Pioneer 9 was the fourth in a series of five probes designed to collect data on electromagnetic and plasma properties of interplanetary space from widely separated points in heliocentric orbit over at least six passages of solar activity centers.
In its 297.55-day orbit at 0.75 × 0.99 AU, the cylindrical, spin-stabilized spacecraft obtained valuable data on the properties of the solar wind, cosmic rays, and interplanetary magnetic fields.
The Delta launch vehicle also carried the Test and Training Satellite known as Tetr 2 (TETR-B) which was put into Earth orbit to test ground-based communications systems in support of the Apollo program.
By May 1969, NASA announced that the mission had already achieved all its objectives, having transmitted more than 6 billion bits of data. NASA maintained contact with Pioneer 9 until May 19, 1983.
Subsequent attempts to use Search for Extraterrestrial Intelligence (SETI) equipment to establish contact with the probe on March 3, 1987 failed, and the Agency officially declared the spacecraft inactive.
Siddiqi, Asif A. Beyond Earth: A Chronicle of Deep Space Exploration, 1958-2016. NASA History Program Office, 2018.