Location, location, location. Tantalizingly close to Earth though notoriously treacherous, Mars continues to lure planetary scientists.
Thirty years ago, NASA's Viking landers made history by descending from orbit to the surface of the Red Planet. They were the first probes from Earth to land intact on Mars -- and the first American spacecraft to land on another planetary body since Apollo.
Viking 1 and its sister ship Viking 2 revealed an alien world comprised of sterile soil and eerie salmon-colored skies. They answered the probing question: No life on Mars -- at least not in the two areas where the spacecraft landed.
Working steadily for years beyond their planned life expectancy, the landers accumulated 4,500 up-close images of the Martian surface. Their partner spacecraft high above -- the Viking orbiters -- snapped over 50,000 images, mapping 97 percent of the Martian globe. Successful in so many ways, Viking is best remembered for the first space probes to conduct on-the-spot biological tests for life on another planet.
Viking blazed the trail for future Mars missions. Mars Pathfinder, the Sojourner rover, Mars Global Surveyor and Mars Odyssey continued to prepare the way. The darlings of Mars exploration, the twin rovers, Spirit and Opportunity, captured the attention of young and old with their antics. The most recent mission, the Mars Reconnaissance Orbiter, launched in August 2005, arrived at the Red planet in March making it the fourth active satellite currently focused on Mars.
NASA's march across the sparse yet alluring planet continues. More inquisitive craft are on the drawing boards. Perhaps one will solve the mystery of what happened to the planet's water. Perhaps one will deliver the ultimate scientific explorer: humans.
And when humans finally arrive on Mars, they’ll find a pair of dusty pioneers already there -- the Viking landers that blazed the trail for all to follow.