What is the main reason JWST will be in a halo orbit around the Sun-Earth L2 LaGrange point? There have been questions on this site that ask Why won't JWST deploy in LEO where it is potentially serviceable?, however why not keep it in LEO, or MEO (Medium Earth Orbit). Keeping it in a Low Earth Orbit will mean it can be serviced if there is a flaw, such as what happened with the Hubble Space Telescope.

This NASA FAQ article says that it's not in LEO because of the debris that resides there. But there are many telescope in LEO and can't JWST be in a higher altitude (while still being in a LEO). So what's the main reason NASA decided that it's better for it to be 1.5 million km from the Earth instead of LEO?

Related: Why are so many space telescopes placed in LEO instead of at Lagrange Points? And why do we hear about Hubble more than any Langrange-orbit telescope?

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    $\begingroup$ The two reasons I've personally heard (from sites like NASA) are: Constant Sunlight / Reliable Solar Energy and no high-to-low temp swings like the ISS gets when orbiting Earth. But I'm sure it's far more thought out than just those points. $\endgroup$ Commented Aug 27, 2019 at 20:46
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    $\begingroup$ @MagicOctopusUrn that's a good point, see What are the sources of light at L2? How will the James Webb telescope be powered? However a high sun-synchronous orbit or something like the orbit TESS uses might also solve that problem. $\endgroup$
    – uhoh
    Commented Aug 27, 2019 at 23:20
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    $\begingroup$ JWST could conceivably stare for days at a time at any point in a huge portion of the sky. The same could not be said of a telescope in LEO. In LEO, almost all of the sky will be regularly eclipsed by Earth. The only part of the sky an LEO telescope could observe completed unobstructed for long durations would be directly along the axis of its orbit. An LEO telescope would also almost certainly have to deal with Earthshine when pointed pretty much anywhere else.. $\endgroup$
    – Anthony X
    Commented Aug 28, 2019 at 2:22
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    $\begingroup$ I think the answer to this question is in the NASA FAQ linked in the question. The telescope needs to be kept very cold to observe faint objects in the infrared. It has to keep Sun, Earth, and Moon constantly on the other side of its heat shield to stay cold enough. The closest place that meets those requirements which is also an orbit (minimal station-keeping fuel) is Sun-Earth L2. $\endgroup$
    – Anthony X
    Commented Aug 28, 2019 at 2:37
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    $\begingroup$ @AnthonyX looks like that is a good answer. Can you write it up as such. $\endgroup$ Commented Aug 28, 2019 at 9:05

1 Answer 1


NASA's explanation is that the Sun, Earth, and Moon are all too hot for the telescope, so they all need to be on the same side of the sun-shield to protect the telescope. Those three objects are all in roughly the same direction from the perspective of the L2 point.

By putting the telescope near the L2 point, they can use very little fuel to keep the telescope in a place where the Sun, Earth, and Moon are always blocked by the sun-shield. It's also close enough to the Earth that tracking the telescope and communicating with it is relatively easy and quick (only about a ten second round-trip signal delay).

As to why a halo orbit rather than parking it right at the L2 point, I found this answer to a related question, which explains it pretty well. By putting the telescope in a halo orbit, the telescope can get more power from fewer solar panels (since it won't be in the Earth's shadow), and it can "hear" radio signals from the Earth more easily. The Sun produces a lot of radio noise (that's one source of static on an AM radio) and with the telescope in a halo orbit, it can point the high-gain communications antenna at the Earth without also pointing it straight at the Sun.

  • $\begingroup$ Excellent answer $\endgroup$
    – Freddo411
    Commented Dec 30, 2021 at 21:10

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