  Space-borne optical frequency references based on spectroscopy of atomic
vapors may serve as an integral part of compact optical atomic clocks, which
can advance global navigation systems, or can be utilized for earth observation
missions as part of laser systems for cold atom gradiometers. Nanosatellites
offer low launch-costs, multiple deployment opportunities and short payload
development cycles, enabling rapid maturation of optical frequency references
and underlying key technologies in space. Towards an in-orbit demonstration on
such a platform, we have developed a CubeSat-compatible prototype of an optical
frequency reference based on the D2-transition in rubidium. A frequency
instability of 1.7e-12 at 1 s averaging time is achieved. The optical module
occupies a volume of 35 cm^3, weighs 73 g and consumes 780 mW of power.
