entered Sun-Earth rotating frame, and a Moon-centered Earth-Moon rotating frame.  In the Earth-Moon rotating frame, on the right, we can see the familiar NRHO shape, albeit with 15 years of variations.

In the Sun-Earth rotating frame, on the left, we can see how the LSR orbit accomplishes eclipse avoidance:  by setting up a repeating geometrical pattern in the Sun-Earth frame, which consistently avoids passing through the Earth’s shadow (along the axis opposite the Sun). Note that the elliptical shapes on this axis are projections of the Earth’s outer shadow, or penumbra.

The 9:2 Lunar Synodic Resonance indicates that the orbit will make, on average, 9 revolutions for every 2 lunar
months.  The resulting orbit has a perilune radius that varies from 3196 to 3557 km, with an average of 3366 km.  The average orbit period of the reference trajectory is 6.562 days, closely matching the value that would be expected by applying the resonance ratio to the mean lunar synodic period.

Avoiding eclipses by the Earth was a primary design goal for this reference trajectory, as these eclipses can approach 5 hours in overall duration, and include periods of total eclipse of over 2.6 hours, durations well beyond current hardware limits for the Gateway and Orion spacecraft.  The reference trajectory is successful in avoiding eclipses by the Earth, with the exception of two grazing partial eclipses during the 14th and 15th years.
(See Figure 3.)  It is actually expected that a new reference trajectory would be developed before this point.

Eclipses by the Moon, while relatively frequent with several occurring every year, are always less than 80 minutes in duration, within current hardware limits.

If Gateway were not utilizing an LSR orbit for eclipse avoidance, occasional eclipse avoidance maneuvers would be required, and Gateway operations would require ongoing predictions of future eclipse risks.  These avoidance maneuvers would have some finite propellant Whitley, F.

[1]  M.

McGuire, "Power & Propulsion Element (PPE) Spacecraft Reference Trajectory Document," NASA John H.

Glenn Research Center, Vols.

PPE-DOC-0079, no.

Rev B, pp. 4-5, 2018.
[2]  K.

C.

Howell and J.

V.

Breakwell, "Almost Rectilinear Halo Orbits," Celestial Mechanics, vol. 32, no. 1, pp. 29-52, January 1984.
[3]  K.

C.

Howell, "Three-Dimensional Periodic Halo Orbits," Celestial Mechanics, vol. 32, no. 1, pp. 53-71, 1984.
[4]  D.

C.

Davis, S.

A.

Bhatt, K.

C.

Howell, J.-W.

Jang, R.

L.

D.

Clark, D.

Guzetti, E.

M.

Zimovan and G.

H.

Barton, "Orbit Maintenance and Navigation of Human Spacecraft at Cislunar Near Rectilinear Halo Orbits," in 27th AAS/AIAA Space Flight Mechanics Meeting, San Antonio, TX, February 2017.
[5]  E.

M.

Zimovan, K.

C.

Howell and D.

C.

Davis, "Near Rectilinear Halo Orbits and Their Application in Cis-Lunar Space," in 3rd IAA Conference on Dynamics and Control of Space Systems, 2017.
[6]  D.

C.

Davis, S.

M.

Phillips, K.

C.

Howell, S.

Vutukuri and B.

P.

McCarthy, "Stationkeeping and Transfer Trajectory Design for Spacecraft in Cislunar Space," in 2017 AAS/AIAA Astrodynamics Specialist Conference, Stevenson, WA, 2017.
[7]  C.

P.

Newman, D.

C.

Davis, R.

J.

Whitley, J.

R.

Guinn and M.

S.

Ryne, "Stationkeeping, Orbit Determination, and Attitude Control for Spacecraft in Near Rectilinear Halo Orbits," in AAS/AIAA Astrodynamics Specialists Conference, Snowbird, Utah, 2018.
[8]  R.

Whitley and R.

Martinez, "Options for Staging Orbits in Cislunar Space," in IEEE Aerospace 2015, Big Sky, Montana, 2015.
[9]  R.

J.

Whitley, D.

C.

Davis, L.

M.

Burke, B.

P.

McCarthy, R.

J.

Power, M.

L.

McGuire and K.

C.