(CNN) -- Since the first close-up picture of Mars in 1965, probes sent to the red planet have revealed a world that is rocky and cold beneath a hazy, pink sky. Images sent back by satellite hint that Mars used to be a volatile world where volcanoes raged, meteors plowed deep craters, and flash floods rushed over the land.
But one discovery has especially piqued the curiosity of scientists and space buffs: Clues that liquid water -- the key to life -- may still be present in the martian soil.
Even if Mars has never had life before, it could have life in the future if humans choose to travel there.
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Between now and 2005, all the crafts sent to the Red Planet will be disposable
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To discover the possibilities for life on Mars, NASA has adopted a "follow the water" strategy for exploration.
That means exploring features that look like dry riverbeds and rock types that only form when water is present. The space agency also will search for hot springs, hydrothermal vents and eventually try to bring back a sample of martian soil for study on Earth.
The optimal window for launching probes to Mars (determined by its orbit in relation to Earth's orbit) opens every 26 months. NASA plans to launch a craft during every available window between now and 2005.
