The retrograde geosynchronous orbit, or RGSO, is prized for a specific operational advantage. Because it runs counter to Earth’s rotation, a satellite in this orbit sweeps across the entire geosynchronous belt roughly every 12 hours, a cadence that makes it exceptionally valuable for space surveillance and space situational awareness missions. Yet the very property that makes the orbit useful also makes it costly to attain. The direction of Earth’s rotation, together with the tracking, telemetry, and control constraints of launch sites, means that a direct launch into RGSO from Earth must overcome an additional velocity increment of more than approximately 4 kilometers per second. That figure translates into enormous fuel consumption and significant engineering difficulty. Conventional workarounds rely on lunar gravity assists to move satellites from low Earth orbit or geosynchronous transfer orbit into RGSO, but even those routes demand relatively high velocity increments and offer limited transfer paths, leaving engineers with few good options.

The Beihang team’s answer begins with a careful choice of dynamical model. Rather than the classical circular restricted three-body problem, which treats only Earth and the Moon, the researchers adopted the bicircular restricted four-body problem, or BCR4BP. In this framework, Earth and the Moon revolve in circular orbits around their common barycenter, while the Sun moves in a coplanar circular orbit around the Earth-Moon barycenter. The addition of solar gravity matters enormously for the transfers in question, because some of the most fuel-efficient routes deliberately exploit the Sun’s perturbing influence. The BCR4BP also provides a closer approximation to the true ephemeris than the CR3BP, which makes solutions found within it more likely to survive the transition to high-fidelity numerical models used in actual mission planning.

Three initial orbits anchor the study. The first is a 2:1 stellar-resonant distant retrograde orbit, the nominal orbit once planned for NASA’s Asteroid Redirect Mission. The other two are near-rectilinear halo orbits: an 11:3 synodic-resonant orbit around the L1 point and a 9:2 synodic-resonant orbit around L2, the latter serving as the nominal orbit for the Lunar Gateway space station. These cislunar orbits are natural staging points because spacecraft stationed there already inhabit the sensitive gravitational regime of the three-body problem, where small maneuvers can produce large changes in trajectory. The destination is fixed and demanding: a retrograde geosynchronous orbit at 36,000 kilometers altitude, inclined at 180 degrees, moving retrograde in the equatorial plane.

To design the transfers, the researchers built a two-stage method. For two-impulse transfers, they integrated trajectories forward from the cislunar orbit and constructed an initial guess database using the departure point position, the solar initial phase, and a velocity ratio as search variables. A nonlinear programming stage then optimized the total fuel consumption of the departure and insertion impulses. For three-impulse transfers, the method adds a perilune impulse: trajectories are integrated forward and backward to build pre-perilune and post-perilune Poincaré section sub-databases, which are then matched to divide the transfer into two segments that are optimized separately, yielding a total of three impulses. This database-plus-optimization architecture allows the team to populate an entire solution plane rather than chasing individual trajectories one at a time.

The resulting solution space reveals four transfer types, each reversing the spacecraft’s orbital direction by a different mechanism. Direct transfers keep the apocenter below three times the Earth-Moon distance and achieve orbital reversal through what the authors call apocenter reversal, tracing a semi-elliptical pattern in the time-versus-velocity-increment plane. Weak stability boundary transfers, by contrast, lift the apocenter beyond three Earth-Moon distances into the weak stability boundary region, where sensitive solar gravitational dynamics lower the perigee altitude. These WSB transfers take more than 60 days but cut fuel consumption to approximately 1.3 kilometers per second. Lunar gravity-assisted transfers use a powered flyby near perilune to reverse the orbit, bringing the total velocity increment below 2.2 kilometers per second for transfer times under 60 days. Finally, combined WSB-plus-lunar-gravity-assisted transfers merge both mechanisms and further optimize the Pareto frontier of the time-fuel tradeoff.

One of the study’s clearest findings is the complementary role of the third impulse. Three-impulse solutions fill gaps in the solution space that two-impulse transfers leave empty, effectively rounding out the Pareto frontier of optimal tradeoffs between transfer time and fuel consumption. Across all three initial orbits, the shortest transfers complete within 8 days, and regardless of the starting orbit, the minimum fuel consumption consistently remains below 1.5 kilometers per second. Set against the more than 4 kilometers per second penalty of direct launch from Earth, those numbers represent a dramatic reduction, and they suggest that a spacecraft already stationed in cislunar space could be repositioned into RGSO with a fuel budget that no ground-launched mission could match.

Designing trajectories in an idealized model is one thing; flying them is another. To test engineering feasibility, the team converted the four transfer types to the ephemeris model, using a multiple shooting method to transform the state vector from the Earth-Moon rotating frame to the Earth-centered J2000 inertial frame while simultaneously accounting for the multi-revolution constraints of the initial orbit and the insertion constraints of the quasi-retrograde geosynchronous orbit. This step is where many elegant three-body designs fail, because the real solar system is not perfectly circular or coplanar. The conversion results therefore serve as a critical reality check on the entire approach.

The epoch sensitivity analysis exposes a fundamental tradeoff among the transfer types. Direct transfers, tested under daily initial epochs in January 2025, showed total velocity increment fluctuations of more than 100 meters per second and transfer time variations within 1 day, but feasible results could be obtained for any initial epoch. Lunar gravity-assisted transfers behave similarly, showing low sensitivity to departure timing. Weak stability boundary transfers tell a different story: examined across daily initial epochs in the first two months of 2025, they offer only two consecutive transfer windows per month, with gray regions in the analysis marking failed transfers or cases where the velocity increment exceeded an upper limit of 2 kilometers per month. The combined WSB-plus-lunar-gravity-assisted transfer is the most constrained of all, offering just one window per month with a narrower duration, because it depends on a specific Sun-Earth-Moon angular configuration that occurs only once per synodic period of the Moon.

Taken together, the results confirm that deploying retrograde geosynchronous orbit satellites from cislunar orbits is feasible and provide a new pathway around the prohibitively high fuel consumption of direct launch from Earth. The practical picture that emerges is one of options rather than a single prescription: direct transfers and lunar gravity-assisted transfers offer high engineering flexibility and can depart at nearly any time, while WSB-type transfers deliver the lowest fuel costs but demand patience and precise scheduling around monthly windows. As cislunar infrastructure grows and spacecraft accumulate in distant retrograde orbits and near-rectilinear halo orbits, the ability to redeploy those assets into surveillance-rich retrograde orbits could transform them from fixed-position platforms into a mobile fleet, with the gravitational interplay of Earth, Moon, and Sun doing much of the work that rockets would otherwise have to do.

Subject of Research: Low-energy trajectory design for spacecraft transfers from cislunar orbits to the retrograde geosynchronous orbit

Article Title: Trajectory design for transfers from cislunar orbits to the retrograde geo-synchronous orbit

Article References: Trajectory design for transfers from cislunar orbits to the retrograde geo-synchronous orbit. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: cislunar space, retrograde geosynchronous orbit, distant retrograde orbit, near-rectilinear halo orbit, bicircular restricted four-body problem, weak stability boundary, lunar gravity assist, trajectory design, velocity increment, space situational awareness, three-body problem, Beihang University