The Active Cavity Radiometer Irradiance Monitor satellite, or AcrimSat, which was launched in December 1999, carries the Acrim III instrument. The instrument is the third in a series of solar-monitoring tools built by NASA's Jet Propulsion Laboratory, Pasadena, Calif., and launched over the past 25 years. The goal is to study the Sun-Earth connection by measuring solar irradiance, the Sun's energy that reaches our planet.

Scientists use data from the instruments to learn how solar energy affects Earth's winds, heats the land, and drives ocean currents, all of which affect Earth's weather and climate. The data help researchers create global climate models and study solar physics.

The experiments also found a drastic drop in solar irradiance levels when Venus transited between the Earth and Sun in June 2004. The decrease was equivalent to all the energy used by humans in 2003.

"The satellite's measurements of total solar irradiance have been the most precise ever collected," said Roger Helizon, AcrimSat project manager/scientist at JPL. "The mission has provided a wealth of data for its relatively small cost of 30-million dollars."

More information on the mission is available at http://acrim.jpl.nasa.gov.

AcrimSat is funded by NASA through the Earth Science Programs Office at NASA's Goddard Space Flight Center, Greenbelt, Md. The AcrimSat project office at JPL managed the design, fabrication, and test of the Acrim III instrument and the subcontract for the spacecraft, built by Orbital Sciences Corporation, Dulles, Va. The spacecraft is controlled by a dedicated ground station located at JPL's Table Mountain Facility in Wrightwood, Calif. JPL is managed for NASA by the California Institute of Technology in Pasadena.

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