The five Lagrange points

There are locations around a planet’s orbit where the gravitational forces and the orbital motion of the Sun and planet interact to create a stable location, from where a spacecraft can reside with little effort from the operators on the ground to keep it in place. These points are known as Lagrangian or ‘L’ points, after the 18th century Italian astronomer and mathematician Joseph-Louis Lagrange (born Giuseppe Luigi Lagrancia).

As seen from the Sun, the L4 and L5 points lie at 60 degrees ahead of and behind Earth, close to its orbit. Unlike the other Lagrange points, L4 and L5 are resistant to gravitational perturbations. Because of this stability, objects such as dust and asteroids tend to accumulate in these regions.

At L4 or L5, a spacecraft is truly stable, like a ball in a large bowl.