What Are Lagrange Points?

Lagrange points are stable gravitational points in space where an object could be placed and it would remain at a fixed position with respect to a body orbiting the Sun, such as the Earth. Every orbiting planet has five Lagrange points. Earth's Lagrange points are called L₁, L₂, L₃, L₄, and L₅. These points often feature in discussions about space or science fiction because they would be ideal locations to build space stations.

Of the Lagrange points, L₁ and L₂ are located closest to the planet, just 1.5 million km (930,000 mi) closer to the Sun and away from the Sun from the Earth, respectively. This is about five times as far away from the Earth as the Moon is, and would require a trip of roughly two weeks in a modern spacecraft. As the Earth rotates around the Sun, these two points rotate around the Sun with it, always staying in the same place with respect to the planet. These stable orbits are being eyed as places to deploy satellites that observe the Earth-Moon system.

Two additional points — L₄ and ₅ — are located on either side of the Earth with respect to the Sun, lie 60° ahead of the Earth and 60° behind the Earth in its orbit around the Sun, and contain interplanetary dust. The Earth-Moon system also has these points, which may contain interplanetary dust grouped into what are called Kordylewski clouds. L₄ and ₅ in the Sun-Earth system are also called the triangular Lagrange points or Trojan points. The latter name comes from the Trojan asteroids at the Sun–Jupiter L₄ and L₅ points. Every planet has its own Trojan points, and the larger the planet, the more interstellar dust will be found trapped in them.

The last Lagrange point is L₃. This point is located the farthest away from the Earth, on the opposite side of the Sun. Until the development of interplanetary probes which could travel to areas of the solar system outside the Earth and Moon, L₃ could never be observed due to its position. This led some mystics to postulate the existence of an "anti-Earth" there, though this proved false. Billions of years ago, a Mars-sized object, named Theia, is believed to have formed in L₃. Its orbit became unstable, until eventually it collided with the Earth. The impact was so great that it ejected millions of cubic kilometers of molten magma into orbit. This cooled and became the Moon.