In any system with two large orbiting bodies — the Sun and Earth, or the Earth and Moon — there are five points where a third, small object can maintain a stable position relative to the two larger ones. Spacecraft use these points constantly, for reasons that have nothing to do with fuel savings alone. Some of the most important observatories in astronomy live there.
The Basic Idea
In a rotating two-body system, gravity and centripetal force balance in specific geometric configurations. The Italian-French mathematician Joseph-Louis Lagrange worked out where in 1772. There are five such points per system, labelled L1 through L5. A small object placed at one of them experiences, to first order, zero net force in the rotating frame: it can hold station indefinitely, at least in the idealised case.
Three of the five points (L1, L2, L3) are saddle points — stable in some directions, unstable in others. A spacecraft there needs occasional small station-keeping burns to stay on station. The other two (L4, L5) are genuinely stable: a spacecraft slightly displaced from them will drift around them rather than away. In the Sun–Jupiter system, L4 and L5 collect actual asteroid populations over billions of years — the Trojan asteroids that lead and trail Jupiter's orbit.
The Sun–Earth Points
The Sun–Earth Lagrange points are what most missions mean when they say "L-point." Each is about 1.5 million km from Earth, a hundredth of the distance to the Sun.
Sun–Earth L1
Directly between Earth and the Sun. Any spacecraft here sees the Sun continuously and gets an early view of solar wind and coronal mass ejections on their way to Earth. L1 is the home of solar monitors like SOHO (a joint ESA/NASA observatory still operating after nearly 30 years) and DSCOVR, which provides real-time solar-wind data to the U.S. Space Weather Prediction Center. An L1 observatory is one of the cornerstones of operational space weather forecasting.
Sun–Earth L2
Directly opposite the Sun, on Earth's far side, in Earth's shadow. This is ideal for infrared and deep-field astronomy: the Sun, Earth, and Moon are all on the same side, a sunshield can block all three sources of thermal radiation at once, and the spacecraft can radiate waste heat into clean sky. L2 currently hosts the James Webb Space Telescope, ESA's Euclid, and the Gaia star-mapping observatory. The Chinese Chang'e 5/6 relay satellites have used an L2-halo orbit around the Earth–Moon system for far-side lunar communications. Comet Interceptor will wait at Sun–Earth L2 before intercepting its target.
Sun–Earth L3
Directly opposite Earth, on the far side of the Sun. No operational spacecraft has ever used L3, mainly because the Sun is permanently in the way for communications. It is a useful site in principle for observations of the Sun's far side, but current solar physics manages with orbital observatories closer in.
Sun–Earth L4 and L5
60 degrees ahead of and behind Earth in its orbit around the Sun. Both are stable, both are empty of significant natural debris, and both are excellent vantage points for looking back at the Sun from a different angle than Earth. A future L5 mission is the leading proposal for improving space weather warning times (see the Sun article): a spacecraft at L5 sees coronal mass ejections in profile days before they reach Earth, giving operators much more lead time to safe-mode satellites and reroute aviation.
The Earth–Moon Points
Lagrange points also exist in the Earth–Moon system. They are smaller in scale and closer in, and they become operationally important in the Artemis era.
- EML1 and EML2: Between Earth and the Moon, and on the far side of the Moon, respectively. A relay spacecraft in a halo orbit around EML2 has a direct line of sight both to Earth and to the lunar far side — which is how China's Queqiao relay satellite enables communications for Chang'e 4's far-side lander. Any sustained far-side operation needs a relay in this region.
- EML4 and EML5: Stable points 60 degrees ahead and behind the Moon in its orbit. Neither currently hosts spacecraft, but both appear in long-term space-settlement literature as candidate locations for free-flying habitats (see the megastructures article).
Near-Rectilinear Halo Orbit
A near-rectilinear halo orbit (NRHO) is not technically a Lagrange point, but it is a closely related concept and worth mentioning. An NRHO is a highly elliptical orbit around EML1 or EML2 that swings close to one lunar pole and far from the other. It is stable with small station-keeping costs, has continuous line of sight to Earth, and is accessible from Earth-return trajectories without needing a large lunar-orbit-insertion burn.
The Lunar Gateway will be placed in an NRHO around EML2, which is what makes Gateway practical as a staging point for lunar surface missions: Orion can arrive and depart without the enormous propellant cost of entering low lunar orbit, and the orbit's long periods over the south polar region align with Artemis's surface-mission focus.
Why Lagrange Points Matter
Lagrange points are not destinations in the Mars-or-the-Moon sense. Nobody is going to visit an L-point for the experience. But they are where a surprising amount of space infrastructure already lives, and where more of it will end up. Space-based observatories that need thermal stability go to Sun–Earth L2. Solar monitors go to L1. Far-side lunar relays go to EML2 halo orbits. Any mission that needs to "park somewhere useful in cislunar space and not move much" will, sooner or later, end up in orbit around one of these points.