NASA’s flagship James Webb Space Telescope has arrived at its destination. After unfolding the carefully packed observatory during its monthlong journey, controllers today briefly fired Webb’s thrusters to put it into a “halo orbit” around L2, a gravitational balance point 1.5 million kilometers from Earth. Far from the heat and hubbub of low-Earth orbit, L2 will be Webb’s home for at least the next decade.
But another 5 months of work remains before Webb is ready to beam down images of its targets in the infrared universe, from the first galaxies that formed after the big bang to the atmospheres of exoplanets that could hold clues to how friendly they are to life. For the next several days, Webb will continue to cool behind its protective sunshield to its operating temperature of –235°C. Then operators will begin the long and intricate process of aligning the 18 hexagonal segments of Webb’s 6.5-meter main mirror so they form a single reflector.
“Everything we’re doing is about getting [Webb] ready to do transformative science,” says Jane Rigby, operations project scientist at NASA’s Goddard Space Flight Center.
For the journey to the launch site in French Guiana and into orbit on an Ariane 5 rocket, the mirror segments were locked in place for safety. Operators have just finished releasing them so they can pivot freely, says Matt Greenhouse, a Webb project scientist at Goddard. Behind each 1.3-meter mirror segment are seven actuators, tiny motors that can adjust the segments’ position, tilt, and even curvature.
First, operators will point Webb at a single, bright, isolated star in the constellation Ursa Major, chosen for its stability and the lack of other bright stars nearby. When viewed with one of Webb’s detectors, the Near Infrared Camera (NIRCam), operators are expecting to see 18 separate dots, one from each segment. “It will be very misaligned,” says Lee Feinberg, Webb telescope manager at Goddard.
Each segment will be wiggled to see which dot it is producing. Then begins the process of adjusting the tilt of each segment until the dots are all stacked on top of each other in the sensor’s view. At the same time, operators will adjust the curvature of the segments to make each spot as small and sharp as possible. “It’s a painstakingly detailed sequence of steps,” Feinberg says.
The final process is “phasing,” ensuring that the light is not only focused, but also in tune, with the peaks and troughs of the waves from all 18 segments coinciding. To ensure the light paths are all the same length, operators will adjust the segments’ distance from the secondary mirror by fractions of a wavelength of light—billionths of a meter. Only then will the 18 separate segments have the resolution of “a beautiful monolithic primary mirror,” Feinberg says, and “a star will look like a star.” It will be the start of a lifelong process: Operators will continue to check Webb’s optics every 2 days and will tweak the segment positions, as required, every 2 weeks. “If they drift by one-twentieth of a wavelength, we care about it,” Feinberg says.
After tuning the mirror with the help of NIRCam, operators will check that light passes cleanly into two other two detectors, the Near Infrared Spectrograph and the Near Infrared Slitless Spectrograph. A fourth sensor, the Mid-Infrared Instrument (MIRI), operates at much lower temperatures than the other three, just 6.4° above absolute zero. It requires a mechanical cryocooler which, because it emits waste heat, must sit on the warm side of the sunshield and pipe its coolant through to MIRI—one of Webb’s most challenging systems to build. Once MIRI is fully cooled in early April, it, too, will be optically aligned.
By early May, operators expect to be testing all 17 observing modes of the instruments. Commissioning involves viewing a range of reference objects, such as stars with precisely known brightness or star fields with exactly measured positions. Researchers want to understand how a sensor’s output relates to the influx of light and whether the telescope’s internal optics distort the positions of stars. Operators will also point the telescope at starless areas of sky to understand the “dark current” created by thermal noise in the instrument itself. “We don’t want the fingerprints of the instrument on the science,” says Scott Friedman, Webb commissioning scientist at the Space Telescope Science Institute.
He and his colleagues want to be sure the telescope can stay on target, too. “Positioning has to be exquisite,” Friedman says. They will test Webb’s Fine Guidance Sensor, which enables a laserlike lock on moving targets such as the moons of Jupiter, or anything that requires long exposures, such as dim exoplanets or distant galaxies. In the final weeks, operators will test the observatory’s thermal stability when pointed in different directions. Although the sunshield keeps the mirror and instruments in permanent shadow, sunlight heats the spacecraft differently at its pointing extremes, and one-tenth of 1°C can have an effect. “Those sort of temperature changes matter,” Feinberg says.
At the end of all of that, 180 days after its Christmas Day 2021 launch, science operations can begin, and astronomers will get to see what Webb is capable of. Friedman says: “This is what we’ve worked for, for years and years.”