Halo orbit
Periodic, non-planar orbit around Lagrange points.
A halo orbit is a periodic, non-planar orbit associated with one of the L1, L2 or L3 Lagrange points in the three-body problem of orbital mechanics. It can be orbited by a spacecraft and results from an interaction between the gravitational pull of two planetary bodies and the Coriolis and centrifugal force. Halo orbits exist in any three-body system, such as a Sun–Earth–orbiting satellite system or an Earth–Moon–orbiting satellite system, and continuous families of both northern and southern halo orbits exist at each Lagrange point. Because halo orbits tend to be unstable, station-keeping using thrusters may be required to keep a satellite on the orbit.
- named by
- Robert W. Farquhar
- notable missions
- SOHO, Queqiao, James Webb Space Telescope, Euclid, Aditya-L1
- associated Lagrange points
- L1, L2, L3
Lore & Background
Robert W. Farquhar advocated using spacecraft in such an orbit beyond the Moon (Earth–Moon L2) as a communications relay station for an Apollo mission to the far side of the Moon. In 1984, Kathleen Howell showed that more precise trajectories could be computed numerically and found that for most values of the ratio between the masses of the two bodies there was a range of stable orbits.
Reader's Guide
Halo orbits are significant because they enable continuous communication and observation in locations where a spacecraft would otherwise be blocked by a planetary body. Although many missions use Lissajous orbits, halo orbits remain a key concept for missions requiring continuous line-of-sight and periodic motion.
Frequently Asked Questions
What exactly is a Halo orbit?
A halo orbit is a periodic, three-dimensional (non-planar) trajectory that loops around one of the L1, L2, or L3 Lagrange points in a three-body gravitational system. It arises from the combined effects of two bodies' gravity, the Coriolis force, and the centrifugal force, producing a figure-eight-like path rather than a flat ellipse.
Why can't a spacecraft just sit in a halo orbit forever?
Halo orbits are inherently unstable, meaning tiny perturbations grow over time and the spacecraft will drift away from the intended path. Operators must therefore perform periodic station-keeping burns to keep the craft locked in its halo trajectory.
Do halo orbits only exist around the Sun and Earth?
No—halo orbits are a general feature of any three-body system, so they appear in the Earth–Moon pair, the Sun–Earth pair, and even around binary asteroid systems. At each of the L1, L2, and L3 points there are continuous families of both northern and southern halo orbits to choose from.
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