Earth-Moon Near Rectilinear Halo and Butterfly Orbits for Lunar Surface Exploration

May 31, 2023

How Are Cislunar Experts Planning for Exploration of the Lunar Surface?

NASA is planning the next phase of crewed spaceflight with a set of missions designed to establish a human-tended presence beyond low Earth orbit. The current investigation focuses on the staging post potential of cislunar infrastructure by seeking to maximize performance capabilities by optimizing transits in the lunar vicinity for both crewed and robotic spacecraft. In this paper, the current reference orbit for the cislunar architecture, known as a Near Rectilinear Halo Orbit (NRHO), is examined alongside the bifurcated L2 butterfly family for their use as a staging location to the lunar surface.


NASA has proposed the Gateway1 concept as a testing ground for deep-space technologies, a staging point for missions beyond cislunar space, and a platform for Earth, lunar and space science. Currently, assembly of the Gateway components is expected to occur on orbit; tended by crew transported to the Gateway via the Orion spacecraft. Careful selection of orbit architecture is required to ensure that constraints are met while fulfilling the goals of the Gateway mission.2 One objective of the Gateway is to serve as a staging post for lunar surface access, both for crewed and robotic applications. With the capability to stage crewed missions, the Gateway will support the return of humans to the surface of the Moon, a NASA objective outlined recently in Space Policy Directive 1.3

The reference orbit for the Gateway is a Near Rectilinear Halo Orbit (NRHO).4 NRHOs comprise a subset of the halo families of orbits in the Earth-Moon system, characterized by close passages of the Moon and nearly stable behavior. More specifically under consideration is an L2 southern NRHO in a 9:2 synodic resonance with the Moon’s orbit around the Earth that completes an orbital period about every 6.5 days. As a destination for the Gateway, the NRHO offers distinct advantages. These include relatively low transfer costs from Earth that fit within the capabilities of the Orion spacecraft, low orbit maintenance costs, and favorable communications opportunities to both Earth and the lunar south pole.5 NRHOs also offer opportunities for transfers to other orbits within cislunar space, including other members of the halo families, butterfly orbits, and DROs. In addition, a spacecraft in an NRHO can serve as a potential staging point for transfers to the lunar surface.

Current architecture designs for crew landings on the lunar surface envision a multiple-launch scenario. It is this fact that makes NRHO attractive as a staging location to the surface. For example, a dual launch scenario in which a crewed lunar lander is delivered months in advance on a ballistic lunar transfer could save significant total propellant prior to rendezvous with a crewed vehicle when it arrives at the NRHO on a separate launch. The lunar surface mission cost is then primarily a function of the cost to transfer from NRHO to different landing sites and the capability to perform aborts back to NRHO. Previous work provided limited analysis on the relative cost for these transfers, focusing on a single reference NRHO.2 In this current investigation, multiple sizes of NRHO and L2 butterfly orbits, which bifurcate from the halo family, are considered.

Future missions to the lunar surface will feature both crewed and robotic spacecraft. The two categories of missions involve different sets of constraints and challenges. Crewed missions from Earth to the NRHO and from the NRHO to the Lunar surface require short transfer times and must allow for abort opportunities throughout the transfer sequence. Crewed missions are round trip with relatively heavy spacecraft, generally powered by chemical propulsion to allow for short times of flight. Uncrewed transfers are less constrained by flight time and abort planning, and they may be one-way voyages of lightweight spacecraft. They may employ chemical propulsion or low-thrust solar electric propulsion (SEP). Short crewed transfer options can differ significantly from longer, uncrewed transfers in flight time, cost, and appearance.

The current investigation first explores transfers from the Earth to the NRHO. Examples of crewed transfer options6 are compared to a new exploration of ballistic lunar transfers for cargo delivery to the NRHO and to low thrust cargo delivery results.7 Then, transfers from the NRHO to the surface are explored to enable robotic science missions as well as human access to the Moon. Finally, butterfly orbits are investigated as an alternative starting location for transfers to the Lunar surface.


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