Ask Astro: How and why do satellites orbit Lagrange points?

The Queqiao satellite orbits the L2 Lagrange point located behind the Earth and the Moon to relay communications from Chinese lunar landers to Earth. How is it able to orbit around this point if the gravitational forces are balanced there?

Michael G. Blazeski

Dillonvale, Ohio

Lagrange points are locations around a two-body system where the gravitational forces of the two objects, in this case the Earth and the Moon, are equal to the centrifugal force. This allows a satellite to stay in place with relatively little movement at any of Lagrange’s five locations. However, the gravitational forces for L1, THE2, and me3 are only well balanced in a radial direction, which means that these points are unstable.

You can think of it as if you have a bullet to your head. Your head will prevent the ball from falling directly to the ground, but it will roll quickly to one side unless you balance it by moving around a bit.

Likewise, for Queqiao to remain on L2, it would need to have exactly the right speed and location. However, it can easily drift in a plane perpendicular to the imaginary line connecting the Earth and the Moon. If the satellite goes slightly past the Earth-Moon line, the Moon’s gravity pulls it back and slows it down. Conversely, when Queqiao begins to fall behind, the Moon pulls him forward and accelerates him again. This gravitational dance only requires slight adjustments from the spacecraft’s thrusters to keep it roughly in orbit around L2.

Heino falcke

Professor of Astroparticle Physics and Radio Astronomy, Radboud University, Nijmegen, The Netherlands

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