Initially, this launch season was to see the second launch of a new lander design that was first employed on Mars Polar Lander. Its mission ended in humiliating failure following an equally embarassing failure only weeks before with the Mars Climate Orbiter.

So the real space odyssey in 2001 will not be the same as that famous motion picture promised. And the hard reality of just how difficult Mars exploration truly is, can be seen in the sharply reduced -- but vastly more realistic and well-planned -- new design of NASA's Mars program for this decade.

Instead of the opening phase of the first sample-return mission to Mars being launched in 2003, the mission that year will be a pair of long-range "Mars Exploration Rovers" scheduled for launch on May 30 and June 27, 003 -- which will land on Mars on Jan. 4 and Feb. 8, 2004, using the Martian beachball technique that was so successful in the 1996 Mars Pathfinder mission.

After each Pathfinder-based lander rolls to a stop and props itself upright by unfolding three side petals, the 150-kg rover nestled between them will scan its surroundings, and shortly afterwards roll away on a voyage of discovery that will last three to six months and stretch a kilometer or more across the surface

Eventually airborne dust will settle and accumulate on the rover's solar panels, slowly reducing its power supply enough to stop it. Nonetheless this first klick across the surface of Mars will dwarf the local putterings of Pathfinder's tiny rover that got no more than 10 meters from its lander.

The purpose of each rover is to use a set of five scientific instruments to photograph, and chemically and geologically analyze, local Martian surface features. This will enable scientists to "determine the aqueous, climatic, and geologic history of a site on Mars where conditions may have been favorable to the preservation of evidence of possible biotic or pre-biotic processes."

This will most likely be an area where orbital observations have provided strong evidence that liquid water existed on the surface of ancient Mars, during the planet's early days when it still apparently had a dense atmosphere and enough warmth from either the greenhouse effect or local volcanism to allow surface liquid water to exist for at least a moderately long period.

There is still a tremendous amount of debate over the extent, and the length of time, in which liquid water did exist on ancient Mars. Some theorists believe that the planet may have had large lakes and rivers -- perhaps even oceans -- of water for hundreds of millions of years, before its initially dense atmosphere collapsed and the surface temperature and air pressure became too low for water to exist in any forms other than solid or vapor.

One proposed variation of the wet Mars theory is the possibility that there may have been occasional repeated episodes -- perhaps as recently as a hundred million years ago -- when Mars briefly regained its dense atmosphere and surface liquid water.

Some scientists though, think that ancient Mars, even though it had a dense carbon dioxide atmosphere, was always below freezing on its surface -- so near-surface liquid water was limited to relatively small, local geothermally heated flows, which may in fact always have been buried under a layer of ice or even several dozen meters of ground.

It's quite possible that Mars' famous "valley networks" -- which are very sparse compared to ancient riverbeds on Earth, and look somewhat different in structure -- may have been carved not by water flowing across Mars' surface, but by smaller amounts of water tunneling out the ground beneath the surface until the tunnel's roof caved in.

And a few theorists -- such as Australia's Nick Hoffman -- take the even gloomier view that even early Mars was never anywhere near warm enough to maintain surface or near-surface liquid water, and that the erosive evidence of it is really due to flows of some more exotic substance, such as eruptions of liquid or gaseous CO2 from underground.

It's now become clear that orbital observations alone are not enough to answer these questions. We need "ground truth" studies of these apparent aqueous phenomena to decide which interpretations of them are correct. And we also need ground studies to decide just how promising these various features of Mars -- and the sedimentary rocks they may contain -- are as hunting grounds for fossil evidence of any ancient Martian microbes.

Hence the two 2003 rovers. But just what sites should they be aimed at? The process of decision is already under way, and the first major step has just been completed with the first "Landing Site Workshop" for the missions, held Jan. 24-25 at the Ames Research Center.

Click For Part Two

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