Relative to each other.
Any measurement value re: the speed of something, is relative to the speed of the measurement frame.
Michalson and Morley proved that there were no interference patterns for beams going with each other, against each other or at 90 degrees to each other.
Earth orbit was chosen for the experiment because it was pretty darn handy, and everything else wasn't.
Once again, I'm just a cook, exploring the mysteries of physics.
/johnny