|Mission type||Solar astronomy|
|Operator||NASA / JPL|
|Mission duration||Planned: 5 years; |
Achieved: 13 years, 11 months and 23 days 
|Launch mass||120 kilograms (260 lb)|
|Start of mission|
|Launch date||21 December 1999, 07:13UTC|
|Launch site||Vandenberg LC-576E|
|End of mission|
|Last contact||14 December 2013|
|Semi-major axis||7,067.28 km (4,391.40 mi)|
|Perigee altitude||675 km (419 mi)|
|Apogee altitude||717 km (446 mi)|
|Epoch||5 December 2013, 12:18:57 UTC|
The Active Cavity Radiometer Irradiance Monitor Satellite, or ACRIMSAT is a defunct satellite carrying the ACRIM-3 (Active Cavity Radiometer Irradiance Monitor 3) instrument. It was one of the 21 observational components of NASA's Earth Observing System program. The instrument followed upon the ACRIM1 and ACRIM2 instruments that were launched on multi-instrument satellite platforms. ACRIMSAT was launched on 20 December 1999 from Vandenberg Air Force Base as the secondary payload on the Taurus rocket that launched KOMPSAT. It was placed into a high inclination, 700 km. sun-synchronous orbit from which the ACRIM3 instrument monitored total solar irradiance (TSI). Contact with the satellite was lost on 14 December 2013.
ACRIM3 made measurements of the TSI since the start of its mission in April 2000. It extended the TSI measurement database begun by earlier ACRIM instruments on the NASA Solar Maximum Mission (ACRIM1: 1980–1989) and Upper Atmosphere Research Satellite (ACRIM2: 1991–2001).
ACRIMSAT/ACRIM3 tracked TSI during a 2004 transit of Venus and measured the 0.1% reduction in the solar intensity caused by the planet's shadow. It also recorded data for the 2012 Transit of Venus.
On 14 December 2013, ACRIMSAT went silent, with attempts to reestablish contact proving unsuccessful. The most likely cause has been attributed to the failure of aging batteries. The mission was determined to be unrecoverable and officially terminated 30 July 2014.
The defunct spacecraft will remain in orbit for approximately 64 years before returning to Earth.
Richard C. Willson was the principal investigator and led the science team. Willson designed the active cavity radiometer type of sensor used by self-calibrating satellite TSI monitoring experiments. The ACRIM3 instrument was a collaboration between Willson, original JPL/ACRIMSAT Project Manager Ronald Zenone and ACRIM3 Instrument Scientist Roger Helizon. The Mission was controlled using the ACRIMSAT tracking station at the Jet Propulsion Labs (JPL) Table Mountain Observatory in Southern California. Co-Investigators were: Nicola Scafetta (climate impact of solar variability), Hugh Hudson (solar physics) and Alexander Mordvinov (solar physics).
ACRIMSAT was a spin-stabilized, single-purpose satellite constructed by Orbital Sciences Corporation. Its total cost, including the instrument, launch, ground station, operations, and science team activities during its 14-year mission was less than $50 million.