The second of NASA's Discovery missions, Mars Pathfinder will demonstrate whether the trimmed-down space agency's "faster, better, cheaper" concept of sub-scale spacecraft will provide both engineers and scientists with the kind of results once generated by the more expensive spaceships of the previous age of space.

The Pathfinder craft, consisting of the rover, lander, and entry shield, was launched last December aboard a Delta II rocket from Cape Canaveral, Florida. Attached to the base of the saucer-shaped shield for the seven month flight has been a cruise stage containing instruments and control systems for the spacecraft. Friday morning at about noon Eastern Daylight Time, the disc-shaped shield will separate from the stage and begin a fiery descent through the Martian atmosphere. The planet below will be cloaked in nighttime darkness. As the disc speeds through the Martian night, it will end its 309 million mile trek from Cape Canaveral streaking through the sky at 16,600 mph. Then, a mortar at the base of the shield will fire to deploy a 40-ft parachute, breaking is fall towards the surface. The shield, its work protecting the lander from the searing heat of planetary entry complete, will fall away. Now comes the part of the Pathfinder's flight never before tried in space. Instead of riding a landing stage down to a soft touchdown, an assembly of automobile-style airbags will deploy around the lander, in essence enveloping the three-foot tall craft in an inflated cocoon. Just 100 feet above Mars, a burst from a solid fuel rocket will slow the craft so that it literally bounces down onto the surface at about 50 mph. Bouncing repeatedly - perhaps as high as 100 feet - as it comes to a rest.

Three hours after touchdown - or "bouncedown" - the landing air bags will be deflated, and three petals will unfold, like a spring flower on Mars. At the center will be the lander's base camp complex of instruments and antennas, sending out the first detailed data about its landing site, the Ares Vallis ancient dried river bed. By now, it should be about 5pm Eastern time US. If NASA believes the craft to be in good shape, the first photos of the area around the lander will be taken by the lander's camera, and sent back to Earth. The first pictures should be available to NASA and the public around 7pm EDT US. If all looks safe, the space agency is expected to give approval to deploy a ramp leading off of the lander's petals. There, about midnight EDT US, the two foot long, one foot tall microrover named Sojourner will roll off of the lander and begin a week's worth of exploration of the Martian soil. Powered by solar energy, however, Sojourner will await the local sunset, which will occur about 2am July 5, EDT US, before resuming its explorations the next Martian "morning".

The rover will be capable of climbing over rocks and boulders, and will contain sensors to analyze the rocks and soils that it or its Earth-bound monitors deem of interest. The basecamp of the lander can operate for a Martian month of 24.6 hour days, the microrover about a week.

The Mars Pathfinder isn't the only Earth visitor headed for Mars. Next October, the Mars Surveyor is to swing into orbit for detailed analysis and mapping of the planet. Both the Pathfinder and Surveyor missions are looked at as precursor engineering tests to pave the way for a more ambitious roving mission that NASA may dispatch to Mars in 2001. On that future flight, descendants of Sojourner will extract a sample of Martian soil, and return it to Earth for study.

But for now, the first test of microrover concept is Pathfinder, drawing nearer to the Red Planet all the time. For the U.S. space program, though, landings on Mars on Independence Day isn't new. On July 4, 1976 Viking 1 did just that, beginning the exploration of Mars Pathfinder will continue.

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