What Is the James Webb Space Telescope? An Infrared Eye a Million Miles Out

The universe is expanding, and light from a distant object is stretched on its way to us, so that the ultraviolet and visible light of the first galaxies arrives as infrared. A telescope built to see the earliest stars must therefore see in the infrared, which brings two problems: the atmosphere absorbs most of it, so the instrument must be in space, and every warm object glows in it, so the telescope must be cold or it will be blinded by its own heat. Infrared also passes through dust, which visible light does not, so the same instrument can look inside the clouds where stars are forming and see the disc around a young star, and it can read the chemistry of a planet's atmosphere from the wavelengths that its molecules absorb.

Every unusual feature of the telescope follows from the need to be large and cold:

• •The mirror: 6.5 metres across, six times the collecting area of Hubble's, made of eighteen hexagonal segments of beryllium coated in gold, which reflects infrared well, each segment on actuators that adjust its position by nanometres
• •Folding: the mirror and the sunshield had to fold to fit the rocket and unfold in space, which is why the deployment was the riskiest part of the mission
• •The sunshield: five layers of a thin polymer the size of a tennis court, separated by vacuum gaps, which keep the telescope side at about minus 233 degrees while the sun side reaches 85
• •The orbit: not around the Earth but around the second Lagrange point, 1.5 million kilometres out on the far side from the Sun, where the Sun, Earth and Moon are all in the same direction and can be blocked by one shield
• •The instruments: four of them, cameras and spectrographs covering wavelengths from 0.6 to 28 micrometres, with the longest-wavelength one cooled further to 7 degrees above absolute zero by its own refrigerator
• •No servicing: at that distance no astronaut can reach it, so everything had to work the first time

The first images, released in July 2022, showed a cluster of galaxies with the light of more distant ones bent around it, and the discoveries since have come faster than expected. The telescope has found galaxies at redshifts above 14, seen as they were less than 300 million years after the Big Bang, and found more of them, and brighter, than the models of galaxy formation predicted, which is currently the field's most interesting problem. It has measured the atmospheres of exoplanets, detecting carbon dioxide, water, sulphur dioxide and, in one contested case, a possible signature of a biological gas, and it has imaged the discs of dust and gas around forming stars, the aftermath of a spacecraft deliberately striking an asteroid, the auroras of Jupiter and the rings of Neptune, which had not been seen clearly since Voyager passed in 1989.

The project was proposed in 1996 for a 2007 launch at half a billion dollars and launched in 2021 at ten billion, a record even by the standards of large science projects, and it nearly died in Congress in 2011. The reasons were the usual ones for a first-of-its-kind machine: a design that required inventing ten new technologies, including the folding mirror, the sunshield material and the detectors; a management structure across NASA, the European and Canadian agencies and a prime contractor; and the requirement that nothing could be repaired, which meant testing everything to destruction on the ground, including a shake table failure in 2018 that tore the sunshield and set the schedule back two years. Whether the cost was worth it is a question about what else the money would have funded, and the astronomers who spent two decades waiting are mostly answering yes.

The mission was designed for five years and hoped for ten, limited by the fuel it needs to hold its position and point; the launch was so accurate that the telescope used far less fuel than budgeted to reach its orbit, and the current estimate is that it has propellant for twenty years or more. The end will come when the fuel runs out and the telescope drifts out of position, after which it will remain in orbit around the Sun, unreachable and inert, carrying a gold mirror that spent two decades looking at the beginning of things.

The James Webb Space Telescope is a 6.5-metre folding infrared observatory at the second Lagrange point, a million and a half kilometres from Earth, shaded from the Sun by a five-layer shield the size of a tennis court so that it can cool to forty degrees above absolute zero. Infrared lets it see the stretched light of the earliest galaxies, look through dust into star-forming clouds, and read the chemistry of exoplanet atmospheres; it launched in December 2021 after twenty years and ten billion dollars, and has fuel for two decades.