WEEKLY WHINE

Lagrangian points of interest

There now follows a ranking of all libration points in the corotating reference frame of one massive body orbiting another.

1. L₁: Located along the line between the two objects, just inside the orbit of the less massive object. The Sun-Earth L₁ point is commonly used for spacecraft that observe the Sun or Earth. The Earth-Moon L₁ point would be a convenient location for a space station that would help transport humans and objects between Earth and the Moon.
2. L₂: Located along the line between the two objects, just outside the orbit of the less massive object. The Sun-Earth L₂ point is ideally suited for space observatories; the James Webb Space Telescope will be deployed to a Lissajous orbit around this point. The Earth-Moon L₂ point could be used for communications satellites that cover the Moon’s far side.
3. L₃: Located along the line between the two objects, on the opposite side of the more massive object. The Sun-Earth L₃ point has been proposed as a site for spacecraft observing the Sun’s far side, to provide early warning of sunspots before they rotate into view of Earth.
4. L₄: Located along the less massive object’s orbit, sixty degrees ahead. Asteroids orbiting at a system’s L₄ point are called trojans; in the case of the Sun-Jupiter system, the L₄ trojans are called the Greek camp, and the asteroids are named after Greek participants in the Trojan War. Several thousand asteroids in the Greek camp are known so far.
5. L₅: Located along the less massive object’s orbit, sixty degrees behind. Asteroids orbiting at a system’s L₅ point are called trojans; in the case of the Sun-Jupiter system, the L₅ trojans are called the Trojan camp, and the asteroids are named after Trojan participants in the Trojan War. Several thousand asteroids in the Trojan camp are known so far.