Explorer S-66 (also called BE-A, acronym of Beacon Explorer-A), was a NASA satellite launched on 19 March 1964 by means of a Thor-Delta B launch vehicle, but it could not reach orbit due to a vehicle launcher failure.

[1] Beacon Explorer-A was a small ionospheric research satellite instrumented with an electrostatic probe, a 20-, 40-, and 41-Hz ionospheric radio beacon, a passive laser tracking reflector, and a navigation experiment.

Its primary objective was to obtain worldwide observations of total electron content between the spacecraft and the Earth.

[1] During the third stage operation, a malfunction of unidentified origin prevented successful orbit.

Satellite and third stage descent were in the south Atlantic Ocean with no useful scientific data obtained.