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A halo orbit is a periodic, three-dimensional orbit near one of the L1, L2 or L3 Lagrange points in the three-body problem of orbital mechanics. Although a Lagrange point is just a point in empty space, its peculiar characteristic is that it can be orbited. Halo orbits can be thought of as resulting from an interaction between the gravitational pull of the two planetary bodies and the Coriolis and centripetal acceleration on a spacecraft. Halo orbits exist in any three-body system, e.g., the Sun–Earth–Orbiting Satellite system or the Earth–Moon–Orbiting Satellite system. Continuous "families" of both Northern and Southern halo orbits exist at each Lagrange point. Because halo orbits tend to be unstable, stationkeeping is required to keep a satellite on the orbit.
Most satellites in halo orbit serve scientific purposes, for example as space telescopes.
Robert W. Farquhar first used the name "halo" for these orbits in his 1968 Ph.D. thesis. Farquhar advocated using spacecraft in a particular halo orbit on the far side of the Moon (Earth–Moon L2) as a communications relay station for an Apollo mission to the far side of the Moon. A spacecraft in such a halo orbit would be in continuous view of both the Earth and the far side of the Moon. In the end, no relay satellite was launched for Apollo, since all landings were on the near side of the Moon.
The first mission to use a halo orbit was ISEE-3, a joint ESA and NASA spacecraft launched in 1978. It traveled to the Sun–Earth L1 point and remained there for several years. The next mission to use a halo orbit was Solar and Heliospheric Observatory (SOHO), also a joint ESA/NASA mission to study the Sun, which arrived at Sun–Earth L1 in 1996. It used an orbit similar to ISEE-3. Although several other missions since then have traveled to Lagrange points, they typically have used the related non-periodic variations called Lissajous orbits rather than an actual halo orbit.
In May 2018, Farquhar's original idea was finally realized when China placed the first communications relay satellite into a halo orbit around the Earth-Moon L2 point. On 3 January 2019, the Chang'e 4 spacecraft landed in the Von Kármán crater on the far side of the Moon, using the Queqiao relay satellite to communicate with the Earth.
This is the first-ever lunar relay satellite at this location.