Launched on 27 April 1961 by a Juno II, the satellite returned data until 17 November 1961, when power supply problems ended the science mission.

In addition to detecting gamma rays, Explorer 11 was designed to map their direction with emphasis on the plane of the galaxy, the Galactic Center, the Sun, and other known radio noise sources; to relate the measurements to the cosmic-ray flux density and the density of interstellar matter; and to measure the high-energy gamma-ray albedo of the Earth's atmosphere.

Explorer 11 was constructed so that its stable motion was an end-over-end tumble about the transverse principal axis that had the largest moment of inertia.

The orientation of this axis in space was determined to approximately 5° by means of optical aspect detectors and the use of the known radiation pattern of the vehicle antenna.

Telemetry was provided only in real time by two PM transmitters, since the onboard tape recorder failed at launch.

The gamma-ray experiment was designed by researchers at the Massachusetts Institute of Technology (MIT) primarily to detect cosmic gamma rays greater than 50 MeV.

Also, the "last" and "total" components of the signals from the photomultiplier that viewed the sandwich detector afforded a method of distinguishing neutron and gamma-ray induced events.

[5] The Explorer 11 telescope, developed at Massachusetts Institute of Technology (MIT), used a combination of a sandwich scintillator detector along with a Cherenkov counter to measure the arrival directions and energies of high-energy gamma rays.

An improved gamma-ray telescope, also developed at MIT, was flown on the Orbiting Solar Observatory 3 (OSO 3), which was launched in 1967.