Sky & Telescope - April 2025 - 17

distant galaxies; gas glowed around young stars still hidden
in their natal cocoons. Earth-orbiting satellites streaked overhead
like a car driving through raindrops.
WISE delivered on its promised science almost immediately:
Before long, the first new brown dwarf was discovered
by searching for the telltale signs of methane absorption in
the 3.4- and 4.6-micron images. In the classic sequence of
stellar spectral types, articulating the progression of stellar
temperature from hottest to coolest, stars belong to type
O, B, A, F, G, K, or M. But astronomers had since added
two more categories for brown dwarfs, L and T, with a third
proposed called Y. That first new WISE-discovered brown
dwarf was a T dwarf with a chilly temperature of 600° above
absolute zero. Soon after, scientists found Y dwarfs with even
more contrasting infrared colors, indicating temperatures
cooler than Earth's.
Although we have turned up a couple of new brown dwarf
systems within 10 light-years of Earth, the hunt is still on
for a brown dwarf closer to the Sun than Proxima Centauri.
Scientists suspect that it likely does not exist.
Finding Things That Go Bump in the Night
Asteroid discoveries continued to pour in. Armed with the
NEOWISE software augmentations, we quickly determined
that WISE was particularly good at spotting asteroids with
incredibly dark surfaces, like the first one it discovered.
We could also distinguish asteroids' sizes. To visible-light
telescopes, objects that are large but dark have the same
brightness as objects that are small but lighter colored. But
using the asteroids' heat signatures, we could tell the two
apart. WISE's infrared eyes allowed us to make robust size
measurements for the asteroids zipping through Earth's
neighborhood.
Still, although WISE could pick out the candidate moving
objects, we needed follow-up by ground-based observers using
visible-light telescopes to obtain solid orbital predictions. This
proved challenging because the candidates were so dim, often
requiring large 4- to 8-meter telescopes. We made friends all
around the world as we sought help from the international
community to chase our NEO candidates: Planetary defense
truly is a global sport.
By mid-July 2010, we had successfully made the first
complete survey of the entire sky, fulfilling the baseline
mission requirement. The solid hydrogen in one tank was
exhausted in August 2010, immediately resulting in the loss
of the 22-micron channel as it became blinded by its own
heat. The hydrogen in the next tank held out longer; the
12-micron channel continued to operate, along with the
3.4- and 4.6-micron channels. NASA's Astrophysics Division,
with additional support from its Planetary Science Division,
granted us permission to continue to survey until February
1, 2011, long enough to complete a survey of the main
asteroid belt's inner edge. Our last smidgen of hydrogen-ice
coolant disappeared by October 2010, blinding the 12-micron
channel. The remaining 3.4- and 4.6-micron detectors
Hollywood Cameo
Comet NEOWISE served as the prototype for the
comet in the 2021 movie Don't Look Up, in which
astronomers find a comet on course to hit Earth
within six months, but most people don't believe
them. Such short notice is standard for comets. We
discovered the real Comet NEOWISE on March 27,
2020, and by July 3, 2020, it made its closest
approach to the Sun. (Although unlike its fictional
counterpart, Comet NEOWISE had no chance of
Earth impact.)
warmed to 75K but continued to operate normally.
With only these two shorter-wavelength channels, we were
less sensitive to asteroids in the main belt between Mars and
Jupiter. But NEOs tend to spend much of their time closer to
the Sun. Their warmer temperatures make them bright even
at shorter infrared wavelengths. So we continued to discover
and detect NEOs.
On the first day after losing the 12-micron channel, we
discovered an asteroid that proved to be in a most unusual
orbit. Dragged by Earth's gravity into our planet's path
around the Sun, it is locked into a resonant orbit that keeps
it perpetually ahead of us. This so-called Earth Trojan was the
first such companion discovered for our planet. While Jupiter
is known to have thousands and possibly millions of Trojan
asteroids trapped by its powerful gravity, no one had ever
found an Earth Trojan until the discovery of 2010 TK7.
Because of its orbit, 2010 TK7 perpetually lurks in the
near-Sun sky, a difficult region for Earth-based observers to
search. But that's precisely where our telescope pointed.
February 1, 2011, came quickly. While funding for NASA's
program devoted to discovering and studying potentially
hazardous asteroids had risen somewhat from $4 million per
year, the agency still did not have enough to extend our mission
again. We had to shut down the spacecraft.
Running out of funding was sad, but we felt satisfied that
we had exceeded our mission's requirements. We placed the
spacecraft in hibernation, turning off the instrument and
leaving the telescope pointed continuously at the north ecliptic
pole. Because its aperture cover was ejected after launch,
WISE now viewed the warm Earth for half of each orbit, and
the instrument's temperature quickly shot up to roughly
200K, blinding even the 3.4- and 4.6-micron channels.
We set about analyzing the archival data. We saw hundreds
of NEOs, enabling study of their statistical properties.
By searching for asteroids using their heat signatures rather
than the sunlight they reflect, we could obtain a fair census
of how many NEOs have coal-dark surfaces versus lighter
stony or metallic ones. We found that there was a sizeable
population of dark NEOs that visible telescopes often missed.
Roughly 40% of the NEO population turns out to be made of
dark, carbon-rich material, likely scattered in from the outer
solar system.
sk yand tele scope .o r g * APRIL 2025 17
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Sky & Telescope - April 2025

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