Lagrange point
A Lagrange point is a spot where the gravity of two large bodies balances so that a small object can stay in step with them.
In the system of two massive bodies, such as the Sun and Earth, there are five places where a small object can stay in a fixed position relative to both. These are Lagrange points, named after the eighteenth-century Italian-French mathematician Joseph-Louis Lagrange.
L1, L2 and L3 lie on the line through the two bodies and are unstable, so a spacecraft there needs occasional small rocket burns to stay in place. The Sun–Earth L1 point, about 1.5 million kilometres sunward, is used by solar observatories. L2, the same distance on the far side, hosts the James Webb Space Telescope.
L4 and L5 lie 60 degrees ahead of and behind the smaller body in its orbit. They are stable, so natural objects can collect there over billions of years. These are the homes of Trojan asteroids, most famously Jupiter's two swarms.
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