Some of the most important science missions do not orbit the Earth at all. They sit near special locations in space called Lagrange points, where the gravity of the Sun and the Earth combine in a useful way. This article explains what Lagrange points are and why missions such as SOHO and the James Webb Space Telescope use them.
What a Lagrange point is
In a system of two large bodies, such as the Sun and the Earth, there are five points where a small spacecraft can keep its position relative to both bodies. At these points, the gravitational pull of the two bodies and the motion of the spacecraft are in balance. They are named L1 to L5 after the mathematician Joseph-Louis Lagrange.
The five points of the Sun Earth system
- L1: about 1.5 million kilometres from Earth towards the Sun. It offers an uninterrupted view of the Sun and is used for solar observation and space weather monitoring.
- L2: about 1.5 million kilometres from Earth on the side facing away from the Sun. The Sun, Earth and Moon are all behind the spacecraft, which makes L2 ideal for telescopes that need a cold, stable environment.
- L3: on the far side of the Sun, opposite the Earth, and not used by missions so far.
- L4 and L5: ahead of and behind the Earth along its orbit. Unlike L1 to L3, these points are stable.
Halo orbits and station keeping
L1, L2 and L3 are unstable, which means a spacecraft slowly drifts away unless it corrects its position. Missions therefore fly in large orbits around these points, such as halo orbits, and perform regular small manoeuvres. The required propellant is modest, but it limits the lifetime of the mission.
Examples
The ESA and NASA Solar and Heliospheric Observatory, SOHO, has observed the Sun from L1 since the 1990s. The James Webb Space Telescope operates near L2, where its large sunshield can block the light and heat of the Sun, Earth and Moon at the same time.
Why this matters for launch
Missions to Lagrange points need launchers or upper stages that can deliver the spacecraft onto an escape trajectory, which requires considerably more energy than a low Earth orbit. Rideshare opportunities to these destinations are rare, although some science missions share launches when their targets are compatible.
Related reading
- Mars Lagrange Points: What They Are and Why They Matter for Mars Communications
- Falcon Heavy Launches NASA's Nancy Grace Roman Space Telescope to Hunt Dark Energy
- LEO, MEO and GEO Satellites Compared: Altitudes, Uses and Examples
Sources
Cover image: Artist's concept of the SOHO spacecraft. Credit: NASA (PIA18170), public domain.