The Pioneer Venus Multiprobe, also known as Pioneer Venus 2 or Pioneer 13, was a spacecraft launched in 1978 to explore Venus as part of NASA's Pioneer program. This part of the mission included a spacecraft bus which was launched from Earth carrying one large and three smaller probes, which after separating penetrated the Venusian atmosphere at a different location, returning data as they descended into the planet's thick atmosphere. The entry occurred on December 9, 1978. In context There was also an orbiter launched in 1978, part of the overall Pioneer Venus project along with this entry probe mission. Whereas the probes entered the atmosphere in 1978, the Pioneer Venus Orbiter would stay in orbit throughout the 1980s and the early 1990s. The next major mission was the Magellan spacecraft, which was an orbiter capable of mapping Venus by seeing through its opaque clouds with radar. Spacecraft The Pioneer Venus Multiprobe bus was constructed by the Hughes Aircraft Company, built around the HS-507 bus. It was cylindrical in shape, with a diameter of and a mass of . Unlike the probes, which did not begin making direct measurements until they had decelerated lower in the atmosphere, the bus returned data on Venus' upper atmosphere. The bus was targeted to enter the Venusian atmosphere at a shallow entry angle and transmit data until destruction by the heat of atmospheric friction. The objective was to study the structure and composition of the atmosphere down to the surface, the nature and composition of the clouds, the radiation field and energy exchange in the lower atmosphere, and local information on atmospheric circulation patterns. With no heat shield or parachute, the bus made upper atmospheric measurements with two instruments: BIMS – an ion mass spectrometer to determine the origin and long-term development of the Venusian atmosphere, the dynamics of the upper atmosphere layers, its energy balance and the effect of solar radiation and interplanetary space on those layers. This instrument had a range of 1 to 46 u, used 6 W of power and weighed . BNMS – a neutral mass spectrometer. This made measurements of the interaction between the solar wind and Venus, the photochemistry of the upper layers of and heat distribution in the Venusian atmosphere. It had a range of 1 to 60 u, weighed , and used ~1 W of power. The spacecraft operated down to an altitude of about