James Webb's cooling system(Cryocooler)

The cooling device for the Mid-Infrared Instrument(MIRI), is one of the James Webb Space Telescope's four instruments. The MIRI requires a lower operating temperature than Webb's other instruments, the cryocooler accommodates this requirement.

On May 24, MIRI's cooler officially passed its pre-ship review. Its main portion, called the cryocooler compressor assembly, was shipped on May 26 to its next destination: the Northrop Grumman Aerospace Systems facility in Redondo Beach, California. There, the cooler will be united with the body of the Webb spacecraft. The MIRI instrument itself is currently at NASA's Goddard Space Flight Center in Greenbelt, Maryland, where it is part of the integrated telescope and instruments. Eventually, those components will make their way to Northrop Grumman too, where the whole observatory will come together in preparation for its momentous 2018 launch.

MIRI is a joint project of Europe and the United States, with the U.S. portion being managed by JPL. The MIRI cooler was developed by Northrop Grumman, and then later sent to JPL for testing to demonstrate its performance and verify its readiness for spaceflight. The Webb telescope mission itself is managed by NASA Goddard.

MIRI will be the coldest instrument onboard the telescope, operating at beyond-frostbite temperatures of no more than 6.7 degrees above absolute zero, or minus 448 degrees Fahrenheit. Why so cold? MIRI sees what is known as mid-infrared light, which is given off by objects at around room temperature. Desks, people, and the air we breathe, for example, are aglow with mid-infrared light that we can't see with our eyes. Specialized instruments like MIRI are designed to pick up this mid-infrared glow, but they must be chilled to avoid background infrared light that can drown out what astronomers want to see.

Cryocooler
Image Credits: NASA/JPL-Caltech

Other infrared telescopes, such as NASA's Spitzer Space Telescope and Wide-field Infrared Survey Telescope (WISE), used thermos-bottle-like coolers filled with coolants, such as liquid helium and solid hydrogen, to chill their instruments. But those systems can be large and heavier to launch. Their biggest downside is that they have finite lifetimes, warming up when their coolants run out.

MIRI started out with a design like this but was later changed to an active cooling system, which works more like a common refrigerator. The MIRI cooler, also called a cryocooler, can chill the instrument without the need for a consumable coolant.

Engineering Challenge

Being an exquisitely sensitive infrared astronomical observatory, the James Webb Space Telescope's optics and scientific instruments need to be cold to suppress infrared background "noise." Moreover, the detectors inside each scientific instrument, that convert infrared light signals into electrical signals for processing into images, need to be cold to work just right. Typically, the longer the wavelength of infrared light, the colder the detector needs to be to do this conversion while also limiting the generation of random "noise" electrons. 

Passive Cooling

Three of Webb's four scientific instruments "see" both the reddest of visible light as well as near-infrared light such as light with wavelengths from 0.6 microns to 5 microns. These instruments have detectors formulated with Mercury-Cadmium-Telluride (HgCdTe), which work ideally for Webb at 37 kelvin. We can get them this cold in space "passively," simply by virtue of Webb's design, which includes a tennis court-sized sun shield.

Active Cooling

The basic principle of active cooling is to compress a gas, then let it expand a process that cools the gas. The same thing happens in refrigerators and air conditioners, which are heat pumps that move heat from a colder place to a warmer place, in reverse of what occurs naturally. A gas or “refrigerant” is compressed by a pump, then allowed to expand where you want the cooling to happen. The process of expansion absorbs heat, and the expanded gas is pumped away and its absorbed heat is dumped away by a radiator. The gas is then recycled and recompressed and the process begins anew. 

However, Webb's fourth scientific instrument, the Mid-infrared Instrument, or MIRI, "sees" mid-infrared (MIR) light at wavelengths from 5 to 28 microns. By necessity, MIRI's detectors are a different formulation (Arsenic-doped Silicon (Si: As)), which needs to be at a temperature of less than 7 kelvin to operate properly. This temperature is not possible on Webb by passive means alone, so Webb carries a "cryocooler" that is dedicated to cooling MIRI's detectors.

