With two new vehicles and a yen to be near water, they're looking at three promising Martian destinations: Meridiani, Isidis Planitia and the Gusev Crater.

Each has its geologic charms. Meridiani is thought to have rich deposits of hematite, an iron oxide that usually forms from heated water. Isidis Planitia shows evidence of small channels. And the Gusev Crater might be a dry lake bed.

Mission planners at the space agency are poring over data from the orbiting Mars Global Surveyor before they make a selection.

"The sites will be ones at which there is clear evidence that liquid water was once present . . . ," according to a mission Web site.

Water is the key in the search for extraterrestrial life, but it also is critical for future Mars exploration and colonization because it is necessary for astronauts to produce food and fuel.

The twin rovers will be launched next spring or early summer on separate Delta II rockets, targeted to land at two Martian sites in January 2004.

In 1997 the Pathfinder mission had instruments both on the lander and on its 23-pound rover, Sojourner.

This time all the instruments -- cameras, spectrometers, microscopes and cutters -- will be on 400-pound rovers. The vehicles will have greater range, as much as a half-mile or more, compared with Sojourner's maximum range of 33 feet.

In addition to Geoffrey Landis, a physicist and engineer at the NASA Glenn Research Center in Cleveland is represented on the science team by Ron Li, an Ohio State University professor of geodetic science who developed the navigational and mapping software that will guide the rovers.

Maps of the search area will be derived from pictures taken from the Global Surveyor as well as from the lander as it descends and the rover on the ground.

Cameras on the rovers will allow maps to be updated as the vehicles move, said Kaichang Di, an engineer working with Li on the project.

This also means the views transmitted to Earth will constantly change.

Di, however, said he doesn't expect the scenery to be remarkably different than the flat desert landscape captured by Pathfinder five years ago.

Professor Li said he and the rest of the project planners have different views on where to land.

"The engineers want to land somewhere where it's flat, but the scientists would like to see some of the features. For us, we favor a flat terrain because it's easier for us to navigate."
 
 

dlore@dispatch.com 
 

Caption: (1) Graphic with Map
(2) NEAL C. LAURON | DISPATCH
Geodetics professor Ron Li leads an Ohio State group developing the navigational software that will guide the Martian rovers. 
 

Sept. 25, 1992 _ Mars Observer launched to map Martian surface from orbit. 

Aug., 1993 - contact lost. 

Nov. 7, 1996 - Mars Global Surveyor launched. 

Dec. 4, 1996 - Mars Pathfinder launched. 

July 4, 1997 - Pathfinder lands on Mars and begins a successful mission. 

Sept. 11, 1997 - Global Surveyor arrives at Mars; begins mission that continues today. 

Spring, 1998 - Due to engineering and funding problems, launch of the Athena Mars Rover Payload is delayed until 2003. A stationary lander will go to Mars in 2001. 

Dec. 11, 1998 - Mars Climate Orbiter launched. 

Jan. 3, 1999 - Mars Polar Lander launched. 

Sept., 1999 - Mars Climate Orbiter fails to achieve orbit due to a navigation error. 

Dec. 3, 1999 - Mars Polar Lander fails, apparently due to a software error. 

May, 2000 - Mars 2001 lander mission cancelled. NASA restructures program. 

July-August 2000 - Mars Exploration Rover mission selected for 2003. Two identical rovers will be flown. 

April 7, 2001 - Mars Odyssey orbiter launched; will aid in landing site selection. 

May 30-June 16, 2003 - MER-A launch period. 

June 27-July 14, 2003 - MER-B launch period. 

Jan. 4, 2004 - MER-A lands. 

Feb. 8, 2004 - MER-B lands. 

Jan.-May, 2004 - Rovers operate on Mars.