Mission Type: Orbiter
Launch Vehicle: Thor-Delta E-1 (no. 55 / Thor no. 489 / DSV-3E)
Launch Site: Eastern Test Range / launch complex 17B, Cape Canaveral, USA
NASA Center: Ames Research Center, Jet Propulsion Laboratory
Spacecraft Mass: 65.36 kg
Spacecraft Instruments: 1) single-axis fluxgate magnetometer; 2) plasma analyzer; 3) cosmic-ray telescope; 4) radio-wave propagation experiment; 5) cosmic-ray gradient detector; 6) electric field detector; 7) cosmic dust detector; and 8) celestial mechanics experiment
Spacecraft Power: solar cells and batteries
References:
Deep Space Chronicle: A Chronology of Deep Space and Planetary Probes 1958-2000, Monographs in Aerospace History No. 24, by Asif A. Siddiqi

National Space Science Data Center, http://nssdc.gsfc.nasa.gov/

Pioneer 8, like its two predecessors, was sent to heliocentric orbit to study interplanetary
space, particularly to collect information on magnetic fields, plasma, and cosmic rays. Although the spacecraft carried a different complement of scientific instruments from those of Pioneers 6 and 7, its findings were correlated with those of the other two probes.

The spacecraft was launched into a path ahead of Earth to provide the vehicle with added velocity in solar orbit in order to move out beyond Earth's orbit at about 1.0 x 1.1. AU. It arrived at Earth's magnetospheric bounds at 19:00 UT on 15 December 1967.
Later, on 18 January 1968, Pioneer 8, the Sun, and Earth were perfectly aligned to allow investigation of Earth's magnetic tail in detail. (Pioneer 7 conducted similar experiments in September 1968.) Controllers have intermittently maintained contact with the spacecraft for nearly 30 years, although only one instrument, the electric-field detector, remained operational past 1982.

During tracking on 23 July 1995, NASA was unable to switch on Pioneer 8's transmitter, probably because the spacecraft was too far away from the Sun to charge the solar panels. On 22 August 1996, contact was re-acquired via a backup transmitter. The electric-field detector remains functional as of June 2001, nearly 36 years after launch.