The MIRI instrument. MIRI operates at temperatures of no more than 6.7 degrees above absolute zero or minus 448 degrees Fahrenheit.
Credit: NASA/Chris Gunn

Cryocooler Advancements 

Webb's cryocooler has advanced the state of the art in spaceflight cryocoolers of this power and temperature class in two ways. The first one is the precooler uses three stages of pulse-tube cooling vs. heritage systems that have only two stages, and the second one is the separation between the precooler and the JT cooling hardware; typically this separation is centimeters, not several meters.

The Cryocooler Electronics during testing.
Image Credits: NASA/JPL-Caltech

Low Vibration

Moreover, one of the cryocooler's most challenging requirements is low vibration. Vibration levels need to be very low to preclude jitter (induced shaking) of the optics and resultant blurred images. The pulse tube cooling in the precooler in the CCA and the Joule-Thomson effect cooling in the CHA has no moving parts. The only moving parts in the cryocooler are the two 2-cylinder horizontally opposed piston pumps in the CCA, and by having horizontally-opposed pistons that are finely balanced and tuned and move in virtually perfect opposition. 

To avoid excess heat and vibrations affecting MIRI, the Webb telescope's designers had to place the majority of the cooler behind the telescope's massive sun shield. Webb's telescope and main instrument module are protected from the heat of the sun by a shade about as big as a tennis court. With the pumping portion of the cooler on the other side of the shield, a pair of refrigerant lines -- one feed line and one return line, each roughly one-sixteenth of an inch in diameter -- are used to connect it to MIRI. In total, the cooling system involves roughly 67 feet (20 meters) worth of thin tubing that snakes delicately throughout the observatory, carrying the recirculating helium coolant.

Life Span

Being a refrigerator and a "closed" system, the cryocooler does not consume coolants like an ice chest full of ice or a big container which is known as dewar of liquid helium does, and so its life is limited only by wear in its moving parts (the pumps) or the longevity of its electronics, all of which should last for many years.

In-Depth

The MIRI cooling system has four stages, chilling gas down successively to lower and lower temperatures. The first three stages make up the majority of the cooler and take place in the cold compressor assembly -- the largest portion of the cooler. That compressor, as well as its controlling electronics, recently passed cold and vibration tests at JPL. Engineers first fitted the compressor and their electronics into a special cold chamber and tested it, then they vibrated the compressor to mimic the effects of a rocket launch, and finally tested it once again in the cold chamber, checking out its full range of performance.

The Webb MIRI cryocooler is basically a sophisticated refrigerator with its pieces distributed throughout the observatory. The primary piece is the Cryocooler Compressor Assembly (CCA). It is a heat pump consisting of a precooler that generates about 1/4 Watt of cooling power at about 14 kelvin (using helium gas as a working fluid), and a high-efficiency pump that circulates refrigerant (also helium gas) cooled by conduction with the precooler, to MIRI. The precooler features a two-cylinder horizontally-opposed pump and cools helium gas using pulse tubes, which exchange heat with a regenerator acoustically. The high-efficiency pump is another two-cylinder horizontally-opposed piston device that circulates a different batch of helium gas separate from the precooler's helium.

The CCA is located in the heart of the spacecraft bus, on the sun-facing "warm" side of the observatory, and it precools and pumps cold helium gas through plumbing to MIRI, which is roughly 10 meters away in the integrated science instrument module (ISIM). The CCA is controlled by the Cryocooler Control Electronics Assembly (CCEA), which is a collection of electronics boxes mounted in the spacecraft bus inside the port-side equipment panel. The CCA is connected to the ISIM via the Cryocooler Tower Assembly (CTA), which is a pair of gold-plated stainless steel tubes (a feed line and a return line), each about 2 millimeters in diameter, held every foot or so by a series of delicate suspension assemblies (called Refrigerant Line Supports, or RLSs), mounted to the outside of the observatory structure. The CTA connects to the final piece of the cryocooler called the Cryocooler ColdHead Assembly (CHA), which resides in the ISIM. Within the CHA plumbing, inside a gold-plated cylinder, roughly the size and shape of a large coffee can is a small (less than 1 millimeter) orifice that the cooled helium refrigerant passes through, resulting in expansion and final cooling of the helium gas down to about 6 kelvin, care of the Joule-Thomson (JT) effect. This coldest of refrigerated helium gas passes through more than 2-millimeter tubing to a palm-sized copper block fastened to the backside of the MIRI detectors. This is where the target heat is exchanged, resulting in the cooling of MIRI's detectors to nominally around 6.2 kelvin. The CHA also contains valves that allow helium to bypass the JT restriction when the observatory and MIRI are in cooldown mode (such as shortly after launch during deployment and commissioning). The CCA, CTA, and CHA tubing are connected together with pairs of 7/16 inch fittings that on the outside resemble automotive hydraulic brake line connections.

