Sky & Telescope - April 2025 - 18

Infrared Eye
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WISE ROUNDUP This " top-down " view of
the solar system shows asteroids and comets
discovered by WISE through February 1, 2011.
Main-belt asteroids are shown in black. NearEarth
objects discovered by other surveys are
purple, while those discovered by WISE are
red. The squares are comets: Previously known
comets are blue, and comets discovered by
WISE are yellow. When the spacecraft's two
solid hydrogen tanks were exhausted, the craft's
own heat blinded the 12- and 22-micron bands,
but the survey continued at 3.4 and 4.6 microns.
The drop-off in detections at the figure's top is
due to that reduced sensitivity.
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construction would be expensive, more
than $100 million, while delivering only
similar capability as a simpler and much
cheaper option: reactivating the WISE
spacecraft. For an annual budget of
roughly $5.5 million, we could efficiently
characterize NEO sizes and reflectivities.
This cost fit the limited budget of
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Astronomical units
We also learned that the NEO surveys had done a good job
of finding the largest near-Earth asteroids, objects big enough
to cause global extinctions. But the community had not
found the majority of smaller asteroids still large enough to
cause severe regional disasters.
Our small-body discoveries extended beyond near-Earth
space. The previous all-sky infrared survey, IRAS, had
detected roughly 2,000 asteroids. Now, we had measurements
of diameters and reflectivities for more than 140,000, mostly
in the main belt. We found that main-belt asteroids tend to
break up into families that produce distinctive clumpings of
bright and dark objects. We studied Jupiter's Trojan asteroids,
ancient objects likely captured early in our solar system's
history, and showed that the cloud leading Jupiter has more
members than the cloud trailing in the giant planet's wake.
Comets found by the mission were revealed to have dark
nuclei consistent with a carbon-rich composition.
The rest of the astronomical community quickly dove into
the data, and a steady stream of new results turned into a
flood of papers.
Episode IV: A New Hope
In 2012, NASA issued a request for proposals to build a new
camera mounted on a geostationary satellite to search for
NEOs. I put together a study team, but we noted that new
18 APRIL 2025 * SKY & TELESCOPE
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NASA's Near-Earth Object Observations
program, and they accepted our proposal.
By December 2013, we once again
pointed the spacecraft away from Earth,
beginning survey operations shortly after
the telescope cooled back down to 75K.
In honor of its new mission, we renamed
the spacecraft NEOWISE.
We continued to find new NEOs,
most of which were dark. We also supported a planetary
defense campaign to simulate the discovery of the asteroid
99942 Apophis, a 340-meter-wide space rock that will not
impact Earth but will come hair-raisingly close - underneath
our geostationary satellites - in the year 2029.
Although Apophis was discovered long ago, the international
community engaged in an exercise to treat its 2020-21 close
approach as if it was an unknown new object, helping to
hone astronomers' responses to future discoveries. The NEOWISE
infrared observations of Apophis provided a diameter
estimate that quickly ruled out sizes consistent with global
devastation, limiting its destructive potential to " merely "
regional catastrophe.
In 2020, at the height of the pandemic, we found a fuzzylooking
object in the images: the naked-eye comet C/2020
F3 NEOWISE (S&T: Nov. 2020, p. 14). I remember driving
along a remote mountain road seeking a glimpse of it, only to
find hundreds of other people lining the roadside, skygazing
together while social distancing. Seeing the comet that had
been a dot on my computer screen now in the sky with my
own two eyes embodied the power and beauty of science.
We kept expecting to eke one to two more years out of
NEOWISE. Extensions were meted out accordingly by NASA.
The unexpected boon was that the Sun remained unusually
quiet. The increased influx of radiation during the peak of the
Astronomical units
ADAPTED FROM A. MAINZER ET AL. / ASTROPHYSICAL JOURNAL 2011 (731:53)
Sky & Telescope - April 2025

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