Webb's Big Chill
One of the telescope's most prominent features, Webb's sunshield protects the telescope from the heat and light of the Sun, Moon, and Earth. The sunshield, about the size of a 20-car parking lot, 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. Coatings of aluminum help to 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 coated on the sunward side with silicon, which reflects and radiates away the incoming sunlight.
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. Webb will be directed via rocket to the Second Sun-Earth Lagrange Point (L2), 940,000 miles (1.5 million km) away from Earth. L2 is one of five points in the solar system where gravitational forces allow an object to remain in a fixed position relative to Earth, and will keep Webb accessible for operations and communications. As Earth orbits the Sun, its gravity will drag Webb along with it, keeping it in this optimal location. It will take a full month for Webb to reach its new home beyond the Moon.
The need to keep Webb cold affects its design dramatically. For instance, the telescope's odd appearance 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.
Hot and Cold Sides
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 -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.
The infrared detectors in most of Webb's instruments need temperatures of about 40 Kelvin to operate correctly. The detectors themselves give off heat when they are in use, so operators will have to "read" the detectors continuously rather than periodically, to always keep them at a stable temperature.
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.