Scientists Say: Lagrange point

Lagrange point, (noun, “luh-GRAINJ”)

A Lagrange point is a position in space near two large space objects, such as the sun and Earth. When a third, smaller object — such as a satellite —  is placed at a Lagrange point, it will tend to stay in place.

A two-object system, such as the Earth-sun system, has five Lagrange points. These points are called L1, L2, L3, L4 and L5.

At these points, the large objects’ pull of gravity holds the satellite in place relative to them. As a result, the satellite will move with the larger objects as they move.

If a satellite or space telescope is placed at a Lagrange point, it will maintain a fairly steady orbit. If placed outside one of these points, the satellite’s orbit tends to change over time. Without an occasional rocket boost, the satellite’s orbit may slip so that it falls toward Earth. It would then burn up in the atmosphere. But a satellite at a Lagrange point needs very little rocket boost to maintain orbit. At very stable points — L4 and L5 — they may need no boost at all. That helps save fuel.  

The James Webb Space Telescope (JWST) orbits at the L2 point. Note that it doesn’t just remain stuck at this point. This is an ideal location for a space telescope. Here’s why. 

First, it keeps the telescope close to Earth. That allows for easy communication between the telescope and the people and programs on Earth that run it. Second, the telescope can turn its solar panels toward the sun. That allows for solar power. Third, this position gives the telescope a clear view of outer space. Here, the telescope can turn its mirrors outward to peer into the cosmos.

In a sentence

Many stars in a galaxy orbit in stable positions called Lagrange points.

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