Complex sentence: It was intended to go into Mars orbit and return images and data, but a launch vehicle failure prevented Mariner 8 from even achieving an Earth orbit and the spacecraft reentered into the Atlantic Ocean shortly after launch.
Split into simpler sentences: 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.