The main Centaur engine was ignited 265 seconds after launch, but the upper stage began to oscillate in pitch and tumbled out of control.

The Centaur stage shut down 365 seconds after launch due to starvation caused by the tumbling.

Investigation proceeded quickly and the problem was soon discovered to be the result of a malfunction in the pitch rate gyro amplifier.

A diode intended to protect the system from transient voltages was thought to have been damaged during repairs/installation of the pitch amplifier's printed circuit board, something that would not have been detected through bench tests.

As of 2024[update], Mariner 8 is the most recent US planetary probe to be lost in a launch vehicle malfunction.

Approximately 70 percent of the planetary surface was to be covered, and temporal as well as spatial variations would be observed.

Two sets of 6 attitude control nitrogen jets were mounted on the ends of the solar panels.

Attitude knowledge was provided by a Sun sensor, a Canopus star tracker, gyroscopes, an inertial reference unit, and an accelerometer.