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55 Years Ago: Mariner 2 First to Venus

In 1962, Mariner 2 completed the first successful close-up observations of another planet when it flew by Venus.  The spacecraft made a number of important scientific discoveries not only about the planet but also about interplanetary space during its transit.  The road to Venus, however, was anything but easy.  Relatively little was known about Venus at the time; because it was roughly the same size as the Earth, scientists often called the two sister planets.  Since it orbited closer to the Sun, most scientists assumed Venus was the warmer of the two, and some believed its climate was similar to Earth’s tropics.  A young scientist named Carl Sagan, however, proposed that the known high concentration of carbon dioxide in Venus’ atmosphere created a runaway greenhouse effect, leading to extremely high temperatures at the surface.

Launch of Mariner 2
Launch of Mariner 2.

The Jet Propulsion Laboratory (JPL) in Pasadena, California, planned, developed and executed the first mission to Venus in about one year.  To save development time, JPL adapted the Ranger lunar probe that it was working on to build the Venus explorer.  The 447-ppound spacecraft carried seven scientific instruments weighing 49 pounds to study Venus’ atmosphere and temperature, search for a possible magnetic field, and to study cosmic rays, the solar wind and cosmic dust during the trip to the planet.  Mariner 2 sent data back to Earth at the then-blistering rate of 8-1/3 bits per second. 

Artist’s impression of the Mariner 2 spacecraft
Artist’s impression of the Mariner 2 spacecraft.

Planned as a two-spacecraft project, Mariner 1 launched first on July 22, 1962, but its Atlas-Agena rocket veered off-course and was destroyed in the first few minutes of flight – the fault was traced partly to a missing hyphen in communications software.  Mariner 2 followed on August 27 and this time the launch was successful and the spacecraft entered solar orbit on its way to Venus.  During the 110-day journey, Mariner 2 sent back valuable information about its interplanetary environment including confirming the existence of the solar wind, a stream of charged particles emanating from the Sun.  The spacecraft encountered several hardware anomalies, many of which inexplicably fixed themselves.  By the time Mariner 2 approached Venus, one of its solar arrays had failed and the vehicle came dangerously close to overheating, but it remained healthy enough to complete its scientific mission.

On December 14, Mariner 2 passed within 21,564 miles of Venus, and continued on in solar orbit, sending back the data it had collected.  It measured the temperature at Venus’ surface at 300oF to 400oF, confirming Sagan’s greenhouse effect prediction, and the atmospheric pressure at 20 times greater than on Earth.  Future spacecraft would measure even higher temperatures and pressures.  Previous conceptions of Venus as a tropical haven had to be discarded.  Unlike Earth, Mariner 2 found that Venus has no appreciable magnetic field and therefore no protective trapped radiation belts, meaning the planet is constantly bombarded by cosmic rays.  The spacecraft sent its last transmission on January 3, 1963, having completed the first robotic exploration of another planet.

Jack James, Project Manager at JPL, later said of Mariner 2, “There will be other missions to Venus, but there will never be another first mission to Venus.”  As for JPL, Mariner 2 was the first of dozens of missions that explored all the planets from Mercury to Neptune.

For more on Mariner 2, see