DRO

A distant retrograde orbit is a closed path around the Moon in which a spacecraft travels backwards, relative to the Moon's own orbit around Earth. The spacecraft orbits the Moon in the retrograde direction, meaning it moves opposite to the Moon's motion around Earth. This counterintuitive configuration keeps the spacecraft in a stable gravitational pocket where the combined pull of Earth and Moon reinforces containment rather than fighting against it.

DROs sit roughly 60,000 kilometers from the lunar surface, far enough out that Earth's gravity becomes significant. At this distance, the spacecraft experiences a balance point in the three-body system of Earth, Moon, and spacecraft. The orbital period typically ranges from 14 to 16 days. Unlike the lower, prograde lunar orbits used for landing missions, which require constant fuel burns to maintain altitude, a DRO demands minimal station-keeping maneuvers once established.

Why retrograde, and why distant

The retrograde direction is not incidental. Direct orbits at such distances become dynamically unstable; lunar gravity alone cannot hold a spacecraft in a fixed orbit so far out. Retrograde motion exploits a computational resonance where the spacecraft's backwards motion around the Moon interacts with Earth's gravitational tug to create a persistent halo. The distant altitude ensures Earth's gravity field contributes meaningfully to this stability.

NASA's Artemis program and other lunar missions have selected DROs as stations for gateway habitats and fuel depots. The Lunar Gateway, a planned orbital outpost, is designed to occupy a near-rectilinear halo orbit, a variant of the DRO family. The low fuel cost to maintain station and the stability of the orbit make DROs tactically valuable for long-duration lunar operations without continuous resupply burns.

Entry and exit from a DRO differ from low lunar orbit operations. Transfer trajectories leverage the Moon-Earth gravitational saddle point, reducing the velocity changes needed compared to direct lunar descent profiles. This efficiency translates directly to payload or fuel savings for vehicles that use the gateway as a refueling point or crew rest station before descending to the lunar surface.