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ACRIMSAT Mission Status
April 3, 2000

NASA’s Active Cavity Irradiance Monitor Satellite (ACRIMSAT)
today began taking raw science measurements of the Sun’s total
energy output 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.