The Cryocooler Compressor Assembly. This photo shows the flight cryocooler installed "upside-down" in a vacuum chamber for testing before the chamber was closed.
Image Credits: NASA/JPL-Caltech

MID-INFRARED INSTRUMENT (MIRI)

The Mid-Infrared Instrument (MIRI) has both a camera and a spectrograph that sees light in the mid-infrared region of the electromagnetic spectrum, with wavelengths that are longer than our eyes see.

The optical module (OM), containing the imager, spectrometers, and coronagraphs, is within the JWST integrated science instrument module (ISIM) with a nominal 40K surrounding temperature. The OM and focal plane modules (FPMs) are brought to a lower temperature by a pulse-tube-based mechanical cooler, with compressors (CCA) and control electronics (CCE) in the spacecraft and refrigerant lines (RLDA) to bring cold gas to a Joule-Thompson (JT) expander near the OM. The instrument mechanisms are controlled by the instrument control electronics (ICE), and the focal planes are operated by the focal plane electronics (FPE), both of which are in region 2, a warm module placed near the ISIM.  

Credits: http://ircamera.as.arizona.edu

MIRI covers the wavelength range of 5 to 28 microns. Its sensitive detectors will allow it to see the redshifted light of distant galaxies, newly forming stars, and faintly visible comets as well as objects in the Kuiper Belt. MIRI's camera will provide wide-field, broadband imaging that will continue the breathtaking astrophotography that has made Hubble so universally admired. The spectrograph will enable medium-resolution spectroscopy, providing new physical details of the distant objects it will observe. 

Installation of MIRI into the instrument module.
Credits: https://webb.nasa.gov

JWST Team Photo with Completed Flight Instrument module
Credits: https://webb.nasa.gov

MIRI was built by the MIRI Consortium, a group that consists of scientists and engineers from European countries, a team from the Jet Propulsion Lab in California, and scientists from several U.S. institutions

The MIRI has three Arsenic-doped Silicon (Si: As) detector arrays. The camera module provides wide-field broadband imagery, and the spectrograph module provides medium-resolution spectroscopy over a smaller field of view compared to the imager. The nominal operating temperature for the MIRI is 7K. This level of cooling cannot be attained using the passive cooling provided by the Thermal Management Subsystem. Webb carries an innovative "cryocooler" that is dedicated to cooling MIRI detectors. Instead, there is a two-step process: A Pulse Tube precooler gets the instrument down to 18K, and a Joule-Thomson Loop heat exchanger knocks it down to 7K.

MIRI Engineering Diagram(Labeled)
Credits: https://webb.nasa.gov

MIRI Engineering Diagram
Credits: https://webb.nasa.gov

Image collection of Pillars of Creation

Pillars of Creation (MIRI Image)

Credits: NASA, ESA, CSA, STScI
Image Processing: Joseph DePasquale (STScI), Alyssa Pagan (STScI)

NASA’s James Webb Space Telescope’s mid-infrared view of the Pillars of Creation strikes a chilling tone. Thousands of stars that exist in this region disappear and apparently endless layers of gas and dust become the most important thing.

The detection of dust by James Webb’s Mid-Infrared Instrument (MIRI) is extremely important dust is a major component of star formation. Many stars are actively forming in these dense blue and gray pillars. When knots of gas and dust with enough mass form in these regions, they begin to fall down under their own gravitational attraction, slowly heat up, and eventually form new stars.

Although the stars appear missing, they aren’t. Stars typically do not discharge much mid-infrared light. Instead, they are easiest to detect in ultraviolet, visible, and near-infrared light. In this MIRI view, two types of stars can be identified. The stars at the end of the thick, dusty pillars have recently eroded the material surrounding them. They show up in red because their atmospheres are still enveloped in cloaks of dust. In contrast, blue tones indicate stars that are older and have shed most of their gas and dust.

