This is NASA's DC-8 "Airborne Laboratory". It is used in a wide range of atmospheric and space related research. On the other side of the plane on the lower side of the fuselage and aft of the wing are two Synthetic Aperture Radar (SAR) antennas resembling large flat printed circuit boards (microstrips actually).
The SAR in this DC-8 is very similar to the latest Shuttle Imaging Radar. Of course, flying at 35,000 ft instead of orbiting at 100+ miles allows for much better resolution in ground imaging. Recent tests were conducted in which buried land mines were detected with this DC-8's SAR.