How dark is space?
Could two spaceships travelling in the deep dark Universe see each other?
Space is dark. That much is obvious to anyone. Look up on a clear night in the middle of winter and all that appears to be surrounding the stars is inky blackness.
At the centre of our Solar System lies the Sun, which casts brilliant light and warmth onto the planets of the Solar System.
And, of course, there are all the other billions of stars whose light should surely be illuminating the night sky. Why hasn't their light reached us yet?
This is an age-old problem that astronomers refer to as Olbers' paradox, named after the German astronomer Heinrich Olbers (although many earlier astronomers like Johannes Kepler have advanced the notion).
No-one has sufficiently answered the problem of Olbers' paradox. It could be that stardust is obscuring star light from the human eye; perhaps the Universe is still too young for all the starlight to have reached us.
But let's ignore Olbers' paradox for a moment.
How dark is outer space beyond the Solar System? Would two spaceships flying together in the deep, dark Universe be able to see each other?
It's difficult to know for sure, but there are some inferences that can be made.
One astronomer attempted to answer this by collating the individual brightnesses of known stars and galaxies listed in catalogues, and in doing that the answer converges at a visual magnitude of –6.5, or 0.3% as bright as a full Moon.
Obviously, this average brightness would not stay the same as you journey through the Galaxy: near the galactic centre, far more stars are packed closely together.
Now, whether two space ships could see each other solely by this dim starlight would depend on the shininess of the spacecraft (mirrored or matt black) and whether they carried a telescope on board.
But think how difficult it is to see anything on a dark, moonless night on Earth.
Ocean-going ships passing within just a few miles would struggle to see each other if it weren’t for their navigational lights.
This article originally appeared in the March 2010 issue of BBC Sky at Night Magazine.