India’s Aditya-L1 probe will arrive in a few days at a patch of space between Earth and the sun, almost a million miles away.
The location is remote yet isn’t very lonely. Four active spacecraft are already in orbit near the same spot—known as the Earth-sun system’s first Lagrange point, or L1—and others are parked nearby.
It’s a privileged place where the gravity of our planet, the gravity of the sun and the centrifugal force of a spacecraft’s orbit almost exactly cancel one another out, creating an “island” of comparative stability amid the solar system’s ever shifting gravitational fields, which constantly change as the planets move. The result is that spacecraft orbiting the sun near L1—actually a region a few hundreds of thousands of miles across—stay fixed in relation to Earth without having to expend much fuel.
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“The first Lagrange point is a great place if you want to observe the sun,” says astrophysicist Neil Cornish of Montana State University, whose work on the subject has informed NASA’s definitive explanation of the Lagrange points. “You don’t have Earth in the way at any point in the orbit—you can just sit there, staring at the sun.”
Solar Sentinel
Aditya-L1 isn’t set to arrive at its final destination until the first week of January, but the probe has already begun its observations of our home star with its first images of the solar disk. It will soon enter a “halo” orbit around L1, which will allow the probe to steadily circle the sun, maintaining its trajectory via small bursts from its thrusters every few weeks. That nearly stable region is so vast, Cornish explains, that the many spacecraft near L1 never even see one another, let alone experience close encounters. “There’s just no danger at all of running into anything out there,” he says.
The most tenured tenant of L1 is NASA and the European Space Agency’s (ESA’s) Solar and Heliospheric Observatory (SOHO), an instrument-packed probe that arrived in 1996 to study different aspects of our star. Aditya-L1, too, will image the sun in visible, ultraviolet and x-ray wavelengths of light to give researchers further insight into the dynamics of the solar atmosphere.
According to India’s space agency, the probe will also study “space weather” that results from solar storms using four instruments pointed at our star itself and three others aimed elsewhere to monitor the solar wind and the effects of outbursts on the sun’s magnetic field.
Although Aditya-L1’s primary mission is set to last only five years, its L1 locale means the spacecraft could have a much longer operational lifetime. SOHO, for example, has operated at L1 for over 25 years, although it was originally planned to last just two; and a review a few years ago extended its mission through the end of 2025.
The Lagrangian Archipelago
L1 is not the only island of comparative stability in space. A system of Lagrange points accompanies each planet around the sun. And moons and planets that co-orbit the sun—including our own moon and Earth—have them, too.
Scientists have known of such points since the 1760s, when Swiss mathematician Leonhard Euler presented three of them as solutions to a special “three-body problem” arising from Isaac Newton’s laws of gravity. Italian-French astrophysicist Joseph-Louis Lagrange expanded on Euler’s work and, by 1772, had discovered five such points created by the gravitational pull between the sun and Earth. They are now known as Lagrange points in his honor.
The third Lagrange point, or L3, is directly on the far side of the sun and a little bit farther out than Earth’s orbit. Earth’s view of this Lagrange point is always blocked by the sun, preventing direct communications to and from our planet, so no spacecraft are stationed there.
The fourth and fifth Lagrange points, or L4 and L5, share our planet’s orbit around the sun but are exactly 60 degrees in front of and behind Earth, respectively. Observations show both L4 and L5 are occupied by transient populations of asteroids that piggyback on Earth gravity. Such space rocks are known as “Trojan asteroids,” and similar Trojans are found at the fourth and fifth Lagrange points of other planets, such as Jupiter.
The real gem of all the Earth-sun Lagrange points is L2, which lies about a million miles from Earth but outside our planet’s orbit, in the opposite direction of L1. Looking sunward from L2, Earth, the moon and the sun always appear clustered together in the heavens, allowing spacecraft to easily block science-scuttling stray light that any of the three might emit. Consequently, L2 has become the orbital destination of choice for several probes, including the James Webb Space Telescope. The point’s latest resident is ESA’s Euclid, a space telescope that arrived at L2 last year to measure the cosmic effects of dark energy and dark matter.
ESA’s director of science, astrophysicist Carole Mundell, says L2 allows Euclid to be visible at all times from ground stations on Earth and offers the spacecraft an unobstructed view. “The orbit is the best for radiation environment, thermal stability and availability of the entire sky,” she says. “These advantages combined are ideal for a high-precision survey mission like Euclid.”
An Interplanetary Superhighway
For Martin Lo, a spacecraft trajectory expert at NASA’s Jet Propulsion Laboratory, the Lagrange points are gateways to an “interplanetary superhighway” that extends throughout the entire solar system.
There are seven major Lagrange points within 1.2 million miles of Earth, he notes: the L1 and L2 of the Earth-sun system and five “lesser” Lagrange points of the Earth-moon system. Because all seven of these nearby regions share similar orbital energies, a spacecraft needs only a small “nudge” to move from one to another—a bit like a person swinging from bar to bar on a jungle gym, Lo says.
These Lagrange points’ prospects for allowing high-efficiency orbital transfers have shaped Lo’s work on trajectories for NASA’s Artemis missions, which aim to return astronauts to the moon and to establish a crew-supporting lunar space station that orbits near the first Earth-moon Lagrange point. And he’s currently studying the complex trajectories that exist between the Lagrange points of Saturn and its many moons. One of these moons, Enceladus, may be the best place in the solar system to look for extraterrestrial life.
“Enceladus emits icy plumes near its south pole, and we’re using these trajectories to determine how we get in orbit around it and capture [material from] them”—a matter of using the gentlest nudges possible to be at the right place, speed and time, he says.