"One of the rules for building spacecraft is to have as few moving parts as
possible," Andy Bunker, Oxford University astrophysicist, grins. "And that's
why we have built something that has a quarter million shutters."
Bunker is talking about NIRSpec, the Near Infrared Spectrograph, one of the
four instruments on the James Webb Space Telescope, that are just getting
ready to open their eyes to the cosmos. He is one of seven European
scientists that have shaped the design of NIRSpec (which is funded by the
European Space Agency), and is now eagerly awaiting the moment when the
instrument starts delivering data.
"I've always been interested in pushing the limit for the most distant known
object," Bunker told Space.com. "That goes beyond simply holding the record.
It's about understanding the early stages of the universe, when the first
stars and galaxies formed. And we're trying to chase this."
With its giant 21.6-feet-wide (6.5 meters) mirror, Webb was built to
study the oldest and most distant galaxies that emerged in the young
universe from dust and gas after the dark ages following the Big Bang. It
will do that by observing infrared light, the heat-carrying part of the
electromagnetic spectrum with longer wavelengths than visible light.
Scientists know that although those earliest stars emitted visible light,
because of their vast distance and the expansion of the universe, this light
shifted into the infrared part of the spectrum, an effect known as the
redshift.
NIRSpec, with its rule-breaking quarter of a million microshutters, will
give Webb's capabilities a boost. In fact, NIRSPec will exceed more than one
hundred times the capability of a similar instrument flown on the Hubble
Space Telescope, which is, in many ways, considered Webb's predecessor
(although the two will work in parallel, for some time).
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| Spectrographs, like NIRSpec, split the incoming starlight into spectra, which enables scientists to see what the stars are made of. (Image credit: ESA) |
Galactic fingerprints
NIRSPec might not be the instrument that will produce the most eye-catching
images like the famous snapshots of the pillars of creation and
awe-inspiring deep fields from Hubble. That will be the task of the NIRCam
and MIRI cameras.
NIRSpec, however, will deliver an unprecedented amount of information about
not only the galaxies, stars and planets photographed by NIRCam and MIRI,
but hundreds and thousands of others.
As a spectrograph, NIRSpec doesn't take images. It splits the incoming light
into individual components of the light spectrum. This spectrum, like a
fingerprint, reflects the light-absorbing properties of the imaged objects
and thus their chemical composition. Every chemical element present in the
observed body absorbs light in a certain way, which shows as a distinct line
in the captured spectrum. By capturing an object's spectrum, researchers can
determine what chemical compounds may be present.
"Scientifically, spectra are enormously valuable," said Bunker. "There's a
lot of information encoded in them. We can chart how chemical elements build
up in galaxies, but also determine the distance and properties of galaxies,
such as the rate at which they turn their gas into stars."
To take the fingerprints of these objects accurately, spectrographs need to
block all other light from their field of view. Conventional spectrographs,
like those on Hubble, do that using a slit, a narrow opening in a metal
plate through which they target only the studied object.
"The slit allows you to get as sensitive as you can," Bunker said. "But the
limitation is doing just one object at a time. It's very ineffective,
particularly if you look at some of the deep fields where you have very high
densities of potentially interesting objects."
Add to it that some of the objects Webb will study are so distant and faint
that the telescope will have to stare at them for hundreds of hours to
collect enough light, and the limitation of such an approach becomes
obvious.
Spectrography amplified
This is where NIRSPec's microshutters come in. Each only as wide as a human
hair, these micro-shutters can open in various patterns, flexibly creating a
multitude of slits that will allow astronomers to observe and measure a
hundred (or more) galaxies at the same time, said Bunker.
The first spectrograph of its kind flown in space, NIRSpec features a range
of innovative technologies. The microshutter array itself, developed by
engineers at NASA's Goddard Space Flight Center, is arranged in four
rectangular sections, each having 365 by 171 microshutters.
An electrically controlled magnet sweeps at the back of these arrays. By
selectively applying an electrical current to every single microshutter, the
ground control teams determine, which shutters open and which remain shut.
Engineers, however, can't open microshutters that are too close to each
other as the spectra of the observed objects would overlap. That leaves the
possibility to study around a hundred objects at the same time.
"These microshutters are quite fragile so we expect that a fraction of them
will fail," said Bunker. "That's not a big deal because we will still be
able to use the others. Some may also get stuck open, which would add some
extra background light, but that would only be very few."
A game changer
The vast quantities of stars, galaxies, clusters, planets and other bodies
NIRSpec is going to look at will enable scientists to start answering the
big questions not about individual stars and galaxies, but the entire
universe.
"NIRSpec will be a game changer," Bunker said. "The number of objects that
we will be able to tackle will enable us to start seeing how various
variables depend on each other. For example how the rate at which stars are
forming in galaxies varies with the mass of the galaxy or its age."
With the help of NIRSpec, astronomers will not only be able to see the first
stars and galaxies that formed in the universe, but also to know what they
were made of and how, upon their death, they gave rise to other chemical
elements that gradually populated the universe as we see it today.
"We know that hydrogen and helium formed in the Big Bang," said Bunker. "But
all the heavier elements formed in stars either during their lifetime or
when they died. So as a function of time, the number and fraction of these
heavier elements should increase in galaxies. And we hope to measure that
directly."
Opening its shutters
Since Webb reached its target destination in January, the so-called Lagrange
Point 2 (L2) some 930,000 miles (1 million kilometers) away from Earth, the
telescope has been cooling down to its operating temperature of minus 369.4
degrees Fahrenheit (minus 223 degrees Celsius). Since Webb chases infrared
light and since infrared light is essentially heat, any warmth emitted by
the telescope itself would dazzle its super sensitive detectors.
With Webb as cold as it needs to be, its four instruments are gradually
opening their "eyes" (or micro-shutters), allowing the scientists to, for
the first time, test their performance not in a lab, but in the actual
environment of space.
"We have a number of activities for NIRSpec over the coming three months,"
said Bunker. "We are quantifying how many shutters are usable and what is
their sensitivity. But I think in general, the telescope is performing
extremely well."
The public, however, will have to wait until at least early July, to finally
get a glimpse of the universe through the eyes of the most complex and most
expensive space telescope ever built.
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