# Would space-based telescopes need to remain in the shadow of Earth to avoid light interference from the sun?

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.

• 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 gives an idea of how cool it can operate in this way (minus 225 Celsius).
– Andy
Jul 1, 2016 at 9:07
• Even in Earth's shadow, you will have Zodiacal light to contend with: en.wikipedia.org/wiki/Zodiacal_light Jul 1, 2016 at 14:19

## 4 Answers

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.

• 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. Jul 4, 2016 at 12:25
• I agree. A spacecraft can only be in a penumbral shadow at L2. Jul 4, 2016 at 13:52

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.

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/

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.

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.