The huge NASA rover speeding toward an Aug. 5 landing on Mars may be the most capable and complex Red Planet explorer ever launched, but it's far from the first.
The 1-ton Curiosity rover — which will search for evidence that Mars is, or ever was, capable of supporting microbial life — represents humanity's 40th effort to explore the Red Planet over the last half-century.
The huge number of attempted Mars missions may seem surprising, especially since many of our solar system's other planets and moons remain relatively unstudied. But the Red Planet keeps calling us back — and for good reason, experts say.
Fifty years of Mars exploration
The Mars exploration era began in October 1960, when the Soviet Union launched two probes four days apart. The spacecraft, known in the West as Marsnik 1 and Marsnik 2, were designed to perform flybys of the Red Planet, but neither even reached Earth orbit.
The United States got in the game in 1964, launching the Mariner 3 spacecraft on an intended Mars flyby. The mission failed, but Mariner 4 succeeded, cruising past the Red Planet in July 1965 and sending 21 photos back to Earth.
The nation built on that accomplishment, sending a series of orbiters, landers and rovers to Mars over the following five decades.
Notable NASA successes include the Viking 1 and Viking 2 missions, which sent orbiters and landers toward the Red Planet in 1975; the Spirit and Opportunity rovers, which landed in January 2004; the Mars Reconnaissance Orbiter, which arrived at the Red Planet in 2006; and the Phoenix lander, which discovered subsurface water ice in 2008.
But failure remains a regular part of Mars exploration. NASA setbacks include the Mars Polar Lander and Mars Climate Orbiter, two missions that were lost upon arrival at the Red Planet in late 1999. And none of the 19 Mars efforts the Soviet Union/Russia has launched over the years achieved its goals in full.
Overall, the success rate for Mars missions is south of 50 percent.