Originally built as a backup to the Mariner 4 spacecraft, which successfully journeyed to Mars in 1965, Mariner 5 was modified to fly by Venus and collect data on the planet's atmosphere, radiation and magnetic field.

The spacecraft's flight path following its Venus encounter brought it closer to the sun than any previous mission.

Scientific Instrument(s)

- Two-frequency beacon receiver
- Interplanetary ion plasma probe
- Trapped radiation detector
- Helium magnetometer
- Ultraviolet photometer