The primary mission of LunarSail is to deploy a CubeSat into Earth orbit, deploy a thin-membrane solar sail and utilize the sail for propulsion and navigation to propel the spacecraft out of Earth orbit, into a lunar intercept trajectory and then execute orbital insertion around the Moon. However, we will consider the mission a success if, at a minimum, LunarSail navigates itself sufficiently in order to be captured in the Moon’s gravitational well.
The mission design presumes LunarSail will be delivered to either low or highly-elliptical Earth orbit as a secondary payload of a primary satellite launch. After separation from the launch vehicle, a ground control station will establish contact with LunarSail and evaluate its health. After a period of spacecraft checkout and monitoring, LunarSail will be commanded to begin on-orbit operations intitiate the sail deployment sequence.
After the sail is successfully deployed, the spacecraft will enter a sail checkout period to ensure proper deployment and operation of the sail and boom steering mechanism. During this time, mission planners will determine the correct time to begin LunarSail’s slow transition from parking orbit to insertion on a lunar intercept trajectory.
Once it has reached an Earth escape trajectory, LunarSail will enter the second phase of the translunar portion of the mission during which time the spacecraft will use the sail primarily for navigation. Its steering mechanism will alter the orientation of the sail with respect to the Solar wind in order to guide itself on a trajectory to intercept lunar orbit near the point where the Moon will be in the vicinity at the time of crossing.
One of the challenges in reaching the Moon lies in the fact that the propulsive power of a solar sail is directly proportional to the strength of the solar wind against the sail wing. Following Newton’s Second Law, F=ma, the force imparted by the solar wind is calculated from the velocity of the solar wind and the total mass of particles hitting the sail material. Because of the dynamic nature of the Sun, the local strength of the solar wind is directly related to events such as coronal mass ejections, which send massive amounts of particles into the solar wind at extremely high velocities. This can increase the effectiveness of a solar sail by giving it greater acceleration, much like a sailboat can travel faster when the wind is stronger. Conversely, during times of a “quiet Sun”, the solar wind is calmer and a sail accelerates more slowly, which increases travel time to a destination.
Because of these factors, it will be impossible to precisely determine when LunarSail will exit the grip of Earth’s gravity. While controllers on the ground will have some control of the timing and location, it could reach this critical region at any point in its orbit. This will present a real-time challenge to placing the spacecraft on an optimal trajectory for rendezvous with the Moon. If the LunarSail achieves escape velocity while traveling in the opposite direction, it will be necessary to use the sail to steer the craft toward the desired trajectory.
Once LunarSail is captured by the Moon’s gravity, the spacecraft’s systems will transition from using the sail for forward propulsion and will utilize the drag of the large-area sail material to slow down and enter lunar orbit.