Distant Retrograde Orbit
Distant Retrograde Orbit (DRO) is a highly stable orbit that has been researched for several decades, but as of 2014, no spacecraft have utilized such an orbit for an actual flight.
Stability of a DRO orbit 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."
Space concepts proposed to utilize a DRO
By 2014, a DRO lunar orbit was the preferred alternative under consideration for the NASA-proposed Asteroid Redirect Mission. 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. NASA subsequently cancelled work on ARM in 2017 and never funded the build of flight hardware nor space launch contracts..
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.
In the Daniel Suarez novel Delta-v, a 560-tonne crewed asteroid-mining ship The Konstantin was constructed in a lunar DRO approximately 40,000 km (25,000 mi) above the Moon.
- 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.
- Foust, Jeff (14 June 2017). "NASA closing out Asteroid Redirect Mission". SpaceNews. Retrieved 3 November 2019.
- Suarez, Daniel (2019). Delta-v. New York: Penguin Random House. pp. 189–198. ISBN 978-1524742416.