The James Webb Space Telescope (JWST) is approaching its new home. On January 24, it will arrive at a point in space that scientists call Lagrange point 2, or L2.
This is the technical name for a delicate gravitational tipping point. JWST is bound for where, in the Earth-sun pair, the gravitational pull of Earth perfectly balances out the much stronger gravity of the sun. JWST’s designers planned for their telescope to drift there, because there, the telescope can work without gravity nudging it out of place.
“We knew that we needed to keep JWST at L2,” says Stefanie Milam, a NASA planetary scientist on JWST.
Each pair of gravitationally bound objects—a sun and its planet, say, or a planet and one of its moons—has five Lagrange points. An asteroid or a spacecraft, for instance, can live at one of those five points without falling out of orbit.
In 1765, the mathematician Leonhard Euler crunched gravitational equations to find the first three points. Those three—L1, L2, and L3—form a straight line. Take Earth and the sun. L1 lies between the Earth and the Sun: to be more precise, about 930,000 miles (1.5 million kilometers) from Earth. L2 is tucked away on Earth’s far side: it’s also about 930,000 miles from us, facing the outer reaches of the solar system.
Several years after Euler, in 1772, one of his close correspondents—another mathematician named Joseph-Louis Lagrange—ran through those equations again and realized that two additional points exist: L4 and L5. They’re located in Earth’s orbit, with L4 a bit ahead of us and L5 an equal bit behind us.
L4 and L5 are more stable than their counterparts—they can pull in gas, dust, and even larger objects. Astronomers have discovered at least two asteroids at the sun and Earth’s L4 and L5, and they’re hunting for more. Other sun-planet pairs have captured objects at these points, too. The sun and Jupiter’s L4 and L5 points are home to whole groups of asteroids, known as trojans, which NASA’s Lucy probe will visit.
L3 is the odd point without a counterpart. To find it, you’d have to go all the way to the other side of the sun, close to the opposite point in our orbit (but, because Earth’s gravity subtly pulls at the sun, not quite exactly the opposite point). Predictably, it’s a little impractical for spacecraft to easily get there; no known spacecraft has ever called L3 home.
But L1 and L2 are much easier for us to visit, each less than four times the distance from Earth to the moon. L1, the perch facing the sun, has been an ideal stomping ground—or stomping space—for missions designed to observe our star or the streams of particles in its solar wind.
L2, on the other hand, lies in Earth’s shadow. This site is ideal for craft that peer out beyond the solar system and into the vast cosmos beyond. Currently calling it home are ESA’s Gaia, which is measuring the distances to the stars, and the X-ray observatory Spektr-RG. On January 24, JWST will join them.
From the beginning of JWST’s planning, decades ago, its designers and planners decided that L2 was its right place. JWST is an infrared telescope. Heat is also infrared, so being in Earth orbit—and constantly having to revolve into sunlight—is not ideal. Even heat from our own planet could throw off the telescope’s extremely finicky observations.
“Any heat from the Earth or the moon would be something that we would have to fight with, and we’re trying to detect the faintest signals of galaxies and stars across the universe,” says Milam.
Placing JWST at L2, far from Earth, circumvents that problem. Being in Earth’s shadow also means that the telescope can use a sunshield, rather than being wrapped inside a tube like the Hubble Space Telescope is. This is part of the reason JWST can use its colossal mirror.
There are other advantages to being at L2, Milam says. For instance, being out of Earth’s orbit means dodging other spacecraft, as well as most of the space junk that could slam into the telescope and damage it.
But there is a catch. JWST is too far from Earth to easily conduct maintenance. That’s in contrast to Hubble, whose location in Earth orbit meant that it was easy for NASA to conduct repairs, including its famous mirror job.
JWST won’t be fixed into place at L2, but actually in a fine-tuned orbit around it. Its orbit won’t be completely stable, either. The force of the sun’s radiation pushing down on the sunshield will gently push JWST out of place. There, says Milam, “unlike Hubble, we don’t get Earth’s gravity to help us” keep the spacecraft’s momentum in check.
Thus, JWST will need constant adjustments to stay in place. Figuring out what that will require, Milam says, is something JWST’s operators will be playing with over the next several months. After the telescope arrives in place Monday, they’ll begin checking to ensure that all of its instruments function. Then, observations begin .
Once JWST is settled near its companions at L2, all may be quiet—for a few years, at least.
Over a half-dozen other missions are slated to go there. The Nancy Grace Roman Space Telescope, for instance, is set to head there sometime later in the 2020s. So will Euclid, ESA’s dark-matter-watcher; PLATO and ARIEL, two ESA telescopes that will peer at exoplanets; and LiteBIRD, a Japanese mission to try to peer into the very earliest days of the universe.