  The exploration of the lunar poles and the collection of samples from the
martian surface are characterized by shorter time windows demanding increased
autonomy and speeds. Autonomous mobile robots must intrinsically cope with a
wider range of disturbances. Faster off-road navigation has been explored for
terrestrial applications but the combined effects of increased speeds and
reduced gravity fields are yet to be fully studied. In this paper, we design
and demonstrate a novel fully passive suspension design for wheeled planetary
robots, which couples for the first time a high-range passive rocker with
elastic in-wheel coil-over shock absorbers. The design was initially conceived
and verified in a reduced-gravity (1.625 m/s${^2}$) simulated environment,
where three different passive suspension configurations were evaluated against
steep slopes and unexpected obstacles, and later prototyped and validated in a
series of field tests. The proposed mechanically-hybrid suspension proves to
mitigate more effectively the negative effects (high-frequency/high-amplitude
vibrations and impact loads) of faster locomotion (~1\,m/s) over unstructured
terrains under varied gravity fields.