Mid-infrared light also details dense regions of gas and dust. The red region toward the top, which forms a delicate V shape, is where the dust is both disseminate and cooler. And although it may seem like the scene clears toward the bottom left of this view, the darkest gray areas are where the densest and coolest regions of dust lie. Notice that there are many fewer stars and no background galaxies popping into view.

Mid-infrared data from James Webb will help researchers determine exactly how much dust is in this region and what it’s made of. These details will make models of the Pillars of Creation far more precise. Over time, we will begin to more clearly understand how stars form and burst out of these dusty clouds over millions of years.

Pillars of Creation (MIRI Compass Image)

Credits: NASA, ESA, CSA, STScI
Image Processing: Joseph DePasquale (STScI), Alyssa Pagan (STScI)

This image of the Pillars of Creation, captured by Webb’s Mid-Infrared Instrument (MIRI), shows compass arrows, a scale bar, and a color key for reference. It lies within the Eagle Nebula, which is also known as Messier 16 (M16).

The north and east compass arrows show the orientation of the image on the sky. The relationship between north and east in the sky as seen below is flipped relative to direction arrows on a map of the ground as seen from above.

The scale bar is labeled in light-years, which is the distance that light travels in one Earth-year. It takes 2 years for light to travel a distance equal to the length of the scale bar. One light-year is equal to about 5.88 trillion miles(9.46 trillion kilometers). The field of view shown in this image is approximately 7 light-years across.

This image shows invisible mid-infrared wavelengths of light that have been translated into visible light colors. The color key shows which MIRI filters were used when collecting the light. The color of each filter name is the visible light color used to represent the infrared light that passes through that filter.

Pillars of Creation (Hubble and Webb Images Side by Side)

Credits: NASA, ESA, CSA, STScI, Hubble Heritage Project (STScI, AURA)
Image Processing: Joseph DePasquale (STScI), Anton M. Koekemoer (STScI), Alyssa Pagan (STScI)

NASA's Hubble Space Telescope made the Pillars of Creation famous with its first image in 1995 but revisited the scene in 201 to reveal a sharper, wider view in visible light, shown above at left. A new, near-infrared-light view from NASA’s James Webb Space Telescope, at right, helps us peer through more of the dust in this star-forming region. The thick, dusty brown pillars are no longer as opaque, and many more red stars that are still forming come into view. While the pillars of gas and dust seem darker and less penetrable in Hubble’s view. But they appear more diaphanous in James Webb’s.

The background of this Hubble image is like a sunrise, beginning in yellows at the bottom, before transitioning to light green and deeper blues at the top. These colors highlight the thickness of the dust all around the pillars, which obscures many more stars in the overall region.

In contrast, the background light in James Webb’s image appears in blue hues, which highlights the hydrogen atoms, and reveals an abundance of stars spread across the scene. By penetrating the dusty pillars, Webb also allows us to identify stars that have recently – or are about to – burst free. Near-infrared light can penetrate thick dust clouds, allowing us to learn so much more about this incredible scene.

Both views show us what is happening locally. Although Hubble highlights many thick layers of dust and James Webb shows more of the stars, neither shows us the deeper universe. Dust blocks the view in Hubble’s image, but James Webb’s major role is played by the interstellar medium. It acts like thick smoke or fog, preventing us from peering into the deeper universe, where countless galaxies exist.

The pillars are a small region within the Eagle Nebula, a vast star-forming region 6,500 light years from Earth.

Pillars of Creation (NIRCam Compass Image)

Credits: NASA, ESA, CSA, STScI
Image Processing: Joseph DePasquale (STScI), Alyssa Pagan (STScI)

This image of the Pillars of Creation, captured by Webb’s Near-Infrared Camera (NIRCam), shows compass arrows, a scale bar, and a color key for reference. It lies within the Eagle Nebula, which is also known as Messier 16 (M16).

This image shows near-infrared wavelengths of light that have been translated into visible-light colors. The color key shows which NIRCam filters were used when collecting the light. The color of each filter name is the visible light color used to represent the infrared light that passes through that filter.

James Webb's Observations on Red Planet

On September 5, 2022, the first images of Mars, captured by James Webb’s Near-Infrared Camera (NIRCam), revealed reflective and thermal properties of the planet with sufficient sensitivity and resolution to explore localized occurrences. 

