On Monday, Jan. 24, engineers plan to instruct NASA’s James Webb Space Telescope to complete a final correction burn that will place it into its desired orbit, nearly 1 million miles away from the Earth at what is called the second Sun-Earth Lagrange point, or “L2” for short.

Mathematically, Lagrange points are solutions to what is called the “restricted three-body problem.” Any two massive, gravitationally significant objects in space generate five specific locations – Lagrange points – where their gravitational forces and the centrifugal force of the motion of a small, third body such as a spacecraft are in equilibrium. Lagrange points are labeled L1 through L5 and are preceded by the names of the two gravitational bodies that generate them (the big one first).

While all Lagrange points are gravitational balance points, not all are completely stable. L1, L2, and L3 are “meta-stable’ locations with saddle-shaped gravity gradients, like a point on the middle of a ridgeline between two slightly higher peaks wherein it is the low, stable point between the two peaks, but it is still a high, unstable point relative to the valleys on either side of the ridge. L4 and L5 are stable in that each location is like a shallow depression or bowl atop the middle of a long, tall ridge or hill.

So why send Webb to orbit Sun-Earth L2? Because it is an ideal location for an infrared observatory. At Sun-Earth L2, the Sun and Earth (and Moon, too) are always on one side of space, allowing Webb to keep its telescope optics and instruments perpetually shaded. This enables them to get cold for infrared se