Adistant retrograde orbit (DRO), as most commonly conceived, is a spacecraftorbit around a moon that is highly stable because of its interactions with twoLagrange points (L1 andL2) 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 orbit isretrograde, that is, moving in the direction opposite to the direction in which the moon orbits the planet. The orbit isdistant in the sense that it passes above the Lagrange points, rather than being near the moon. Considering more and more distant orbits, thesynodic 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. Thesidereal 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 withEuropa has a sidereal period about eight times the orbital period of Europa.[1]
DROs have been researched for several decades. In April 2022, CNSA'sChang'e 5 orbiter[2] became the first to enter the orbit, followed by NASA'sOrion Spacecraft during theArtemis 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 theYZ-1S upper stage.[4] Despite the earlier problems, by August 2024 the DRO A and B appeared to have reached their orbit.[5]
The stability of a DRO is defined in mathematical terms as having very highLyapunov stability, where an equilibrium orbit is "locally stable if all solutions which start near the point remain near that point for all time".[1]
After dropping off return samples for Earth, China'sChang'e 5 (CE-5) orbiter first moved to Sun-Earth Lagrange point 1 (L1) in March 2021 for solar observations.[6] In January 2022, CE-5 left L1 point for the lunar distant retrograde orbit (DRO) to conductvery-long-baseline interferometry tests in preparation for the next stage of China'slunar exploration program.[6][7] According toThe Space Review (TSR), this maneuver was depicted in Chinese government and academic documents.[2] In February 2022, multiple amateur satellite trackers observed CE-5 had entered DRO, making it the first spacecraft in history to utilize the orbit.[6]
On 16 November 2022, theSpace Launch System was launched from Complex 39B as part of theArtemis 1 mission carrying Orion to the Moon.[8][9] On 25 November it entered DRO and orbited the Moon in that orbit.[10][11]
Yuanzheng 1S upper stage failed to deliver these twoCAS spacecrafts (not underCLEP) into correct orbit. The satellites were intended to test Distant retrograde orbit.[12] Tracking data appears to show China is attempting to salvage spacecraft and they appear to have succeeded in reaching their desired orbit.[13][14]
A distant retrograde orbit was one of the proposed orbits aroundEuropa for theJupiter Icy Moons Orbiter—principally for its projected stability and low-energy transfer characteristics—but that mission concept was cancelled in 2005.[1]
A distant retrograde orbit was considered to be used for the proposed Asteroid Redirect Mission. Although the mission would end up getting cancelled, the research done with DRO in-mind, lead to the orbit being used forArtemis 1.[15]
Two system requirements for the NASALunar Gateway, as published in the Baseline DSG-RQMT-001[16] published in June 2019, mention the use of lunar DROs. 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-energycislunar orbit transfers between anear-rectilinear halo orbit (NRHO) and a distant retrograde orbit (DRO) and orbit maintenance for a period of 15 years between refueling". Although the selected orbit for the Gateway has been confirmed to be NRHO[17] instead of DRO.
In the 2019Daniel Suarez novelDelta-v, a 560-tonne crewed asteroid-mining shipKonstantin is constructed in a lunar DRO approximately 40,000 km (25,000 mi) above the Moon.[18]
This article incorporates text from this source, which is in thepublic domain.
