Hmmmmmmmmmmmmmm. You're right, not much reason to put him on the surface.

The Ashtar can't pin down a precise location from purely psionic interaction; they need psytech to translate that information into real coordinates. So, they'd know that their guy is around [i]somewhere[/i], but can't pin it down enough to find him. Perhaps have the Observer or another ship hide out in the asteroid belts with him? Could lead to some submarine movie style cat and mouse. Bear in mind, their objective is to kill the prisoner so he can just reincarnate; it's much easier than a rescue op.

Although...if Sigma's alright with it, it could be fun to escalate the conflict a bit, have the Ashtar launch an assault on Aer. I'm game if you are (and if Sigma is).