Distant retrograde orbit

The Earth-Moon Lagrange points
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) can then 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. In April 2022, CNSA's Chang'e 5 orbiter[2] became the first to enter the orbit, followed by NASA's Orion Spacecraft during the Artemis 1 mission which entered in November 2022.[3] Two more CNSA spacecraft, DRO A and B, attempted in 2024, but were left in lower orbits due to a failure of the YZ-1S upper stage.[4]

  1. ^ Cite error: The named reference johnson2014 was invoked but never defined (see the help page).
  2. ^ Cite error: The named reference tsr20220411 was invoked but never defined (see the help page).
  3. ^ Foust, Jeff (25 November 2022). "Orion enters lunar distant retrograde orbit". SpaceNews.
  4. ^ Jones, Andrew (28 March 2024). "China appears to be trying to save stricken spacecraft from lunar limbo". SpaceNews. Retrieved 2 July 2024.