The Design of a Drag-Free CubeSat and the Housing for its Gravitational Reference Sensor

A Drag-Free CubeSat mission has been proposed to demonstrate the feasibility of a Gravitational Reference Sensor (GRS) with an optical readout for a 3 units (3U) spacecraft. A purely drag-free object is defined by the absence of all external forces other than gravity. The Drag-Free CubeSat is designed to shield a 25.4 mm spherical test mass (TM) from external non-gravitational forces and to minimize the effect of internal generated disturbances. The position of the TM relative to the spacecraft is then sensed by the GRS with an advanced LED-based Differential Optical Shadow Sensor (DOSS). This position is used in a control system to command a micro-propulsion system and to constrain the CubeSat orbit to that of the TM. In principle, the TM is then freed of all forces but gravity and the hosting spacecraft also follows a purely geodesic orbit. However, the purity of the orbit depends on the spacecraft’s capacity to protect the TM from disturbances. Several of them are passively reduced by the design of the TM housing. This system is a thick-walled aluminum box that holds the shadow sensors and shields the TM. The housing has an effect on the mechanical, thermal and magnetic environment around the TM. All of them have been analyzed. The mechanical vibrations have to fit the launch environment and the modes have to be outside of the measurement range (10^-4 – 1 Hz). The magnetic field has to be reduced by a 0.01 factor. The temperature difference between internal opposing surfaces, determining pressure on the TM, has to be below 10-3×(1 mHz/f)1/3 K Hz-1/2. The housing, together with the TM, the sensors and the UV LEDs for charging control, constitutes the GRS, which would then fit into a 1U. The other 2Us are occupied by the caging mechanism that constraints the TM during launch, the thrusters, the Attitude Determination And Control System (ADACS) and the electronics. Next generation GRS technology for navigation, earth science, fundamental physics, and astrophysics has been under development at Stanford University since 2004. The Drag-Free CubeSat will be the result of the combined efforts of Stanford, University of Florida, KACST and NASA and will be the first drag-free mission with an optical readout and the first GRS designed within the limits of a 3U small satellite. In the first section, this paper briefly updates on the main characteristics and systems of the project. Particular emphasis is then given to the recently designed housing, its expected performance and the open issues.