Why are eclipses of the James Webb Space Telescope (JWT) by the Earth or Moon not permitted during the mission?

In Seasonal variations of the James Webb Space Telescope orbital dynamics it is stated

Shadows, or eclipses of the spacecraft by either the Earth or the Moon, are not permitted at all during the mission… these constraints conspire to limit the maneuverability of the spacecraft and couple the orbital dynamics to the rhythms of the Earth, Moon, and Sun.

How come? The cold “science” side of the spacecraft is already eclipsed by the sunshield. Communications interruption should not be an issue since communications are intermittent anyway.

The Earth's umbra extends 1,400,000km and the JWT is 1,500,000km from Earth, so the telescope would never be completely eclipsed in the worst case. Since it is in a 800,000km halo orbit with a period of 6 months, it would take about 18 hours to traverse one Earth diameter or 36 hours to pass through the penumbra.

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    $\begingroup$ What is Mkm? Megakilometer? Is it really used instead of gigameter anywhere? $\endgroup$
    – Ruslan
    Dec 4, 2021 at 8:44
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    $\begingroup$ A 1000 m is 1 km, 1000 km is 1 Mm, 1000 Mm is 1 Gm. The combination of M and k is not allowed for metric units. $\endgroup$
    – Uwe
    Dec 4, 2021 at 21:04
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    $\begingroup$ @CuteKItty_pleaseStopBArking "Greater than 3-sigma launch vehicle dispersions or other operations contingency scenarios could result in an eclipse from the Moon during the first 10.5 years. " - preprint linked in my answer. Wonder how many sigmas? $\endgroup$ Dec 5, 2021 at 5:00
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    $\begingroup$ @OrganicMarble this means the variation of solar input due to a Lunar "Eclipse" is less than the annual variation due to the ellipticity of the earth's orbit (6.25% vs 6.8%). One wonders why this level of variation could possibly be of concern. $\endgroup$ Dec 5, 2021 at 8:40
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    $\begingroup$ @VladimirF no, anything is not possible. Kg is the only base metric unit to use an SI prefix and it still doesn’t allow for prefixing the prefix. It’s not a kkg, it’s a Mg. $\endgroup$
    – Tim
    Dec 6, 2021 at 10:41

5 Answers 5


From the JWST Mission Operations Concept Document

Some families of Lissajous orbits can result in the spacecraft crossing the line connecting the two bodies. In the Sun-Earth orbit, this can result in crossing the Earth shadow, which must be avoided because shadow crossing could last longer than battery charge capacity.

While the JWST does have rechargeable lithium-ion batteries, those batteries are scoped to address two key operations concepts: Providing vehicle power prior to and during unfurling of the solar arrays, and providing power during brief operational periods where peak energy utilization exceeds the power output of the solar arrays.

The batteries are not scoped to provide power during extended eclipses. Losing both solar power and battery power could potentially represent an end of mission type of event. The easiest way to avoid this potentially catastrophic event is to never let it happen.

  • $\begingroup$ At the end of the same paragraph that you cited from the linked Mission Operations Concept document, it can be read "A L2 Lissajous orbit with a semi-major axis of 800,000 km has been selected". Do we have to conclude that JWST orbit is not necessarily a halo orbit? $\endgroup$
    – Ng Ph
    Dec 3, 2021 at 21:31
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    $\begingroup$ @NgPh Halo orbits are a subclass of Lissajous orbits. $\endgroup$ Dec 3, 2021 at 23:13
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    $\begingroup$ @NgPh In addition to the above, this document was from 2006. That would have given NASA over a decade to refine the JWST operations concepts. What that decade wouldn't have given NASA was the ability to allow eclipse crossings. Once that decision had been made to disallow eclipses, the sizing of the batteries was pretty much set in stone. $\endgroup$ Dec 3, 2021 at 23:46
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    $\begingroup$ true. Also li-ion batteries do not like sub-zero temperatures. So, the decision to disallow eclipse is a wise and well-justified, IMO. I think I have understood the general strategy in designing the JWST orbit: they did not start from an "ideal halo" template. This is because the achievable orbit is very dependent of the date and time of launch, as it is explained in a paper by Brown et. al. (2015). See comments after space.stackexchange.com/questions/56043/… $\endgroup$
    – Ng Ph
    Dec 4, 2021 at 9:20
  • $\begingroup$ The easiest way to avoid running out of power is to avoid eclipses? How about sizing the battery to handle these short duration events? Sure, it's an engineering trade off, but it's really weird to not address this option. $\endgroup$
    – Freddo411
    Dec 26, 2021 at 14:49

