The James Webb Space Telescope and Lagrangian Points

By Betty Robinson,

The James Webb Space Telescope. Credit: NASA

The James Webb Space Telescope (JWST) is currently scheduled for launch on October 31, 2021. JWST is an infrared telescope that will carry on from Hubble. It will detect the infrared, or heat, signals from distant objects in space. JWST will operate at L2. L2 is a Lagrangian point.

Lagrangian points are locations in space associated with one small and two large bodies in a system, for example, the Sun, Earth, and a satellite. At a Lagrangian point, the gravitational attraction of the two large bodies equals the centripetal force required for the smaller object to be stationary relative to the other two bodies. So a satellite (or other object) at a Lagrangian point stays in the same spot relative to the two large bodies.

A two-body system has five Lagrangian points. The Swiss mathematician Leonhard Euler predicted the existence of L1, L2, and L3 around 1750. French astronomer Joseph Louis de Lagrange predicted the existence of the other two, L4 and L5, in 1772.

The points L1 and L2 are the same distance from Earth, about 1.5 million kilometres.

The Lagrangian points are useful for space exploration. As you can see in the diagram, L1 is between the Sun and Earth (the Moon is included with Earth). Satellites at L1 can continuously observe the Sun, so a number of Sun-observing satellites are positioned here, for example, the Solar and Heliospheric Observatory (SOHO). At L2, satellites can continuously observe deep space, and they are far enough away from Earth’s magnetosphere to avoid interference but close to enough to be able to communicate with Earth. JWST will be at L2. The WMAP observatory is located at L2, and Planck is currently there. (WMAP is the Wilkinson Microwave Anisotropy Probe used to study cosmology, and the Planck space probe also studied cosmology. Both are currently inactive.)

There are no uses for L3 at the moment because it is always behind the Sun for us. However, there have been suggestions for observations of the Sun at this point. For example, a satellite that monitors evolving sunspots could provide valuable advance notice before the sunspots rotated to the Earth side, about 7 days later (assuming there are some communications satellites to support it).

Points L1, L2, and L3 are unstable, and all satellites at these points must orbit the points in space (called halo orbits) and make course corrections to stay there. However, points L4 and L5 are stable. Because of their stability, debris such as dust and asteroids tend to collect at L4 and L5. (They don’t actually sit at the point; they librate around the point in space.)

Asteroids that settle at Lagrangian points are called Trojan asteroids. There are several thousand Trojan asteroids at L4 and L5 of the Sun–Jupiter system. Mars, Neptune, and some of Saturn’s moons have Trojan asteroids. There is only one known Trojan asteroid in the Sun–Earth system: asteroid 2010 TK7 was discovered at L4 in 2010 by astronomers using data from the space telescope WISE (Wide-field Infrared Survey Explorer).