Sofia is the next major airborne observatory. It is a former Pan Am/United 747SP. I believe it is in storage right now at Marana... though it could have been moved.
My boss, was in charge of the old Kupier Airborne Observatory, a joint venture between NASA and the University of Arizona Lunar and Planetary Lab. They fitted a small, 2-meter (?), telescope with a spectrometer and all the related gadgetry into the prototype Lockheed C-141. This was all based from Moffet. Before they had the C-141 they used two CV-990s (one crashed) and several LearJets. The C-141 was decommissioned in 1990 I believe and nothing has been doing that sort of work since then.
Stop drop and roll will not save you in hell. --- seen on a church marque in rural Virginia