Mission Type: Flyby
Launch Vehicle: Juno II (no. AM-11)
Launch Site: Cape Caneveral, United States, launch complex 5
NASA Center: Jet Propulsion Laboratory
Spacecraft Mass: 5.87 kg
Spacecraft Instruments: 1) photoelectric sensor trigger and 2) two Geiger-Mueller counters
References:
Deep Space Chronicle: A Chronology of Deep Space and Planetary Probes 1958-2000, Monographs in Aerospace History No. 24, by Asif A. Siddiqi

National Space Science Data Center, http://nssdc.gsfc.nasa.gov/

This mission was the first of two U.S.Army launches to the Moon.

Pioneer 3 was a spin-stabilized probe (up to 700 rpm) whose primary goal was to fly by the Moon. Two special 0.21-ounce weights were to spin out on 1.5- meter wires and reduce spin to 12 rpm once the mission was under way.

The spacecraft carried an optical sensor to test a future imaging system. If the sensor received, from a source such as the Moon, a collimated beam of light that was wide enough to pass through a lens and fall simultaneously on two photocells, then the sensor would send a signal to switch on an imaging system (not carried on this spacecraft).

In the event, the main booster engine shut down 4 seconds earlier than planned due to propellant depletion. Once put on its trajectory, Pioneer 3 was about 1,030 kilometers per hour short of escape velocity. It eventually reached 102,322 kilometers and burned up over Africa 38 hours 6 minutes after launch.

The spacecraft contributed to the major scientific discovery of dual bands of radiation around Earth.