  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, which are shielded by the spacecraft. In a
real case, the TM will still be affected by disturbances. Several of them are
passively reduced by the design of the TM housing. This system is a
thick-walled aluminium 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 analysed. The mechanical vibrations have
to fit the launch environment and the modes have to be outside of the
measurement range (0.0001 - 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. 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.
