<p>The mindboggling GAIA spacecraft-telscope <em>sort of</em> does what you suggest! That's kind of the closest concept to what you describe.</p>

<p>Just as explained in JamesS answer, if you were that far away you wouldn't be in orbit around the Earth.  But, GAIA's L2-like orbit is kind of the closest thing to what you say.</p>

<p><a href="https://i.sstatic.net/e5zUn.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/e5zUn.png" alt="enter image description here"></a></p>

<p>Explanation: <a href="https://en.wikipedia.org/wiki/Gaia_(spacecraft)#Launch_and_orbit" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/Gaia_(spacecraft)#Launch_and_orbit</a></p>

<p>Note that <strong><em>just as you say</em></strong>, light interference is a huge problem for instruments this insanely sensitive.</p>

<p>GAIA has a problem where, apparently some tiny little bits of the sun-shade (like small hairs) were poking around, and that is enough to cause stray light; also some ice particles seem to be causing stray light.  <a href="http://blogs.esa.int/gaia/2014/04/24/commissioning-update/" rel="nofollow noreferrer">http://blogs.esa.int/gaia/2014/04/24/commissioning-update/</a></p>

CC BY-SA 3.0
2016-07-01T13:37:27.340Z
2016-07-03T14:39:04.383Z
2016-07-03T14:39:04.383Z
Fattie
<p>As Joe Blow explains, there is an orbit, at the L2 point, 1.5 million km directly further away from the Sun/Earth, where a satellite could remain permanently in the Earth's shadow, though it would not be completely shadowed - the Sun would still appear as a narrow ring around the blocking Earth</p>

<p>This would be a great advantage in terms of cooling the spacecraft and in terms of avoiding scattered light entering the instrument. It would allow a greater fraction of the sky to be observed at any one time, though there would still be a zone of avoidance close to the Earth/Sun.</p>

<p>However, all the satellites (WMAP, Planck, Herschel, Gaia) that have exploited L2 have been specifically placed in "Lissajous orbits" that orbit around the L2 point and are <em>never</em> in the Earth's shadow. The reason is that you need the sunlight for power.</p>

<p>Neverthless, L2 still offers a very stable thermal environment and it is still the case that a much larger proportion of the sky is viewable than from a lower Earth orbit.</p>

CC BY-SA 3.0
2016-07-04T11:51:00.407Z
2016-07-04T13:53:37.237Z
2016-07-04T13:53:37.237Z
ProfRob
<p>Light pollution isn't as much of a problem in space as it is on Earth because you're in a vacuum.  Imagine making a small black disk and holding it up at arms-length to cover the Sun....it's not going to simulate the effects of an eclipse, you're not going to get to see the corona and background stars because the rest of the sky is still flooded with light - or, more accurately, the Earth's <em>atmosphere</em> is flooded with light as it scatters off water vapour, dust and the like.</p>

<p>On the other hand, you <em>can</em> cover the Sun up in space and see the corona etc in exactly that way, the STEREO solar observatories do it. As long as you're careful about blocking <strong>direct</strong> sunlight from hitting your lens, and make sure there's no reflective bits of spaceship in the frame, you're not going to get light pollution in the same way you would on Earth.</p>

CC BY-SA 3.0
2016-07-04T14:06:24.563Z
2016-07-04T14:06:24.563Z
The Geoff
<p>Its a nice idea, but it wouldn't work.</p>

<p>To stay in the Earth's shadow a telescope would need to orbit the Earth in the same time the earth orbited the Sun so that it always stayed on the opposite side. For its orbit to be this long it would need to be a long way out, remember geosynchronous orbit (one orbit in 23h 56min 4sec) is at a distance of 42,164 km. For a 1 year orbit the telescope would be so far away that its would not be in orbit around the Earth but the Sun, as it would have a much larger gravitational influence. It would also be too far away to be in the Earth's shadow cone.</p>

CC BY-SA 3.0
2016-07-04T12:25:32.753Z
L2 is about 1.5 million km from Earth, the Earth's shadow cone is less then 1.4 million km long. Therefore an object at L2 cannot be in the Earth's shadow cone.
James Screech
CC BY-SA 3.0
2016-07-04T13:52:00.370Z
I agree. A spacecraft can only be in a penumbral shadow at L2.
ProfRob
CC BY-SA 3.0
2016-07-01T07:25:35.423Z
2016-07-01T07:25:35.423Z
James Screech
<p>So I am thinking about orbiting telescopes tonight and I was thinking that it would probably be beneficial to keep them in the shadow of the Earth or some other space body to avoid light interference.</p>

<p>Specifically, I am thinking about looking off in all directions around Earth. I would imagine staying in a shadow would help the most. But that at the very least there would be a range of degrees around the direction of the sun that you could not observe until the Earth changed orbital position so that the Sun's light was no longer interfering when looking in a specific astronomical direction.</p>

<p>Am I correct in any of these assumptions? I believe there are telescopes that don't sit in the shadow of a body, but I do think that would greatly limit the range of area it could look at during a given timeframe.</p>

CC BY-SA 3.0
2016-07-01T09:07:55.487Z
Not a direct answer, but: the James Webb scope will orbit "outside" Earth's orbit (at the L2 point) and achieve cooling by a sunshade and by being shielded from Earth itself. It won't actually be in Earth's shadow, but being kept out of sight of the Sun and Earth helps. [This page](http://jwst.nasa.gov/orbit.html) gives an idea of how cool it can operate in this way (minus 225 Celsius).
Andy
CC BY-SA 3.0
2016-07-01T14:19:57.883Z
Even in Earth's shadow, you will have Zodiacal light to contend with: https://en.wikipedia.org/wiki/Zodiacal_light
Wayfaring Stranger
<orbit><telescope><photography><space-telescope>
Would space-based telescopes need to remain in the shadow of Earth to avoid light interference from the sun?
Patrick
# Would space-based telescopes need to remain in the shadow of Earth to avoid light interference from the sun?

