if it was James Webb Space Telescope Is to work – looking so far, and thus far back in time that he can see the formation of the first galaxies after the Big Bang – he will have to photograph objects so faint that they barely stand out from the cold around them. The world will begin to discover how well the observatory works As soon as next weekwhen JWST is expected to release its first set of scientific images and spectroscopic data.
So, for the sake of argument, let’s suppose All indications so far Does it actually indicate a successful start to the (long and crammed) scientific data-collection phase of the Webb mission? How, then, did the engineers and designers of this massive telescope manage to cool the telescope enough — all at a distance of four times the distance from Earth to 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 less than 40 degrees Kelvin (-233 degrees Celsius), cool enough to see the early universe more than since 13.5 billion years old. Remarkably, most of the cooling was done passively, by shielding the telescope from the sun and letting physics take care of the rest.
“Webb is not just a product of a group of people. It is not the product of some smart astronomer—Webb is really a product of the capacity of our entire universe,” he says. Keith ParrishHe is a captain on the Webb team at NASA Goddard Space Flight Center in Maryland. “Overall, Webb is really the result of our complete knowledge of how to build complex machines.”
Parrish joined the project in 1997, eventually becoming commissioning manager through the years of design, assembly, testing, and delays, finally launching it on December 25, 2021. Almost everything about it says about it—its shape, location, and materials made—is dictated by the need for an observatory that can survive for years at degrees Super cold heat.
In this photo, the JWST five-layer sun visor is opened and inspected 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, 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 a file Hubble telescopewas in low Earth orbit, had no protection from the sun, and most of its targets would be sunk due to the sun and Earth, and 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 Flynndirector of sun shield in Northrop GrummanWebb’s main contractor.
So Webb was sent to circle a place in space called L2, 1.5 million km, opposite the Sun, one of the locations known as Lagrange points. These “L” points are where the gravity of the Earth and the Sun perfectly conspire to keep it in a relatively stable and “fixed” orbit with respect to the Earth as it makes its way around its 365,256 day orbit around the Sun. 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] Maybe he did the job. Five gave us a little insurance policy. I’d 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,” he says. Alexandra Lockwooda project scientist in Space Telescope Science Institute, which runs Webb. “It’s specifically designed the way he wanted it because they wanted to do intense infrared science.”
It is made for an unobtrusive looking vessel in many designs, with the telescope assembly, intentionally open to space to prevent heat build-up, attached to the silver sun hood, about 14 meters wide and 21 meters long, with five layers of insulating film to keep the telescope in complete darkness Approximately.
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 a 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 armor 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 the temperatures are below 40 K (-233 °C). Or in other words: more than 200 kilowatts Solar energy It falls on the first layer, but only 23 milliwatts makes it to the fifth layer.
Why five layers? There were a lot of computer models, but it was difficult to simulate the thermal behavior of the shield before flight. “Maybe four did the job. Five gave us a bit of an insurance policy,” says Parrish. “I would say it was a lot more complicated than that, but that wasn’t what it was at all.”
The ability to Cool the telescope naturally, 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, cooled by a multi-stage freezing cooler, which pumps cold helium gas through pulse tubes to draw heat away from the instrument’s sensor. Uses Jules Thompson effect, to reduce the temperature of helium by making it expand after being forced through a 1 mm 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 visor needed a lengthy redesign after testing, when the Kapton ripped and the fasteners came loose.
“We got away a little more than we can chew,” Parrish says now. “That’s exactly what NASA has to do. It has to push the envelope. The problem is that Webb is finally getting 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 wonderful product of what it means to be an advanced civilization,” he says.
Update: July 26, 2022: The story has been updated to show that the gravity at the L2 Lagrangian point does not “cancel” (as the story previously mentioned) but actually adds up to keep an object in L2 orbit in the exact same orbital period as in this case, Earth—that is, 365,256 days.
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