Goals: Like its predecessor, Mariner 2, Mariner 5 was designed to conduct only scientific measurements of Venus and not attempt to photograph the planet. Mariner 5 was programmed to fly much closer to Venus to improve the chances of detecting a magnetic field and any interaction with the solar wind.
Accomplishments: The radiation team, led by James van Allen (who discovered Earth's radiation belts with Explorer 1) found no similar belts around Venus. The planet was determined not to have a magnetic field more than 1percent as strong as that of Earth. Observing how much Venus' gravity changed Mariner 5's trajectory put the planet's mass at 81.5 percent that of Earth. Seeing how Venus' atmosphere affected radio signals that were transmitted while the spacecraft was behind the planet enabled scientists to calculate temperature and pressure at the planet's surface of 527°C and 75 to 100 Earth atmospheres. These figures disagreed with readings from the Soviet Venera 4 mission and led Soviet scientists to conclude that their probe had not survived its descent to the surface as originally thought.