Just landing at all was a victory. The Jet Propulsion Laboratory engineers who built the Mars Exploration Rovers remembered all too well that they'd blown it last time, that over the course of 10 weeks in 1999 they'd lost two spacecraft and in the process nearly scuttled NASA's Mars exploration progr^n. So when the identical twin rovers, Spirit and Opportunity, rolled to a stop inside their protective airbags on opposite sides of the planet on January 4 aud January 25, then sent back panoramic photos confirming their safe landing, half the battle was already won. The other half was finding evidence that water once existed on Mars. Life requires water, biologists tell us, and planetary scientists have made water the central preoccupation of Martian research. Spirit landed in the vast, nearly flat Gu-sev Crater, which is thought to have long ago been a lake (see "Next Stop: Gusev Crater," Dec. 2003/Jan. 2004). Opportunity was sent to a plain named Meridiani to search for hematite, a mineral that on Earth typically forms in the presence of water. With their collection of high-resolution cameras, microscopic imagers, drills, and spectrometers, the rovers were well equipped for what project scientist Steve Squyres of Cornell University called "the coolest geologic field trip in human history."
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