Near-rectilinear halo orbit

A near-rectilinear halo orbit (NRHO) is a halo orbit with slightly curved – or nearly straight – sides between close passes with an orbiting body. The 2022 CAPSTONE mission is the first such orbit in cislunar space, and this Moon-centric orbit will serve as a staging area for future lunar missions. The orbit could be used with other bodies in the Solar System and beyond.

A halo orbit is a periodic, three-dimensional orbit associated with one of the L1, L2 and L3 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 (taking maximum diameter as that of the smallest circle that contains the whole of the orbit). In the extreme case all segments have zero curvature with four points with infinite curvature (''i.e.'' a polygon). Such a non-elliptical orbit would require at least two other bodies (''e.g.'' the Earth and Moon), and thus NRHO orbits are one theoretical solution to the classic three-body problem in gravitational mechanics. Of the three bodies, one is taken to be of negligible mass (the spacecraft).

There are four families of NRHO orbits associated with the L1 and L2 Lagrange points, 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.

Planned NRHO usage

By 2018, NASA had begun considering use of a near-rectilinear halo orbit for a future lunar mission,''NASA Shapes Science Plan for Deep-Space Outpost Near the Moon''
space.com, March 2018.
space.com, October 2017, video with refs
and by 2020, an NRHO is the planned orbit for the NASA Lunar Gateway, to be orbiting Earth-Moon in circa 2024. The Gateway orbit is planned to be a highly-elliptical seven-day NRHO around the Moon, which would bring the small space station within of the lunar north pole at closest approach and as far away as over the lunar south pole.Angelic halo orbit chosen for humankind's first lunar outpost
European Space Agency, published by ''PhysOrg,'' 19 July 2019, accessed 15 June 2020.

By 2022, the company Advanced Space built a 12-unit cubesat to fly on a Gateway precursor mission for NASA. Named CAPSTONE (Cislunar Autonomous Positioning System Technology Operations and Navigation Experiment), 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 is to test and verify the calculated orbital stability planned later for the 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 (LRO), without relying on ground stations.

NASA/JPL's Lunar Flashlight, a cubesat intended to search for water near the lunar south pole, will also use a near-rectilinear halo orbit.

References

External links

Angelic halo orbit for Gateway
(video), ESA, 17 July 2019. The planned Lunar Gateway orbit (c. 2024) appears a bit like a halo around the Moon from the point of view of an observer on the Earth.

Three-body orbits
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