of which is only implied by the movie:

    1. I know from reading the moon-hoax debunking about why it is logical to NOT see stars in the background in photos on the moon. But in Space Cowboys there are several shots around the large satellite they're trying to repair in which the background stars are highly visible. Should they not have been?

    2. How does heat propagate in a vacuum? Specifically, I know that if I stand beneath the exhaust of a spaceship when the engine fires, I'll be fried. But what if I stand off to the side just a few feet so that the plume of flame travels next to me but doesn't touch me. Will I still get fried or will no heat travel across the vacuum sideways to me? Did I write this clearly enough?

    Thanks for tolerating the astronomically illiterate such as myself, but this is why I come to this board. I know it's not a big deal to most on this board, but it's because of lurking here that I was able to point out Mars to my kids when we went on vacation this summer.

  2. #2
    Join Date
    Oct 2001
    Posts
    2,683
    Well, you might start with the BA's review of the movie. He mentions the star thing in it.

    http://www.badastronomy.com/bad/movi...ecowboys2.html

    As for number 2, I'm no expert, but I've picked up a lot from this board. Heat in space is almost exclusively tranferred through radiation, which I take to mean mostly infrared light. And it's slow and inefficient. I think that if you are near a firing rocket you should be all right for a long time, but eventually you'll absorb enough IR and other radiation to be fried. (Assuming you actually stay next to the exhaust. Whatever is firing is probably going to start moving away from you pretty quickly. [img]/phpBB/images/smiles/icon_biggrin.gif[/img])

    ...But wait, you'll also be radiating yourself, so I think it would depend on whether there's a net gain or loss of heat energy there. I think some expert should take over now. [img]/phpBB/images/smiles/icon_smile.gif[/img] [img]/phpBB/images/smiles/icon_smile.gif[/img]

  3. #3
    Join Date
    Oct 2001
    Posts
    143
    On 2001-10-29 07:15, Garrette wrote:

    2. How does heat propagate in a vacuum? Specifically, I know that if I stand beneath the exhaust of a spaceship when the engine fires, I'll be fried. But what if I stand off to the side just a few feet so that the plume of flame travels next to me but doesn't touch me. Will I still get fried or will no heat travel across the vacuum sideways to me? Did I write this clearly enough?
    A rocket propels by ejecting material in one direction and that causes the vehicle to go in the other direction. The material being ejected is hot and so carries most of the heat of combustion away with it.

    However, inside the combustion chamber the burning fuel transfers heat to the structure by conduction, convection and radiation. These are the three methods of heat transfer. The only way this heat can leave the vehicle is by radiation. The temperature rises until the heat being radiated equals the heat input from the burning fuel.

    For example, when you sit in front of a fireplace or near a pot-bellied stove your face feels hot and your front gets a lot hotter than your back. This is mostly a result of radiation (electromagnetic waves, like radio waves, in the infra-red wavelength).

    There is a formula called the Stefan-Boltzman law that allows computation of this temperature rise (or fall) for objects that absorb (or radiate) all wavelengths equally well. Modifications of this formula allow estimates for other objects as well.

    Standing, in a vacuum, to one side of the rocket exhaust you would probably still burn up if you were too close. I don't know how close is too close without a bunch of computations. But I wouldn't stand closer than half a mile to the exhaust of, say, a shuttle orbit-insertion motor burn. But then, I'm famous for cowardice.

  4. #4
    Thanks. I got it, I think.

    BA's stuff on the movie was informative, and I'm proud to say that I noticed the majority of those things myself before reading the BA review; maybe I'm learning something here despite my best efforts otherwise.

    But regarding the heat thing, I suppose the idea is that maybe I won't burn up as fast as on earth but ignorance doesn't excuse stupid risks. (Note to self: scratch plans to float next to exhaust ports of shuttle during next mission.)

  5. 2009-Apr-16, 04:33 PM


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