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science.org SCIENCEinflated costs. After Congress threatened
to cancel the project in 2011, NASA set a
new schedule and, for a while, kept to it.
But more delays came when prime contrac-
tor Northrop Grumman discovered faulty
welds, incorrect lubricants, missing bolts,
and tears in the telescope’s giant sunshield.
Now, all that is fixed, but one last hur-
dle remains. In September, engineers in
California packed Webb up in an environ-
mentally controlled shipping container
and sent it by sea to French Guiana, home
to Europe’s spaceport. On 18 December,
Webb will endure the roar and rattle of a
27-minute ride to orbit on a European
Ariane 5 rocket, its folded-up mirror just
barely fitting inside the fairing.
A month of critical maneuvers will fol-
low as the telescope cruises deeper into
space. “It’s 30 days of terror,” says Garth
Illingworth of the University of California,
Santa Cruz (UCSC), who was one of the
project’s early architects. As soon as it’s in
space, Webb will deploy its solar array and
then, 2 hours later, its communications
antenna. On day 3, as it passes the Moon,
its huge sunshield will begin to unfurl. By
day 11, the mirrors will start to unfold and
swing into place. Finally, after 29 days,
Webb’s boosters will make a final burn to
put it in orbit around L2, a gravitational
balance point 1.5 million kilometers from
Earth. Unlike Hubble, Webb will be too
far away to be repaired by visiting astro-
nauts. It must work flawlessly straight out
of the box.
PLANNING FOR Webb began as far back
as 1989, before Hubble had even left the
ground. It was originally nicknamed the
First Light Machine, recalls Marcia Rieke,
an astronomer at the University of Arizona
who served on the project’s working group
in the late 1990s. The goal was to peer back
to the universe’s infancy.
Ground-based telescopes at the time
could barely see halfway back across the
13.8-billion-year history of the universe.
Hubble, with no atmosphere to blur its
view, could take astronomers much closer
to the beginning. In its first “deep-field”
exposure, in 1995, it stared at a seem-
ingly empty patch of sky in Ursa Major for
140 hours. Almost every one of the 3000 ob-
jects that popped into view was a distant
galaxy, shining from times as early as 1.5 bil-
lion years after the big bang.
Subsequent exposures went even deeper
in time. Astronomers assumed the number of
galaxies would fall off sharply at those early
times because gravity had not yet pulled
clouds of gas into stars, let alone assembled
stars into galaxies. But Hubble showed the
galaxies were there, albeit in dwindling
numbers. In 2016, researchers using Hubble
data found a small galaxy, dubbed GN-z11,
that dated, astonishingly, from a time when
the universe was just 400 million years old.
“We had no idea you could see objects at that
time,” Illingworth says.
At that distance, the expansion of the uni-
verse shifts a galaxy’s visible light—where
stars tend to shine brightest—well into the
infrared, which Hubble cannot see. GN-z11
was only visible because it shone brightly in
ultraviolet light, seen as visible light after
redshifting. Many more early galaxies, in-
visible to Hubble, may lurk in the infrared
band. “It’s hard to be sure,” Rieke says.
Webb should be able to tell, thanks to its
huge mirror and infrared detectors, which
are insulated from heat that would degrade
their sensitivity. The multilayer fabric sun-
shield, as big as a tennis court, will create ashadow deep enough to passively chill three
of Webb’s instruments to –234°C, or 39 K.
The cryocooler will refrigerate a fourth in-
strument, meant to peer further into the in-
frared, to 7 K.
Roberto Maiolino, an astronomer at the
University of Cambridge, expects Webb to
find 10,000 galaxies between cosmic dawn—
when the first stars ignited about 200 million
years after the big bang—and cosmic noon,
the peak of star formation roughly 2 billion
years later. The first galaxies probably started
out small and disorganized, nucleating
around clumps of dark matter, unseen stuff
that makes up 85% of the matter of the uni-
verse. By compiling a census of early galaxies,
Webb will show “how these blobs change to
more organized structures,” says Rieke, who
led the development of Webb’s near-
infrared camera.
Mapping out how protogalaxies formed
might also reveal something about the na-
ture of dark matter, Rieke says. “Finding
the first aggregations of stars may tell us
more about what was leftover after the big
bang,” she says. There are other fundamen-
tal questions: Did the galaxies grow simply
by pulling in more gas, or through a series
of mergers, known to spark bursts of star
formation in more recent galaxies? “There
is so much low-hanging fruit, so many
obvious questions ... but we didn’t have
the technical ability to answer them yet,”
Finkelstein says.A MAJOR COUP for Webb would be spotting
evidence for first-generation stars, known
as population III stars, formed from the
primordial hydrogen and helium gas left-
over from the big bang. Later generations
of stars contain heavier elements, forged
in stellar furnaces and scattered by super-
novae, that radiate energy efficiently. Lack-
ing these radiators, population III stars
swell to enormous sizes, and are possibly up
to 1000 times as massive as the Sun. Their
size means they burn fast and furiously, ex-
hausting their fuel in a few million years.
Webb will almost certainly not be able to
see to these stars individually, but a galaxy’s
spectrum can betray their presence. “If we
see a galaxy with a spectrum of only hydro-
gen and helium, that would be a smoking
gun” of population III stars, Maiolino says.
The spectrum would also hold clues to the
stars’ abundance and temperatures, giving
astronomers a picture of early star forma-
tion and how these fast-burning giants, after
exploding in supernovae, delivered the first
smattering of heavy elements to the universe.
Those discoveries could also bear on
the mystery of what ionized the hydrogen
gas that fills the universe, making it trans-
parent to light. Roughly 400,000 years
after the big bang, the universe had cooled
enough for protons to hook up with elec-
trons. The resulting neutral hydrogen
acted as a cosmic fog, absorbing high en-
ergy photons. But half a billion years later,
the fog began to clear as something split
the hydrogen apart again. This “Epoch
of Reionization” continued for another
half-billion years until all the hydrogen
was ionized.
Ultraviolet light from big and hot popula-
tion III stars is an obvious ionizing source.
But were there enough stars to ionize all of
space? The small number of early galaxies
discovered by Hubble would suggest not,
but Webb’s more complete census may ce-
ment the role of starlight. If not, there is
another possibility: the supermassive black
holes at the heart of most galaxies. As ma-
terial sucked into a black hole swirls down
the cosmic drain, friction heats it to such
enormous temperatures that it shines as a
brilliant beacon: a quasar or active galactic
nucleus (AGN).
Astronomers have found quasars less
than 700 million years after the big bang,
but many think they are too few and far be-
tween to evenly ionize all of space. Others
are more hopeful. The quasars spotted so
far are huge, with masses equivalent to 1 bil-
lion Suns, which means many smaller ones
remain to be discovered, Finkelstein says.
“There are probably a lot of 100-million-
solar-mass black holes and even more
10-million-solar-mass black holes,” he says.“We will see the first
chemical fingerprints from
Earth-sized planets.”
Natalie Batalha,
University of California, Santa Cruz808 12 NOVEMBER 2021 • VOL 374 ISSUE 6569