An artist's concept of the ACRIMSAT satellite orbiting Earth. Photo: NASA/JPL

NASA's Active Cavity Irradiance Monitor Satellite (ACRIMSAT) began taking raw science measurements of the Sun's total energy output on Monday following successful pointing of the spacecraft and its instrument.

ACRIMSAT, launched Dec. 20, 1999, is in a Sun-synchronous orbit at an altitude of about 695 kilometers (429 miles). It measures "solar irradiance," or the total amount of the Sun's energy that enter's Earth's atmosphere-ocean system. ACRIMSAT is part of a multi-decade effort to understand variations in the Sun's output and resulting effects on Earth.

"All systems are go and operating within specifications," said ACRIMSAT project manager Ron Zenone of NASA's Jet Propulsion Laboratory, Pasadena, Calif. "We look forward to extending the scientific database on solar measurements."

Since its launch from Vandenberg Air Force Base, Calif., many in-flight characterization tests of the spacecraft and instrument have been conducted. The performance of the spacecraft's attitude-control subsystem, which governs ACRIMSAT's orientation in space, was adjusted through changes to flight software to improve attitude stability.

The program's next milestone is a review of spacecraft operations. The manufacturer of the spacecraft, Orbital Sciences Corp., McLean, Va., is currently operating ACRIMSAT. Following a successful "hand-over" review, satellite operations will be taken over by JPL for the five-year mission.

Previous measurements have demonstrated that the total radiant energy from the Sun was not a constant. However, the solar variability was so slight (0.1 percent) that continuous monitoring by state-of-the-art instrumentation is necessary. Researchers theorize that as much as 25 percent of the anticipated global warming of Earth may be solar in origin. Small changes in total solar irradiance by the Sun over a century or more may cause significant climate changes on Earth.

The ACRIMSAT mission is funded by the Earth Science Programs Office at the NASA Goddard Space Flight Center, Greenbelt, Md. The instrument and spacecraft subcontract is managed by the ACRIMSAT Project Office at JPL. Principal investigator is Dr. Richard Willson of Columbia University's Center for Climate Systems Research, Coronado, Calif., and instrument scientist is Roger Helizon of JPL. ACRIMSAT spacecraft program manager is Tom Itchkawich of Orbital Sciences Corp. JPL is a division of the California Institute of Technology in Pasadena.