The early Mariners were very small cylindrical probes, measuring only 3.04 m in length with a base diameter of 1.52 m.  Mariner 1:  Launched July 22, 1962.  Mariner 1 failed only minutes after launch due to problems with the antenna and then an error in the on-board computer guidance software.  Mariner 2:  Launched August 27, 1962.  Mariner 2 became the first successful planetary flyby mission.  It travelled 109 days in order to have only 35 minutes for instrument scans of Venus.  The probe passes 35,000 km from the planet; atmospheric studies showed little water and very large amounts of carbon dioxide.  Mariner 5:  Launched June 14, 1967.  Mariner 5 measured the surface temperature of Venus and set an upper limit on the magnetic field strength as it flew by.