Expert: Tom Whiting - 6/5/2006

Question
I know that seeing the stars from the Earth's surface depends a lot on background light, atmospheric conditions, etc. What would you be able to see of the stars from space? Say through the visor of a spacesuited astronaut floating out past Mars' orbit and looking away from the Sun? Would it be totally black, or filled with a million specks of light? Thank you.

Answer
Hi John,
 No, an astronaut out past Mars sees the same starry sky
as we do IF we can find a totally dark, non-light polluted sky here on the Earth.....human eyes are limited to objects brighter than about 6.5 magnitude (depending on your age) no matter
where you are in the Universe.
(There are only 2 sites left with zero light pollution east of the Mississippi now; Spruceknob, W. VA, and Cherry Springs State Park, PA....which we go to the latter with our large portable scopes, every new moon weekend for the total darkness!)
And never having been in space, I really can't say how much, if any, an astronaut's visor interferes....but probably not that much as long as there is no extraneous local light pollution or light glare, around him....and he has a clean,
clear visor.)

The reason we put telescopes in space is not for seeing
extra stars...as I said, the human eye is only capable of seeing down to about 6.5 magnitude objects no matter where you are.....
but for resolution....sharpness of extremely fine detail at high
magnifications.  That is what our atmosphere limits us from
doing here on the Earth's surface.
We live at the bottom of a great swimming pool...our
atmosphere, and even when the visibility (clarity) is great,
there is still the "seeing" component; if someone is
swimming up there (the jet-stream overhead)
it causes the atmosphere to waver, so the stars twinkle  violently, and all high resolution is lost.
(Higher powers are out of the question on such a night).
But you can have other nights where their is even a slight
haze but very steady "seeing"....calm air up on top....
and you can run your powers up quite high before image
breakdown occurs.  We astronomers like to have both
worlds on the same night, then you are in for a great night
of observing, but it does not happen very often that you
have both, excellent visibility and excellent steady seeing.

That's why the Hubble telescope is in orbit above the
atmosphere....it is guaranteed of perfect "seeing" all the
time because it does not have to view the night sky up
through "the swimming pool" as we do.

So yes, it would be a very black sky background for an astronaut in space, but that would not cause his pupil size (the limiting magnitude factor) to become any bigger than our eye pupils on the Earth's surface.
So he would see no more stars than we do at a zero-light
polluted observing site.

Hope all this helps,
Clear skies,
Tom Whiting
Erie, PA