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During a discussion of Lagrange points I was asked why we didn't put JWST at SE L4 or L5 since they are less unstable than L2. I thought a bit and came to think that the Earth gravity driven Halo orbit just inside of L2 is more fuel efficient to maintain over long periods of time than trying to park right at L4 or L5. Also, much easier communications. Other, more correct reasons?

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    $\begingroup$ You get dust clouds around Trojan points, and occasionally bigger things. Near SE L4 we have en.wikipedia.org/wiki/2010_TK7 & en.wikipedia.org/wiki/2020_XL5 $\endgroup$
    – PM 2Ring
    Commented Apr 26, 2022 at 20:28
  • $\begingroup$ @PM 2Ring yes! a while ago I was watching a great animation of Jupiter trojans. Quite the mess bumbling about out there! $\endgroup$
    – BradV
    Commented Apr 26, 2022 at 20:55
  • $\begingroup$ en.wikipedia.org/wiki/Claimed_moons_of_Earth#Earth_trojans has anims of those two, showing the paths in the corotating frame from 1600 to 2500 AD. $\endgroup$
    – PM 2Ring
    Commented Apr 26, 2022 at 21:06
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    $\begingroup$ There are certainly clouds at the EM L4 & L5: en.wikipedia.org/wiki/Kordylewski_cloud I can't find info about the relative dustiness at SE L4 & L5. There is a proposed mission to L5, to watch for CMEs: en.wikipedia.org/wiki/ESA_Vigil $\endgroup$
    – PM 2Ring
    Commented Apr 26, 2022 at 21:17
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    $\begingroup$ L4 and L5 can be advantageous locations for some spacecraft, but not for JWST. First, Sun-observing spacecraft can see parts of Sun not observable from Earth. (European Space Agency is considering a project of such spacecraft). Second - radiotelescope can be placed there, to serve as radiointerferometer, as well as for communication with other spacecraft during solar conjunction. But transfer to L4/L5 is problematic - it will need long coasting (over decades) or large delta V at arrival (so most of the spacecraft mass will propellant, with less payload) $\endgroup$
    – Heopps
    Commented May 12, 2022 at 9:00

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Surely a significant reason is that L4 and L5 are much farther away, roughly 150 million km vs 1.5 million. That makes things more difficult in a number of ways, but probably the biggest is that communications bandwidth would be significantly lower. Increasing the distance by 100 times will decrease the signal strength by 10,000 times. That in turn reduces the rate at which data can be transmitted back, which is something that really matters.

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    $\begingroup$ Yes, com distance is factor. Also a factor might be com antenna pointing complications that arise as JWST rotates and pivots to access certain sections of sky. Right now antenna pointing is uncomplicated since relative orientation is fairly stable. $\endgroup$
    – BradV
    Commented Apr 27, 2022 at 15:52
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Others have mentioned the distance and partial shade provided by Earth, but also, the stability advantage of L4 and L5 isn't really relevant. JWST is only expected to operate for 20 orbits around the sun, it doesn't matter if the orbit is stable for millions or even just hundreds of years. A telescope could easily be put into a slightly leading or trailing orbit such as was used for STEREO-A and STEREO-B, and spend its entire lifetime slowly drifting toward or past the L4/L5 points, without having to actually be placed at those locations.

Also note that JWST's stationkeeping requirements are more about maintaining the correct position with respect to Earth's shadow than staying around the L2 point.

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About NWST's orbit, by NASA

The telescope itself will be operating at about 225 degrees below zero Celsius (minus 370 Fahrenheit). The temperature difference between the hot and cold sides of the telescope is huge - you could almost boil water on the hot side, and freeze nitrogen on the cold side!

To have the sunshield be effective protection (it gives the telescope the equivalent of SPF one million sunscreen) against the light and heat of the Sun/Earth/Moon, these bodies all have to be located in the same direction.

This is why the telescope will be out at the second Lagrange point.

Another one from NASA

"A huge advantage of deep space (like L2) when compared to Earth orbit is that we can radiate the heat away," said Jonathan P. Gardner, the Deputy Senior Project Scientist on the Webb Telescope mission and Chief of the Observational Cosmology Laboratory at NASA's Goddard Space Flight Center in Greenbelt, Md. "Webb works in the infrared, which is heat radiation. To see the infrared light from distant stars and galaxies, the telescope has to be cold. Webb's large sunshield will protect it from both Sunlight and Earthlight, allowing it to cool to 225 degrees below zero Celsius (minus 370 Fahrenheit)." For the sunshield to be effective, Webb will need to be an orbit where the sun and Earth are in about the same direction.

With the sun and the Earth in the same part of the sky, the Webb telescope will enjoy an open, unimpeded view of the universe. In comparison, the Hubble Space Telescope is in low-Earth orbit where it goes in and out of the Earth's shadow every 90 minutes. Hubble's view is blocked by the Earth for part of each orbit, limiting where the telescope can look at any given time.

In summary, the sun-earth L2, where the sun and earth are roughly in the same direction, are chosen for (at least) two reasons:

  1. Heat shielding is easier.
  2. Less sky is blocked.
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  • $\begingroup$ That's (specifically the 3 bodies being in the same direction) is why it's not in Earth orbit, it's not a reason not to put it in a more distant solar orbit. JWST would not need to shield itself from the 1 AU-distant Earth at SEL4 or SEL5 any more than it needs to shield itself from the other planets at SEL2. $\endgroup$ Commented Apr 27, 2022 at 15:51
  • $\begingroup$ difference in distance between SE L4 and SE L2 is only 1%, so almost negligible heat load difference. $\endgroup$
    – BradV
    Commented Apr 27, 2022 at 16:04
  • $\begingroup$ 1. You don't need protection against the light and heat of the Earth and Moon if you are 150 million km away! 2. Less sky is blocked relative to Hubble, not relative to L4/L5. This answer is just wrong. $\endgroup$
    – TonyK
    Commented May 10, 2022 at 21:39
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After letting this question 'simmer a bit' and giving myself time to do a bit more research... I've come to fully agree with comments by PM 2Ring as being the best explanation for not putting something like JWST at L4 or L5. Wish PM 2Ring had put into an answer even if not documented/supported.

PM 2Ring commented that there is just too much stuff hanging out at L4/5 such as Trojans and dust clouds. This would interfere with telescope function. Also would raise risk of damage to an unacceptable level.

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