NASA’s Solar Radiation and Climate Experiment (SORCE) Mission Ends

After 17 years in orbit, NASA’s Solar Radiation and Climate Experiment (SORCE) mission ended as planned on February 25, 2020. This highly successful NASA Earth Observing System (EOS) mission provided a groundbreaking data record of total solar irradiance (TSI) and spectral solar irradiance (SSI), two key inputs for atmosphere and climate modeling. The mission length also enabled valuable measurements during two of the Sun’s 11-year cycles. SORCE data provide a unique understanding of how the flow of energy from the Sun varies and how these variations impact Earth’s weather, climate systems, and, ultimately, all life on Earth that depends on solar irradiance.

Launched on January 25, 2003, aboard a Pegasus launch vehicle, SORCE was designed as a five-year mission. After completing its nominal mission, NASA Headquarters extended mission operations to maintain data continuity. Eight years into the mission, SORCE battery degradation started to impact operations, and insufficient power was available to support consistent data collection by the time the mission had been in orbit 10 years. The mission operations team redesigned their operation concept and eventually switched to daytime-only operations in February 2014. This power-saving plan enabled the spacecraft to continue for a full six additional years, allowing sufficient time for NASA to build and launch a replacement instrument to maintain TSI and SSI data continuity.

The spacecraft bus was built by Orbital Sciences Corporation (now known as Northrop Grumman Space Systems) and the spacecraft payload was built by the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado. SORCE carried four instruments: the Solar Stellar Irradiance Comparison Experiment (SOLSTICE), the Spectral Irradiance Monitor (SIM), the Total Irradiance Monitor (TIM), and the X-ray ultraviolet Photometer System (XPS). The SORCE spacecraft and instruments were operated by LASP, which also was responsible for the acquisition, management, processing, and distribution of the science data.

After processing, SORCE data were delivered to NASA’s Goddard Earth Sciences Data and Information Services Center (GES DISC) at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, for archiving and distribution. GES DISC is the NASA Earth Observing System Data and Information System (EOSDIS) Distributed Active Archive Center (DAAC) responsible for data related to atmospheric composition, atmospheric dynamics, global precipitation, and solar irradiance.

SORCE was a highly successful mission. Its TSI measurements extended continuity of the now four-decade-long TSI climate data record. SORCE provided the foundation for the visible and infrared portions of the SSI record, and comparisons of SORCE SSI measurements to similar measurements have led to the production of reference SSI spectra for different phases of the solar cycle.

Other SORCE accomplishments include:

• Implementing next-generation instrumentation of spaceflight radiometers for solar irradiance monitoring with the highest accuracy and precision yet achieved
• Observing two Venus transits and three Mercury transits of the Sun, demonstrating exoplanet detection capabilities and limitations
• Providing total and spectral irradiance inputs to the climate and atmospheric communities, with these inputs used in a wide variety of simulations and models
• Acquiring the first solar flare measurements in TSI and accompanying spectral variations
• Advancing and validating models of the Sun’s total and spectral irradiance variability
• Establishing a new level of TSI that is 4.6 W/m² (0.34%) lower than prior space-based observations
• Acquiring the first continuous measurements of SSI in the 115- to 2400-nm spectral range
• Defining an accurate reference spectrum of the Sun’s spectral irradiance from 0.1 to 2400 nm during very quiet solar conditions
• Seamlessly extending NOAA’s Mg II index of chromospheric activity with greatly improved spectral resolution
• Validating the white dwarf flux scale for absolute calibration of instruments for ultraviolet (UV) astronomy and making the first absolute measurement of disk integrated lunar UV reflectance

SORCE also achieved its final goal of acquiring a minimum of 12 months overlap with NASA’s Total and Spectral Solar Irradiance Sensor 1 (TSIS-1; launched December 15, 2017). The TSI and SSI climate record is now being continued using TSIS-1 measurements.

SORCE’s orbit will slowly decay until it safely re-enters the atmosphere in 2032 after more than 30 years in space. NASA’s Orbital Debris Program Office has determined that only a few small spacecraft components are likely to reach Earth’s surface upon re-entry, and these components will not be capable of causing significant damage or injury.

Learn more about SORCE and explore SORCE data

NASA SORCE page: https://eospso.nasa.gov/missions/solar-radiation-and-climate-experiment

LASP SORCE page: http://lasp.colorado.edu/home/sorce

SORCE data at NASA’s GES DISC: https://disc.gsfc.nasa.gov/datasets?keywords=SORCE&page=1