Stratospheric Observatory for Infrared Astronomy) telescope.
While it may seem unusual to have a telescope hanging out the side of a jet, it actually has its advantages. Flying at altitudes between 41,000 and 45,000 feet, the airplane is able to get above most of the water vapor in the atmosphere, meaning the telescope can soak up infrared radiation that is unavailable to ground-based telescopes. It's also capable of flying anywhere in the world to photograph phenomena, and since it comes down for a landing after each observation, the telescope can be easily modified and replaced as new technology becomes available (unlike space telescopes, which require expensive manned space missions for hardware replacement).
SOFIA's predecessor, the Kuiper Airborne Observatory, flew for 20 years and is credited with the discovery of Uranus' rings and Pluto's atmosphere. Let's hope SOFIA has an equally illustrious career.