Fifty-four years ago this week, NASA’s Pioneer 4 probe flew within 37,000 miles of the lunar surface. In doing so, the tiny spacecraft flew the first successful American lunar flyby mission.
The Pioneer Program involved a series of planetary space missions conducted by NASA between 1958 and 1978. The target of the early missions (1958-1960) was the Moon. The one exception was Pioneer 5 which investigated the interplanetary medium between Earth and Venus. Later Pioneer mission (1965-1978) were devoted to investigation of Jupiter and Saturn.
It was tough sledding in the early days of the Pioneer Program where the primary goal was to orbit the Moon. However, launch vehicle reliability was simply too poor and space trajectory control too crude to meet the lunar orbit goal in the lat 1950’s.
Indeed, none of the ten (10) Pioneer missions flown in the 1958-1960 period managed to achieve a lunar orbit of any kind. Interestingly, the United States would not orbit a spacecraft around the Moon until the Lunar Orbiter 1 mission in August of 1966.
While the lunar orbit goal proved too daunting for the early Pioneer Program, a lunar flyby mission appeared feasible using then-extant technology. The flyby mission simply required the spacecraft to sweep by the Moon (without impacting the surface) as the probe moved along an interplanetary trajectory towards the Sun. Ultimately, the spacecraft would find itself in solar orbit.
The Pioneer 4 spacecraft was a cone 20-inches in length and 9-inches in diameter. It weighed a mere 13.5 pounds. Probe spin-stabilization was effected by imparting an angular rate of 400 rpm about the vehicle’s longitudinal axis. Instrumentation was sparse; just a photoelectric sensor and a pair of radiation sensors.
On Tuesday, 03 March 1959, a Juno II launch vehicle carrying the Pioneer 4 spacecraft lifted-off from Cape Canaveral, Florida at 1711 UTC. The 4-stage Juno II was a modification of the Juno I launch vehicle that orbited America’s first satellite (Explorer 1) on Friday, 31 January 1958.
Pioneer 4 was successfully placed into an interplanetary trajectory that saw the probe pass within 37,000 miles of the lunar surface at 22:25 UTC on Wednesday, 04 March 1959. Considering that the Moon is about 238,000 miles from Earth, the flyby wasn’t all that close. However, at the dawn of the space age, it was indeed a significant accomplishment.
Pioneer 4 was powered by mercury batteries; that is, the spacecraft did not use solar cells. Sensor measurements were telemetered back to Earth at 960.05 MHz via a 0.1-Watt transmitter. Earth-based stations tracked the space probe out to a distance of roughly 407,000 miles from Earth.
At 01:00 UTC on Wednesday, 18 March 1959, Pioneer 4 reached the closest point in its eternal orbit about the Sun. Having done so, Pioneer 4 would forever hold the distinction of being the first American spacecraft to transit interplanetary space and reach heliocentric orbit.