A recent article about the telescope states (emphasis mine):

Unlike Hubble, which orbits the Earth at about 340 miles of altitude, Webb will be sent almost a million miles into space, at a specific location called "L2." It's one of five so-called Lagrange points, specific areas of stability where the gravity from the Earth and the Sun balances out in such a way that putting an object there keeps it in a fixed position relative to the two celestial bodies. The telescope will therefore hitch a ride through space without the need for engines or propulsion, while enjoying an unobstructed view.

The same article also notes:

To protect itself from the warmth of the Sun, the mirror will sit on a 70-ft sunshield -- as long as a tennis court -- made of a special heat-resistant material. It looks like a giant kite and it will keep the mirror at a gelid -370°F, or -223°C, nearly three times colder than the coldest temperature ever recorded on Earth.

From which I gather that the telescope is designed to sit at the L2 Lagrange point while using minimal (if any) propulsion to maintain its position and orientation. However the second paragraph makes it sound like the telescope will sport what essentially sounds like a (quite small) solar sail.

So the question is, if the 'sunshield' is always positioned between the telescope and the sun and therefore always being irradiated by the sun, won't that over time generate enough thrust the push the telescope out of position or alignment, particularly if it's a truly passive system and not able to counteract with its own engines/maneuvering thrusters?

  • 9
    $\begingroup$ It looks like CNN's style writers need to do a better job of consulting CNN's science writers (if they have any). That article, while very stylish (the article is in CNN Style, not CNN Space) has some glaring errors. You've picked out two, hitching a ride without the need for engines or propulsion, and "three times colder than the coldest temperature ever recorded on Earth." $\endgroup$ Jun 12, 2018 at 7:05
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    $\begingroup$ Well I did try the Fox News version. They ignored the story completely and were instead blaming the FBI for the telescope's launch delay, saying they used it to illegally spy on the upper floors of Trump Tower during the 2016 election. :) $\endgroup$
    – aroth
    Jun 12, 2018 at 7:23
  • $\begingroup$ @aroth please tell me that about Fox is a joke... it is, right? It's sad how ridiculous you can go without making that obvious. $\endgroup$ Jun 12, 2018 at 10:04
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    $\begingroup$ @leftaroundabout Yes, it is. Hence the :). One must always be careful to mark absurd claims about Fox News as jests, because you never know when one might turn out to be true. $\endgroup$
    – aroth
    Jun 12, 2018 at 11:05
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    $\begingroup$ @DavidHammen I'd say three - calling a Lagrange point, generally, an "area of stability" is not really correct, especially in the case of L2. $\endgroup$
    – J...
    Jun 12, 2018 at 11:56

1 Answer 1


No it will not

According to this answer, James Webb will require 150 m/s of $\Delta v$ to maintain its orbit for its mission duration (5 year), an overall very small amount See also this question as to why L2 isn't perfectly stable.

It looks like a giant kite

  • This is purely a cosmetic remark, and the heat shield is absolutely not designed to produce any thrust
  • Most $\Delta v$ will be spent counteracting gravitational effects and not light pressure (although we can expect a paper quantifying the solar pressure on James Webb :)).

Doing the maths


  • The solar pressure at L2 is $4.533\times10^{-6}\text{N}/\text{m}^2$

  • James Webb is about 6000kg

  • The shade is $300 \text{m}^2$.

  • The pressure exerted on JW is $300 \times 4.533\times10^{-6}\text{N}$ <=> $0.001\text{N}$

  • Using a calculator it gives 5m/s per year of acceleration.
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    $\begingroup$ The spacecraft will not be partially shadowed by the Earth (it orbits hundreds of thousands of kilometers about L2), and the heat shield will produce the equivalent of thrust. Solar radiation pressure is small but accumulative, and is a major concern for spacecraft in geosynchronous orbits -- and for spacecraft in pseudo-orbits about one of the unstable Lagrange points. $\endgroup$ Jun 12, 2018 at 7:14
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    $\begingroup$ "the heat shield is absolutely not designed to produce any thrust" I get that this is true, however not being designed to produce thrust doesn't (necessarily) mean that it won't. And the rest of the answer seems to imply that it will produce thrust (albeit only a tiny amount)? $\endgroup$
    – aroth
    Jun 12, 2018 at 7:19
  • $\begingroup$ @DavidHammen earth is in the middle. It's not a full eclipse (else the solar panels wouldn't work, but the light is definitely dimmed by earth. Maybe I should use a better technical term ? Shadow != dark so I think it's ok. I'll edit informations about the solar pressure. $\endgroup$
    – Antzi
    Jun 12, 2018 at 8:11
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    $\begingroup$ Also, according to my answer there, which cites Wikipedia, the JWST will likely consume only 2 to 4 m/s of delta-v per year for station keeping. This sounds quite low, but it's because of the aggressive 3-week updates to the orbit. 150 m/s is the total budget and includes a mid-course correction on the way to L2. $\endgroup$
    – uhoh
    Jun 12, 2018 at 8:35
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    $\begingroup$ @Antzi - That would be the case if the Webb was at the Sun-Earth L2 point. From 1.5 million km, the Earth appears to be slightly smaller than does the Sun. The Earth would eclipse a large portion of the Sun at the Sun-Earth L2 point. However, the Webb will never be at the Sun-Earth L2 point. It instead will be in a rather wide pseudo orbit about that point, with the pseudo orbit specifically designed so that neither the Earth nor the Moon ever eclipses even a small part of the Sun throughout the ten year projected lifetime of the Webb. $\endgroup$ Jun 12, 2018 at 8:58

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