A small Mars (or other celestial body) ascent vehicle is unlikely to achieve the necessary propellant fraction required to achieve orbit. Scaling down of liquid propulsion systems, as shown in the figure, is difficult. In the 100-kg class of vehicles, liquid propellant vehicle designers should expect a propellant fraction of only 0.75. In contrast, solid rocket motors (SRM) scale down much easier, so designers should expect a propellant fraction of at least 0.92. To be practical, however, the SRM must operate in extreme low temperature environments, which is difficult for state of the art polybutadiene binders. ASI proposes to develop a new, low temperature binder based upon siloxane. Siloxane polymers have glass transition temperatures below 150K, making them ideal for use on Mars with little or no external heaters required. A siloxane binder SRM-based MAV will easily achieve the propellant fraction needed for a sample return mission.