tl;dr: despite having to drain its batteries to run its electronics heaters, it might survive a short eclipse, but since the mission can easily designed to be eclipse free it's simply good practice to do so.

Why risk it?

For more on this see @DavidHammen's answer.

The spacecraft has no internal "Goldilocks zone" where the electronics would naturally be at a comfortable temperature

From this answer to How can electronics on board JWST survive the low operating temperature while it's difficult to survive lunar nights? we can see that one side of the heat shield is "too hot" and the other side is "too cold" and in order to provide an electronics box inside of which the temperature is "just right" there is a complicated heat management system that depends upon the spacecraft being in its normal thermal state.

From Status of the JWST Sunshield and Spacecraft found in @Antzi 's answer:

The Two Sides of the Telescope

Most of the electronics is on the "hot side" but there needs to be some conventional electronics on the cold side (beside the cooled IR sensor chips).

Small thermal environments on the cold side are equipped with heaters to provide mini-environments at normal operating temperature for these electronic devices.

[...]Thermostatically control heaters are used to maintain equipment above minimum required temperature while under cold conditions. Heater drive electronics (HDE) controlled heaters are used to maintain the +J3 panel, propulsion lines, battery, star tracker, and 1 Hz isolators within the required stability range.

The spacecraft component temperatures are maintained within the required limits by the use of radiators, heat pipes, MLI, and heaters. Thermostat and software controlled heaters are the two types used on this spacecraft. The software control heaters are used to maintain tight temperature control for critical spacecraft components and structures. The heaters are controlled by flight software with temperature feedback control. The flight software enables the ground to modify any TCS mission constants which include on/off heater set-points and failure thresholds.

The electrical heaters will generally run off of solar power.

Once you are in eclipse, the only way to keep the spacecraft alive, awake and communicating with earth is by draining the batteries to run heaters. And of course the faster you drain the batteries, the less time you can run the electronics on the same batteries.

The longer the eclipse, the bigger and heavier the batteries need to be to provide a margin of safety.

The more times you eclipse and cycle the batteries, tradeoffs between battery resiliency, power density and mass kick in.

It might survive a short eclipse, but since the mission can easily designed to be eclipse free it's simply good practice to do so.

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    $\begingroup$ While the spacecraft bus is on the hot side, the avionics equipment and battery are shielded to some extent from that extreme 85 °C environment. The spacecraft bus operates at about 300 K (about 30 °C). $\endgroup$ Dec 4, 2021 at 8:41
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    $\begingroup$ I'd counter that it's risky to design a spacecraft that can't handle being in eclipse more than a tiny amount of time -- vs. scaling the batteries up a bit handle short term periods in eclipse. It's an engineering tradeoff $\endgroup$
    – Freddo411
    Dec 26, 2021 at 14:52
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    $\begingroup$ Planets are big. Nothing in the Solar System is going to meaningfully change for thousands to millions of years. There's no "risk" involved here; if you design the satellite so that it doesn't go through an eclipse, then the highest-likelihood scenario it somehow does anyway involves us miraculously inventing planet-moving engines in twenty years. Meanwhile, the other side involves a heavier payload, more moving parts, and more stuff in orbit. $\endgroup$ Jul 13, 2022 at 23:55

"Power and thermal" but no further details.

enter image description here

Earth and Moon eclipses pose significant hazards to the power and thermal subsystems.


The published paper just says

Thermal and power requirements state that the JWST mission shall not experience any Earth or Moon eclipses during its entire mission lifetime

One might wonder why even the "significant hazards" comment was removed.

