Sites with too many rocks of any size are not desirable either,
because a densely populated rock field can create a treacherous obstacle
course for a rover. "Too many rocks inhibit mobility, but then
again, you're going there to look at the rocks," said Golombek,
pointing out another area where safety and scientific appeal must compromise.
The site evaluation process started in September 2000 when
Golombek and fellow scientist Tim Parker (also at JPL) identified
nearly 200 possible landing sites that met the basic engineering
constraints. Subsequent work and meetings have reduced that to four
prime candidates and two backups. By May of 2002, a region measuring
600 by 900 kilometers will be selected - one for each rover. At that
time, targeting data will be hardwired into the launch vehicles that will
carry each rover . After launch, the two spacecraft will be more finely
targeted during their cruises to Mars based on detailed navigation
measurements taken on the way. At that time, the final landing boundary
will be narrowed to a football-shaped ellipse of about 100 to 200
kilometers long by 20 kilometers wide.
Mars Global Surveyor, an orbiter currently at Mars, has provided
global elevation data through its laser altimeter,
surface temperature and mineralogical readings from the
thermal emission spectrometer,
and images from the camera.
New data collected by these instruments will be used to better
characterize the sites in coming months. In addition, the recently
arrived 2001 Mars Odyssey orbiter will start taking routine scientific
data in early 2002, which will also be used in determining the final
two sites selected.