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Thread: Webb telescope to orbit L2?

  1. #1 Webb telescope to orbit L2? 
    Forum Freshman GreatBigBore's Avatar
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    I just read that the Webb telescope will orbit the L2 point, rather than just sitting at the L2 point. I thought that orbiting would happen only around an object with mass; I had no idea that a body could orbit a point in empty space. How does that work? What keeps it orbiting that point?


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  3. #2  
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    Interesting. I think it meant to say would orbit the sun at L2. L2 aligned with earth as it circles sun but at further distant.


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  4. #3  
    Forum Freshman GreatBigBore's Avatar
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    Quote Originally Posted by Lynx_Fox
    Interesting. I think it meant to say would orbit the sun at L2. L2 aligned with earth as it circles sun but at further distant.
    That's what I thought, until I looked at the NASA website that has an animation showing it orbiting the L2 point. You can see the animation here; click on "orbit" on the left-hand side to see what I mean.

    Edit: it's a little confusing, because it says that Webb will "stay fixed in the same spot with relation to the Earth and the Sun," but then it clarifies that by saying, "Webb will orbit around the L2 point. It won't stay completely motionless at a fixed spot."
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  5. #4  
    Moderator Moderator Dishmaster's Avatar
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    The L2 is not a stable point of equilibrium, rather a saddle. No object can stay there without further help. It more acts like a gravitational hill, not like a sink like L4 and L5 (see image below). This means, one can orbit it like there was a body having a mass.

    See also:
    http://nssphoenix.wordpress.com/2009...agrange-point/

    The JWST is not the only telescope that will be put there. Currently, the Herschel Space Telescope and the Planck satellite are orbiting at at different distances. Previously, the WMAP stellite was there , too.

    The term "fixed" related to Earth and Moon just means that they both are always roughly in the same direction, but not exactly.

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  6. #5  
    Forum Freshman GreatBigBore's Avatar
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    Quote Originally Posted by Dishmaster
    ... This means, one can orbit it like there was a body having a mass.
    Fascinating. I had no idea. Do you perhaps know the size of the orbit of these spacecraft, the distance from the actual L2 point to the vessel? Even a ballpark figure, like is it on the order of 1km or 1000km or even more, that sort of thing. Thanks a lot for the answer!
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  7. #6  
    Moderator Moderator Dishmaster's Avatar
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    Quote Originally Posted by GreatBigBore
    Quote Originally Posted by Dishmaster
    ... This means, one can orbit it like there was a body having a mass.
    Fascinating. I had no idea. Do you perhaps know the size of the orbit of these spacecraft, the distance from the actual L2 point to the vessel? Even a ballpark figure, like is it on the order of 1km or 1000km or even more, that sort of thing. Thanks a lot for the answer!
    I know it for sure only for Herschel. Here it is 700000 km diameter. So hardly AT the L2. From Earth, Herschels orbit has a diameter of around 30 degrees.
    See also:
    http://www.mpia-hd.mpg.de/IRSPACE/herschel/
    Planck's orbit is smaller.
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  8. #7  
    Forum Freshman GreatBigBore's Avatar
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    Quote Originally Posted by Dishmaster
    I know it for sure only for Herschel. Here it is 700000 km diameter. So hardly AT the L2. From Earth, Herschels orbit has a diameter of around 30 degrees.
    Very cool. Thanks so much!
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  9. #8  
    Forum Professor Wild Cobra's Avatar
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    Quote Originally Posted by Dishmaster
    The L2 is not a stable point of equilibrium, rather a saddle. No object can stay there without further help. It more acts like a gravitational hill, not like a sink like L3 and L4 (see image below). This means, one can orbit it like there was a body having a mass.
    You lost me there. All orbits are like a Saddle, or are you using the term in a manner I'm not understanding? It doesn't matter what orbit you are in, once you leave the sweet spot, correction is required.

    One more thing. The L1 to L3 can be a single point, or "halo" orbits:

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  10. #9  
    Moderator Moderator Dishmaster's Avatar
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    Not all Langrangian points are potential saddles. L4 and L5 (see edit of my previous post) are potential wells, where e.g. asteroids can accumulate as the trojans.

    Herschel's orbit is a halo orbit around L2:
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