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Identical to Pioneer 6, Pioneer 7 was put into heliocentric orbit at 1.01 × 1.125 AU with a period of 402.95 days to study magnetic fields of solar origin, measure various characteristics of the solar wind, and distinguish between solar and galactic cosmic rays.

On Aug. 17, 1966, Pioneer 7 flew through Earth’s magnetic tail region at the 3.5 million mile (5.6 million kilometer) range from Earth, and discovered long periods when the solar wind was completely or partially blocked out, suggesting that its instruments were monitoring the end of an organized tail region.

On Sept. 7, 1968, the spacecraft was correctly aligned with the Sun and Earth to begin studying Earth’s magnetic tail.

In 1977, 11 years after its launch, Pioneer 7 registered the magnetic tail 12 million miles (19.3 million kilometers) out, three times further into space than previously recorded.

At 23:36 UT on March 20, 1986, the spacecraft flew within 7.5 million miles (12.1 million kilometers) of Halley’s Comet -- the closest a U.S. spacecraft approached the comet -- and monitored the interaction between the cometary hydrogen tail and the solar wind.

Like Pioneer 6 and Pioneer 8, NASA maintained intermittent contact with Pioneer 7 in the 1990s, more than 30 years after its mission began (with data returned from its cosmic ray detector and plasma analyzer in Feb. 1991, for example).

On March 31, 1995, the plasma analyzer was turned on during 2 hours of contact with the ground, this being the final contact made with the spacecraft.

Source

Siddiqi, Asif A. Beyond Earth: A Chronicle of Deep Space Exploration, 1958-2016. NASA History Program Office, 2018.

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