Lagrangian point

Definition

A Lagrangian point is a point of interplanetary gravitational stability in a two body orbital configuration.

Positions of the Lagrangian points

Example: The position of the L₁, L₂, L₃ and L_4,5 points in the Mars-Sun system.

The simplest Lagrangian point to understand is the "first Lagrangian" (or $L_{1}$) 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 ($g_{Earth}+g_{Sun}=0$) creates an island of gravitational stability where space observatories, or indeed space stations, can be positioned.

$L_{2}$ is located on the opposite side of the smallest orbital body (in this case, the Earth) to $L_{1}$. 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.

$L_{3}$ 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.

$L_{4}$ and $L_{5}$ 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 $L_{1}$ and $L_{2}$

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:

$r\approx R{\sqrt[{3}]{\frac {M_{2}}{3M_{1}}}}$

where $r$ is the distance of $L_{1}$ or $L_{2}$ from the orbiting body, $R$ is the distance between the bodies and $M_{2}$ and $M_{1}$ are the masses of the smaller and larger bodies respectively.

Current missions using the Lagrangian points of the Earth-Sun system

• $L_{1}$

• The Solar and Heliospheric Observatory (SoHO) - A multi-instrument observatory contantly observing the Sun.
• Advanced Composition Explorer (ACE) - An in-situ observatory measuring the properties of the solar wind and solar radiation.

• $L_{2}$

• Wilkinson Microwave Anisotropy Probe (WMAP) - Surveying the sky, observing the microwave radiation left over by the Big Bang.

Uses of the Mars $L_{1}$ point

• An "early warning system" to notify settlers about the onset of solar storms.