According to “https://webb.nasa.gov/content/about/orbit.html”, NASA plans to exploit a quarter million mile radius “halo” orbit at L2.

And Webb will orbit around L2, not sit stationary precisely at L2. Webb's orbit is represented in this screenshot from our deployment video (below), roughly to scale; it is actually similar in size to the Moon's orbit around the Earth! This orbit (which takes Webb about 6 months to complete once) keeps the telescope out of the shadows of both the Earth and Moon. Unlike Hubble, which goes in and out of Earth shadow every 90 minutes, Webb will have an unimpeded view that will allow science operations 24/7.

I understand the semi-stable utility to this 3BP solution, but I don’t understand why the radius NASA is showing is so large. Since L2 is beyond the Sun-Earth umbra, there is no chance of hiding from the Sun, but why such a large radius? Is this radius a fuel expenditure compromise? Any smaller, you burn more fuel and any larger, you lose L2 lock?

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    $\begingroup$ closer requires more frequent adjustments of the orbit, and more fuel use. No, i have no idea why this works so, but start here which explains why small halo orbits turn to tangled spaghetti space.stackexchange.com/questions/15167/…₁s-and-l₂s-preferred-over-small-orbits-for-reaso $\endgroup$ Commented Jan 9, 2022 at 17:12
  • $\begingroup$ Thanks. Great reference. $\endgroup$
    – Bob Buxton
    Commented Jan 9, 2022 at 19:42

2 Answers 2


Answer: to avoid eclipses by the Moon.

See: Why are eclipses of the James Webb by the Earth or Moon not permitted during the mission? . It discusses NASA's decision not to allow any lunar eclipses of JWST.

This NASA document ntrs.nasa.gov/citations/20190028885 states, "Thermal and power requirements state that the JWST mission shall not experience any Earth or Moon eclipses during its entire mission lifetime."

Because L2 is only 2 Moon orbital diameters away from Earth, geometry dictates a "big fat halo" with major axis in the plane of Moon's orbit.

The box in illustration c) indicates potential positions of the Moon which need to be avoided.

enter image description here

The halo orbit's large diameter restricts aiming of telescope. At extreme Y axis excursion, aiming in the ecliptic plane is limited to prevent Earthlight from peeking around the sunshade.

  • $\begingroup$ I don't believe this answer yet. First, you are speculating and cite no sources. 1) Transits of the Moon seen at JWST's location in a smaller halo orbit will be rare and might easily be avoided completely for halo orbits sub-synchronous with the Moon's apparent motion. 2) a drop of 5 to 10% in solar power could easily be managed (it's NOT an eclipse) by not doing the most electrical power-intensive operations during those times, if it ever did happen, which again, can be easily avoided even for small halo orbits. 3) You cite NO sources to back up your speculative assertion. -1 $\endgroup$
    – uhoh
    Commented Jan 10, 2022 at 21:06
  • $\begingroup$ @ Uhoh ----- I agree with 1) and 2) . It makes no sense to me that JWST needs to avoid lunar "eclipses", but that is exactly what NASA chose. The reference to NASA's decision is in the first line of the posted question. $\endgroup$
    – Woody
    Commented Jan 10, 2022 at 22:05
  • $\begingroup$ Ya I've added a block quote to the question now. The last sentence of the question is so flawed that it calls the veracity of the entire web page into question. It is a non-technical page and so was probably written by a science writer and not an engineer associated with the mission. "Unlike Hubble, which goes in and out of Earth shadow every 90 minutes, Webb will have an unimpeded view that will allow science operations 24/7." conflates several different concepts; The Hubble certainly maintains "science operations 24/7", eclipses don't interfere with operations. $\endgroup$
    – uhoh
    Commented Jan 10, 2022 at 22:42
  • $\begingroup$ There is a rotation of available targets every 365 days for JWST and every 1/16th day for Hubble; the cadence of rotation of FOV or FOR is different but they both have access to roughly half the sky at any moment. I don't feel that page is an authoritative source at all. I'm pretty sure the larger orbit requires less delta-v per year for station keeping for a given observation interrupting maneuver cadence (every two weeks I think) $\endgroup$
    – uhoh
    Commented Jan 10, 2022 at 22:44
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    $\begingroup$ @Uhoh ---- Here is a NASA document ntrs.nasa.gov/citations/20190028885 which states, "Thermal and power requirements state that the JWST mission shall not experience any Earth or Moon eclipses during its entire mission lifetime." $\endgroup$
    – Woody
    Commented Jan 10, 2022 at 23:32

the leading factor in the NASA citation is... THERMAL!

it is of utmost importance that a super stable thermal environment is provided to the mirror system and instruments. popping in and out of shadows, even the very weak moon shadow, would cause fluctuations in thermal loading to the overall system. Those very minor temperature variations would screw everything up. Primary Mirrors would need constant adjustments. Instrument calibrations would be off. Also, solar wind pressure exerted on the sun-shield would vary, causing more station keeping fuel usage, decreasing useable telescope lifetime.

  • $\begingroup$ JWST can point at inclinations varying 85^{o} to 135^{o}, and also rotate 360^{o} around the axis of rotation. Rotating around a circle has minimal effect on the overall solar radiation, but wouldn't changing the inclination have some effect? Or does the 5 layer solar shade cancel out any minor temperature variations due to changing inclination? $\endgroup$
    – Sheldon
    Commented Jan 22, 2022 at 22:49
  • $\begingroup$ @Sheldon: changing the inclination would certainly have some effect, not least in the direction and amplitude of the solar wind pressure. I think that's what the mysterious trim flap is for. $\endgroup$
    – TonyK
    Commented Jan 24, 2022 at 0:57
  • $\begingroup$ @sheldon true, inclination changes will be made, but because of the boat shape of the sun shade, as one end increases its exposure the other end will decrease exposure resulting in same overall gain. Although I have not read anything specific to this I suspect that this is one of the reasons for the boat shape. $\endgroup$
    – BradV
    Commented Jan 24, 2022 at 4:54

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