Spirit's destination is the Columbia Hills, a group of seven low hills that rise up from the floor of Gusev Crater about 1.7 kilometers (about 1.2 miles) from the rover's landing site.

The Columbia Hills are a tempting target because, scientists believe, they are older than the crater-floor material that Spirit has encountered so far. The hills may preserve a record of an earlier time in Gusev's history, a time when perhaps the crater was filled with water.

"We believe that the hills provide the best opportunity to look deeper in to the history of Gusev by looking at older rocks, certainly the best opportunity we have with the remainder of this mission," says David Des Marais of NASA Ames Research Center in Moffett Field, California.

This older material, he adds, may have been uplifted and tilted to its present position. Or it may be comprised of hard-to-weather rocks left behind by erosion that, over time, ground away the surrounding region. "We're going to know one way or the other what they are when we get there," says Des Marais.

But getting there will take some time. Spirit began its journey to the Columbia Hills in early May. Mission controllers expect it to reach its goal, the nearest spur, in mid-June. And although the rover's focus has been on covering ground as quickly as possible, Spirit also is conducting a unique set of scientific experiments along the way.

Every four sols - a sol is a martian day - the rover completes a cycle of imaging and spectroscopic experiments. Over the course of its excursion to the Columbia Hills, Spirit will complete this cycle 10 times. It already has completed several such cycles.

One of the primary goals of these experiments is to provide "ground truth" data for observations made from orbit. Satellites orbiting Mars can "look down at this piece of ground that we've crossed and get the spectral information back and be able to correlate that with features that we actually observed on the ground as we traversed that area," says Des Marais.

NASA's orbiting spacecraft, MGS (Mars Global Surveyor) and Odyssey, contain thermal emission spectrometers that read spectral information about the surface (radiation emitted by the surface at various infrared wavelengths) and that use that information to calculate surface temperature.

The orbiters take these readings at different times during the day to determine how well different surface materials retain heat. That information enables scientists to form general impressions about the types of materials that make up the surface rocks and dust. Solid rock retains heat overnight better than dust or soil.

But the instruments aboard the orbiters are relatively low-resolution. Moreover, they look down at the ground through the dust-filled martian atmosphere, which can superimpose its own spectral signature on that of the surface, thereby distorting orbital measurements.

But Spirit also contains a thermal emission spectrometer, Mini-TES. It's readings, taken at ground level, provide a useful reality check for readings taken from orbit.

During Spirit's journey to the Columbia Hills, on the afternoon of the first sol of each 4-sol cycle, when the martian surface is relatively warm, Mini-TES examines the ground in front of the rover.

Then, early the next morning, after the surface has cooled overnight and before it is warmed back up by the sun, Mini-TES measures the same region again. These readings can then be compared to similar readings taken from orbit.

On several occasions during Spirit's trip to the hills, Mini-TES is making a set of measurements more tightly coordinated with an orbiting spacecraft as it flies directly over the rover's location. During these overflights, Mini-TES points up to take spectral readings of the atmosphere and then down to take readings of the ground.

Simultaneously, the orbiting spacecraft takes readings of the ground through the atmosphere. These synchronized measurements will help scientists understand more precisely how the atmosphere affects readings taken from orbit and correct for these inaccuracies.

Pancam, the rover's panoramic camera, will also get a workout during Spirit's traverse. One of its jobs will be to capture hi-resolution images, looking both forward and back before each segment of the rover's drive.

These images will later be used to construct a digital 3-D simulation of the entire 1.7-kilometer stretch of terrain. "That could be useful to explain things that maybe we don't even discover until we've analyzed data after the mission's over," says Des Marais. No doubt, it will also make for an entertaining and popular Web download.

Another task for Pancam is studying "ventifacts," the effects of wind sculpting, in the rocks that the rover drives past. On the afternoon of sol 1 of each cycle, Pancam takes an image of the rocks that lie just ahead.

Because the rover is heading east, these rocks are well lit in the afternoon. On sol 2 of the cycle, Spirit drives past the rocks imaged on sol 1. Then, on the morning of sol 3, with the morning light illuminating the backsides of the same rocks, Pancam looks back and takes another image. By comparing the ventifacts on the fronts and backs of the rocks, geologists will be able to learn more about wind patterns in the region.

Other experiments include: documenting the diversity of rocks in the area, noting how they change as the rover gets closer to the hills; studying the mist clouds that form each morning, and looking, as days get shorter and winter approaches, for ice clouds; and examining the ground before and after the rover has driven over it, to compare materials at the surface with those just below it.

And, of course, if these broad surveys discover a rock or some soil that appears particularly interesting, Spirit may stop for a sol and use its full suite of instruments - the M�ssbauer and APX spectrometers and the microscopic imager - to take a closer look.

And all this is merely data collected en route to what could be Spirit's greatest discovery: a close-up look at the Columbia Hills. With any luck, Spirit should begin that phase of its mission within a couple of weeks.

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