NASA Measures ‘Moonlight’ to Improve Earth Observations

In March 2025, NASA pilots took the agency’s ER-2 science aircraft on a series of night flights over NASA’s Armstrong Flight Research Center in Edwards, California, as the Moon increased in visible size. The Airborne Lunar Spectral Irradiance, or air-LUSI, mission observed the Moon at different phases and measured the sunlight reflected by the lunar surface. Specifically, the instrument tracked the amount of light reflected at different wavelengths. This information enables scientists to use the Moon as a calibration tool for Earth-observing sensors. The air-LUSI project is a collaboration between scientists and engineers from NASA, the National Institute of Standards and Technology, the U.S. Geological Survey, the University of Maryland Baltimore County, and McMaster University in Ontario.

What is said in the film

Air LUCI is something that actually helps not just a handful but all Earth-observing satellites that can use the Moon as a calibration reference. And this actually greatly can enhance the capacity for actually making accurate measurements of the Earth. Well, for one thing, it does give us the opportunity to make those improvements to our ability to observe the Earth. It's also something that hasn't been done before at this level of accuracy. So this is the most accurate measurements that have ever been made of moonlight.

And moreover, up until our ability to do this, we really have not been able to predict accurately the amount of light coming off the moon at any given time within several percent. Our measurements are down less than a 1% accuracy, probably down to maybe as far as a half a percent accuracy. So essentially, Air Lucy allows us to use the Moon as a benchmark, which allows sound lights to make sure their measurements are accurate. They can turn and look at the Moon and make sure that what they read from that, which is well known, will then reflect in terms of the accuracy of what they measure from Earth. Basically, measure the amount of light that's coming at different wavelengths, which is pack full of information about the surface of our Earth.

So, Air Lucy will help facilitate making more accurate measurements using the moon. The moon is a pretty much perpetual reference that doesn't change past, present or future. Any measurements that are made by satellites in the past or ones made in the future can be inter-consistent over time. That means if they're consistent over time, we can do long-term measurements of the Earth that are consistent and don't have spurious trends. So those measurements actually now become a very good representation of changes in climate and our Earth system.

The Euro 2 is an excellent aircraft for this particular exercise, in particular because it can fly above 95% of the atmosphere. That gives us basically a view of the Moon that is very similar to what satellites see from orbit. And we don't have to worry about as much the effects of the atmosphere and our measurements. Previous measurements that have been made of the Moon to date have been done from the surface of the Earth, more recently from mountaintops. But you still have a lot of atmosphere to go through.

Air Lucy on the Euro 2 gives us an opportunity to be near space. Aerolucie is kind of non-intuitive in the sense that it actually helps build our capacity to measure how the earth is changing and how those processes are actually going on a global scale. So if we better understand how climate is changing, if we better understand how systems interact with each other in terms of weather, ocean currents, ecologies and any type of process on the surface of the earth, these lead to things that affect humans on a daily basis. We are affected by weather. We want more accurate predictions of weather and how weather may be changing with climate.

We want to know how this might be affecting agriculture, forest ecology, ocean ecology. And if we can make better accurate and more stable measurements from space, then Aerolucie is actually contributing to a lot, indirectly, to people's everyday lives. So interestingly enough what's really beneficial about Erlucie is we're kind of pulling together existing technologies such as the ER2 and you know NIST spectrometers and all those equipment that is very very well understood and and well known and bringing it to bear and this sense we're doing something new and innovative because we haven't made this accurate a measurement of the moon before. I would mention that you know we're using the best you know spectrometers from instrument systems these have been vetted by the National Institute of Standards and Technology. We're using National Institute of Standards and Technology sources so we're basically making everything internationally traceable in terms of making very accurate measurements but the beauty about what we're doing is we're taking the best of our existing technology and bringing it to bear to make this special measurement.

Oh, I mean, the applications are great. It helps us learn more about the climate. It gets used to calibrate satellites that are looking at climate, and it's really fun to come out here to NASA, fly on this beautiful plane. So it's been a fun project that way too. Air Lucy's a telescope attached to a spectrograph.

So the telescope collects the light, and then the spectrograph splits it into different colors or wavelengths of light. And then we can calibrate that instrument so that we know the amount of light very precisely. And the idea is then to fly that in an airplane, have it point at the moon, and measure the light from the moon very precisely. So the data we collect primarily impacts the ability for the satellites that are looking at the Earth to make high-quality measurements that are used from everything from weather to climate change measurements. And really being able to improve those, you know, billion-dollar instruments with a relatively inexpensive measurement so that they can make climate quality measurements is really exciting.

