A distant retrograde orbit (DRO), as most commonly conceived, is a spacecraft orbit around a moon that is highly stable because of its interactions with two Lagrange points (L1 and L2) of the planet-moon system.
In more general terms, an object of negligible mass can be in a DRO around the smaller body of any two-body system, such as planet–Sun or exoplanet–star.
Using the example of a spacecraft in a DRO around a moon, the craft would orbit in the direction opposite to the direction in which the moon orbits the planet. The orbit is "distant" in the sense that it passes above the Lagrange points, rather than being near the moon. If we consider more and more distant orbits, the synodic period (the period between two moments when the craft passes between the planet and the moon) gets longer and approaches that of the moon going around the planet, so that the sidereal period (the time it takes for the craft to come back to a given constellation as viewed from the moon) can become much longer than the orbital period of the moon. An example with Europa has a sidereal period about eight times the orbital period of Europa,
DROs have been researched for several decades, but as of 2021, no spacecraft have used such an orbit for an actual flight.
The stability of a DRO is defined in mathematical terms as having very high Lyapunov stability, where an equilibrium orbit is "locally stable if all solutions which start near the point remain near that point for all time."
By 2014, a lunar DRO was the preferred alternative under consideration for the NASA-proposed Asteroid Redirect Mission (ARM). This orbit would have had a lunar orbital altitude of approximately 61,500 km (38,200 mi), a distance somewhat greater than the distance from the Moon to either of the Earth-Moon L1 or L2 Lagrangian points. NASA subsequently cancelled work on ARM in 2017 and never funded the build of flight hardware nor issued any space launch contracts.
A distant retrograde orbit was one of the proposed orbits around Europa for the Jupiter Icy Moons Orbiter—principally for its projected stability and low-energy transfer characteristics—but that mission concept was cancelled in 2005.
Two system requirements for the NASA Lunar Gateway mention[when?] the use of lunar DRO's. Requirement L2-GW-0029, Single Orbit Transfer, states "the Gateway shall be capable of performing a single round trip transfer to Distant Retrograde Orbit (DRO) and back within 11 months." Requirement L2-GW-0026, Propulsion System Capability, states "the Gateway shall provide a fuel capacity that would support performing a minimum of two round-trip uncrewed low-energy cislunar orbit transfers between a near-rectilinear halo orbit (NRHO) and a distant retrograde orbit (DRO) and orbit maintenance for a period of 15 years between refueling."