No planet in the solar system holds more mystery and fascination than Mars. And now a bargain-basement spacecraft called the Mars Pathfinder is writing the latest entries in the Martian chronicles. Pathfinder is the first successful mission to Mars since the two Viking probes landed there in 1976.

In a technological tour de force, Pathfinder parachuted through the thin Martian atmosphere, inflated a protective cocoon of airbags and bounced to a soft touchdown just after 1 PM EDT on the 4th of July. After a six-month voyage, it landed precisely on target and began performing nearly flawlessly. Shortly after reaching the surface, the spacecraft began sending its first images of the Red Planet. The few minor glitches encountered so far have been quickly corrected by commands radioed from Earth.

SOJOURNER EMERGES. On July 5, 1997, this small rover rolled down the ramp and planted its six wheels on Martian soil. Creeping at a snail's pace, the mobile robot will collect data on soil and rocks.

By July 5, a six-wheeled "micro-rover," dubbed Sojourner, had rolled down its flexible ramp onto the surface of Mars. About the size of a household microwave oven, Sojourner is an experiment in automated robotics, designed to pilot itself between the rocks, study the composition of the soil and rocks and send the data back to the Earth.

At a press conference on July 6, excited scientists at NASA's Jet Propulsion Laboratory, which manages the project, announced the first scientific results from the Pathfinder mission. Already, data sent back to the Earth seem to confirm earlier thoughts that the landing site, in a region known as Ares Vallis, appears to be an ancient floodplain. Although images of Ares Vallis broadly resemble those from the Viking landing site some 850 kilometers away--both look like an arid, ruddy desert strewn with rocks of various sizes--the two sites are actually quite different.

VIKING'S VIEW. Images recorded by the Viking landers look similar to those transmitted by Pathfinder. But from a geological perspective, the two sites are very different.

The Viking probes landed in an ancient lava flow; all the surrounding rocks were of similar, igneous composition. But in Ares Vallis, project scientists believe they are seeing what they had hoped for: a variety of rocks that were most likely transported to the site from great distances by flowing water. The hills near Pathfinder hold some other intriguing features. Preliminary images reveal a series of horizontal bands that could be layers of sedimentary rock or benches cut into the sides of the valley by rushing water.

If the bands prove to be layers of sedimentary rock, they would be just the place to look for fossil evidence of ancient life on Mars. Such a finding would confirm the sensational recent report of possible fossilized bacteria in a meteorite found in Antarctica that is believed to have originated on Mars. Sojourner will not be able to conduct a search for fossils, but NASA hopes to return Mars rocks to the earth for detailed analysis early next century.

What Sojourner will discover remains to be seen as the tiny rover begins its slow crawl over the rocky terrain. At top speed, it can cover a mere 0.4 meters each minute. Even at that speed, however, Sojourner should be able to examine a far greater variety of rocks and soil than have been studied so far. It will transmit its data to Pathfinder, which will relay it to the earth. Pathfinder also functions as a weather station, sending back meteorological data on the thin Martian atmosphere.

Scientists are not alone in their excitement at the return to Mars. The Pathfinder website is reeling from tens of thousands of Internet users seeking the latest information on the intrepid craft. And in Washington, D.C., long lines of people snaked through the Smithsonian Institution's National Air and Space Museum to glimpse the first pictures snapped by Pathfinder.

Pathfinder's early success is another feather in the cap for NASA's Discovery program, which aims to conduct important space science experiments for less than $150 million. (Pathfinder's budget was less than one fifteenth the price of the Viking program, when adjusted for inflation.) Only a week ago, another Discovery mission, the NEAR probe, made a similarly impressive showing when it sent back high-resolution images of the asteroid Mathilde.

In the wake of Pathfinder's success, the world should not have to wait another 20 years for the next visit to Mars. Pathfinder is the first round of NASA's Mars Surveyor Program, a series of missions to be launched every 26 months over the next decade. These spacecraft should vastly expand our understanding of the Red Planet and may answer whether it was once an abode for life.

Sojourner is preparing the way for a more sophisticated rover that was recently being tested in the Mojave Desert, here on the Earth. Called Rocky 7 (while Sojourner was under construction it was known as Rocky 4), it is scheduled for missions to Mars in 2001 and 2002. In the recent shake down, Rocky 7 maneuvered over one kilometer, taking more than 500 photographs and placing scientific instruments along the way.

In comparison, Sojourner is fairly primitive. Its ability to avoid dangerous terrain, such as craters and rocks, is limited because it depends on the lander's imaging system to plan its course. Rocky 7 will navigate on its own using small stereo cameras mounted on its front.

For now, it is tiny Sojourner's turn. The results of its preparatory ramblings are likely to make Mars seem a little less mysterious--but, quite possibly, even more fascinating.