Lunar Halo Orbits
What is a Halo Orbit?

Coined by Robert W. Farquhar in his 1968 Ph. D. thesis, the term ”halo” orbits describes a unique orbit around the L1, L2, or L3 Lagrange points in a 3-body astrodynamical problem. While a halo orbit does not actually orbit the libration point, it does fly in a periodic trajectory around it (4). In the graphic below, the gravitational pull from the two planetary bodies (blue and gray) exert forces on the spacecraft that keep it traveling in an orbit behind between the secondary and the L2 point.

Figure 3: A Halo Orbit at the L2 Lagrange Point (click for cite)

These orbits are considered dynamically unstable, meaning that active station keeping is necessary to to keep the satellite in the orbit.

Halo Orbits in History

The first mission to ever use a halo orbit was the International Sun-Earth Explorer (ISEE-3) satellite launched in 1978. ISEE-3 was a joint venture between NASA and ESA to explore the relationhip between Earth’s magnetic field and solar winds. ISEE-3 utilized the L1 point for many years in the Earth-Sun system, show below.

Figure 4: The ISEE Mission Trajectory (9)

ISEE-3 proved that suspension between gravitational fields in a 3-body system was not only theoretically possible, but also feasible in practice (9).

A more recent halo orbit was flown by the Genesis spacecraft, below, in 2001.

Figure 5: The Genesis Spacecraft (click for cite)

Genesis was sent to the Earth-Sun L1 point to collect solar wind particles beyond the orbit of the sun. Genesis’ mission profile is shown below:

Figure 6: The Genesis Mission Trajectory (9)

After 5 revolutions around the L1 point the Genesis spacecraft was sent Earthwards on a low energy return trajectory (3).