ABSTRACT
Lagrange points Five locations within a three-body system where a small object will always maintain a fixed orientation with respect to the two larger masses though the entire system rotates about the center of mass. If the largest mass in the system is indicated by M1 and the second largest mass is M2, the five Lagrange points are as follows: in a straight line with M1 and M2 and just outside the orbit of M2 (usually called the L1 point); in a straight line with M1 and M2 and just inside the orbit of M2 (L2); in a straight line with M1 and M2 and located in M2’s orbit 180◦ away from M2 (i.e., on the other side of M1) (L3); and 60◦ ahead and behind M2 within M2’s orbit (L4 and L5). In the sun-Jupiter system, the Trojan asteroids are found at the L4 and L5 locations. In the Earth-sun system, the Lagrangian points L1 and L2 are both on the sunEarth line, about 236 RE (≈ 0.01 AU) sunward and anti-sunward of Earth, respectively. The other points are far from Earth and therefore too much affected by other planets to be of much use, e.g., L3 on the Earth-sun line but on the far side of the sun. However, L1 (or its vicinity) is a prime choice for observing the solar wind before it reaches Earth, and L2 is similarly useful for studying the distant tail of the magnetosphere. Spacecraft have visited both regions — ISEE-3, WIND, SOHO and ACE that of L1, ISEE-3 and GEOTAIL that of L2. Neither equilibrium is stable, and for this and other reasons spacecraft using those locations require on-board propulsion.
