Some ESA scientific missions (e.g. GOCE, SMART-2) require an accurate control of the attitude of the spacecratf. Many European factories have developed small thrusters capable of delivering very small forces (3 mN).
Those thrusters have to be characterized in order to determine their thrust and its noise.
IMGC has developed, as required by Alenia Spazio, an instrument able to perform those measurements, the "Nanobalance". The instrument is mainly constituted by two Cu-Be plates, hold by a Zerodur spacer. In the upper part of the plates a hinge has been realized, in order to let them move under the action of the thrusters. Two mirrors are mounted on the plate, constituting a resonant optical cavity. The length of the optical cavity is varied under the action of the thrusters. The variation of the length is measured by the beat frequency between a Nd:YAG laser, whose frequency is locked to the resonant frequency of the cavity, and a second laser referenced to a transition of molecular iodine. Two calibrated voice-coils are used in order to perform instrument calibration.
The experiment is performed in vacuum, in order to get rid of the refractive index fluctuations and to improve acoustic and thermal isolation.
Preliminary measurements showed that a sensitivity better that 1 mN is achieved, limited by seismic noise.
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