Pioneer 6 was the first of several NASA spacecraft designed for launch at six-month intervals to study interplanetary phenomena in space in heliocentric orbits similar to that of Earth. These spacecraft successfully provided simultaneous scientific measurements at widely dispersed locations in heliocentric orbit.
The so-called Improved Thrust Augmented Delta launch vehicle’s third stage burned for 23 seconds to boost Pioneer 6 into heliocentric orbit. Initial solar orbit for the spacecraft ranged from 0.814 AU (perihelion) to 0.985 AU (aphelion) with a period of 311.3 days.
By March 2, 1966, Pioneer 6 had transmitted about 250 million readings from its six scientific instruments. In the fall of 1969, JPL and UCLA scientists reported the results of a solar occultation performed from Nov. 21 to 24, 1968. This was the first time that a spacecraft had been tracked while passing behind the Sun, allowing scientists, despite an unfavorable signal-to-noise ratio of data transmission, to examine the solar corona during this passage.
Scientists used instruments on Pioneer 6, in coordination with those on Pioneer 7, in November and December 1969 to measure solar wind particles and to carry out long-distance communications experiments.
Five years after launch, at the end of 1970, Pioneer 6 had orbited the Sun six times, and had passed by the Sun’s far side (relative to Earth), sending back new information on the solar atmosphere and regular solar weather reports. One of the two radio receivers was still operational, and although some of the solar cells had been damaged by solar flares, the spacecraft was still getting sufficient power to operate satisfactorily.
Pioneer 6 returned the first data on the tenuous solar atmosphere and later recorded the passage of Comet Kohoutek’s tail in 1974 (in conjunction with Pioneer 8) from a range of about 62.1 million miles (100 million kilometers) from the comet’s nucleus.
Along with Pioneers 7, 8, and 9, the spacecraft formed a ring of solar weather stations spaced along Earth’s orbit. Measurements by the four Pioneers were used to predict solar storms for approximately 1,000 primary users including the Federal Aviation Agency, commercial airlines, power companies, communication companies, military organizations, and entities involved in surveying, navigation, and electronic prospecting.
By December 1990, Pioneer 6 had circled the Sun 29 times (travelling 15.4 billion miles or 24.8 billion kilometers) and had been operational for 20 straight years -- a record for a deep space probe. Its original slated lifetime had been only six months, achieved on June 16, 1966.
Of the spacecraft’s six scientific instruments, two (the plasma Faraday cup and the cosmic ray detector) functioned well into the 1990s.
NASA maintained contact with the spacecraft once or twice each year during the 1990s. For example, one hour’s worth of scientific data was collected on July 29 and Dec. 15, 1995, although the primary transmitter failed the following year.
Soon after, on March 31, 1997, NASA officially declared the mission complete largely due to the costs associated with continuing communications sessions. Despite the decision, contact was established with the backup transmitter on Oct. 6, 1997 as part of a training exercise for the Lunar Prospector spacecraft. By this point, the probe’s solar arrays had deteriorated, although the transmitter could still be turned on at perihelion when the solar flux was strong enough to provide sufficient power.
On Dec. 8, 2000, ground controllers established successful contact for 2 hours to commemorate the 35th year of operation. This, however, proved to be the very last contact made with the probe.
Siddiqi, Asif A. Beyond Earth: A Chronicle of Deep Space Exploration, 1958-2016. NASA History Program Office, 2018.