 pioneer 3 was a spin stabilized spacecraft launched at 05:45:12 utc on 6 december 1958 by the u.s. army ballistic missile agency in conjunction with the national aeronautics and space administration, using a juno ii rocket. this spacecraft was intended as a lunar probe, but failed to go past the moon and into a heliocentric orbit as planned, but did reach an altitude of 102,360&nbsp;km before falling back to the earth. the revised spacecraft objectives were to measure radiation in the outer van allen radiation belt using two geiger-mÃ¼ller tubes and to test the trigger mechanism for a lunar photographic experiment. == spacecraft design == pioneer 3 was a cone-shaped probe 58&nbsp;cm high and 25&nbsp;cm diameter at its base. the cone was composed of a thin fiberglass shell coated with a gold wash to make it electrically conducting and painted with black and white stripes to maintain the temperature between 10 and 50 degrees celsius. at the tip of the cone was a small probe which combined with the cone itself to act as an antenna. at the base of the cone a ring of mercury batteries provided power. a photoelectric sensor protruded from the center of the ring. the sensor was designed with two photocells which would be triggered by the light of the moon when the probe was within about 30,000&nbsp;km of the moon. at the center of the cone was a voltage supply tube and two geiger-mÃ¼ller tubes. a transmitter with a mass of 0.5&nbsp;kg delivered a phase-modulated signal of 0.1 w at a frequency of 960.05&nbsp;mhz. the modulated carrier power was 0.08 w and the total effective radiated power 0.18 w. a despin mechanism consisted of two 7&nbsp;gram weights which could be spooled out to the end of two 150&nbsp;cm wires when triggered by a hydraulic timer 10 hours after launch. the weights would slow the spacecraft spin from 400 rpm to 6 rpm and then weights and wires would be released. == mission == [[image:pioneer-3-4.gif|thumb|right|pio
Lunar_flybys Missions_to_the_Moon Pioneer_program Spacecraft_launched_in_1958 Spacecraft_which_reentered_in_1958