Sputnik

Mission Log

A Record of Spaceflight, 1957–Present
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SORCE patch
Solar Irradiance Satellite

SORCE

Precisely measured the Sun's energy output to separate solar from human influences on climate.

NASA / LASP Launched 2003-01-25

SORCE measured how much energy the Sun actually delivers to the Earth, a quantity known as total solar irradiance and one of the most fundamental inputs to any climate calculation. It is also very difficult to measure absolutely, and the mission's precision was its whole purpose.

The central result is that solar output varies by only about 0.1 per cent over the eleven-year cycle — enough to matter, but far too little to account for observed warming. Establishing that number precisely removed a significant uncertainty from climate attribution and constrained one of the more persistent alternative explanations.

SORCE prelaunch image
SORCE prelaunch image — NASA

It also measured the spectral distribution of that energy, which turns out to vary much more than the total. Ultraviolet output changes by several per cent across the cycle, and since ultraviolet drives stratospheric ozone chemistry, the atmospheric response to solar variability is more structured than a simple brightness change would suggest.

From: http://lasp.colorado.edu/sorce/images/SORCE_litho.jpg Credit: either "NASA" [1] or "NASA / LASP" [2] depending on the page.
From: http://lasp.colorado.edu/sorce/images/SORCE_litho.jpg Credit: either "NASA" [1] or "NASA / LASP" [2] depending on the page. — See below

The mission continued a measurement series stretching back to the late 1970s, and maintaining that continuity required overlapping instruments — any gap would break the record irreparably, since absolute calibration between different radiometers is not reliable at the necessary precision.

Its battery degraded badly after 2011, and the team devised a hybrid operating mode in which the spacecraft powered down during eclipse and took measurements only in sunlight, extending the mission by nearly a decade. It was retired in 2020 once its successor was safely established on the Space Station.

Gallery

Image of the SORCE spacecraft with transparent background. SORCE prelaunch image From: http://lasp.colorado.edu/sorce/images/SORCE_litho.jpg Credit: either "NASA" [1] or "NASA / LASP" [2] depending on the page.