(Asked by: username_0 on 2016-07-01)

So I am thinking about orbiting telescopes tonight and I was thinking that it would probably be beneficial to keep them in the shadow of the Earth or some other space body to avoid light interference.

Specifically, I am thinking about looking off in all directions around Earth. I would imagine staying in a shadow would help the most. But that at the very least there would be a range of degrees around the direction of the sun that you could not observe until the Earth changed orbital position so that the Sun's light was no longer interfering when looking in a specific astronomical direction.

Am I correct in any of these assumptions? I believe there are telescopes that don't sit in the shadow of a body, but I do think that would greatly limit the range of area it could look at during a given timeframe.

**Tags:** orbit, telescope, photography, space-telescope
> **Comments:**
> **username_1**: Not a direct answer, but: the James Webb scope will orbit "outside" Earth's orbit (at the L2 point) and achieve cooling by a sunshade and by being shielded from Earth itself. It won't actually be in Earth's shadow, but being kept out of sight of the Sun and Earth helps. [This page](http://jwst.nasa.gov/orbit.html) gives an idea of how cool it can operate in this way (minus 225 Celsius).  
> **username_2**: Even in Earth's shadow, you will have Zodiacal light to contend with: https://en.wikipedia.org/wiki/Zodiacal_light  

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## 4 answers

### Answer by username_6 on 2016-07-01. Score: 4 | Accepted answer

Its a nice idea, but it wouldn't work.

To stay in the Earth's shadow a telescope would need to orbit the Earth in the same time the earth orbited the Sun so that it always stayed on the opposite side. For its orbit to be this long it would need to be a long way out, remember geosynchronous orbit (one orbit in 23h 56min 4sec) is at a distance of 42,164 km. For a 1 year orbit the telescope would be so far away that its would not be in orbit around the Earth but the Sun, as it would have a much larger gravitational influence. It would also be too far away to be in the Earth's shadow cone.

> **Comments:**
> **username_6**: L2 is about 1.5 million km from Earth, the Earth's shadow cone is less then 1.4 million km long. Therefore an object at L2 cannot be in the Earth's shadow cone.  
> **username_4**: I agree. A spacecraft can only be in a penumbral shadow at L2.  

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### Answer by username_3 on 2016-07-01. Score: 4

The mindboggling GAIA spacecraft-telscope *sort of* does what you suggest! That's kind of the closest concept to what you describe.

Just as explained in JamesS answer, if you were that far away you wouldn't be in orbit around the Earth. But, GAIA's L2-like orbit is kind of the closest thing to what you say.

[![enter image description here](https://i.sstatic.net/e5zUn.png)](https://i.sstatic.net/e5zUn.png)

Explanation: <https://en.wikipedia.org/wiki/Gaia_(spacecraft)#Launch_and_orbit>

Note that ***just as you say***, light interference is a huge problem for instruments this insanely sensitive.

GAIA has a problem where, apparently some tiny little bits of the sun-shade (like small hairs) were poking around, and that is enough to cause stray light; also some ice particles seem to be causing stray light. <http://blogs.esa.int/gaia/2014/04/24/commissioning-update/>


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### Answer by username_4 on 2016-07-04. Score: 1

As Joe Blow explains, there is an orbit, at the L2 point, 1.5 million km directly further away from the Sun/Earth, where a satellite could remain permanently in the Earth's shadow, though it would not be completely shadowed - the Sun would still appear as a narrow ring around the blocking Earth

This would be a great advantage in terms of cooling the spacecraft and in terms of avoiding scattered light entering the instrument. It would allow a greater fraction of the sky to be observed at any one time, though there would still be a zone of avoidance close to the Earth/Sun.

However, all the satellites (WMAP, Planck, Herschel, Gaia) that have exploited L2 have been specifically placed in "Lissajous orbits" that orbit around the L2 point and are *never* in the Earth's shadow. The reason is that you need the sunlight for power.

Neverthless, L2 still offers a very stable thermal environment and it is still the case that a much larger proportion of the sky is viewable than from a lower Earth orbit.


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### Answer by username_5 on 2016-07-04. Score: 1

Light pollution isn't as much of a problem in space as it is on Earth because you're in a vacuum. Imagine making a small black disk and holding it up at arms-length to cover the Sun....it's not going to simulate the effects of an eclipse, you're not going to get to see the corona and background stars because the rest of the sky is still flooded with light - or, more accurately, the Earth's *atmosphere* is flooded with light as it scatters off water vapour, dust and the like.

On the other hand, you *can* cover the Sun up in space and see the corona etc in exactly that way, the STEREO solar observatories do it. As long as you're careful about blocking **direct** sunlight from hitting your lens, and make sure there's no reflective bits of spaceship in the frame, you're not going to get light pollution in the same way you would on Earth.


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