ACRIMSat

Launch data: Designation 26033 / 99070B Launch date 21 Dec 1999 - 07:12 UT Launch site  Va, SLC-576E Launch vehicle  Taurus-2110 (#4) Mission Scientific: sun observation Government: Solar Physics LEO orbit, sun-synchronous Perigee/Apogee 684 x 722 km Eccentricity  3,18E-03 Inclination 98,28° Period 98,89 min

Description:

• ACRIMSAT is a NASA funded minisatellite mission to monitor the amount of total solar energy received (part of EOS program). The objective is to monitor the solar constant, or TSI (Total Solar Irradiation), with maximum precision and provide an important link in the long-term TSI database. ACRIMSAT is part of a multi-decade effort to understand variations in the sun's output and resulting effects on Earth.
• The spacecraft builder/integrator is OSC (Orbital Sciences Corporation) of McLean, VA. The structure is based on the Ministar bus. In stowed configuration the S/C dimensions are: 79 cm in diameter x 69 cm in height (total span with solar arrays deployed = 178 cm). The spacecraft is spin-stabilized and sun pointing. It carries a flight processor used on previous missions. S/C mass = 115 kg, power = 49 W (average). The mission design life is 5 years.
• The mission is managed and operated by the JPL ACRIM team via the ACRIMSAT ground station located at the JPL/Table Mountain Facility. The PI of the mission is Richard C. Willson of Columbia University.

Mission details:

• Monitor total solar irradiance (TSI) with maximum precision; ascertain the extent of solar radiation variability.
• ACRIMSat was a satellite to ascertain the extent of solar radiation variabiliry. It carried an ACRIM-3 (Active Cavity Radiometer Irradiance Monitor) instrument to monitor solar irradiance at high accuracy (< 0.1 %) as a long term followup to the ACRIM-2 (carried on UARS spacecraft since 1991) and ACRIM-1 (carried on the SMM spacecraft in 1980).
  
  
  
• Initial problems of the attitude control subsystem were experienced preventing proper pointing of the S/C toward the center of the sun. However, the pointing issue was resolved two weeks after launch. The science payload was intentionally left in standby mode, from January 16, 2000 through April 4, 2000, in order to assure outgassing of the spacecraft as much as possible; an outgassing lesson learned from previous missions. This has proved to be very effective in initial contamination control, which typically plagues sensor surfaces
• The science mission began April 5, 2000, and has provided an average of 65 minutes of TSI data per orbit.
• The ACRIM-III instrument observed a drastic drop in solar irradiance levels when Venus transited between the Earth and Sun in June 2004.
• The spacecraft and payload (ACRIM-III) are operating nominally as of 2010 (good health of instrumentation with very little degradation in system performance). In Dec. 2009, the mission was in orbit for 10 years. The current mission extension goes through 2011.
• It was the first instrument to clearly demonstrate that the total radiant energy from the sun was not a constant. The solar variability is so slight, however, that its study calls for continuous state-of-the-art monitoring. Theory suggests that as much as 25% of Earth's global warming may be of solar origin. It also seems that even small (0.5%) changes in the TSI over a century or more may have significant climatic effects. ACRIMSAT is part of NASA's EOS (Earth Observing System).
• The small ACRIMSAT satellite which has been studying the total solar output since 1999 failed on 2013 Dec 14 due to battery problems, months after a review recommended its continued operation. The ACRIM series of experiments, critical data for climate change studies, flew on Solar Max (1980), Spacelab-1 (1983), UARS (1991), and ACRIMSAT. The similar TIM instrument on the SORCE satellite (2003) continues operating, but will it survive until a replacement instrument flies on the JPSS satellite no sooner than 2017?

External links:

• EOportal.org Satellite description