Attitude Control System for the Mars Cube One Spacecraft

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CubeSats are small spacecraft based on a 10cm by 10cm by 10cm (1U) cube standard that have historically only been operated in Earth orbit. Mars Cube One (MarCO) is the first CubeSat mission developed for interplanetary operation. MarCO is a technology demonstration mission comprised of two identical, solar powered 6U satellites with several key goals, including that of providing a bent pipe telecom relay to Earth for NASA's InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission during its Entry, Descent, and Landing sequence. MarCO launched on the same rocket as InSight and makes use of the Deep Space Network for communications and ranging. It therefore has an attitude control system and propulsion system suitable for operating in several pointing modes, providing desaturations for reaction wheel momentum buildup, and thrusting to change the spacecraft trajectory. Because the spacecraft design is constrained to the CubeSat standards and because of the distances of the spacecraft from Earth and the Sun, the components used for attitude control and propulsion must meet tight size, mass, and power requirements. Autonomous modes of operation are also critical to ensure that the spacecraft can function safely with periods of several hours occurring between consecutive communication periods. A robust testing sequence was required to ensure that the spacecraft functions were exercised and that the operations team understood how the spacecraft were expected to behave after launch. This paper discusses several elements of the MarCO attitude control and propulsion systems. The paper begins with a discussion of the hardware that was selected for the two systems as well as descriptions of the interface between the attitude control and propulsion systems and the interface between these systems and the rest of the spacecraft's command and data handling system. Next, the paper summarizes the different types of tests that were performed at the system and spacecraft levels. Test data is included for some of these tests which helped define the methods by which the spacecraft is operated in space. Lastly, the paper lists a series of lessons-learned for developing attitude control and propulsion systems for interplanetary CubeSats.