NASA’s DART mission aims to save the world

If the James Webb Space Telescope is to work, looking so far and thus far back in time that it can see the first galaxies forming after the big bang, it will have to visualize objects so faint that they can barely distinguish themselves from the cold around them. The world will begin to discover how the observatory works as soon as next week, when the JWST is expected to release its first collection of scientific images and spectroscopic data.

So, for the sake of argument, let’s assume that all indications so far actually point to a successful start to the (hopefully long and legendary) science-gathering phase of Webb’s mission. How did the engineers and designers of this massive telescope ever allow the telescope to cool enough, all at a distance of nearly four times the distance from Earth to the moon, to do its job?

After more than 25 years of work and untold technological hurdles, Webb’s team launched and positioned its giant observatory in solar orbit and brought its instruments below 40 kelvins (-233 ° C), cold enough to see. the early universe more than 13.5 billion years ago. Surprisingly, most of the cooling was done passively, shielding the telescope from the sun and letting physics take care of the rest.

“Webb is not just the product of a group of people. It is not the product of some intelligent astronomers: Webb is truly the product of the capabilities of our entire world, “says Keith Parrish, leader of the Webb team at NASA’s Goddard Space Flight Center in Maryland.” Overall, Webb is indeed the result. of our entire know-how on how to build complex machines “.

Parrish joined the project in 1997, eventually becoming its commissioning manager over the years of designing, assembling, testing, delaying and finally launching on December 25, 2021. It almost says it all: its shape and location, i materials it is made from — was dictated by the need to have an observatory that would survive super-cold temperatures for years.

Photo of a clean room with five giant sheet-like sheets stacked on top of each other, with three scientists in the distance inspecting the parasolIn this photo, the five-layer JWST sunshade is unfolded and inspected in a clean room. Coated Kapton E layers never touch, minimizing heat transfer from one layer to another. Alex Evers / Northrop Grumman

The 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 lengthens, causing a dramatic red shift. Infrared is also useful for seeing through cosmic dust and gas, and for viewing cold objects such as comets, Kuiper belt objects, and possibly planets orbiting other stars.

But infrared radiation is often best measured as heat, which is why it is important for the Webb to be so cold. If, like the Hubble telescope, it were in a low Earth orbit and had no sunscreen, most of its targets would be suffocated by the sun and the ground and by the heat of the telescope itself.

“If my signal is heat, and infrared is heat, then what I can’t have is other heat sources that are noise in the system,” says Jim Flynn, head of sun protection at Northrop Grumman, principal contractor for Webb.

Then the Webb was sent in a circle around a point in space called L2, 1.5 million kilometers away, facing the sun, one of the places known as Lagrange points. These “L” points are the points where the gravity of the Earth and the sun exactly conspire to keep it in a stable and relatively “fixed” orbit with respect to the Earth as it makes its way around its 365,256-day path in a circle around the sun. . It’s a good compromise: Earth is distant enough not to interfere with observations, but close enough to allow relatively fast communication with the spacecraft. And because the ship doesn’t fly overnight and goes back on every orbit, its temperature is relatively stable. All it needs is a really, really good sunshade.

“Four [layers of sunshield] it probably would have done the job. Five gave us a bit of an insurance policy. I would like to say it was much more sophisticated than that, but it really isn’t what it was. “
—Keith Parrish, NASA’s Goddard Space Flight Center

“Engineering has been pushed above and beyond scientific goals,” says Alexandra Lockwood, a researcher on the project at the Space Telescope Science Institute, which runs the Webb. “It’s designed specifically the way it is because they wanted to do intensive infrared science.”

In many renders it is an awkward-looking ship, with the telescope assembly, intentionally open to space to prevent heat build-up, attached to its silver parasol, approximately 14 meters wide and 21 meters long, with five layers of insulating film. to keep the telescope in near-total darkness.

From its sunlit side, the parasol roughly resembles a kite. The elongated shape, the engineers found, would be the most efficient way to keep the Webb’s optics out of the sun. They considered it a square or octagon, but the final version covers more area without much more mass.

“It’s no bigger than it needs to be to meet science’s field of view requirements, and the result is that unique kite shape,” says Parrish. “Whatever is bigger than it is now, it makes everything more complex.”

The five layers of the shield are made of Kapton E, a plastic film first developed by DuPont in the 1960s and used for the insulation of spacecraft and printed circuit boards. The layers are coated with aluminum and silicone. Each is thinner than a human hair. But the engineers say that, together, they are very effective at blocking 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 flared as you move away from the center of the shield so that heat escapes from the sides.

The result: Temperatures on the sunny side of the shield approach 360 K (87 ° C), but on the dark side they are below the all-important 40 K (-233 ° C). Or, in other words: More than 200 kilowatts of solar energy falls on the first layer, but only 23 milliwatts make it up to the fifth.

Why five layers? There was a lot of computer modeling, but it was difficult to simulate the thermal behavior of the shield before flight. “Probably four would have done the job. Five of them gave us a small insurance policy, “says Parrish.” I’d like to say it was a lot more sophisticated than that, but it really isn’t what it was. ”

The ability to naturally cool the telescope, first calculated in the 1980s as possible, has been a great advance. It meant that the Webb would not have to rely on a heavy and complex cryogenic apparatus, with refrigerants that could leak and shorten the mission. Of its four main scientific instruments, only one, a mid-infrared detector called MIRI, needs to be cooled to 6.7 K. It is cooled by a multi-stage cryocooler, which pumps cold helium gas through pulse tubes to move heat away from the instrument’s sensor. It uses the Joule-Thomson effect, reducing the temperature of the helium by causing it to expand after it has been forced through a 1mm valve. The pressure comes from two pistons, the only moving parts of the cryocooler system, facing in opposite directions so that their movements cancel each other out and do not disturb the observations.

The construction of the telescope proved immensely complicated; it fell behind for years as its budget soared to $ 10 billion. The sunshade needed a long redesign after testing, when the gates and Kapton fasteners came loose.

“We just bit a lot more than we could chew,” Parrish says now. “This is exactly what NASA is supposed to do. It should push the envelope. The problem is that Webb eventually got too big to fail.

But it was eventually implemented, sending data and surprising the engineers who were expecting at least some failures when it started working. Keith Parrish, having finished his work, is moving on to other projects in Goddard.

“I think Webb,” he says, “is just a great product of what it means to be an advanced civilization.”

Update: July 26, 2022: The story has been updated to clarify that the gravity at Lagrange’s point L2 does not “cancel” (as the story had previously stated) but rather adds to keep an object in L2 in orbit in the same precise orbital period as, in this case , the Earth, that is, at 365,256 days.

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