Lagrange Points: An Orbital Parking Spot for Satellites

Satellites and instruments placed at Lagrange points remain in a stable position because the gravitational forces from the Earth and sun balance each other, requiring only minimal energy for adjustments.

Lagrange (L) points are special places in space where the gravitational pull of two large objects, like the Earth and the sun, balances with the centripetal force felt by a smaller object, such as a spacecraft. This balance creates points of equilibrium, often described as “orbital parking spots,” because objects placed there can remain in a relatively stable position with little fuel needed for adjustments.

For any combination of two orbital bodies, there are five Lagrange points within their orbital plane. They are named after the 18th-century mathematician Joseph-Louis Lagrange, who wrote about them in a 1772 paper explaining what he referred to as the "three-body problem." However, the first three of these points (L1, L2, and L3) were discovered earlier in 1722 by Leonhard Euler. 

Each point has unique characteristics:

• Objects at L1, L2 and L3 act like a ball balanced on top of a hill, where small nudges can send them drifting away. Spacecraft stationed here require occasional corrections to maintain their position. Because of this, these locations are considered less stable than L4 and L5.
• Objects at L4 and L5 act like a ball in a bowl, tending to stay put a bit more easily. These points can even trap space debris or asteroids. For this reason, L4 and L5 are considered more stable, and lie along a planet’s orbit, 60 degrees ahead of or behind it. The distance between each of these locations—Lagrange point, planet, and sun—forms equilateral triangles in space.

Fun fact: In 2011, NASA announced the discovery of the first asteroid that shares Earth’s orbit (known as an Earth trojan) at L4, called 2010 TK7. A second was discovered in 2020, called 2020XL5. No Earth trojans have been discovered at L5 (yet). 

Different Lagrange points have different uses. For example: 

• L1 allows for constant, unobstructed monitoring of the sun.
• L2 provides an ideal, unobstructed view of deep space.
  • This is where the James Webb Space Telescope is located.
• L3, located directly opposite the Earth on the far side of the sun, provides a preview of active regions that cannot be seen from Earth.
  • NASA’s STEREO-A and -B spacecraft made these observations in the first part of their missions.
• L4 is an excellent location for long-term scientific instruments due to its stability. While no spacecraft are currently stationed here, future missions may use this vantage point for solar observations or for studying Earth’s trojan asteroids.
• L5 is ideal for solar observations, enabling early detection and tracking of space weather activity.
  • The European Space Agency’s Vigil satellite, scheduled to launch in 2031, will be stationed here. NOAA will be providing a coronagraph (CCOR-3) as part of the  Space Weather Next program and NASA will be providing an extreme ultraviolet imager (JEDI) for the mission. 

In essence, Lagrange points are nature’s solutions to the problem of stationing spacecraft in stable positions, taking advantage of a cosmic balance between gravity and motion.