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    $\begingroup$ In the linked paper by Wayne Yu, can be read "SK maneuver" and "EA maneuver". I guess that "SK" is for station-keeping, but what is the meaning of "EA"? $\endgroup$
    – Ng Ph
    Dec 3, 2021 at 21:47
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    $\begingroup$ @NgPh a wild guess but "eclipse avoidance"? $\endgroup$ Dec 3, 2021 at 22:00

Besides what's already been mentioned -- eclipses have a risk associated with them.

Basically, if the spacecraft doesn't know that this is a planned eclipse, and the systems kick in to try to "find" the sun, this will put the spacecraft into a roll that it might not recover from. (because it's no longer getting solar power to operate).

This is how Yohkoh was lost. It was even sending back images at the time, so you can see it start to rotate.

So you have to disable those routines going into the eclipse ... but that means that if a gyroscope or whatever else it's using for station keeping fails, you risk losing the spacecraft.

This is also related to how STEREO-B was lost. It was intentionally put into a slow roll before it passed behind the sun, but it's believed that the laser gyroscopes (well past their design life) gave it bad information, and it went into a faster roll than expected, from which they couldn't recover it.

  • $\begingroup$ Yohkoh “survived” several previous eclipses in its 10 year operational life. The “fatal” eclipse was due to a combination of factors, including operational errors. en.wikipedia.org/wiki/Yohkoh . Stereo B’s spin was not precipitated by an eclipse. It is in a heliocentric orbit with earth conjunction not occurring until 2023 en.wikipedia.org/wiki/STEREO#Loss_of_contact_with_STEREO-B $\endgroup$
    – Woody
    Dec 29, 2021 at 19:10
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    $\begingroup$ @Woody: Yohkoh failed because someone forgot to tell it an eclipse was coming. STEREO was an example of bad things that happen when you go into a roll (which is what happens when you forget to tell it about an eclipse). And I should’ve mentioned— I worked for the Solar Data Analysis Center & STEREO Science Center from 2004 to 2017, which is why I’m familiar with solar observatories $\endgroup$
    – Joe
    Dec 29, 2021 at 22:42

From JWST Observatory Characteristics/JWST Orbit https://jwst-docs.stsci.edu/jwst-observatory-characteristics/jwst-orbit

Because JWST is solar powered, it cannot pass through the Earth's shadow during the mission. Orbits are selected that avoid shadow crossings, by selecting the launch time for a given launch day.

This diagram of the orbit indicates the only portion of the trajectory which risks an eclipse is the portion of the insertion which is radial to the Earth’s orbit. Once JWST is in halo orbit, the whole eclipse shadow thing is a non-issue. enter image description here

I suspect the "no eclipse permitted" is a vestigial statement, put in early documentation and perpetuated ever since. Since it is a complete non-issue once the telescope is in halo orbit, no one has ever edited or qualified the statement.

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    $\begingroup$ I can't understand why this is an answer to your "why" question. $\endgroup$
    – Ng Ph
    Dec 6, 2021 at 23:06
  • $\begingroup$ Recall that the requirements just give the maximum allowable Z and Y swings. Nothing can tell us (at least me!) that smaller orbits within the allowed "box" would not give some desirable features (freedom of launch window? stability?...). So "non-issue" is a bit far-reaching conclusion. "Not very constraining" would be a correct qualification for the no-eclipse-allowed requirement. Also, the displayed orbit is just one representative example, not the actual one nor the "targetted" one. $\endgroup$
    – Ng Ph
    Dec 7, 2021 at 20:22
  • $\begingroup$ And even if, for the sake of argumentation, it could be easy to understand that it is "not very constraining" (still not obvious for me, to be honest), the "why" in your question is not answered by such triviality, even if demonstrated. It can only be answered by the observation, as pointed out by @David Hammen, that some parts must remain warm. Apparently, this (tbs) thermal goal will not be met if the spacecraft remains in a penumbra for a given (tbd) duration. Such a demonstration would be a full answer to your question. $\endgroup$
    – Ng Ph
    Dec 7, 2021 at 22:41

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