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Why we keep going back to Mars (+video)

The Mars Curiosity Rover scheduled to touch down on Aug. 5 represents mankind's 40th attempt to explore the Red Planet over the past 50 years. What is it about the Mars that keeps calling us back?

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"Mars wins most of the time," Doug McCuistion, director of NASA's Mars Exploration Program, told reporters earlier this month. [History of Robotic Mars Missions (Infographic)]

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Too interesting to ignore

Scientists are deeply interested in Mars partly because of its perceived past potential to host life as we know it. The Red Planet is cold, dry and desolate today, but Spirit and Opportunity have found plenty of evidence that it was once far warmer and wetter.

"When you look at geology, atmospheres, chemistry, and so forth and rack up your reasons to explore, anything that has to do with the possible origins of life on another world is always the first among equals," Hubbard said. "It's such a fundamental question. It goes to this 'Are we alone?' uber-question, or super-question."

Mars is not the only solar system body that may have been capable of supporting life at some point. For example, organisms might thrive today in the subsurface oceans of Jupiter's moon Europa and Saturn's moon Enceladus, some scientists say.

But these two ice-covered bodies are much farther away from Earth than Mars is, meaning they'd be much more difficult — and expensive — to reach. So the Red Planet's proximity is another big reason why so many spacecraft have visited it over the years. (Planetary alignments make Mars missions feasible every 26 months, and a probe can get there in eight months or less.)

Mars' status as a prime target for future human colonization also helps drive more robotic missions to the Red Planet, Hubbard said. After all, a thorough understanding of the planet — including whether or not it ever hosted life — is necessary before sending astronauts there.

"If Mars already has life, you have to understand the effects on humans," McCuistion said in April. "So this is a critical question — not just the innate human question of 'Are we alone?' but also safety of humans on the surface of the planet."

Finally, NASA's long history at Mars has built up momentum that helps push future missions along. NASA structures its planetary exploration efforts in stages, Hubbard said. Flybys come first, followed by orbiters, then landers and/or rovers. A sample-return mission is the last step in this robotic chain.

"We are now at the phase of Mars exploration where, as the National Academies have said, we're ready to do a sample return," Hubbard said.

By contrast, "we are just now getting to the point of doing a flyby of our poor little dwarf planet Pluto," he added, referring to NASA's New Horizons mission, which is slated to cruise past Pluto in July 2015.

Follow senior writer Mike Wall on Twitter @michaeldwall or @Spacedotcom. We're also onFacebook and Google+.

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