Distant retrograde orbit

The Earth-Moon Lagrange points

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. Considering 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. The sidereal period (the time it takes for the craft to come back to a given constellation as viewed from the moon) then can become much longer than the orbital period of the moon. A hypothetical example with Europa has a sidereal period about eight times the orbital period of Europa.^[1]

DROs have been researched for several decades and as of 2022 only CNSA's Chang'e 5 orbiter^[2] and NASA's Orion Spacecraft during the Artemis 1 mission have entered the orbit.