Yeah, so it's these measurements are used to help calibrate the instruments that are making the measurements of the earth and climate. So those instruments spend most of their time looking at the light reflected off the earth, but to calibrate them we want them to turn and look at the moon. So if we can measure the moon precisely, then they can get a good calibration off the moon and go back to looking at the earth and make great measurements. Oh boy, it's, so usually we do this kind of work in a very controlled laboratory environment. And to come in here to an aircraft hangar and make a high quality calibration measurement, you know, you can see the lights coming in, the temperature is not controlled.

So that's a real challenge for us. And then of course the instrument itself is going to fly up at 70,000 feet. It's minus 40 or 60 degrees Celsius up there. So designing the instrument so it can operate in that environment has also been a real challenge for us. The idea is, so this, our telescope usually looks at the moon, so we make a light source that kind of mimics the moon, and then we brought some instrumentation that can measure the light coming from that source very precisely, and then the telescope's going to look at that source.

And that's kind of how we say, "transfer the calibration. " So we have our well-calibrated instrumentation. It looks at the source. Our telescope looks at the source, and we can take our well-calibrated instrumentation and kind of transfer that level of accuracy to the telescope instrument. So the ER-2 is a high-altitude aircraft.

It'll fly up above 65,000 feet. And one of the challenges of making these measurements, like we, you know, everyone can see the moon, why not measure it from down here on Earth? And the reason is that the atmosphere scatters some of the light coming from the moon before it gets down here to the Earth. And figuring out how much light you've lost on that path is hard. So that kind of degrades our ability to measure from down here.

Putting it on this plane, we're flying above 95% of the Earth's atmosphere. So it's much less scattering. That little bit that's left is the accuracy which we can estimate that is good enough to make our measurements. So yeah, getting above the atmosphere is a real important thing for us. And the satellites are of course above all of the atmosphere.

So we want to know the amount of light that's up at the top of the atmosphere, not what we're getting down here. So the instrument itself needs to, like we can't communicate directly with it during the flight. It's operated by the pilot and he's just got a few on-off switches that he can push to operate the instrument. He's busy, you know, mostly busy flying the plane. He can't do a whole lot.

So he had to program the instrument so that the pilot just right before takeoff, he pushes one switch to let the instrument know we're taking off. When he gets to altitude, he pushes another switch that deploys the telescope. Then it automatically finds the moon and stays tracked on the moon while we collect the data. Then after about 20 minutes, we've got enough data. He pushes another switch to tell it, you know, to shut down.

So it's all operated on just a few switches and a lot of good programming from our computer guy. So the interesting thing about the Moon is its reflectance is very constant. So you look up, you know, any night in the sky, the amount of light is changing all the time. But its reflective properties, it's just reflecting sunlight off its surface and we're seeing that. That reflectance is very constant.

So if we can make a few really good measurements and then make a model using the reflectance, then you can use that model to predict at any time where you are knowing where you are and the Moon is and the Sun is, how much light you would see. So, it's really more on the calibration side. We've used, and it's some stuff we've been doing at NIST over the last, say, 20 years, using tunable lasers to calibrate the spectrograph and developing techniques to get the spectrograph calibrated well that have really been key to making a high-quality measurement. Yeah, so right now the way satellites try to maintain their calibration is to bring up a solar reflector of their own. So they bring up kind of a white plaque that the satellite can put in front and bounce some sunlight off of to do the calibration.

That takes up space, it takes up mass, it's not the most expensive part of the satellite. But particularly as people are going towards smaller satellites and CubeSats and stuff like that, having something like that on board is way too much for them. So if they could just turn and look at the moon instead of having to carry up some equipment of their own to maintain their calibration, that would be a big advance for those. Then you could have smaller, more cost-effective satellites in the future. The challenges are...

So another thing that usually doesn't happen back in the lab is no one says, "Okay, at 102, you need to start your calibration and be done in an hour. " But here, the plane's just been fueled, we've turned on the instrument, it's warming up. In about 15 minutes, we need to start calibrating, and it needs to be finished an hour later, and then they tow the plane out. Something goes wrong, then the whole schedule goes off. So doing this all on schedule is another challenge for us, and makes things really exciting.

We're just hoping for some good weather and some good luck and really looking forward to being up in the air.

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