Moving on Sand - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 258

The mission of the Spirit rover came to an end when it
got stuck, not in a ripple, but in a patch of soft dust
(Fig. 22.12 ). As in previous temporary stoppages, images
were examined, the configuration of the rover with respect
to the soil was modeled (Fig. 22.13 ) and extraction proce-
dures developed and tested with an engineering model rover
in the lab (Fig. 1.14 ).
Diagnosing the state of the rover is carried out first with
telemetry on the number of wheel revolutions (which can be
compared with imaging showing how much the rover has
moved, and thus how much wheel slippage there has been).
Second, where cameras can show the configuration of the
wheels and the soil around them, images can be analyzed to
evaluate the depth of sinkage. Most cameras tend to be
mounted on the upper part of a rover for a good overall
view, but where they are placed on articulated elements
such as sampling arms, the wheels and underside can be
inspected. Sometimes the news from this perspective is not
good—as Fig. 22.14 shows, the wheels of the Spirit rover
did not just sink into soft ground, but the vehicle got high-
centered on a rock, preventing the wheels from providing as
much purchase as they otherwise could.
Extensive efforts were made to recover Spirit, and there
were some encouraging signs of movement in their later
stages. But Martian winter drew in and, with the sun low in
the sky, without being able to move to a slope oriented
favorably to get power from its solar panels, Spirit's bat-
teries became cold and depleted. Despite hopes that it might
revive the following spring, Spirit did not respond to
commands from Earth and thus, at least indirectly, the
mission was ended by getting stuck in soft ground.

Given the unforgiving nature of getting stuck on other
planets, considerable effort is devoted to understanding the
limitations of the chosen vehicle's wheels, motors, mass
distribution and steering performance ahead of time. The
larger Curiosity rover (also known as the Mars Science
Laboratory) was tested extensively on Earth to learn the
limits of its mechanical systems (Fig. 22.15 ). Tests were
done with a 'Scarecrow' mockup of the rover: although
Scarecrow's wheels and motors are the same as Curiosity,
Scarecrow is otherwise stripped down so that it weighs
about the same on Earth as Curiosity does in the lesser
gravity of Mars. At the time of writing, Curiosity had lan-
ded successfully at the Gale Crater on Mars (see Fig. 12.22 )
and may encounter sand dunes on its way to its target, Mt.
Sharp, at the center of the crater.
With Mars, in the present epoch at least, one is relatively
assured of encountering only dry sand or dust, which limits
to a manageable range the types of environment in which
one needs to do testing. Roving on Venus seems a rather
forbidding prospect for a variety of reasons: getting stuck in
what little sand there is may be the least of an engineer's
problems, given the high temperatures, the corrosive envi-
ronment and rugged rocky terrain. As for Titan, the chem-
ical composition of Titan sand is of course rather different,
and perhaps also its mechanical properties, especially if
there has been a recent methane rainstorm. The prospect of
dealing with wet organic mud presents vehicle designers
and operators with a whole new set of interesting but dif-
ficult-to-test challenges, which has in part led to an
emphasis in Titan exploration of considering aerial plat-
forms such as balloons or aircraft (see Fig. 13.14 ).