Computer illustration of Near Rectilinear Orbits between EML1 and EML2 (Credit: NASA).
NASA appears to have settled on a Near Rectilinear L2 Halo Orbit (NRO) for its future Deep Space Habitat (DSH). NROs are a subset of of L1 or L2 halo orbits. NRO's have large amplitudes over either the north or
south lunar poles with shorter periods that pass closely to the opposite pole. Station keeping at an NRO would require a delta-v of only 5 m/s per year. With an impulsive departure from LEO at about 3.124 km/s, a crewed spacecraft would reach an L2 NRO in about 5.33 days. Orbital capture would require a delta-v of 0.829 km/s.
An EML1 location for a DSH would only require a delta-v of 3.77 km/s and four days of travel time. But 2 days of travel time would be required for a journey from EML1 to Low Lunar Orbit (LLO). An NRO location, however, would only require 12 hours of travel time to LLO. So the surface of the Moon could be accessed from a NRO located Deep Space Hab in just 12 hours.
Possible Cis-Lunar Locations for a DSH (Deep Space Habitat)
EML1(Earth-Moon Lagrange Point One):
Travel time to and from LEO: ~4 days (3.77 km/s)
Station keeping: < 10 m/s per year
Travel time to and from LLO: ~ 2 days (0.750 km/s)
EML2 (Earth Moon Lagrange Point Two):
Travel time to and from LEO:~ 8 days from LEO (3.43 km/s)
Station keeping < 10 m/s per year
Travel time to and from LLO:~ 3 days to LLO (0.8 km/s)
DRO (Distant Retrograde Orbit):
Travel time to and from LEO: ~ 6 days
Station keeping: 0 m/s per year
Travel time to and from LLO: ~ 4 days (0.83 km/s)
NRO: (Near Rectilinear Halo Orbit):
Travel time to and from LEO:~5 days from LEO (3.95 km/s)
Station keeping: 5 m/s per year
Travel time to and from LLO:~ 12 hours to LLO (0.730 km/s)
Significantly shorter flight times from LEO to NRO could be achieved
with higher delta-v levels that could easily be achieved by future
reusable LOX/LH2 fueled spacecraft such as the ULA's XEUS and Lockheed
Martin's MADV which could be used for round trip journeys to the lunar
surface from a NRO and for transporting crews between LEO and NRO.
"The knowledge that we have now is but a fraction of the knowledge we must get, whether for peaceful use or for national defense. We must depend on intensive research to acquire the further knowledge we need ... These are truths that every scientist knows. They are truths that the American people need to understand." (Harry S. Truman 1948).