Stratospheric Observatory For Infrared Astronomy

In 2010 a joint program led by NASA and the German Aerospace Center (DLR) began observing the sky with a 2.5-meter infrared telescope mounted in the aft fuselage of a modified Boeing 747SP. The program is known as the Stratospheric Observatory For Infrared Astronomy or SOFIA (no relation to this blog, though I once worked with a guy who spent a summer internship working on the project), and you can read more about it here:
SOFIA

Infrared astronomy is difficult to conduct on Earth since the planet's ambient warmth and water vapor in the atmosphere tend to drown out much of the most interesting parts of the infrared band of the electromagnetic spectrum. One solution is to use space telescopes, such as the current Spitzer and the planned (and epically over-budget) James Webb Space Telescope. Observing from space eliminates the obscuring effects of Earth's temperature and atmosphere, but requires expensive design, assembly, and launch operations, and limits the practicality of upgrading the instrument once it's in service. Ground-based telescopes are much cheaper to build and get to, but lack the capability of their spaceborne counterparts. SOFIA sought to split the difference between these approaches by lifting the telescope to 45,000 feet, above most of the atmosphere, but retaining the ability to inspect and service the telescope each morning after  SOFIA returns to her base at the Dryden Flight Research Center in the California desert.

The modifications carried out on the mothership to allow astronomy from the back of a 747 were not trivial, and this really is a remarkable, if relatively obscure program that NASA and the DLR are operating. Cutting a large section out of the fuselage does nasty aerodynamic things to the aircraft, inducing vibration that could potentially destroy the quality of any data obtained from the telescope. Evidently the engineers found a way around this, and they deserve great kudos for it.

The 747SP is a shortened, vaguely cartoonish-looking version of Boeing Commercial Airplanes' 1960s-vintage flagship. Originally intended to compete with the trijets offered by Lockheed and McDonnell Douglas, the 747's fuel burn was too high to be truly competitive with these smaller twin-aisle liners, and the 45 built proved more popular as airborne transonic luxury yachts for wealthy oil barons than long-haul money makers for the airlines. Doing science that once could only be done in space on the (relative) cheap goes a long way toward redeeming the airplane in my book.