If you were asked to name the coldest spot in the solar system, chances are pretty good you’d think it would be somewhere as far as possible from the ultimate source of all the system’s energy – the Sun. It stands to reason that the further away you get from something hot, the more the heat spreads out. And so Pluto, planet or not, might be a good guess for the record low temperature.
But, for as cold as Pluto gets – down to 40 Kelvin – there’s a place that much, much colder than that, and paradoxically, much closer to home. In fact, it’s only about a million miles away, and right now, sitting at a mere 6 Kelvin, the chunk of silicon at the focal plane of one of the main instruments aboard the James Webb Space telescope makes the surface of Pluto look downright balmy .
The depth of cold on Webb is all the more amazing given that mere meters away, the temperature is a sizzling 324 K (123 F, 51 C). The hows and whys of Webb’s cooling systems are chock full of interesting engineering tidbits and worth an in-depth look as the world’s newest space telescope gears up for observations.
Not Cold Enough
Probably the first most obvious question regarding cryocoolers in space is: Why in the world does Webb even need a cryocooler? Isn’t space, especially the area around Webb’s halo orbit around Lagrange point L2, already cold enough? In a word, no – the infrared astronomy Webb’s instruments are designed for, space is nowhere near cold enough. But what’s so special about infrared astronomy, and why does it require such low temperatures?
From its earliest designs, what would become the James Webb Space Telescope was always conceived as an infrared telescope. This is because the objects Webb was intended to study are among the oldest objects in the universe, and Hubble’s Law tells us that the farther away an object is, the faster it is moving away from Earth, the light from them will be dramatically red- shifted thanks to the Doppler effect. This means that the light from pretty much everything Webb will be pointed at lies somewhere in the infrared part of the spectrum. Webb’s four imaging and spectrographic instrument packages can cover from the very edge of the visible portion of the spectrum, around 0.6 μm wavelength, to the mid-infrared wavelengths around 28 μm. For reference, microwaves start at about 100 μm wavelengths, so the frequency of the light that Webb is designed to study isn’t that far above the radio part of the electromagnetic spectrum.
The problem with infrared astronomy is that the sensors used to pick up the light are easily overwhelmed by the heat of their surroundings, which radiates in the infrared region. Also, the photosensors used in infrared telescopes are susceptible to dark current, which is a current flow in the sensor even in the absence of any light falling on it. Dark current is primarily caused by the thermal stimulation of electrons within the sensor material, so keeping the sensor as cold as possible goes a long way to reducing noise.
There’s Cold, and Then There’s MIRI Cold
As stated earlier, Webb has four main instruments. Three of them-the Near-Infrared Camera (NEARCam), the Near-Infrared Spectrograph (NEARSpec), and the Fine Guidance Sensor and Near-Infrared Imager and Slitless Spectrograph (FGS-NIRISS)-all operate in the near-infrared part of the spectrum, as their names suggest. The near-infrared is just below the visible part of the spectrum, around 0.6 to 5.0 μm. The sensors for these wavelengths use an alloy of mercury, cadmium, and tellurium (Hg: Cd: Te), and require cooling down to around 70 Kelvin to be usable.
For Earth-based near-IR telescopes, cooling Hg: Cd: Te sensors is usually done with liquid nitrogen. On Webb, though, another option is available, thanks to the massive, five-layer sunshade that protects the observatory from the blazing light of the Sun, as well as the light reflected off the Earth, which thanks to the telescope’s halo orbit is always in view. The layers of Webb’s aluminized Kapton sunshade are spaced out such that incident IR bounces between adjacent layers and eventually radiates out into space more or less perpendicular to the sunshade, rather than penetrating through the layers to the sensitive optics on its dark side. The sunshade receives on the order of 200 kW of energy on the hot side, while allowing only 23 mW to pass through to the cold side. This keeps the instruments located there are a frigid 40 K, which is plenty cold enough for the three near-IR instruments.
