Editing Pioneer program (section)

=== Interplanetary weather ===
The spacecraft in Pioneer missions [[Pioneer 6, 7, 8, and 9|6, 7, 8, and 9]] comprised a new interplanetary [[space weather]] network:
* Pioneer 6 (Pioneer A) – launched December 1965
* Pioneer 7 (Pioneer B) – launched August 1966
* Pioneer 8 (Pioneer C) – launched December 1967
* Pioneer 9 (Pioneer D) – launched November 1968 (''inactive since 1983'')
* Pioneer E – lost in launcher failure August 1969
Pioneer 6 and Pioneer 9 are in solar orbits with 0.8 [[Astronomical unit|AU]] distance to the Sun. Their orbital periods are therefore slightly shorter than Earth's. Pioneer 7 and Pioneer 8 are in solar orbits with 1.1 AU distance to the Sun. Their orbital periods are therefore slightly longer than Earth's. Since the probes' orbital periods differ from that of the Earth, from time to time, they face a side of the Sun that cannot be seen from Earth. The probes can sense parts of the Sun several days before the Sun's rotation reveals it to ground-based Earth orbiting observatories.