NASA's Total and Spectral Solar Irradiance Sensor – 2, or TSIS-2, will measure the Sun's energy input to Earth. Since 1978, various satellites have measured the Sun’s brightness above Earth’s atmosphere. TSIS-2 will add solar irradiance measurements to four decades of continuous data records. Unlike its predecessor TSIS-1, which operates from the International Space Station, TSIS-2 will ride on a free-flying spacecraft.