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Mars has been bereft of visitors from Earth for almost two decades, but that's about to change.

Next year, two U.S. and one Russian spacecraft will be launched toward our sister planet. All will arrive in 1997 to begin a campaign of exploration using cameras, a roving vehicle, a variety of instruments to study soil and rock chemistry, and even a balloon to travel above the craggy Martian surface.The most detailed information about the planet so far came from the spectacularly successful Viking mission: four spacecraft - two satellites and two landers - that reached Mars in the summer of 1976.

Since then, there have been a few other attempts. A pair of Russian spacecraft were supposed to visit the planet. One, which was to compile a detailed map from orbit in 1993, failed mysteriously just before it reached the planet.

The missions scheduled for launch next year are far less ambitious than those recent failures, but they are just the vanguard of a fleet of planned missions. Because the Martian year, at 687 days, is almost two Earth-years long, the two planets come closest about every two years, and those encounters provide the optimum launch opportunities. NASA plans to send craft toward Mars during several upcoming two-year intervals.

One of next year's U.S. missions is called Mars Pathfinder and is largely planned as a test of technology for sending remote-controlled roving vehicles across the Martian surface. Its tiny rover - about the size and weight of a cat - will likely range only a few hundred yards from the landing site, and is to send back pictures and detailed chemical data on interesting rocks it encounters.

The second U.S. mission, Mars Global Surveyor, is an orbiter to make detailed maps of the planet.

The Russian craft will launch a French- and U.S.-built balloon that will soar across the landscape by day and land each night to sample the soil at different locations.

Two years later, the Russians hope to send another craft, carrying a U.S.-built instrument to test soil chemistry - an experiment that could answer questions raised by Viking's life-detection experiments.

NASA-Ames scientist Christopher McKay says this experiment might, in fact, provide clear evidence for the existence of living microbes on Mars.