A Lagrangian point is a point of interplanetary gravitational stability in a two body orbital configuration.
Positions of the Lagrangian points
The simplest Lagrangian point to understand is the "first Lagrangian" (or ) point between the Earth and the Sun. The point at which the gravitational pull of the Sun and the gravitational pull of the Earth cancels out () creates an island of gravitational stability where space observatories, or indeed space stations, can be positioned.
is located on the opposite side of the smallest orbital body (in this case, the Earth) to . This very stable region is also useful to space observatories observing the cosmos. The Earth in this case will be constantly eclipsing the Sun, allowing sensitive optics to operate free of noise emitted from the Sun.
is a less-stable Lagrangian point on the far side of the Sun. In this case, the Earth's gravitational force is negligible, allowing other planet's orbits to interfere with the gravitational stability of the region.
and are points leading and trailing the orbiting body at an angle of approximately 60° from the Earth-Sun line. These points are also known as "Trojan points" where asteroids (known as Trojan asteroids) become captured by the relative gravitational stability and orbit with the orbital body.
Calculating the position of and
If the mass of the larger body is massively greater than that of the orbiting body (as is the case in the Earth, or Mars, around the larger Sun), the following equation can be applied:
where is the distance of or from the orbiting body, is the distance between the bodies and and are the masses of the smaller and larger bodies respectively.
Current missions using the Lagrangian points of the Earth-Sun system
Objects observed in the Mars-Sun and points
Asteroids in the and Mars-Sun Lagrangian points are often called Mars Trojan asteroids. A handful of asteroids are in stable solar orbits, leading () and following () the path of Mars including an asteroid named "1999 UJ7" (at ) and "5261 Eureka" (at ).