Mariner 5 was originally built to be the backup for Mariner 4 to Mars, but was never needed for that purpose. Thus, it was refurbished and modified to go to Venus instead. It flew by Venus at a distance of 3,990 kilometers (2,480 miles), and with its more sensitive instruments than aboard Mariner 2, revealed new information about Venus’ atmosphere, including its composition of 85-99% carbon dioxide. Mariner 5 also studied the interplanetary space in the vicinity of Venus and furthered the art and science of building and operating interplanetary spacecraft.
Radio occultation data from Mariner 5 helped to understand the temperature and pressure data returned by the Venera 4 lander, which arrived at Venus shortly before it. After these missions, it was clear that Venus had a very hot surface and an atmosphere even denser than expected.