When the Apollo astronauts brought back the first images of Earth as a disk in space, poet Archibald MacLeish wrote of it as “that beauty shining in the eternal cold”. It wasn’t far away. Deep space has a temperature of 2.7 Kelvins—just 2.7 degrees above absolute zero.
If the James Webb Space Telescope is to work — looking so far, and therefore so far back in time that it can see the formation of the first galaxies after the Big Bang — it will have to photograph objects so faint that they barely stand out from the cold surrounding them. The world will begin to discover how well the observatory is working as soon as next week, when JWST is expected to release its first set of scientific images and spectroscopic data.
So, for the sake of argument, let’s assume all indications so far do in fact point to a successful start to the (long and packed) scientific data-collection phase of the Webb mission. So how did the engineers and designers of this massive telescope manage to cool the telescope enough — all at a distance of four times the distance between Earth and the Moon — to possibly do its job?
After more than 25 years of work and countless technological hurdles, the Webb team launched its giant observatory and put it into a heliocentric orbit — bringing its instruments to below minus 40 degrees Celsius (minus 233 degrees Celsius), cool enough to see the early universe more than 13.5 a billion years ago. Remarkably, most of the cooling was done passively, by shielding the telescope from the sun and letting physics take care of the rest.
“Web is not just a product of a group of people. It’s not the product of some smart astronomers — Webb is really a product of the capacity of our entire universe,” says Keith Parrish, Team Leader Webb at NASA’s Goddard Space Flight Center in Maryland. “Webb, as a whole, is really the result of our complete knowledge of how to build complex machines.”
Parrish joined the project in 1997, eventually becoming commissioning director through the years of design, assembly, testing, and delays, and finally launching it on Christmas Day 2021. He says just about everything about it—its shape, location, materials used to make—dictated by the need for an observatory that could survive. For years in frigid temperatures.
In this photo, the JWST five-layer sun visor is opened and examined in a clean room. The coated Kapton E layers never touch, which reduces heat transfer from one layer to the next. Northrop Grumman / Alex Evers
Webb is an infrared observatory for many reasons, not the least of which is that as the universe expands, the wavelength of light from distant objects is lengthened, causing a large redshift. Infrared is also useful for seeing through cosmic dust and gas, and for imaging cold objects such as comets, Kuiper Belt objects, and possibly planets orbiting other stars.
But infrared is best measured as heat, which is why it’s important for Webb to be extremely cold. If, like the Hubble telescope’s coil, was in low Earth orbit, and had no protection from the sun, most of its targets would be sunk by the sun and Earth, and by the heat in the telescope itself.
“If my signal is heat—and infrared is heat—what I can’t get are other heat sources that are causing noise in the system,” Jim Flynn, director of Sunshield at Northrop Grumman, Webb’s main contractor.
So Webb was sent to circle a place in space called L2, 1.5 million kilometers away, opposite the Sun, one of the named Lagrangian Points where the gravity of the Earth and the Sun almost cancel each other out. It’s a good compromise: Earth is far enough away that it doesn’t interfere with observations, but close enough that communication with the spacecraft can be relatively fast. Since the ship does not fly from day to night and returns in every orbit, its temperature is relatively stable. All you need is a really good umbrella.
“four [layers of sunshield] They would probably do the job, five of them gave us a little insurance policy. I would say it was more complicated than that, but that wasn’t what it was at all.”
—Keith Parrish, NASA Goddard Space Flight Center
“Engineering has been pushed even further to achieve scientific goals,” says Alexandra Lockwood, a project scientist at the Space Telescope Science Institute, who operates Webb. “It’s specifically designed the way he wanted it because they wanted to do intense infrared science.”
It is made for an unflattering-looking vessel in many designs, with the telescope assembly, intentionally open to space to prevent heat build-up, attached to a silver sun hood, about 14 meters wide and 21 meters long, with five layers of insulating film to keep the telescope in almost complete darkness .
From its sunlit side, the sunvisor almost resembles a kite. The engineers found that the elongated shape would be the most effective way to keep Webb’s optics out of the sun. They were considered square or octagonal, but the final version covers a larger area without much more mass.
“It’s no bigger than it needs to be to meet the demands of the scientific field of view, and this unique shape of the kite is the result,” Parrish says. “Anything bigger than it is now, makes everything more complicated.”
The five layers of shield are made of Kapton E, a plastic film first developed by DuPont in the 1960s and used to insulate spacecraft and printed circuits. The layers are aluminum and silicone coated. Each is thinner than a human hair. But the engineers say that, together, they are very effective at keeping out the sun’s heat. The first layer reduces its strength by about an order of magnitude (or 90 percent), the second layer removes another order of magnitude, and so on. The layers never touch, and are slightly glowing as one moves away from the center of the shield, so that heat escapes from the sides.
The result: temperatures on the sunny side of the shield are close to 360 K (87 °C), but on the dark side temperatures are below 40 K (-233 °C). Or in other words: more than 200 kilowatts of solar energy fall on the first layer, but only 23 milliwatts to reach the fifth layer.
NASA/STScI
Why five layers? There was a lot of computer modeling, but it was difficult to simulate the thermal behavior of the shield before flight. “Four of them were likely to do the job, and five gave us a bit of an insurance policy,” Parrish says. “I would say it was more complicated than that, but that wasn’t what it was at all.”
The ability to naturally cool the telescope, which was first calculated in the 1980s to be possible, was a major advance. This meant that Webb would not have to rely on a heavy, complex cooling device, with refrigerant that could leak and shorten the job. Of its four main science instruments, only one, an infrared detector called the MIRI, needs cooling to 6.7 K and is cooled by a multi-stage cooler, which pumps cold helium gas through pulse tubes to draw heat away from the instrument. sensor. It uses the Joule-Thomson effect, to reduce the temperature of helium by causing it to expand after being forced through a 1mm valve. The pressure comes from two pistons – the only moving parts in the refrigerant system – facing opposite directions, so their movements will cancel each other out and not disturb the feedback.
The construction of the telescope proved to be quite complex; It slipped years while its budget swelled to $10 billion. The sun visor needed a long restyling after testing, when the Kapton gates and stabilizers separated.
“We got a little bit further away than we could chew,” Parrish says now. “This is exactly what NASA should do. You must pay the envelope. The problem is that Webb eventually became too big to fail.”
But it was finally deployed, sending data, and surprising engineers who had expected at least some failures when it got going. Keith Parrish, his completed work, moves on to other projects at Goddard.
“I think Webb is just a great product of what it means to be an advanced civilization,” he says.
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