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Thread: BBC article on Oumuamua & 2I/Borisov

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    BBC article on Oumuamua & 2I/Borisov

    I was reading through this article, not expecting any real accuracy due to the intended audience.
    However, one statement, repeated several times struck me as wrong.
    It's perfectly normal for comets to accelerate as they travel back out from a close encounter with the Sun, but only because they are being powered by their tails – the gases being ejected give them a kick, like the engine on a rocket.
    I was always under the impression that a comets tail points away from the sun, because it's driven by the solar wind. In which case the tail would be actively working against any acceleration away from the sun.

    Where have I gone wrong, or what am I missing?

    https://www.bbc.com/future/article/2...isited-our-sun

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    Quote Originally Posted by headrush View Post
    I was reading through this article, not expecting any real accuracy due to the intended audience.
    However, one statement, repeated several times struck me as wrong.


    I was always under the impression that a comets tail points away from the sun, because it's driven by the solar wind. In which case the tail would be actively working against any acceleration away from the sun.

    Where have I gone wrong, or what am I missing?

    https://www.bbc.com/future/article/2...isited-our-sun
    I think you are right, according to all previous pictures, but the sun is pushing on the comet with a small force all the time quite separately from gravity. The solar wind and radiation generate the tail which like the flag on a boat, points away from the wind. Thus while small there must be a force there. But the writer has it wrong, the tail is always away from the sun.
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    The journalist is correct, but skips a few stages. The acceleration always comes from the sunlit side of the comet nucleus, because that's where most material is being ejected from the surface into space. (The acceleration vector doesn't point exactly away from the sun, though--it's offset slightly by the rotation of the comet nucleus.)
    The direction in which this ejected material subsequently streams to form the visible tail is irrelevant--the material that forms the tail has already been ejected, has applied its force to the nucleus, and is now following its own trajectory under the influence of gravity and radiation pressure. The nucleus doesn't care where the tail points, only where it comes from.

    Grant Hutchison

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    Also technically all deceleration is acceleration so technically that part is correct as well. *adjusts pince-nez*
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

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    Quote Originally Posted by Noclevername View Post
    Also technically all deceleration is acceleration so technically that part is correct as well. *adjusts pince-nez*
    I think I’m just not paying attention well enough, so when you say “that part” what do you mean? Is it referring to a paragraph in the linked article? Could you specify what you mean by “that part”?


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    Quote Originally Posted by Jens View Post
    I think I’m just not paying attention well enough, so when you say “that part” what do you mean? Is it referring to a paragraph in the linked article? Could you specify what you mean by “that part”?


    Sent from my iPhone using Tapatalk
    Sorry for the mix up. I was referring to the quoted statement in the OP:
    It's perfectly normal for comets to accelerate as they travel back out from a close encounter with the Sun, but only because they are being powered by their tails – the gases being ejected give them a kick, like the engine on a rocket.
    The Sunward face of the comet head would be thrusting mainly toward the Sun, so decelerating on the way in and accelerating on the way out (from the Sun).

    I was always under the impression that a comets tail points away from the sun, because it's driven by the solar wind. In which case the tail would be actively working against any acceleration away from the sun.
    Acceleration, in an opposite alignment.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

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    Quote Originally Posted by grant hutchison View Post
    The journalist is correct, but skips a few stages. The acceleration always comes from the sunlit side of the comet nucleus, because that's where most material is being ejected from the surface into space. (The acceleration vector doesn't point exactly away from the sun, though--it's offset slightly by the rotation of the comet nucleus.)
    The direction in which this ejected material subsequently streams to form the visible tail is irrelevant--the material that forms the tail has already been ejected, has applied its force to the nucleus, and is now following its own trajectory under the influence of gravity and radiation pressure. The nucleus doesn't care where the tail points, only where it comes from.

    Grant Hutchison
    Ah, that will be it. Thanks. I still take exception to the "being powered by their tails" comment though as the tail is not the source of the power, merely the visible result of it. It's like suggesting a jet aircraft is being powered by its con-trails.

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    Quote Originally Posted by headrush View Post
    Ah, that will be it. Thanks. I still take exception to the "being powered by their tails" comment though as the tail is not the source of the power, merely the visible result of it. It's like suggesting a jet aircraft is being powered by its con-trails.
    The tail is certainly not causing any thrust, in either direction. The tail particles are independent after they've been ejected from the nucleus.

    However, the tail has some mass, albeit tiny, and the nucleus will be attracted by its gravity. So it will have an apparent acceleration towards a leading tail. Very likely undetectable given the mass difference, but just sayin.

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