halo orbit

A halo orbit is a type of orbit around an L1, L2, or L3 Lagrangian point. It is periodic, in that it follows same path each cycle other than the slow degradation due to the point's instability (which can be handled by stationkeeping). Such an orbit can allow a line of sight to the further body; for example, an object can be in a halo orbit around the Earth-Moon L2 point (on the far side of the Moon) such that it has a line of sight to Earth. They also allow multiple spacecraft to operate "at" (in the vicinity of) the same point, such as all the spacecraft currently at Earth-Sun L1 and L2.

Two other kinds of orbits are possible for use at the L1-L3 points: a Lissajous orbit, which is not periodic in the same sense. With each cycle, the path is different, but I believe they remain in a cylinder-like thin curved volume. Lissajous orbits are now more commonly used than halo orbits. A Lyapunov orbit is also aperiodic, but remains in a plane. I suspect halo orbits and Lyapunov orbits can be taken as special cases of Lissajous orbits, but I haven't seen this stated.

I also suspect some or all of these can be used with L4 and L5, which definitely support orbits of some kind: spacecraft haven't commonly used L4 and L5, and discussions of these orbits generally don't mention them. However, L4 and L5 orbits are discussed regarding asteroids, e.g., Trojan asteroids at Sun-Jupiter L4 and L5, but other terms are used, such as horseshoe orbit and tadpole orbit. Such orbits of L4-L5 have been observed and analyzed for quite some time, but consideration of possible orbits around L1-L3 began in the latter half of the 20th century when the notion of putting space probes at L1 or L2 were first considered.

The term halo orbit is also occasionally used in another sense: to refer to orbits of astronomical objects within the Milky Way halo.