Any two bodies that are gravitationally bound, such as Earth and the Sun, have five regions of gravitational stability. In these regions, the forces of gravity from the two bodies balance out, and anything located at these regions, known as Lagrangian points (or simply Lagrange points) will remain stationary.
Lagrangian points are fascinating, and it just so happens they are incredibly useful for space exploration. To understand how they occur, let’s examine the Earth-Sun system. Earth, as we know, is in a stable orbit around the Sun. Our planet and the star are pulling on one another. As you travel away from Earth, the gravity of the planet pulls you back.
At the same time, however, the gravity of the Sun pulls you inward into the Solar System. Travel far enough from Earth in the direction of the Sun, and you will be pulled into the Sun. Conversely, if you travel only a short distance from Earth with a speed that is not great enough to escape the planet’s gravity, you will be pulled backwards.
However, there are points around the Earth-Sun system, or indeed any such system, such as Venus and the Sun, for example, where the gravity of each body essentially ‘balances out’. In the diagram you are able to see the areas where these points occur.
The interesting thing about Lagrangian points is that an object placed on them will remain stationary, if it is not already moving, unless acted upon by something else. This makes Lagrangian points crucial stopovers for many spacecraft. Over the past few decades many spacecraft have made use of Lagrangian points in the Earth-Sun system, specifically L1 and L2, for a number of reasons.
L1 is an area of stability between Earth and the Sun. It is a prime location for Sun-observing telescopes, as they can get full views of the entire Sun over the course of a year without interference from Earth. In addition, the don’t need to exhaust much fuel to remain in position.
L2, on the opposite side of Earth, is a good location for space observatories as they can get views of the universe without any obstruction from Earth. It has also been touted as a possible location for a future space station that could be used as a ‘pitstop’ for manned spacecraft venturing further into the Solar System.
Meanwhile, Lagrangian points L4 and L5 are known to play host to numerous asteroids and could be a viable destination for future asteroid hunting spacecraft.