Lunar Sunrise Orbits and Applications to NASA CLPS Missions

* NASA’s Commercial Lunar Payload Services (CLPS) initiative has stimulated the lunar economy with recent and planned landings on the Moon by U.S. companies.

* Primary Purpose: discover, and analyze Lunar Sunrise Orbits (LSOs), sun-synchronous and near repeat ground orbits at the Moon that are phased for a spacecraft to encounter a landing site at sunrise.

* Lunar sunrise orbits (LSOs) are defined as orbits with near repeat ground tracks that pass over a specified lunar surface site at successive lunar sunrise times. CLPS landers generally land at (or shortly after) local lunar sunrise to ensure that most of the lunar day can be used for surface operations given the challenges of surviving the cold lunar night.

* For an arbitrary staging orbit, if a lunar descent is delayed for any reason, the spacecraft will approximately return to the landing site after the moon completes 1 synodic revolution. However, the return geometry defined by the spacecraft, landing site, and terminator is not exact due to inertial rotation of the terminator, and due to perturbations from the Earth and the non-spherical gravity of the Moon.

* By utilizing one or more of these LSOs, mission resiliency can be achieved by ensuring existence of backup landing windows at lunar sunrise in case the descent orbit initiation (DOI) and/or associated maneuvers are missed for any reason (e.g., temporary loss of communication, thruster malfunctions, maneuver execution errors, orbit determination problem, off-nominal attitude profile, landing site problems, etc.).