Imagine you have a space telescope that had temperature sensitive instruments. (Like the Spitzer Space Telescope).

In order to maintain a cool temperature - you want it always to orbit in the shadow of the Earth. (Ie a permanent Solar eclipse due to the Earth blocking it).

If every satellite needs to orbit at such a high orbit the Earth will always be too small to have an eclipse - I'll take that as an answer.

In addition, if you're that close to the earth that this causes the opposite temperature effect to what is desired, that's a valid answer too.

My question is: Can a Space Telescope Orbit so it was always in the shadow of the Earth?

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    $\begingroup$ You wouldn't want to operate constantly in the earth eclipse - your satellite will almost certainly rely on solar panels for power. $\endgroup$ Jul 19, 2019 at 16:30
  • $\begingroup$ You would need another source for electric power as the usual solar panels. Using a nuclear powered RTG with a lot of excess heat does not make sense if you want to maintain a cool temperature by an orbit in the shadow of the Earth. Using primary cells to be discharged one time only would limit operation to a very short time of some weeks or months but not years. $\endgroup$
    – Uwe
    Jul 20, 2019 at 13:54

1 Answer 1



The orbit you are talking about is one of the Lagrangian Points, the L2 point to be more precise.

As a matter of fact, a few satellites are there - and, most relevant to this question, the James Webb Telescope will be placed there.

Note that the L2 point is a point of gravitational equilibrium, but isn't actually a stable point. That means that unlike the L4/L5 points, you need a bit of thrust every now and then to stay there.

Edit: You can read more about the L2 point here

Second Edit:

Actually, I am partially wrong. Yes, the orbit will always be in the shadow of the earth, but it's so far away from earth that the sun won't be fully eclipsed.

So if you want to be completely shielded from the sun, the answer is no!

Still, the L2 is a good position for a Telescope.

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    $\begingroup$ you can add a link to this answer if you like. Also, most spacecraft "at L2" or L1 are really in big halo orbits that never closer than a few 100,000 km from the Lagrangian points themselves. See for example this answer. $\endgroup$
    – uhoh
    Jul 19, 2019 at 13:54
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    $\begingroup$ Those satellites are rarely, if ever, in the Earth's shadow. The Webb is a good example. Its pseudo orbit about the Sun-Earth L2 point is specifically designed to keep it out of the Earth's shadow. $\endgroup$ Jul 19, 2019 at 15:50
  • $\begingroup$ @davidhammen it kind of needs the sun right? Solar panels need love hah... it has solar power not rtg right? Those big "sails" make its own shadow so the telescope just needs to point those towards the sun. $\endgroup$ Jul 19, 2019 at 21:42
  • $\begingroup$ @Magic Octopus Urn That's not a super-strict requirement, could also be powered by a nuclear reactor. $\endgroup$
    – Infrisios
    Jul 22, 2019 at 6:13
  • $\begingroup$ @infrisios I was talking specifically about JWST, which does use solar, I just checked and probably should've before. $\endgroup$ Jul 22, 2019 at 13:51

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