### halo orbit

(type of orbit around an instable Lagrangian point)

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
orbits can allow a line of sight around the body in the middle,
such as an orbit beyond the Moon that always has a line of sight
to stations on Earth. They also help allow multiple spacecraft
to operate "at" (near) the same point, such as all the spacecraft
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 looked at as special
cases of Lissajous orbits, but I haven't seen this stated.

Lissajous orbit | aperiodic | not within a plane |

Halo orbit | periodic | not within a plane |

Lyapunov orbit | aperiodic | within a plane |

I also suspect some or all of these can be used with L4 and L5,
which haven't been as popular for use by spacecraft, and
less is written about their possible orbits.

(*orbits,space*)
**Further reading:**

http://en.wikipedia.org/wiki/Halo_orbit

http://en.wikipedia.org/wiki/Lissajous_orbit

http://en.wikipedia.org/wiki/Lyapunov_stability

**Referenced by pages:**

Lagrangian point

Index