Mariner-H, also commonly known as Mariner 8, was (along with Mariner 9) part of the Mariner Mars 71 project. It was intended to go into Mars orbit and return images and data. Due to a malfunction in the Centaur stage shortly after launch, the spacecraft never achieved Earth staging orbit.
The Mariner Mars 71 project consisted of two spacecraft (Mariners H and I), each of which would be inserted into a Martian orbit, and each of which would perform a separate but complementary mission. Either spacecraft could perform either of the two missions. The two spacecraft would have orbited the planet Mars a minimum of 90 days, during which time data would be gathered on the composition, density, pressure, and temperature of the atmosphere, and the composition, temperature, and topography of the surface. Approximately 70 percent of the planetary surface was to be covered, and temporal as well as spatial variations would be observed. some of the objectives of the Mariner-H mission were successfully added to the Mariner-I (Mariner 9) mission profile.