The James Webb Space Telescope has to be kept at bitterly cold temperatures for its sensitive instruments to work. Each step of the way, engineers must counter Webb's ordinary, everyday heat production with a method of dropping the temperature ever lower
WHY AN INFRARED TELESCOPE?
The Webb telescope sees in infrared light, a wavelength that warm objects emit in large amounts. We look for infrared light for several reasons.
First, the light from the most distant objects in the universe — objects that existed just 200 million years after the Big Bang — has been stretched as it travels through the expanding universe. The stretching changes the light, moving it from the visible range into invisible infrared.
Second, infrared light can pierce the dust that blocks visible light emissions, allowing us to peer through clouds of dust to the warm objects within. Finally, warm objects that don't emit their own visible light, such as planets, do emit infrared, making them viewable with the right tools.
But infrared light also comes with an inherent complication — it's emitted by everything made up of atoms. When the atoms in an object collide or vibrate, their electrons are bumped up to a higher energy level. As those electrons return to their normal energy level, they give off radiation. Much of this radiation escapes as infrared. Warm objects, which have a great number of excited electrons, emit lots of infrared. The more infrared there is nearby, the harder it is to pick out distant sources of infrared.
If Webb were on Earth, it would be constantly awash in the planet's own infrared haze. We avoid that by deploying the telescope in space. But even that doesn't solve the problem entirely, because Webb itself — being made, after all, of atoms — will emit infrared radiation. The solution: Keep the Webb telescope as cold as possible, so the infrared produced by its own excited atoms doesn't overwhelm the distant emissions it's designed to detect.
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Webb's distant orbit performs two functions. It places the telescope in a frigid environment, and makes it possible to position the telescope so it always has the Sun, Earth and Moon on the same side, behind its sunshield. The second Sun-Earth Lagrange point, one of five points in the solar system where gravitational forces allow an object to remain in a fixed position relative to Earth, is 940,000 miles away from Earth. As the Earth orbits the Sun, its gravity will drag Webb along behind it, keeping it in this optimal location. It will take a full month for Webb to reach its new home beyond the Moon.Back to top
The need to keep Webb cold affects its design dramatically. For instance, the telescope's odd structure is the result of it not being enclosed in a tube. Telescopes are usually enclosed in a tube or a dome to block out nearby, unwanted light and radiation. But Webb needs to be open to space in order to keep itself cool enough for its infrared detectors to work properly. A tube would both radiate infrared and trap too much heat inside the telescope, while the open design allows the heat to pour harmlessly off into space. So rather than use a tube, Webb stops unwanted visible and infrared light with its giant sunshield and “baffles,” small, dark barriers that block light at strategically placed points.
The open design is the only way to cool Webb to the right temperatures and still maintain its large size. If the Webb telescope relied mainly on coolant like other infrared telescopes, it would need many tons of coolant and its life would be significantly shorter, for once the coolant ran out the telescope would be unusable.
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DIVIDE AND CONQUER
Webb has two sections — a hot side and a cold side, divided by its sunshield. On the hot side of the sunshield, the region exposed to sunlight, parts of Webb will reach temperatures near boiling, as high As 358 Kelvin (185 degrees Fahrenheit, or 85 degrees Celsius). This is where Webb's ambient-temperature equipment, like its solar panel, antennae, computer, gyroscopes and navigational jets are kept. The warm side of the telescope is, overall, where the electronics and navigational system resides.
On the cold side, Webb will be about 40 Kelvin (-388 degrees Fahrenheit, -233 degrees Celsius). In contrast, the coldest temperature ever recorded on Earth, at the Russian Vostok station in Antarctica, was minus-129 degrees Fahrenheit — far too toasty for Webb's needs. The cold side of the sunshield is where the science happens, and it contains the parts of Webb most sensitive to infrared radiation: its microshutter array, mirror and mirror actuators, filter wheels, and, of course, the infrared detectors.Back to top
Webb's mirror will reach about 40-50 Kelvin (-390 degrees Fahrenheit, -230 degrees Celsius) once it's deployed. The mirror is made of polished beryllium, a metal that has the benefit of contracting less in extreme cold than the glass that typically makes up a telescope mirror. A mirror that contracts dramatically produces a deformed image. Engineers design the Webb mirror by running computer models that show exactly how the beryllium will deform as it cools. The engineers then design the mirrors so that when they deform, they deform into the right shape, and test and test until they get them perfect. Without the chill of space, these incorrectly shaped mirrors would be useless to the telescope.Back to top
CHILLING THE INSTRUMENTS
But even more dramatic cooling goes on around the camera and spectrograph in the Mid-Infrared Instrument (MIRI). MIRI sees farther into the infrared than the other instruments, and thus has to be kept spectacularly cold.
Webb boasts a two-stage cryocooler that works like the world's most effective refrigerator, pumping a warmth-absorbing gas through the instrument. The first stage brings MIRI's temperature down to 18 Kelvin, and the second stage brings the MIRI detectors to 7 Kelvin — that's just seven degrees above absolute zero, the theoretical temperature at which all motion freezes, even the movement of atoms.
The infrared detectors in Webb's other instruments need temperatures of about 40 Kelvin (-375 degrees Fahrenheit or -233.3 degrees Celsius) to operate correctly. The detectors themselves give off heat when they are in use, so operators will "read" the detectors continuously rather than periodically, to always keep them at a stable temperature. This allows them to tell the difference between the internal infrared of the detectors and the infrared of distant objects.Back to top
Webb is protected from the heat and light of the Sun, Moon and Earth by its huge sunshield. The sunshield, large enough for the telescope to remain protected even as it tilts in different directions, consists of five layers, each about the thickness of plastic wrap. The layers do not touch, because that would allow for the transfer of heat from one layer to the next, and coatings of aluminum help further cut down on warmth transmission. The layers are made of a material called Kapton, also used in spacesuits and circuitry, which has the convenient properties of being heat resistant and strong. The sunshield is also coated on the sunward side with silicon, which reflects and radiates away the incoming sunlight.
Because the telescope is so far away from Earth, it lacks the protection of our planet's magnetic field. This means it will be bombarded by high-energy cosmic rays, which can interfere with signals or even build up enough of an electrical charge to cause small lightning strikes on the telescope. The sunshield's coatings of aluminum and a grid of conducting strips on the sunshield layers control the electricity and prevent voltage arcs from damaging the telescope.
With heat banished, pushed away and driven out to the fullest extent possible, Webb will wait in its silent, chilly expanse of space for the faint, distant signals from the early universe. As it gazes into the darkness with cold clarity, the infrared universe will reveal itself to us, its invisible light weaving images that human eyes will see for the first time.Back to top