Veteran concept artist and art director Peter Konig has done a lot of work for Valve over the past few years. His resume lists work on Dota 2, Portal 2 and Left 4 Dead 2. According to a photograph of a wall of concept art in Valve's offices that appeared earlier this year, he has also been drawing spaceships for them.
Several of those wall images match artwork in Peter Konig's online portfolio , sparking speculation on Facepunch that Valve have worked on, or are working on, something set in space. All hands on deck, man the rumour cannon.
The Facepunch post picked up by Kotaku references an unspecified inside source claiming that Valve are making an open, privateer-esque space adventure called Stars of Barathrum. There's no supporting evidence, though, so you could find a salt planet, mine it dry and still lack the salt with which to take that rumour. Still, it's easy to imagine some of the other space scenes featured on Konig's portfolio page being part of the same project, and they look gorgeous.
Here are some of the pieces that were spotted on the walls of Valve's offices, taken from Konig's image pages.
And here are a few more spacey shots from Konig's site.