But as cold as 40 Kelvins above absolute zero may be, it’s still far too hot for the sensors in the fourth of Webb’s primary instruments. The Mid-Infrared Imager, or MIRI, is designed to take images and make spectrographic observations from 5 to 28 μm, which requires an entirely different sensor than its near-IR cousins. Rather than Hg: Cd: Te, MIRI’s sensor is based on arsenic-doped silicon (Si: As), which needs to be cooled to very close to absolute zero-less than 7 Kelvin.
Sounds Pretty Cold
In the original Webb designs, the ultra-cold temperature needed for MIRI was going to be provided by a Dewar flask containing a cryogenic substance: solid hydrogen. The choice for a stored cryogenic system was made based on the immaturity of space-rated active cryocooling systems capable of reaching 6 K at the time. However, Webb’s now-infamous delays allowed cryocooler technology to develop, and in light of the weight savings an active cryocooler offered, not to mention the potential to use MIRI longer-the instrument would be useless once the solid hydrogen had all boiled off-the decision was made to replace the cryogenic Dewar.
This wasn’t without engineering challenges, of course. Chief among these were the ability to hit the target temperature while staying within power and weight constraints, and not adding undue mechanical vibration to the sensitive optics. Both of these specifications were particularly challenging given the sheer size of Webb, and the physical layout of the observatory, which made it necessary to spread the cryocooler assemblies over three different areas of the spacecraft, each with different thermal regimes to deal with.
The warmest region, designated Region 3, is located in the spacecraft bus. It’s on the hot side of the sunshield, which means it can expect to see temperatures up to 300 K or so. The assembly that’s mounted in this region consists mainly of the cryocooler compressor assembly (CCA) and its associated control electronics. The CCA is the “precooler” of the whole system, using a three-stage pulse tube design to achieve temperatures of about 18 K. Pulse tube cryocoolers have no moving parts aside from the pistons used to generate the pressure waves, making them excellent for low-vibration applications like this.
The pulse tube refrigeration process relies on thermoacoustics to transfer heat. In thermoacoustics, a standing wave is set up within a working gas (helium in the case of Webb’s cryocooler) within a sealed tube. A porous plug, called a regenerator or recuperator, sits within the tube, close to one of the nodes of the standing wave. As the working gas is compressed and expanded, a temperature gradient sets up across the regenerator. The hot end of the pulse tube radiates heat out into space via a heatsink, while the cold end is used to remove heat from a closed-loop heat exchanger, also charged with helium. The video below has an excellent demonstration of the principle of thermoacoustic cooling.
The cooled helium, now at around 18 K, enters Zone 2, which is within the tower that supports Webb’s primary mirror. The temperature in this region is between 100 K and almost 300 K, and the supercold helium has to pass through about two meters of tubing to reach the instruments at the telescope’s focus, so a great deal of engineering went into making sure there would be no unwanted heat transfer.
At the end of its trip through Zone 2, the refrigerant reaches the heart of Zone 1 – the focal plane of MIRI itself. This zone is already at about 40 K thanks to the passive cooling steps outlined before, but to drive the refrigerant down to its final 6 K temperature, it passes through what’s known as a Joule-Thomson valve. The JT valve makes use of the Joule-Thompson Effect to cool the helium working fluid even further.
Joule-Thomson says that when the pressure of a gas is reduced, its temperature is also reduced. It’s something we’ve all seen before, as when frost forms on the outside of a dusting air can, or the cloud of water droplets that form when an air cannon lobs a projectile into the air. In Webb’s Cold Head Assembly (CHA) inside MIRI, a special valve allows the pressure of the supercold helium to drop suddenly, causing it to drop to around 6 K and cooling a copper block upon which the MIRI sensors are mounted. The helium is piped back through the JT valve and back down the tubing to the CCA, in a closed-loop system.
So far, Webb’s cryocooler system is hitting all its marks and keeping MIRI happy. As of this writing, the temperature at the MIRI focal plane has been steadily holding below the 7 K setpoint for more than 14 days, with the other near-IR instruments holding well below their 40 K target. Here’s hoping that we get to see results from these instruments soon.
And just for the record, the coldest natural spot in the solar system might actually be the “double-shadowed craters” on the Moon’s south pole, at only 25 K. Poor Pluto-never any respect.