Distant retrograde orbit

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.

The Earth-Moon Lagrange points

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]

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

Chang'e 5 spacecraft with orbiter at bottom

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.^[5] 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.^[5][6] 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.^[5]

Orion spacecraft

Main article: Orion (spacecraft)

Orion spacecraft prior to arriving at the Moon, where it would later get into DRO

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.^[7][8] On 25 November it entered DRO and orbited the Moon in that orbit.^[9][10]

DRO A/B

Yuanzheng 1S upper stage failed to deliver these two CAS spacecrafts (not under CLEP) into correct orbit. The satellites were intended to test Distant retrograde orbit.^[11] Tracking data appears to show China is attempting to salvage spacecraft and they appear to have succeeded in reaching their desired orbit.^[12][13]

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.^[14]

NASA Lunar Gateway

Two system requirements for the NASA Lunar Gateway, as published in the Baseline DSG-RQMT-001^[15] 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^[16] 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.^[17]

See also

• Free-return trajectory

References

1. 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. 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. 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.
5. Jones, Andrew (15 February 2022). "A Chinese spacecraft is testing out a new orbit around the moon". Space News.
6. "Chang'e-5: China's Moon sample return mission". Planetary.
7. 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.
8. NASA (8 November 2022). "NASA Prepares Rocket, Spacecraft Ahead of Tropical Storm Nicole, Re-targets Launch". Retrieved 8 November 2022.
9. 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.
10. Foust, Jeff (2022-11-25). "Orion enters lunar distant retrograde orbit". SpaceNews. Retrieved 2022-11-29.
11. Jones, Andrew (2024-03-14). "Surprise Chinese lunar mission hit by launch anomaly". SpaceNews. Retrieved 2024-03-14.
12. Jones, Andrew (2024-08-20). "Chinese spacecraft appear to reach lunar orbit despite launch setback". SpaceNews. Retrieved 2024-08-20.
13. Jones, Andrew (2024-03-28). "China appears to be trying to save stricken spacecraft from lunar limbo". SpaceNews. Retrieved 2024-03-29.
14. NASA [@NASA] (19 November 2022). "@JVendl @NASA_Orion We first studied the DRO to support the proposed Asteroid Redirect Mission (ARM) which paralleled early SLS and Orion development. The plan for ARM was to capture a near Earth asteroid and redirect it to a lunar DRO. (1/4)" (Tweet). Retrieved 2 December 2022 – via Twitter.
15. 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.
16. Zaid, Christina (2022-05-16). "A unique halo orbit is the road less traveled around the Moon". NASA. Retrieved 2022-11-29.
17. Suarez, Daniel (2019). Delta-v. New York: Penguin Random House. pp. 189–198. ISBN 978-1524742416.