The magnetic field is a fundamental physical quantity, and its accurate measurement to 1 nT or better is required for many future planetary mission. Future missions targeted here are foremost the Jupiter Europa Orbiter / Europa Clipper, which seeks to resolve induction signals a few nano-Tesla in amplitude during a series of flybys executed over an extended period of time to characterize the Europa sub-surface ocean, and the Uranus Orbiter and Probe, which seeks characterize Uranus' unique offset multi-pole magnetic field. In addition, such an instrument would also support other future missions, such as Enceladus Orbiter and Lunar Geophysical Network, both of which require measurement of small-amplitude magnetic perturbations with good long-term stability. Fluxgate magnetometers alone cannot deliver the required performance because their calibration can drift so that long-term stability is not guaranteed. The proven solution is to partner the high-heritage fluxgate instruments with an absolute reference magnetometer, which serves as an in-flight calibration source. The fact is, however, that the mass and power of existing space-flight absolute magnetometers exceed the capabilities of many planetary missions. To solve the resource problem, The Johns Hopkins University Applied Physics Laboratory (JHU/APL) and the National Institute of Standards and Technology (NIST) have developed a novel miniature absolute scalar magnetometer sensor based on a micro-fabricated alkali-metal vapor cell. In the assembled sensor, the micro-fabricated rubidium vapor cell is illuminated with a vertical-cavity surface-emitting laser (VCSEL), and the resonant response of the atoms, which is related to the ambient magnetic field strength, is detected by a photodiode. The present breadboard experiment has a total mass of ~500 g (sensors and electronics), uses 0.5 W of power, and, operating in Mx mode, achieves a sensitivity of 15 pT/√Hz at 1 Hz, or 0.1 nT rms. The objective of the proposed work is to solve the following fundamental technical challenges toward a space-qualified miniature absolute scalar magnetometer so that maturation of this absolute magnetometer may be proposed in response to the Maturation of Instruments for Solar System Exploration (MatISSE) Program: (a) testing key sensor components for the Europa radiation environment and incorporating mitigation steps in the sensor design; (b) building breadboard electronics for the Mz mode to eliminate the sensor heading error; and (c) quantifying absolute accuracy, sensitivity, and long-term stability of magnetometers based on micro-fabricated vapor cells. These development steps are critical toward achieving and demonstrating the targeted absolute accuracy and sensitivity are 0.1 nT and 0.01 nT, respectively, with a long-term stability of 0.1 nT over 1.5 years, commensurate with the performance and durations required to measure Europa's induction response and to map Uranus' magnetic field. The significance of the miniature scalar magnetometer is that it resolves ambiguities in fluxgate offset and gain drift by providing continual absolute knowledge of the total magnetic field to 0.1 nT and thereby retires performance risks of high-heritage, flight-proven fluxgate instruments flown in tandem configuration for future planetary missions.