The January, 2014 issue of Shindig! magazine is a special edition focusing on the Space rock genre of music. The issue contains an article by Marco Rossi where he explores the notion of whether prog-rock pioneers Rush and Yes can be classified as space rock bands. John over at Cygnus-X1.net has transcribed the article and made it available online here. Rossi argues that despite Rush's many space-themed songs and lyrics, they probably shouldn't be considered as space rock:

... the point I'm laboriously trying to get to is that space-referencing lyrics don't automatically make something into spacerock... even if I've yet to be convinced that Rush and Yes comfortably fit into the spacerock pigeonhole - or indeed black hole - it doesn't diminish what either band achieved. As frontline prog progenitors, what they did have in common with spacerockers was the restless, implacable urge to forge ahead, to explore, to discover. ...