Webb’s unique observation post nearly a million miles away at the Sun-Earth Lagrange point 2 which is known as L2, provides a view of Mars’ observable disk (the portion of the sunlit side that is facing the telescope). As a result of that, James Webb can capture clearest images and spectra with the spectral resolution needed to study short-term phenomena like dust storms, weather patterns, seasonal changes, and, in a single observation, processes that occur at different times such as daytime, sunset, and nighttime of a Martian day. 

Because it is so close, the Red Planet is one of the brightest objects in the night sky in terms of both visible light and infrared light that James Webb is designed to detect. This causes special challenges for the observatory, which was built to detect the extremely faint light of the most distant galaxies in the universe. Webb’s instruments are so sensitive that without special observing techniques, the bright infrared light from Mars is blinding, causing a phenomenon known as “detector saturation.” Astronomers adjusted for Mars’ extreme brightness by using very short exposures, measuring only some of the light that hit the detectors, and applying special data analysis techniques.

Image Credits: NASA, ESA, CSA, STScI, Heidi Hammel (AURA), Mars JWST/GTO Team

A reference map of Mars from NASA and the Mars Orbiter Laser Altimeter(MOLA) shows the view from James Webb, with the orientation and lighting of the planet at the day and time of the observation. On September 5, 2022, during summer in Mars’ southern hemisphere, the observation was made. The central longitude is approximately 80 degrees east. The axis is tilted 25 degrees from perpendicular to the orbital plane. The eastern section of the disc is in the evening shadow. The map shows geographic features and surface coloring typically visible in reflected sunlight. Three features are labeled: Syrtis Major, a dark-colored volcanic region; the Huygens Crater, a complex impact crater; and the Hellas Basin, the largest preserved impact structure on Mars. 

Image Credits: NASA, ESA, CSA, STScI, Heidi Hammel (AURA), Mars JWST/GTO Team

A Near-Infrared Camera image shows 2.1-micron (F212 filter) reflected sunlight, revealing surface features similar to those seen in the base map, including the dark volcanic rocks of Syrtis Major, light rings of the Huygens Crater, and a section of the Hellas Basin, which may be coated in a layer of light-colored dust. The field of view is outlined in blue on the base map. 

Image Credits: NASA, ESA, CSA, STScI, Heidi Hammel (AURA), Mars JWST/GTO Team

Another NIRCam image captured simultaneously, shows ~4.3-micron (F430M filter) discharged light that reveals temperature differences with latitude and time of day, as well as darkening caused by atmospheric effects. The measured brightness is highest near the subsolar region. It is shown in the above image with yellow color. It decreases noticeably toward the northern hemisphere which is experiencing winter and toward the western hemisphere which is in night-time darkness. However, the measured brightness of the Hellas Basin is less than its surroundings not because it has a lower temperature, but rather because it is deeper and experiences higher air pressure. It is more than 4 miles, or 7 kilometers, below the surrounding terrain. This dampens the thermal emission at this wavelength: Some of the 4.3-micron light emitted from the basin is absorbed by molecules like carbon dioxide in Mars’ atmosphere.

Mars Atmosphere Composition
Image Credits: NASA, ESA, CSA, STScI, Heidi Hammel (AURA), Mars JWST/GTO Team

The spectrum shows a combination of sunlight reflected from Mars’ surface and atmosphere, and light emitted by the planet as it gives off heat. Wavelengths between 1 and 3 microns are dominated by reflected light. Wavelengths between 3 and 5 microns are dominated by emitted light. Both lights pass through Mars’ atmosphere, affecting the brightness of various wavelengths and the shape of the spectrum in various ways.

The deep valleys are absorption features caused when specific wavelengths are blocked by gases such as carbon dioxide, water, and carbon monoxide. Other details, like the broad shape of the spectrum and the slope of the curve at different points, reveal information about dust, clouds, and surface features. 

The data were collected using six different high-resolution grating modes (spectroscopy modes), each of which covers a different wavelength range. The white line is not continuous because there are small gaps in coverage. The best-fit model takes into account the data as well as other known properties of Mars. Constructing a best-fit model using a tool such as the Planetary Spectrum Generator makes it possible to estimate the abundance of given molecules in the atmosphere.