Sky & Telescope - April 2025 - 14

ELECTROMAGNETIC SPECTRUM: TERRI DUBÉ & BEATRIZ INGLESSIS / S&T; ATOMS: CREATIVE STALL / THE NOUN PROJECT; MOLECULES:
DELWAR HOSSAIN / THE NOUN PROJECT; HUMAN CELLS: TERRI DUBÉ / S&T; GRAINS OF SAND: DENNIS / THE NOUN PROJECT; INSECTS:
VECTORSTALL / THE NOUN PROJECT; HUMANS: ADRIEN COQUET / THE NOUN PROJECT; BUILDINGS: UN·DELIVERED / THE NOUN PROJECT
Infrared Eye
10-12
Gamma rays
106
10-9
X-rays
103
10-6
UV Visible
100
WISE
Wavelength (meters)
10-3
Infrared
Microwaves
Photon energy (eV)
10-3
10-6
100
103
Radio waves
10-9
10-12
106
Atoms
Molecules
Human
cells
Grains
of sand
Insects
Humans Buildings
ELECTROMAGNETIC SPECTRUM The human eye can see only a tiny fraction of the electromagnetic spectrum. Observing infrared radiation,
which humans perceive as heat, greatly expands what we know about the universe. (Objects with sizes similar to the wavelengths are also shown.)
leagues likens observing infrared sources using ground-based
telescopes to trying to observe the sky at visible wavelengths
during the daytime while using a telescope made out of fluorescent
light bulbs.
It's better to go into space, where there's no atmosphere
and no day or night. There, we can cool the telescope and
camera down to temperatures approaching absolute zero,
achieving an orders-of-magnitude improvement in sensitivity.
Even a modest-size infrared telescope becomes vastly more
powerful once it's put into space and cooled so that the heat
from the optics and sky become negligible.
The Revolution Will Be Electronic
Before WISE, the whole sky had not been surveyed in midinfrared
light since the 1983 launch of the Infrared Astronomical
Satellite (IRAS), which used a then-groundbreaking
62-pixel camera to create the first all-sky infrared image.
More modern, powerful infrared detectors were trained on
the sky with the launch of the Spitzer Space Telescope in
2003. Spitzer covered similar wavelengths as IRAS, with a
larger and more sensitive telescope, but it was designed to
make highly detailed observations of small regions of the sky.
The (then upcoming) launch of the James Webb Space Telescope
(JWST) made the creation of a modern all-sky infrared
map all the more important. JWST's gigantic 6.5-meter mirror
would offer spectacular sensitivity and capability beyond
Spitzer, but with an even smaller field of view, effectively peering
through a tiny straw. Scientists would need a " finder map "
to identify the most interesting targets for JWST.
WISE was designed to provide that map. Professor Edward
" Ned " Wright (University of California, Los Angeles), the
WISE principal investigator, had been refining the mission
concept since 1994. NASA finally confirmed WISE for flight
through its Medium-class Explorer program in 2006.
Ned and his colleagues developed a survey concept elegant
in its simplicity. The 40-cm (16-inch) WISE telescope would
orbit roughly 525 km (325 miles) above Earth in a so-called
Sun-synchronous orbit, always traveling above Earth's day14
APRIL 2025 * SKY & TELESCOPE
night line while looking outward at the sky. The spacecraft
would continuously scan as it moved along its orbit, using a
small mirror about the size of a makeup compact to " freezeframe "
the image onto the detectors. Such a survey pattern
would result in an image taken every 11 seconds, efficiently
building a complete sky map in just six months.
Making this map in the infrared was the first and most
important mission goal, at wavelengths of 3.4, 4.6, 12, and
22 microns. The team selected these wavelengths in order to
study two primary science targets: the nearest stars to the
Sun and the most luminous galaxies in the universe.
We chose the two shortest wavelengths to exploit a feature
of brown dwarfs, objects intermediate in mass and temperature
between Jupiter and a star whose internal pressure and
temperature are too low to sustain nuclear fusion. Like Jupiter,
brown dwarfs are rich in methane, a gas that absorbs powerfully
at 3.4 microns but is transparent at 4.6 microns. The
cooler the object, the stronger the signal of methane absorption.
By comparing an object's brightness in the 3.4- and
4.6-micron channels, astronomers could pick out the unusual
infrared colors of methane-rich, ultracool brown dwarfs.
Given that small, cool stars vastly outnumber warmer
Sun-like stars inside our galaxy, scientists predicted that a
brown dwarf might lie even closer to our Sun than our current
nearest stellar neighbor, Proxima Centauri. If such an
object existed, WISE would find it.
At the other end of WISE's vision, the longer wavelength
channels at 12 and 22 microns were designed to pick up
dust emission from so-called ultraluminous infrared galaxies
- galaxies faint in visible light but immensely bright in
infrared. Galaxies like our Milky Way exist today in relative
calm. However, in an earlier era long ago, massive collisions,
mergers, and near-misses were much more common.
As galaxies shredded one another or merged, the pileup and
mixing of massive clouds of dust and gas set off huge waves
of star formation, resulting in a blaze of infrared light that
WISE could see.
Other science objectives included examination of the

Sky & Telescope - April 2025

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