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In about a year, a new NASA instrument designed and built in Colorado will ride on the International Space Station (ISS). From there, it will look down at Earth to measure the light reflecting off our planet鈥檚 puffy clouds, expansive ice sheets, bodies of water, forests, deserts听and other land surfaces.

The mission鈥攌nown as the 鈥攚ill measure sunlight reflecting from Earth with up to 10 times more accuracy than existing space-based sensors. The almost $70-million effort seeks to lay the groundwork for future space instruments to track the pace of climate change with unprecedented accuracy.听

鈥淲e鈥檙e entering a new era of high-accuracy climate observations that will supply a lot of information to the people that set policy on climate change, mitigation and adaptation,鈥� said Peter Pilewskie, professor at the (LASP) at CU Boulder and lead scientist for CLARREO Pathfinder.

LASP is developing the sensor, and NASA鈥檚 Langley Research Center in Hampton, Virginia, leads the overall mission. CLARREO鈥檚 target is 鈥渞eflected sunlight鈥濃�攐r the incident solar radiation that is redirected back into space by Earth鈥檚 atmosphere and surface. Some of that light is visible, but CLARREO can also measure what scientists call near-infrared radiation.

鈥淲e can鈥檛 see near-infrared radiation. We can鈥檛 sense it with our eyes,鈥� Pilewskie said. 鈥淏ut it makes up a large fraction of the radiative energy Earth receives from the sun.鈥�

That light is also critical to understanding how humans are changing Earth鈥檚 climate, he said. As a pathfinder mission, CLARREO packs a limited, but impactful, scope: The mission will demonstrate new approaches to measuring reflected sunlight that have never before flown in space and also help existing space missions deliver more accurate data on Earth鈥檚 climate.听

CLARREO Pathfinder will spend nearly a year mounted to the exterior truss of the ISS, taking turns peering at the sun and Earth.听

Daniel Baker, director of LASP, explained that CLARREO Pathfinder is the latest in the institute鈥檚 decades of work to better understand the planet on which we live.

鈥淒eveloping ever more sensitive sensor systems to assess Earth鈥檚 radiation budget is one of our lab鈥檚 highest callings,鈥� Baker said. 鈥淗aving 鈥榥ew eyes鈥� on our fragile atmosphere will give policy makers information to judge whether our actions are helping or hurting as we seek to avert further climate change.鈥�

Clouds, land and ice

Humans have long been fascinated by reflected sunlight鈥攑icture light shining from behind clouds during sunset or the alpenglow made by reddish light illuminating mountain peaks.

But there鈥檚 also a lot happening in Earth鈥檚 atmosphere that humans can鈥檛 see. As solar radiation bathes the planet, Earth鈥檚 atmosphere, land and oceans absorb听some of that energy, while the remainder makes its way back to space. The balance of incoming versus outgoing radiation helps to shape the planet鈥檚 climate, contributing to shifts in weather and temperature at the ground.听

At any point, the light that reflects from Earth back into space is roughly 1,000,000 times less bright than the light coming from the sun. CLARREO Pathfinder can sort that radiation into different wavelengths so that scientists can identify what constituents, such as clouds, vegetation or ice, are modifying the reflected light.听

鈥淲e鈥檙e confident that Earth鈥檚 climate is changing, and we鈥檙e confident about the source of that change,鈥� Pilewskie said. 鈥淏ut climate is very complicated. Our ability to make measurements at high accuracy will help us to better understand those underlying causes of climate change.鈥�

So far, researchers haven鈥檛 been able to record reflected sunlight with that level of accuracy.听

CLARREO Pathfinder could change that. Engineers at LASP are currently testing the sensor in Boulder to make sure that it will perform as promised in space. Once it launches, CLARREO will map where reflected sunlight is coming from on Earth to within about half a kilometer, or one-third of a mile.

Stable sun

The sensor owes its incredible accuracy to a novel calibration approach, said Greg Kopp, senior research scientist at LASP and instrument principal investigator for the mission.

鈥淐LARREO will use the best-known in-space light source there is to do on-orbit calibrations,鈥� he said. 鈥淭hat light source is the sun.鈥�

Kopp explained that the sun is an extremely dependable source of light鈥攐ver the span of more than a decade, the star鈥檚 output of energy only changes by about one-tenth of 1%, on average. To make sure CLARREO Pathfinder is collecting accurate data on Earth鈥檚 brightness, the sensor will first point toward the sun, narrowing its aperture down to about half a millimeter wide. Using a much larger aperture, it will then swivel back to Earth to measure the planet鈥檚 reflected light as a percentage of the total light coming from the sun.

鈥淚t鈥檚 like having your favorite calibrator already there waiting for you when you get to space,鈥� Kopp said.

For Pilewskie, next year鈥檚 launch will bring his career full circle. He first measured reflected sunlight more than 35 years ago as a graduate student by observing the sides of cumulus clouds from the ground. Now, he鈥檚 following similar phenomena from space with a new level of accuracy.听

鈥淏eing able to take these kinds of measurements to space is really a career goal of mine,鈥� Pilewskie said.