For the James Webb Space Telescope to work, it must be able to see far enough to see the first galaxies that formed after the Big Bang, so it must image objects so dark they are barely noticeable from the cold. around them. Next week, when JWST is expected to release its first stack of scientific images and spectroscopy data, the world will start to see how well the observatory is doing.
So, for the sake of argument, let’s assume that all the signs so far actually point to a successful start to the (long and anecdotal) scientific data-collection phase of the Webb mission. Could the engineers and designers of this giant telescope cool the telescope enough to perform the task at a distance of nearly four times the distance from Earth to the Moon?
After more than 25 years of work and countless technical hurdles, the Webb team launched the Mammoth Observatory and put it into heliocentric orbit. 13.5 billion years ago. Surprisingly, most of the cooling is passive, shielding the telescope from the sun and leaving the rest to physics.
“Web is not just a collective product,” said Keith Parrish, NASA’s Goddard Space Flight Center in Maryland. It’s not the product of some smart astronomer. Webb is just a product of our global capabilities.” “In general, the web is the result of all our experience with how to create complex devices.”
Parrish joined the project in 1997 and eventually became commissioning director after design, build, testing, delay and finally launch on December 25, 2021. dictated by the need for an observatory that survived for years at extremely low temperatures.
In this photo, the JWST 5-layer sun visor is deployed and examined in a clean room. The coated Kapton E layers never touch, which reduces heat transfer from one layer to the next. Alex Evers / Northrop Grumman
Webb is an infrared observatory for many reasons. As the universe expands, the wavelengths of light from distant objects lengthen, 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 planets orbiting other stars.
However, infrared radiation is often measured as heat, so it is important that the web is extremely cold. When it is in low Earth orbit and has no sun shield, like the Hubble telescope, most of its targets are sunk by the heat of the sun, Earth, and the telescope itself.
“If my signal is heat and my infrared is heat, what I can’t get are other heat sources that are making noise in the system.
So Webb was sent into orbit at one of the places known as Lagrange points, on the other side of the Sun, 1.5 million kilometers away in space called L2. These “L” points are where the gravitational forces of the Earth and the Sun specifically conspire to keep the 365,256-day path around the Sun in a stable, relatively “constant” orbit with respect to the Earth. This is a good compromise. Earth is far enough away not to interfere with the observations, but close enough that communication with the spacecraft is relatively quick. Also, since the ship does not fly day and night, its temperature is relatively stable. All you need is a really good shade.
“four [layers of sunshield] Maybe the job is done. Five gave us a little insurance policy. I would say it was a lot more complicated than that, but it really wasn’t. “
—Keith Parrish, NASA Goddard Space Flight Center
“It was specifically designed this way because we wanted to do intense infrared science,” said Alexandra Lockwood, a project scientist at the Space Telescope Science Institute, who directs Webb.
It makes for a poor looking ship in many shows. The telescope assembly is intentionally opened into space to prevent heat build-up and is mounted on a silver canopy 14 meters wide and 21 meters long, with five layers of insulating film to maintain temperature. Telescope in almost complete darkness.
When viewed from the sunlit side, the sun visor almost resembles a kite. Elongated shape engineers have discovered the most effective way to keep Webb’s optics out of the sun. They were thinking square or octagonal, but the final version covers more area without adding much mass.
“It is no larger than required to meet the field-of-view requirements of the flag, resulting in a unique kite shape,” Parrish says. “When you get older than you are now, everything becomes more complicated.”
The shield’s five layers 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 one is thinner than a human hair. But together, engineers say they are very effective at blocking out the sun’s heat, while a second layer cuts it down even more. The layers do not touch and expand slightly away from the center of the shield to allow heat to escape from the sides.
The result: the temperature on the sun side of the shield is close to 360 K (87 °C), but on the dark side it is less than 40 K (-233 °C). In other words, more than 200 kilowatts of solar energy falls on the first layer, but only 23 milliwatts reaches the fifth layer.
Why 5 layers? Much computer modeling was done, but it was difficult to simulate the thermal behavior of the shield before flight. “Maybe four of them did the job,” Barish says. Five of them gave us a little insurance.” “I would say it was a lot more complicated than that, but it wasn’t at all.”
The ability to naturally cool a telescope was first calculated as much as possible in the 1980s, but it was a major advance. This meant that the Web did not have to rely on heavy and complex refrigeration equipment, which can cause refrigerant leakage and shorten the job. Of its four main science instruments, only one, a medium-infrared detector called MIRI, needs cooling to 6.7K. It is cooled by a multistage freezing cooler that pumps cold helium gas through a pulse tube and draws heat from the device’s sensors. The Joule-Thomson effect is used to lower the temperature of helium by pushing it through a 1 mm bulb and then expanding it. Compression comes from two pistons, which are the only moving parts in the cryogenic coolant system. Since these pistons face opposite directions, their movements cancel each other out and do not interfere with monitoring.
The construction of the telescope turned out to be quite complicated. It’s years late with a $10 billion budget. The visor required a lengthy redesign after testing when Kapton ripped and loosened the fasteners.
“We chewed in more ways than we can chew,” Parrish says. “This is exactly what NASA should do. You have to cross the line. The problem is that Webb eventually got too big and failed.”
But it surprised engineers who at least expected some failures when it was finally deployed, data was sent, and work began. Keith Parrish is down and working on other projects at Goddard.
“I think Webb is a great achievement of what it means to be an advanced civilization,” he says.
Update: July 26, 2022: The story is updated so that the gravity at the L2 Lagrangian point (as mentioned earlier in the story) is not “cancelled”, but is actually added so that the body orbits in L2 orbits with exactly the same orbital period. Earth — that’s 365,256 days.
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