Sky & Telescope - April 2025 - 19

SPACE IMAGE: NASA / JPL-CALTECH; COMET OVER LANDSCAPE: SEAN WALKER / S&T
uCOMET NEOWISE Inset: Comet C/2020 F3 appears as a
string of fuzzy red dots in this composite infrared image
taken by NASA's NEOWISE mission, used to discover
the comet on March 27, 2020. Far right: The comet
graces New Hampshire skies on July 9th.
11-year solar cycle causes Earth's atmosphere
to puff up, increasing drag forces
on spacecraft in low Earth orbits. A long
stretch of solar calm reduced atmospheric
drag and kept our orbit in good shape,
allowing us to observe and characterize
thousands more asteroids.
But by 2022, it was clear that solar activity
was picking up again, causing NEOWISE to lose
altitude faster. It was difficult to predict the endgame,
but we decided to propose one final mission extension. We
chose the end of July 2024 as the survey's last day, hoping to
squeeze the last science out before drag forces caused spacecraft
malfunctions.
We collected the last image on July 31, 2024. Reentry
occurred on November 1, 2024, sweeping away any lingering
doubts I had about the survey's end date. We chose the perfect
time: The last image was just as lovely and star-spangled
as the first.
The extended NEOWISE mission gave us a multi-year
view of the entire sky at 3.4 and 4.6 microns, complementing
other time-domain surveys at different wavelengths from
other ground- and space-based observatories. Today, there are
roughly 10,000 refereed citations using the WISE and NEOWISE
data, with more studies being published every day.
The results from NEOWISE also suggested we could do
more. In 2006, I proposed a mission to NASA to make a much
more comprehensive survey of the NEOs, now known as
the NEO Surveyor telescope. It was later accepted by NASA's
Planetary Defense Program and is now finally under construction
and slated to launch in late 2027. By adding many
more pixels and placing the spacecraft farther from Earth to
keep the instrument cool without expendable cryogens like
hydrogen or helium, we will be able to search the dusk and
dawn skies where potentially hazardous asteroids spend much
of their time. We learned a lot about discovering NEOs with a
space telescope from NEOWISE, and its bigger, badder cousin
NEO Surveyor will find and characterize many more.
All space missions are symbols of hope: in the power of
people to dream up creative ways to solve difficult problems,
in their ability to work seamlessly together and build something
that survives the hostile environment of space, and in
their steadfast dedication to see a project through to the end.
I was lucky to be a part of WISE/NEOWISE. Though it took
more than 20 years, the big science that is still coming from
this small mission was worth the wait.
¢ AMY MAINZER is a professor of planetary science at the
University of California, Los Angeles, where she leads the
NEOWISE and NEO Surveyor missions.
sk yand tele scope .o r g * APRIL 2025 19
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Sky & Telescope - April 2025

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