How brightly the moon glows is a mystery, but maybe not for long

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The lunar dark side
may be the moon’s more mysterious face, but there’s something pretty basic scientists
still don’t know about the bright side — namely, just how bright it is.

Current estimates of
the moon’s brightness at any given time and vantage point are saddled with at
least 5 percent uncertainty. That’s because those estimates are based on
measurements from ground-based telescopes that gaze at the moon through the
haze of Earth’s atmosphere.

Now, scientists have sent
a telescope beyond the clouds on a high-altitude airplane in hopes of gauging
the moon’s glow
within about 1 percent or less uncertainty,
the National Institute of Standards and Technology reports in a Nov. 19 news
release.

Knowing the exact
brightness of Earth’s celestial night-light could increase the reliability of
data from Earth-observing satellites that use the moon’s steady glow to check
that their sensors are working properly. Those satellites keep tabs on things
like weather, crop health and dangerous algal blooms.

The new
moonbeam-catching mission
, called the Airborne
Lunar Spectral Irradiance Mission or air-LUSI for short, performed a series of
demonstration flights from late November 12 (right after a full moon) to the
wee hours of the morning on November 17, NASA said in a Nov. 14 news release. During
each flight, a telescope was bathed in moonlight for half an hour while riding
on the wing of a NASA airplane about 21 kilometers above ground — around twice
the cruising altitude of a commercial airliner.

“When we’re up there,
the atmosphere is not an issue,” says air-LUSI team leader Kevin Turpie, a
remote sensing scientist at the University of Maryland, Baltimore County. With
the plane flying above about 90 percent of the atmosphere, “it’s getting much
closer to viewing the moon as you would from space.”

Researchers can’t just
launch a satellite up into space to get a clear view of the moon, because that
probe would be “basically facing the same problems as all the other
Earth-observing satellites” that aim to use the moon’s brightness to check
their function, Turpie says. Namely, instruments degrade in the harsh
environment of space. By sending air-LUSI on a short flight rather than into
orbit, scientists can manually examine the instrumentation after landing to
make sure it continued operating correctly throughout its observations.

Turpie’s team is still
analyzing results from its initial observing run. But if the air-LUSI
measurements are as accurate as expected, observations from those and future
flights could be combined with ground telescope data to create a more accurate
model of the moon’s appearance at various times and locations. Shaving
uncertainty about the moon’s brightness down to about 1 percent or less would
require making air-LUSI observations during different phases of the moon over at
least three years, Turpie estimates.

In the future,
moonlight-seeking experiments that fly even higher than air-LUSI, potentially
on high-altitude balloons, may help measure moonlight with even greater accuracy,
says lunar calibration researcher Hugh Kieffer. He is not involved with
air-LUSI but has modeled the moon’s brightness using ground telescope
observations for the U.S. Geological Survey.

By pointing a
satellite at the moon and comparing its observations with the moon’s true
brightness, scientists can test whether the satellite is seeing things
correctly — and tweak data coming from it to compensate for any errors. It’s
difficult to run the same kind of test using other celestial objects as reference
lights, because “stars are too dim and pointy, and the sun is too bright,” Kieffer
says.

More advanced models
of the moon’s brightness could not only help clean up data from current
Earth-facing satellites, but also refine observations from past satellites that
peered at the moon.

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