Mobile robotic arm for momentum unloading and orbit control

A system for controlling a spacecraft, the system comprising:

• a solar sail having a reflective panel;
• a robotic arm comprising a plurality of rotary joints and at least first and second end effectors, each end effector being configured to latch selectively to any of a plurality of attachment points disposed on an exterior surface of the spacecraft;
• and
• one or more processors configured to execute instructions that cause the processors to:
• (i) determine an amount of solar force available based on a position of the spacecraft;
• (ii) determine a location for the solar sail to provide a resultant force and/or torque required to position the spacecraft based on the amount of the solar force available;
• (iii) determine a position for the robotic arm required to hold the solar sail at the location;
• and
• (iv) control the robotic arm to move to the position by alternately latching the end effectors to the attachment points.

A non-transitory computer readable storage medium storing instructions for controlling a spacecraft, the instructions when executed by a processor cause the processor to execute a method, the method comprising:

• (a) determining, by an onboard controller, a resultant force and/or torque required to achieve a commanded change in at least one of attitude or orbital position of the spacecraft;
• (b) computing, from ephemeris and current-attitude data, a vector quantity representing an amount of solar force available at the spacecraft location;
• (c) selecting, by solving a constrained mixed-integer optimization, (i) an attachment point chosen from a plurality of mechanically and electrically compatible attachment points distributed over an exterior surface of the spacecraft and (ii) a location and orientation for a solar sail such that, when the solar sail is held at the location, a component of the solar force is a vector substantially equals the resultant force and/or torque while maintaining line-of-sight clearance for every spacecraft antenna or sensor;
• (d) determining, from the optimization, a set of joint angles defining a position for a multi-joint robotic arm that is required to hold the solar sail at the selected attachment point;
• (e) controlling the multi-joint robotic arm to move to the position by alternately latching a free end effector of the multi-joint robotic arm to the selected attachment point and unlatching a previously latched end effector, thereby walking the multi-joint robotic arm along the exterior surface;
• and
• (f) when the amount of solar force available is insufficient to achieve the resultant force and/or torque, commanding an onboard thruster to supplement the solar force and retracting the solar sail to a stowed configuration that produces substantially zero net solar force.