Distant retrograde orbit

A distant retrograde orbit (DRO) can be an orbit around a moon that is highly stable because of its interactions with two Lagrangian points (L1 and L2) of the planet-moon system. DROs have been researched for several decades, but as of 2020, no spacecraft have utilized such an orbit for an actual flight.[1]

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]

Space concepts proposed to utilize a DRO

NASA Asteroid Redirect Mission (ARM)

By 2014, a lunar DRO was the preferred alternative under consideration for the NASA-proposed Asteroid Redirect Mission (ARM). This orbit would have had a lunar orbital altitude of approximately 61,500 km (38,200 mi), a distance somewhat greater than the distance from the Moon to either of the Earth-Moon L1 or L2 Lagrangian points.[1] NASA subsequently cancelled work on ARM in 2017 and never funded the build of flight hardware nor issued any space launch contracts.[2].

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]

NASA Lunar Gateway

Two system requirements for the NASA Lunar Gateway mention the use of lunar DRO's. 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."[3]

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. Foust, Jeff (14 June 2017). "NASA closing out Asteroid Redirect Mission". SpaceNews. Retrieved 3 November 2019.
  3. "DSG-RQMT-001 - Gateway Program System Requirements Document (SRD)" (PDF). NASA Technical Reports Server. National Aeronautics and Space Administration (NASA). 2019. p. 25. Retrieved 11 April 2020.
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