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. Two other CNSA spacecraft, DRO A and B, were intended to use DRO orbits, but were left in lower orbits due to a failure of the YZ-1S upper stage.[3]

## Description

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".[1]

## List of objects in distant retrograde orbit

### Chang'e 5 orbiter

After dropping off return samples for Earth, China's Chang'e 5 (CE-5) orbiter first moved to Sun-Earth Lagrange point 1 (L1) in March 2021 for solar observations.[4] In January 2022, CE-5 left L1 point for the lunar distant retrograde orbit (DRO) to conduct very-long-baseline interferometry tests in preparation for the next stage of China's lunar exploration program.[4][5] According to The 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.[4]

### Orion spacecraft

 Main article: Orion (spacecraft)

On 16 November 2022, the Space Launch System was launched from Complex 39B as part of the Artemis 1 mission carrying Orion to the Moon.[6][7] On 25 November it entered DRO and orbited the Moon in that orbit.[8][9]

## Space concepts proposed to use a DRO

### Jupiter Icy Moons Orbiter

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.[1]

### Asteroid Redirect Mission (ARM)

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 for Artemis 1.[10]

### NASA Lunar Gateway

Two system requirements for the NASA Lunar Gateway, as published in the Baseline DSG-RQMT-001[11] 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-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". Although the selected orbit for the Gateway has been confirmed to be NRHO[12] instead of DRO.

## DRO orbits in fiction

In the 2019 Daniel Suarez novel Delta-v, a 560-tonne crewed asteroid-mining ship Konstantin is constructed in a lunar DRO approximately 40,000 km (25,000 mi) above the Moon.[13]

## References

1. ^ a b c Johnson, Kirstyn (18 December 2014). "Understanding NASA's Asteroid Redirect Mission: Distant Retrograde Orbits". Archived from the original on 11 January 2015. Retrieved 3 May 2015.
2. ^ a b Burke, Kristin (11 April 2022). "The Space Review: What is China doing at the lunar distant retrograde orbit?". The Space Review. Archived from the original on 2022-04-12. Retrieved 2022-04-12.
3. ^ Jones, Andrew (28 March 2024). "China appears to be trying to save stricken spacecraft from lunar limbo". SpaceNews. Retrieved 2 July 2024.
4. ^ a b c Jones, Andrew (15 February 2022). "A Chinese spacecraft is testing out a new orbit around the moon". Space News.
5. ^ "Chang'e-5: China's Moon sample return mission". Planetary.
6. ^ Artemis I Launch to the Moon (Official NASA Broadcast) - Nov. 16, 2022. NASA. 16 November 2022. Archived from the original on 29 November 2022. Retrieved 2 December 2022 – via YouTube.
7. ^ NASA (8 November 2022). "NASA Prepares Rocket, Spacecraft Ahead of Tropical Storm Nicole, Re-targets Launch". Retrieved 8 November 2022.
8. ^ NASA (27 November 2015). "The Ins and Outs of NASA's First Launch of SLS and Orion". Archived from the original on 22 February 2020. Retrieved 3 May 2016. This article incorporates text from this source, which is in the public domain.
9. ^ Foust, Jeff (2022-11-25). "Orion enters lunar distant retrograde orbit". SpaceNews. Retrieved 2022-11-29.
10. ^
11. ^ NASA (2019). "DSG-RQMT-001 – Gateway Program System Requirements Document (SRD)" (PDF). NASA Technical Reports Server. p. 25. Archived (PDF) from the original on 11 April 2020. Retrieved 11 April 2020.
12. ^ Zaid, Christina (2022-05-16). "A unique halo orbit is the road less traveled around the Moon". NASA. Retrieved 2022-11-29.
13. ^ Suarez, Daniel (2019). Delta-v. New York: Penguin Random House. pp. 189–198. ISBN 978-1524742416.