Near-rectilinear halo orbit


In orbital mechanics a near-rectilinear halo orbit is a halo orbit that passes close to the smaller of two bodies and has nearly stable behavior. The CAPSTONE mission, launched in 2022, is the first spacecraft to use such orbit in cislunar space, and this Moon-centric orbit is planned as a staging area for future lunar missions. In contrast with low lunar orbit which NASA characterizes as being deep in the lunar gravity well, NRHO is described as being "balanced on the edge" of the gravity well.
The NRHOs are a subset of the halo families. This orbit type could also be used with other bodies in the Solar System and beyond.

Description

Near-rectilinear halo orbits are one theoretical solution to the classic three-body problem in gravitational mechanics, and have been mathematically described for decades.
A halo orbit is a periodic, three-dimensional orbit associated with one of the Lagrange points. Near-rectilinear means that some segments of the orbit have a greater curvature than those of an elliptical orbit of the same maximum diameter, and other segments have a curvature less than that of an elliptical orbit of the same maximum diameter. In the extreme case all segments have zero curvature with four points with infinite curvature.
There are four families of NRHO orbits associated with the, two each in the northern and southern directions. The low perilune orbits are nearly polar. They are nearly stable, minimizing the artificial thrust required for station-keeping.

NRHO in spaceflight

By 2018, NASA began considering a near-rectilinear halo orbit for a future lunar mission,
and by 2020 an Earth-Moon NRHO had become the planned orbit for the NASA Lunar Gateway. The Gateway orbit will be a 9:2 resonant NRHO, with a period of about 7 days and a high orbital eccentricity, bringing the station within of the lunar north pole at closest approach and as far away as over the lunar south pole.
In 2020 NASA refined its plans for the crewed Artemis 3 mission to the lunar surface, removing its dependence on the proposed Lunar Gateway while retaining the planned use of a NRHO for orbital rendezvous between the Orion spacecraft and a lunar lander.

NRHO flown missions

By 2022, the company Advanced Space built a 12-unit cubesat to fly on a Gateway precursor mission for NASA. Named CAPSTONE, the spacecraft became the first spacecraft to operate in an NRHO lunar orbit from 14 November 2022 after launch on 28 June 2022. The mission objective was to test and verify the calculated orbital stability planned later for the NASA Lunar Gateway space station, and the spacecraft will fly the identical orbital parameters planned later for Gateway. It will also test a navigation system that will measure spacecraft position relative to NASA's Lunar Reconnaissance Orbiter, without relying on ground stations.
NASA/JPL's Lunar Flashlight, a cubesat intended to search for water near the lunar south pole, was also planned to use a near-rectilinear halo orbit, but the satellite failed to reach the NRHO following thruster underperformances after launch in 2022.