The Sun is the predominant source of energy input to Earth. Solar radiation ensures the maintenance of the appropriate range of temperatures for the sustenance of life on Earth, by driving land surface heating, plant productivity, and oceanic and atmospheric circulations. It is also the main driver for several water cycle processes, ranging from evaporation and cloud formation to precipitation, although it can also introduce certain adverse influences, such as harmful ultraviolet radiation and production of ozone through photochemistry. Because of the Sun’s dominant influence on Earth’s function, it is important to accurately measure the solar input to Earth or solar irradiance.

Measurement of the total solar irradiance (TSI) is essential for quantifying Earth’s energy budget. NASA has maintained continuous measurement of TSI since 1978 through successive missions: Nimbus-7/ERB, the ACRIMSat series, SORCE, TCTE, and the Total and Spectral Solar Irradiance Sensor-1 (TSIS-1), which was launched to the International Space Station on 15 December 2017. With significant technological improvements in TSIS-1 relative to heritage missions, scientists at NASA and the scientific community worldwide expect to refine our knowledge of how much energy we get from the Sun, both in total and spectrally, and how the Sun affects our Earth and various processes that are important to mankind now and in the future.

The Sun is a variable star, and Earth is a sun-powered planet. The Climate and Radiation Lab (CRL) plays a critical role in developing and operating NASA’s solar radiation missions, which provide fundamental solar irradiance measurements for Sun-Climate research. Satellite observations have revolutionized our view of the Sun, providing the most accurate measurements to study both active and quiet Sun. It remains a challenging question whether the quiet Sun is a time-invariant base level or there exists a secular change in the Sun’s radiative output. Continuing the legacy of the NASA’s Solar Radiation and Climate Experiment (SORCE), the Total and Spectral Solar Irradiance Sensor -1 (TSIS-1) carries the state-of-the-art instruments with the most accurate total and spectral solar irradiance measurements and has been taking solar observations from the International Space Station since 2018. NASA is planning to launch TSIS-2 on a free flyer to continue these solar irradiance measurements.