  Fifty years ago, NASA's Jet Propulsion Laboratory (JPL) built and flew the
first successful spacecraft to another planet: Mariner 2 to Venus. This paper
discusses the context of this mission at a crucial phase in the space race
between the USA and USSR and its results and legacy. As its first major
success, Mariner 2 helped to cement JPL's position as a centre for robotic
planetary exploration. Mariner 2 successfully solved the scientific problem of
the high temperature observed for Venus by ground-based radio telescopes. It
also pioneered new techniques for observing the atmosphere of a planet from
space, which were subsequently developed into the microwave sounding and
infrared sounding techniques for observing the Earth atmosphere. Today these
techniques provide some of the most important data for constraining weather
forecasting models, as well as a key series of data on the Earth's changing
climate.
