ACRIMSAT

ACRIMSAT
Name	ACRIMSAT
Mission type	Solar observation
Operator	NASA / Jet Propulsion Laboratory
COSPAR ID	1999-070B
SATCAT	26119
Mission duration	12 years, 4 months
Spacecraft bus	Orbital Sciences Corporation LEOStar-2
Manufacturer	Orbital Sciences Corporation
Launch mass	115 kilograms
Power	110 watts
Launch date	20 December 1999, 00:50 UTC
Launch rocket	Taurus 2110
Launch site	Vandenberg Space Force Base, SLC-576E
Deployment from	Orbital Sciences Corporation
Entered service	14 March 2000
Last contact	14 December 2013
Decay date	8 September 2024
Orbit reference	Geocentric orbit
Orbit regime	Sun-synchronous orbit
Orbit periapsis	675 km
Orbit apoapsis	699 km
Orbit inclination	98.2 degrees
Orbit period	98.5 minutes
Apsis	gee
Instruments	Active Cavity Radiometer Irradiance Monitor III (ACRIM III)

ACRIMSAT was a dedicated NASA Earth observation satellite that precisely monitored the solar irradiance reaching Earth for over a decade. Launched in late 1999, its primary mission was to continue the critical long-term Total Solar Irradiance (TSI) measurement record initiated by earlier instruments on missions like the Solar Maximum Mission and the Upper Atmosphere Research Satellite. The satellite's single instrument, the Active Cavity Radiometer Irradiance Monitor III, provided essential data for understanding the Sun's influence on Earth's climate system and atmospheric science.

Mission and objectives

The core mission of ACRIMSAT was to provide a continuous, precise, and accurate record of the Total Solar Irradiance, a key variable in climate modeling. This objective directly supported the goals of NASA's Earth Observing System by quantifying the Sun's energy input to the Earth's atmosphere. A primary scientific goal was to determine whether TSI varied in correlation with the solar cycle and to detect any long-term trends that could influence global climate change. The mission aimed to bridge data between the ACRIM II instrument on the Upper Atmosphere Research Satellite and future sensors, ensuring an unbroken climate data record.

Spacecraft design

The spacecraft was based on the standardized Orbital Sciences Corporation LEOStar-2 bus, a compact design used for various low Earth orbit missions. With a launch mass of approximately 115 kilograms, it was a small, cost-effective satellite. It operated in a Sun-synchronous orbit, which allowed its single instrument to maintain a consistent viewing geometry of the Sun. Power was provided by a single solar array generating about 110 watts, and attitude control was maintained using reaction wheels and magnetorquers to ensure precise pointing necessary for its sensitive measurements.

Instrumentation

The sole payload was the Active Cavity Radiometer Irradiance Monitor III (ACRIM III), a third-generation instrument developed by the Jet Propulsion Laboratory. This radiometer used an electrically calibrated, cavity-type detector to achieve extremely high accuracy and stability in measuring the total light energy from the Sun. The instrument's design minimized degradation from ultraviolet radiation and contamination, a challenge for previous sensors. It measured TSI with an absolute accuracy rivaling that of ground-based instruments, making it a crucial reference for cross-calibrating other space-based sensors like those on the Solar and Heliospheric Observatory and SORCE.

Launch and mission timeline

ACRIMSAT was launched on 20 December 1999 aboard a Taurus rocket from Vandenberg Space Force Base in California. It was deployed into its target Sun-synchronous orbit by the launch vehicle's upper stage. Following a successful commissioning phase, the instrument began its primary science observations on 14 March 2000. The mission far exceeded its planned five-year lifespan, collecting data for over twelve years. Communications with the spacecraft were lost on 14 December 2013 after an onboard system failure, ending the mission. The satellite re-entered Earth's atmosphere on 8 September 2024.

Scientific findings

Data from ACRIMSAT's ACRIM III instrument were pivotal in confirming the slight decrease in Total Solar Irradiance during the declining phase of Solar cycle 23. Its measurements were central to ongoing scientific debates regarding the existence and magnitude of a potential upward trend in TSI between the solar minimum periods of 1986 and 1996, a controversy involving comparisons with data from the European Space Agency's ERBS satellite. The mission provided a critical, continuous record that helped scientists separate solar variability signals from other forcings in climate change studies, contributing significantly to reports by the Intergovernmental Panel on Climate Change.

Legacy and impact

ACRIMSAT cemented the value of dedicated, long-term monitoring of solar irradiance for climate science. Its high-quality dataset forms a cornerstone of the multi-decade TSI composite record, which is maintained by institutions like the Jet Propulsion Laboratory and the University of Colorado Boulder. The mission demonstrated the effectiveness of a small, focused satellite within NASA's broader Earth science portfolio. Its legacy is carried forward by subsequent missions such as the Total Solar Irradiance Calibration Transfer Experiment on the PICARD satellite and the current Total and Spectral Solar Irradiance Sensor on the Joint Polar Satellite System.

Category:NASA satellites Category:Earth observation satellites Category:Spacecraft launched in 1999 Category:Solar space observatories