Set to launch on Dec. 24, NASA's James Webb Space Telescope is the largest
space observatory in history, and it has an equally gargantuan task: to
collect infrared light from the distant corners of the cosmos, enabling
scientists to probe the structures and origins of our universe and our place
in it.
Many cosmic objects – including stars and planets, as well as the gas and
dust from where they form – emit infrared light, sometimes called heat
radiation. But so do most other warm objects, like toasters, humans, and
electronics. That means Webb's four infrared instruments can detect their
own infrared glow. To reduce those emissions, the instruments have to be
really cold – about 40 kelvins, or minus 388 degrees Fahrenheit (minus 233
degrees Celsius). But to operate properly, the detectors inside the
mid-infrared instrument, or MIRI, will have to get even colder: less than 7
kelvins (minus 448 degrees Fahrenheit, or minus 266 degrees Celsius).
That's just a few degrees above absolute zero (0 kelvins) – the coldest
temperature theoretically possible, though it's never physically attainable
because it represents the total absence of any heat. (MIRI is not, however,
the coldest imaging instrument ever to operate in space.)
Temperature is essentially a measurement of how fast atoms are moving, and
in addition to detecting their own infrared light, the Webb detectors can be
trigged by their own thermal vibrations. MIRI detects light in a
lower-energy range than the other three instruments. As a result, its
detectors are even more sensitive to thermal vibrations. These unwanted
signals are what astronomers refer to as "noise," and they can overwhelm the
faint signals that Webb is trying to detect.
After launch, Webb will unfold a tennis-court-size sunshield that will block
MIRI and the other instruments from the Sun's heat, allowing them to cool
passively. Beginning about 77 days after launch, MIRI's cryocooler will
spend 19 days lowering the temperature of the instrument's detectors to less
than 7 kelvins.
"It's relatively easy to cool something down to that temperature on Earth,
typically for scientific or industrial applications," said Konstantin
Penanen, a cryocooler specialist at NASA's Jet Propulsion Laboratory in
Southern California, which manages the MIRI instrument for NASA. "But those
Earth-based systems are very bulky and energy inefficient. For a space
observatory, we need a cooler that is physically compact, highly energy
efficient, and it has to be highly reliable because we can't go out and
repair it. So those are the challenges we faced, and in that respect, I
would say the MIRI cryocooler is certainly at the cutting edge."
One of Webb's big science goals will be to study the properties of the first
generation of stars to form in the universe. Webb's Near-Infrared Camera, or
NIRCam instrument, will be able to detect these extremely distant objects,
and MIRI will help scientists confirm that these faint sources of light are
clusters of first-generation stars, rather than second-generation stars that
form later as a galaxy evolves.
By peering through even thicker clouds of dust than the near-infrared
instruments, MIRI will reveal the birthplaces of stars. It will also detect
molecules that are common on Earth – like water, carbon dioxide, and
methane, and those of rocky minerals like silicates – in cool environments
around nearby stars, where planets may form. Near-infrared instruments are
better at detecting these molecules as vapor in much hotter environments,
while MIRI can see them as ices.
"By combining expertise from both the United States and Europe, we have
developed MIRI as a powerful capability for Webb that will enable
astronomers from all over the world to answer big questions about how stars,
planets, and galaxies form and evolve," said Gillian Wright, co-lead of the
MIRI science team and the instrument's European principal investigator at
the UK Astronomy Technology Centre (UK ATC).
The Big Chill
The MIRI cryocooler uses helium gas – enough to fill about nine party
balloons – to carry heat away from the instrument's detectors. Two
electrically powered compressors pump the helium through a tube that extends
to where the detectors are located. The tube runs through a block of metal
that is also attached to the detectors; the cooled helium absorbs excess
heat from the metal block, which in turn keeps the detectors at their
operational temperature below 7 kelvins. The warmed (but still quite cold)
gas then returns to the compressors, where it dumps the excess heat, and the
cycle begins again. Fundamentally, the system is similar to those used in
home refrigerators and air conditioners.
The tubing that carries the helium is made of gold-coated stainless steel
and measures less than one-tenth of an inch (2.5 millimeters) in diameter.
It extends about 30 feet (10 meters) from the compressors, located in a
region called the spacecraft bus, to MIRI's detectors, located in the
Optical Telescope Element, behind the observatory's honeycomb-shaped primary
mirror. Hardware called the Deployable Tower Assembly, or DTA, connects
these two regions. When packed for launch, the DTA is compressed, sort of
like a piston, to help fit the stowed observatory into the protective faring
that rides atop the rocket. Once in space, the tower will extend to separate
the room-temperature spacecraft bus from the much colder Optical Telescope
Instrument, and to allow the sunshield and telescope to fully deploy.
But the elongation process requires the helium tubing to extend along with
the Deployable Tower Assembly. So the tube is coiled like a spring, which is
why MIRI engineers nicknamed this portion of the tube the "Slinky."
"There were a couple of challenges working on a system that spans multiple
regions of the observatory," said Analyn Schneider, MIRI's project manager
at JPL. "Those different regions are led by different organizations or
centers, including Northrop Grumman and NASA's Goddard Space Flight Center,
and we had to interface with everybody. There's no other hardware on the
telescope that requires that, so that was a challenge unique to MIRI. It's
definitely been a long road for the MIRI cryocooler, and we're ready to see
it perform in space."
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