Double-check that your composition went into the render queue and not just the solid.
Also double-check that you're looking through the active camera in your comp.
Otherwise we'll need a lot more info from you. Screenshots would be ideal.
Your workflow seems a bit flawed to me. Why not simply use the keyed layer as a track matte for your space footage?
this may help.
Before screening, I did use the keyed layer as a track matte for the space footage. The problem was that the space footage just filled the e screen instead of just tracking to the characters body.
Imy fairly new to AE, so I'm picking up little things there and here to get better and have better workflow and technique.
If you're new to AE, I'd highly recommend this resource: http://adobe.ly/AE_basics
There's a lot there, but once you pick one of the paths it suggests (I like the Andrew Devis tutorials) and go through it, working in AE will be much less frustrating.
I'll definitely check it out.
...and one of the first things to check is the term, "Guide Layer". I suspect you may have inadvertently turned a layer or two into one. Guide layers don't render.