Mariner 8 was launched on an Atlas-Centaur SLV-3C booster (AC-24). 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. The Centaur and spacecraft payload separated and re-entered the Earth's atmosphere approximately 1500 km downrange and fell into the Atlantic Ocean about 560 km north of Puerto Rico. Read more
Mariner-H (Mariner Mars '71), 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. A launch vehicle failure prevented Mariner 8 from even achieving an Earth orbit and the spacecraft splashed into the Atlantic Ocean shortly after launch. Read more