[HN Gopher] Queqiao: The bridge between Earth and the far side o...
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Queqiao: The bridge between Earth and the far side of the moon
Author : Arunprasath
Score : 54 points
Date : 2021-06-11 09:01 UTC (14 hours ago)
(HTM) web link (www.moondaily.com)
(TXT) w3m dump (www.moondaily.com)
| galuggus wrote:
| QueQiao Que Qiao translates as bridge of magpies
|
| From this folktale
|
| https://en.m.wikipedia.org/wiki/The_Cowherd_and_the_Weaver_G...
| barbegal wrote:
| What they don't mention is that orbiting the L2 point is unstable
| requiring constant manoeuvres to keep the satellite in position.
| You can think of it like going around the top of a hill. The top
| of the hill is relatively flat so it didn't require much energy
| to overcome gravity but after some time you end up rolling down
| the hill unless you apply some force to keep you in position.
| [deleted]
| spockz wrote:
| I thought that the definition of the Lagrange point was that it
| required _no_ energy to remain in that position. Or does that
| happen because the Earth and the moon wobble a bit and
| therefore the exact position of the Lagrange point shifts?
| sp332 wrote:
| Yeah the stable point is a literal geometric point. And
| there's a whole solar system's worth of little gravitational
| disturbances going on.
| biggerfisch wrote:
| L4 and L5 are stable (see Jupiter's asteroid collection), but
| the others involve essentially trying to balance on a
| infinitesimal pinpoint, with any perturbation causing one to
| "fall off" and move away.
| lann wrote:
| I don't know much about Lagrange points specifically, but it
| sounds like this one is an "unstable equilibrium". Even if
| you could park on exactly the right point, any tiny force
| (changing gravitational pull of other planets, dust impact,
| even light pressure from the sun eventually) would nudge you
| off of it and you would start accelerating away from it.
| _Microft wrote:
| L1-3 are unstable, L4 and L5 are not.
|
| https://en.wikipedia.org/wiki/Lagrange_point#Point_stabilit
| y
| [deleted]
| Chris2048 wrote:
| > You can think of it like going around the top of a hill
|
| I always wondered: a ball is not stable on a hill since you
| have to balance it on the top, but a _ring_ is. Could a large
| ring-size space-station be stable at these points by having
| them at their centre?
|
| EDIT: looking here: https://www.youtube.com/watch?v=JZj9zINN3e0
| it looks like more of a saddle. So maybe a space station of 2
| parts tethered together with their centre-of-mass at L2? Since
| space is frictionless (as opposed to an actual saddle surface)
| I guess there will still need to be station-keeping, but only
| in on direction: maybe my allowing synchronised "ballast"
| weights to fall in/outwards.
| pierrec wrote:
| I think the metaphor only sort-of works when you represent
| the object as a point. The farther you are from an unstable
| point of equilibrium, the harder you're getting pushed away
| from it. So if your station gets nudged, the farther part
| will push the station away harder than the closer part.
| Overall it probably changes nothing about how much effort is
| required to keep the station in place.
| saberdancer wrote:
| James Webb space telescope will be at L2 point. My point is
| that station keeping is probably not that hard.
| jagger27 wrote:
| JWST won't sit directly at L2. Instead it will orbit around
| that point in a "halo orbit".
| frumiousirc wrote:
| > This point is known as the Earth-Moon Libration point 2
|
| AKA
|
| https://en.wikipedia.org/wiki/Lagrange_point
| xattt wrote:
| I interpret it as a Sinoist rejection of Western naming
| conventions.
| throwawaybutwhy wrote:
| The concept is also known as Farquhar's orbit around L2. Not
| really new, either.
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