Solar spacecraft completes five-year mission

PASADENA, California, US, June 1, 2005 (Refocus Weekly)

A satellite that measures the solar energy reaching the earth’s atmosphere has successfully accomplished its five-year primary mission.

NASA launched the Active Cavity Radiometer Irradiance Monitor satellite (AcrimSat) in December 1999 to measure variability in the sun’s energy and its impacts on winds and oceans. The Acrim III instrument was the third in a series of solar-monitoring tools built by NASA’s Jet Propulsion Laboratory and launched over the past 25 years.

“The satellite’s measurements of total solar irradiance have been the most precise ever collected,” says Roger Helizon of JPL. “The mission has provided a wealth of data for its relatively small cost of US$30 million.”
Measuring solar irradiance provides data to examine how solar energy affects winds, heats the landmass and drives ocean currents, and is of primary interest for predicting weather and climate. Researchers can also create global climate models and study solar physics.

The Acrim III was a secondary payload on board a Taurus launch and was designed to extend the database first created by Acrim I launched in 1980 on the Solar Maximum Mission spacecraft. Acrim II followed on the Upper Atmosphere Research Satellite in 1991.

Acrim was the first to demonstrate that total radiant energy from the sun was not a constant, although solar variability was so slight (0.1% of full scale) that continuous monitoring was necessary. It is theorized that 25% of the anticipated global warming of the earth may be solar in origin, and that small (0.5%) changes in total solar irradiance over a century may cause significant climatological changes on earth, NASA explains.

AcrimSat found a drop in solar irradiance levels when Venus transited between the earth and sun last June, with a decrease that was equivalent to all the energy used by humans in 2003.

AcrimSat is funded by NASA through the Earth Science Programs Office at NASA's Goddard Space Flight Center in Maryland. JPL manages the fabrication and testing of the Acrim III instrument and the spacecraft built by Orbital Sciences of Virginia.

Data show the daily average of the total solar irradiance at 1 AU (distance from the centre of the sun to centre of the earth) and, at solar maximum, energy output from the sun is highly variable compared to the quiet of solar minimum.

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