One of the biggest challenges Capstone faces is precisely controlling its trajectory using its minimal onboard propulsion system to maintain its highly elliptical orbit around the Moon’s lagrange point Lunar Distant Retrograde Orbit (DRRO). The DRRO is an unstable three-body orbit that requires periodic station keeping to counteract thermal influences, spacecraft dynamics, and other perturbations that could cause its orbit to drift over time.

Maintaining this precarious orbit takes an enormous amount of precise orbital maneuvering. Capstone only carries about 22 pounds of propellant for its cold-gas thrusters, which must carefully control the cubesat’s position and velocity over its planned 6 month demonstration mission. Any propulsion errors could cause the smallsat to go off course and drift out of the desired DRRO orbit. The lack of significant onboard fuel means maneuvers must be extremely efficient and errors are difficult to correct.

The complex natural gravitational forces around the Moon-Earth lagrange point make station keeping in the DRRO quite challenging. Disturbances from the Earth and Moon’s gravity, along with minimal onboard sensors and actuators, mean Capstone’s navigation and attitude control systems must operate with extremely high accuracy to counteract orbital perturbations. Even tiny imbalances or uncertainties in onboard sensors and thrusters could accumulate over time and degrade the orbit.

Thermal influences from variations in sunlight on the spacecraft also perturb its trajectory and must be actively countered. As Capstone orbits in the perpetually changing thermal environment around the lagrange point, solar heating and infrared radiation pressure impart small forces on its structure and components. Changes in the cubesat’s overall density, shape, or center of mass due to minor expansions or movements of its parts in response to thermal swings produce imbalances that require regular trajectory corrections. The lack of an active thermal control system means these thermal disturbances cannot be prevented, adding complexity for maneuver planning.

CommunicationsBlackouts as Capstone passes behind the Moon during each half of its 6 day orbit are also challenging. Navigation depends on tracking radian position from Earth, but loss of signal during the blackout durations degrades onboard state estimates. While stored navigation data helps bridge outages, uncertainties accumulate faster without direct observation and correction. Blackouts reduce the amount of monitoring possible and periods available to assess maneuvers, plan future burns, and redirect the orbit if needed.

The tiny cubesat also faces risks from the space environment around the Moon, such as harmful charged particles in the magnetosphere and unpredictable meteoroid and orbital debris impacts. While Capstone has no moving parts, long term exposure to radiation could potentially compromise electronic systems or navigation sensors and exacerbate station keeping difficulties over its 6+ month mission. The increasing congestion of orbital debris also raises concerns about the potential for high speed collisions that could damage hardware or nudge the orbit off course. Any glitches or anomalies would be difficult to pinpoint and repair on the remote, autonomous smallsat. Maintaining CGPS’s hazardous but precise near-rectilinear halo orbit demands immense precision, planning and risk mitigation from both the spacecraft and ground teams. Even with NASA’s extensive experience, the demonstration provides an opportunity to assess the challenges of operating in this demanding region of space. Lessons from Capstone’s station keeping campaign will help inform strategies for future long term lunar and Mars missions that propose exploiting unstable multi-body dynamics for fuel efficient transit or infrastructure purposes. Precise onboard propulsion, complex orbital dynamics, minimal onboard resources or redundancy, communications gaps, and potential environmental impacts combine to present a considerable ongoing navigation and control problem for the tiny Capstone spacecraft over its six month lunar mission. Careful management of numerous error sources and perturbations will be required to keep the cubesat circling stably in its intended near-rectilinear halo orbit, validating innovative orbital techniques for future exploration.