A NASA orbiter set to start circling Mars later this month has tough shoes to fill -- its predecessor exposed an unprecedented bounty of global data on the planet's water history and suitability for human colonies.

The earlier probe -- Mars Global Surveyor -- and its six scientific instruments have delivered more than 130 CD ROMS-worth of data during its primary mission which started in March 1999.

The incoming spacecraft, called Mars Odyssey, will follow up with even higher quality data also aimed at paving the way for NASA's goal to put humans on Mars.

But the recent successes of Mars Global Surveyor gave scientists their best glimpse yet of what humans will face once they reach the Red Planet. Most typically it will be the stinging winds of a dust storm.

"We need to understand the atmosphere at Mars better before you trust spacecraft with humans in it to go to there," said Arden Albee, the programs project scientist at Caltech.

The main obstacle to human exploration that Mars Global Surveyor has revealed is the dust storms that envelop Mars every summer. A surprise storm of blustery dust could clog the breathing apparatus of astronauts or foul electrical equipment.

Using measurements of temperature, atmospheric density and imagery, the probe has observed the precursors to, and wind patterns of, these potentially dangerous conditions, but the dust storms remain unpredictable. This summer, Mars was wrapped completely in dust two months sooner than it was predicted.

"You cant put Martian weather into neat brackets," Albee said.

Still, Mars Global Surveyor has clued scientists in on Martian weather patterns. Those insights will help engineers understand how, when and where to land roving instruments on the Martian surface to will send back physical samples, something Albee thinks will happen by 2011.

The hardy probe also has pointed scientists to optimal sites for future rovers to roam in the search for signs of life.

An instrument that records data in the infrared spectrum, a measure of heat, has shown that Mars is covered mostly with one type of rock. But the instrument (the thermal emission spectrometer) also discovered that some regions near the equator are rich in the mineral hematite, which forms only in the presence of water.

Hematite forms in water over very long periods of time, even millennia. This gives the hematite locations the upper hand for further landing sites of rover missions.

"Most of the surface of Mars is covered with volcanic rock, and if you're looking for past life, that isn't a good place for it to have been," said Philip Christensen, the principal investigator on the instrument. "Signs of life should be limited to where water was."

Mars Global Surveyor's camera also spied geologic evidence of ancient Martian waters in what appear to be channels and streams. These too are potential sites for further investigation, but Christensen said those features could have formed during a short period of flooding.