ACRIMSAT
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ACRIMSAT, for Active Cavity Radiometer Irradiance Monitor Satellite was a satellite that was part of NASA's Earth Observing System program. Acrimsat was launched on 20 December 1999 from Vandenberg Air Force Base and placed into a sun-synchronous orbit where it continued long-term studies of the solar irradiance.
ACRIMSAT carried the ACRIM III instrument which made precise measurements of the solar irradiance from 2000 to 2005, continuing measurements that had been made by instruments on Solar Max (1980-1989) and the Upper Atmosphere Research Satellite (1991-). ACRIMSAT made measurements during the 2004 transit of Venus, and measured the 0.1% reduction in the solar intensity caused by the shadow of the closer planet.
Richard C. Wilson of Columbia University was the prinicpal investigator for the mission. The single-purpose satellite was constructed by Orbital Sciences Corporation for only $30 million, and weighed only 13 kg.