The Explorer program is the longest running series of NASA missions. Explorer spacecraft have studied the Sun, Moon, Earth and the environment of our solar system. Several of them are still in operation. The first in the series - Explorer 1 - was America's first satellite, a response to Russia's Sputnik 1 launch in 1957. The program continues today with mission operations planned through 2009.

Three Explorers were sent to study the Moon. They were all successful, although Explorer 33 (pictured above) ended up in an Earth orbit and was used to study our home planet instead.

References
http://sse.jpl.nasa.gov/missions/profile.cfm?Sort=Target&Target=Moon&MCode=Explorer_Moon