Good-bye SORCE, Hello TSIS !
The NASA Solar Radiation and Climate Experiment (SORCE) science mission ended on February 25, 2020 after completing more than 17 years of excellent observations of the total solar irradiance (TSI) and spectral solar irradiance (SSI) between 1 nm and 2400 nm. This was a planned passivation event for SORCE now that the new NASA Total and Spectral Solar Irradiance Sensor (TSIS-1) mission has had its required overlap with SORCE. TSIS-1, aboard the International Space Station (ISS), will be continuing the four-decade-long TSI climate data record, as well as continuing the SSI 200-2400 nm climate data record that SORCE initiated for the 400-2400 nm range. These TSI and SSI measurements are key inputs for atmosphere and climate modeling, energy balance modeling, and remote sensing for NASA’s Earth Observing System (EOS). The length of the SORCE mission also enabled measurements in two of the Sun’s 11-year solar cycles, and SORCE observed solar cycle minimum conditions in both 2008 and 2019 that are particularly useful for secular trending of the solar irradiance.
Some of the SORCE science highlights are summarized by NASA’s Earth Observer article in 2013. Several of these science results are from the legacy of the TSI and SSI climate data records, plus SORCE has introduced game-changing radiometer technology for solar irradiance monitoring with the highest accuracy and precision yet achieved. Furthermore, SORCE’s Solar Stellar Irradiance Comparison Experiment (SOLSTICE) has supported space weather operations with its Mg II index of chromospheric activity with greatly improved spectral resolution, highly accurate calibrations of several O and B stars for the astrophysics community, and highly accurate measurements of lunar ultraviolet reflectance for the planetary community and for use in calibrating other Earth climate monitoring instruments. The moderately high spectral resolution measurements of the solar far ultraviolet (FUV: 115-200 nm) irradiance by SOLSTICE are unfortunately not being continued by the TSIS project. There are some lower resolution solar FUV irradiance observations that are continuing with the NOAA GOES-16 and -17 Extreme ultraviolet X-ray Irradiance Sensors (EXIS) and with the NASA Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics (TIMED) Solar EUV Experiment (SEE). Neither of those provide the higher spectral resolution, nor the full coverage of the FUV range, or the stability calibration with UV-stable stars as SOLSTICE used. Regrettably, there will now be a gap for parts of the FUV spectral coverage in the SSI record.
SORCE does not have any on-board propulsion (fuel), therefore, it was not possible to perform a controlled re-entry as part of the decommissioning activities. SORCE was passivated on February 25, 2020 through powering off instruments and most spacecraft systems and disabling the spacecraft reaction wheels and radio communication systems. SORCE will now tumble in its orbit that will slowly decay due to atmospheric drag; it is expected to re-enter and burn up in the atmosphere in 2032.
We thank the dedicated engineers and scientists at the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado (CU) for leading, implementing, and operating the remarkable SORCE mission. The long-standing support for the SORCE mission at NASA HQ and GSFC has been outstanding. We are also deeply indebted to the SORCE operations team at CU-LASP and Northrop Grumman (formerly Orbital Sciences Corp. when they built the SORCE spacecraft bus) for their dedication and inventive solutions for continuing the SORCE well past its expected 5-year lifetime, especially so after anomalies with one of the reaction wheels and battery capacity threatened to end the SORCE mission before TSIS was launched.
Tom Woods and Peter Pilewskie
SORCE PI TSIS PI
