Our solar system is brimming with stunning phenomena: the stormy atmosphere of Jupiter, its clouds coiling like cream poured into hot coffee. The delicate rings of Saturn, the countless pieces of ice and rock arrayed like grooves on vinyl. The aurora borealis on Earth, the collision of solar particles and atmospheric molecules painting the night sky with a ghostly green.
But some of the most mesmerizing marvels in our cosmic neighborhood are, actually, completely invisible.
There are points in space where a quirk of physics borders on sorcery, where Earth and the sun have conspired to produce a special kind of equilibrium. Put something there--an asteroid, a spacecraft, even a cloud of dust--and it will more or less stay there, suspended by the unseen forces of gravity.
Those forces have gifted us five such special points near Earth, and later this month, NASA's brand-new space telescope will head to the one found about a million miles away, four times farther than the moon. For comparison, Hubble, NASA's most famous space telescope, now more than 30 years old, orbits just 340 miles above Earth. The James Webb Space Telescope is the next Hubble, NASA's most important mission in a generation, charged with peering deeper into the universe than ever before, and it has found one of the best parking spots in the cosmos. From here, the telescope will be able to see it all, from the planets in our solar system to the most distant galaxies in the universe. Webb will orbit too far away for any astronauts to swing by if the observatory breaks. But NASA has decided that as far as scenic overlooks go, this one is worth the trip.
The invisible perches are known as Lagrange points, named after one of the mathematicians who worked out their existence in the late 1700s. "Normally, objects closer to the sun than the Earth would orbit more quickly, while objects farther out than the Earth would orbit more slowly," Neil Cornish, an astrophysicist at Montana State University, explained to me. "But the combination of the Earth and sun's gravitational effects at the Lagrange points allows the objects to orbit at the same rate as the Earth." At a Lagrange point, gravity puts an object in lockstep with Earth's journey around the sun.
This is a remarkable phenomenon that human beings have not only figured out exists, but managed to use for their own purposes. Space agencies over the years have anchored all kinds of missions at the first two Lagrange points, known as L1 and L2 for short. "It's a really neat way to use the gravity of the solar system," Michelle Thaller, an astronomer at NASA's Goddard Space Flight Center, told me. (L4 and L5, even farther away, are home to a couple of asteroids nudged there by nature, while L3, on the other side of the sun, is, based on our current understanding, empty.
L1, which sits between the sun and Earth, is the perfect spot for sun-studying spacecraft, and several are hanging out there right now. But for Webb to work properly, the telescope must operate in extremely frigid conditions. So engineers have decided to send the observatory to L2, which sits on the other side of the planet from the sun, where protecting a spacecraft from the sun's glare is far easier. For the orbital-mechanics wonks out there, Webb will reside just slightly off the true L2, which can occasionally become shadowed by the moon, a scenario that NASA wants to avoid. The new observatory has a full tank of gas to fire its thrusters every now and then, making tiny adjustments so that it doesn't drift off, but gravity will still do most of the work to keep it in place.
The telescope will wear one of the most sophisticated sun shields ever constructed, splitting the observatory into two parts: The side facing Earth and the sun will contain propulsion and communications systems that can handle heat, while the side facing space will contain the telescope mirrors and other instruments that require absolute cold. So stark are the differences between these two parts of the observatory that, as NASA explains with science-geek enthusiasm, "you could almost boil water on the hot side, and freeze nitrogen on the cold side!" (That's a swing of about 600 degrees Fahrenheit.)
Since a spacecraft nestled near L2 will remain, from our perspective, in the same spot of the sky, NASA can communicate with Webb without interruption. But if something goes badly wrong, engineers can only send commands, not a crew to help. Unlike Hubble, which has been visited and repaired by astronauts, Webb is too far to reach with any current technology, and it wasn't designed with astronaut-friendly hatches and other parts. But L2 is just too good to pass up, so NASA is willing to take the risk. With the sun and Earth behind them, Webb's gold-plated mirrors will have an unobstructed view of the universe.
The solar system as a whole is sprinkled with Lagrange points. All nature needs to create this invisible scaffolding are two celestial bodies, whether a star and its planet, or a planet and its moon, or even a moon and its moonlet. Mars keeps four asteroids at its own Lagrange points, including one delightfully named Eureka. Tethys, Saturn's fifth-largest moon, keeps two smaller moons at its L4 and L5. Jupiter, ever the record breaker, has collected thousands of asteroids at its Lagrange points, and NASA recently launched a spacecraft to study some of them. The physicist Gerard O'Neill suggested in the 1970s that humanity could someday move into floating homes at a Lagrange point between Earth and the moon.
Cornish suggests a different, more realistic use of Lagrange points. The special properties of gravity make it almost alarmingly easy for spacecraft to move from one Lagrange point to another, nudging themselves along with minimal propulsion. A space agency could, theoretically, bop a telescope from the faraway L2 toward a much closer Lagrange point created by Earth and the moon, which astronauts would have an easier time reaching. "We might have a fix-it shop, a maintenance shed, at L1 of the Earth-moon system and possibly service future missions there and then send them back to L2," Cornish said. NASA has taken advantage of this celestial road before: In the early 2000s, a spacecraft traveled to L1, between Earth and the sun, collected some particles of the solar wind, and then slid over to L2, where it stayed parked until daytime in Utah, when scientists were ready to receive it. (The probe ended up smashing into the desert and shattering into pieces, but that's a different story.)
For Thaller, the existence of Lagrange points provides a visceral reminder of the solar system's fundamental nature--a place in constant motion. "You really feel like you're on these little balls hurtling through space," Thaller said. "They're doing this lovely dance, and ... as they dance, there are these points where the gravity balances." And the fact that humans figured out how to use those points to learn a thing or two about the universe--well, isn't that pretty wild? Cornish thinks of Leonhard Euler and Joseph-Louis Lagrange, the mathematicians who first made sense of these cosmic parking spots. "Imagine telling them that there was going to be this spacecraft out there observing the universe," Cornish said. "If you told them, a couple hundred years ago, that we're actually making use of their mathematical calculations, it would blow their minds."