Major constraints dramatically narrowed down the territory on Mars
that could even be considered. The candidate regions chosen, each
comprising an area about the size of Southern California, exist below a
certain elevation to provide enough atmosphere for the lander's
parachute to descend properly. The sites also sit in a largely equatorial
latitudinal band where enough sunlight shines to keep the solar-powered
rovers supplied with electricity. Areas dominated by steep slopes, such
as ravines or crater walls, are ruled out as hazardous to the lander and rover.
Reducing the Risk to the Airbags
Next to be eliminated were areas with large rocks. A rock larger than
about one-half meter high, or knee-high to most people, is too tall for safety
reasons. If the landing airbag system bounced hard on a rock that size,
the rock might protrude high enough inside the airbags to damage the
lander. Shorter boulders are considered acceptable, because even in the
event of a direct bounce on top of one, the rock would not be tall enough
to impinge on the lander inside.
But using even the highest-resolution images available to search for
sites dominated by right-size rocks, said Golombek, "you can't
guarantee there won't be bigger rocks. You can't eliminate
them." With vigilant study and deduction, however, "you can
try to make smaller the probability of landing on one."
Beware of Stealthy Terrain and 'Foo-foo Dust'
Laser altimeters will gauge the lander's altitude during descent in
order to fire the solid rockets and deploy the parachutes and airbags at
the right time. For those measurements to be made, the landers must
be targeted to areas where the altimeter's radar will bounce back from
the surface. Ruled out as landing sites are so-called "stealth
"Stealth regions" are locales on Mars where the radar
penetrates the surface but doesn't bounce back - a characteristic these
regions share with the military's radar-avoiding stealth technology. In
the case of Stealth fighters and bombers, the aircraft surfaces are made
of a high-tech, radar-absorbing material. In the case of Mars' "stealth
regions," however, the answer isn't known, said Golombek. They
may be covered with a meter or more of "foo-foo dust," a Dr.
Seuss-like term that Golombek uses to describe possibly fluffy
accumulations of Mars' fine iron-oxide dust particles that can pile up in
drifts like red snow.
In addition, "sending a solar-powered spacecraft to a dusty
spot isn't a good idea. The stuff gets on the solar panels and reduces
the power, gets stuck in the wheels and gears and generally gunks
up the works" Golombek said.