The near rectilinear halo orbit, or NRHO, is a special type of halo orbit that was selected by NASA for the Gateway – a small space station that will orbit the Moon and serve as a staging point for Artemis missions. There are several advantages of using an NRHO for the Gateway and future lunar missions compared to other possible orbits.
One major benefit of the NRHO is its stability. Halo orbits around the second Lagrangian point (L2) of the Earth-Moon system are dynamically stable, meaning a spacecraft can remain in this orbit without having to perform complex orbital maintenance maneuvers to counteract perturbations. This allows for long-term dwell of orbital assets like the Gateway. In contrast, low lunar orbits require station-keeping to account for orbital decay over time. The intrinsic stability of the NRHO reduces operational costs and Complexity for missions utilizing the Gateway.
A linked advantage is that the Gateway’s NRHO enables continuous line-of-sight communication with Earth without interruptions from the Moon getting in the way. This “stable remote platform” feature provides mission planners assured and uninterrupted command and control of robonaut or manned sorties from the Gateway to the lunar surface, increasing safety. Low lunar orbits by comparison have intermittent communications blackout periods. Reliable comms through Gateway are crucial for surface missions.
Another key benefit of the Gateway’s NRHO is its free return capability. If engines fail on a spacecraft departing the Gateway for the lunar surface, the craft’s trajectory will return it to the Earth-Moon system without the need for correction. This ensuresBuilt insafe mode return for astronautswithout depleting mission resources. Low lunar orbits lack this fail-safe free return capacity, necessitating precise maneuvers and significant propellant usage for any emergencies.
The phasing properties of the NRHO mean that missions departing from the Gateway can access any part of the lunar surface within a single orbit, offering coverage flexibility for surface sorties, landings or cargo deliveries. This facilitates global access unlike low polar or equatorial orbits which see the same side of the Moon on each pass. The Gateway’s NRHO phasing point allows surface missions to utilize minimal propellant for optimal transit to target locations.
The orbital altitude of the NRHO above the lunar surface, averaging around 70,000 km, also provides an ideal vantage point for long-term scientific observation of the Moon without interference from short-term fluctuations. Platforms in the Gateway will be able to conduct persistent solar astronomy studies as well as high-resolution imaging surveys of the entire lunar farside which remains occluded from Earth-based observation. Long duration monitoring supports rigorous analysis impossible through brief fly-bys alone.
The NRHO actually fosters economical trajectories allowing spacecraft to take advantage of gravity assists from both Earth and Moon, reducing propellant demands. Missions can utilize minimum energy ballistic transfers from low Earth orbit to the Gateway then onward surface excursions. This conserves precious onboard fuel compared to direct transfers and lower orbits. Lower propellant needs cuts spacecraft mass and launch vehicle lift requirements, easing deployment logistics and decreasing costs. Recent studies have shown NRHO transit mass savings can reach 30% compared to lunar surface injection.
The Gateway’s Near Rectilinear Halo Orbit provides unmatched accessibility, communications, crew safety assurances, scientific value, and most importantly – cost effectiveness – through its inherent dynamical characteristics. Its advantages over direct low lunar orbits truly establish it as the optimal orbital choice for establishing a sustainable lunar presence and enabling the long term exploration, development and commercialization of the Moon under the Artemis program and beyond. The decision to position the Gateway in NRHO demonstrates the care and thoroughness that has gone into mission architecture design for enabling sustainable and ambitious human exploration of the lunar surface from this unique vantage point.