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Once every two years the orbits of Earth and Mars are aligned just right so it's possible to send a spacecraft from here to there. That special celestial time is now and today, NASA's Mars Science Laboratory lifted off from Cape Canaveral for an eight-and-a-half month journey to our solar system neighbor. NPR's Joe Palca has this preview of the mission.

JOE PALCA, BYLINE: NASA has some experience with rovers. In 2004, the Mars exploration rovers known as Spirit and Opportunity began their missions on Mars. Astonishingly, Opportunity is still going nearly eight years later. But those rovers are puny compared with the Mars Science Laboratory.

JOHN GROTZINGER: The Mars exploration rovers landed, basically crash-landed softly with airbags. Mars Science Laboratory is so large that we need an active propulsion system.

PALCA: That's John Grotzinger, project scientist for MSL, as the mission is known. The day I interviewed him in 2010 we were standing on a balcony above a large clean room at NASA's Jet Propulsion Laboratory in Pasadena, where the rover was being put together. The six wheels had just been added to the boxy frame.

This rover weighs nearly 2,000 pounds. It's about the size of a small compact car. That active propulsion system that Grotzinger mentioned is a pretty wild scheme. It's a sort of a rocket-powered helicopter. When it gets to about 200 feet above the surface, it lowers the rover down on a cable. With the rover dangling below, it descends slowly until the rover wheels touch the ground.

If you get a chance, check out the animation of the landing on our website npr.org. Like I say, it's wild.

GROTZINGER: The risks are obvious, but the advantages of this are that the rover lands basically intact and there's almost no subsequent setup that has to be done after the rover lands.

PALCA: The rover has a mast with a camera on it and a robotic arm, but Grotzinger says MSL is not just about taking pictures and pulverizing rock.

GROTZINGER: It is a laboratory and so within the belly of the rover are two very important instruments. One of them is an x-ray diffractometer which is the instrument that geologists use on Earth to characterize the mineral content of rocks and soils.

PALCA: The other instrument is called the Sample Analysis at Mars, or SAM for short.

PAUL MAHAFFY: So this is an upside down SAM.

PALCA: Back in 2008, I visited NASA's Goddard Space Flight Center in Greenbelt, Maryland. As you can see, I've been tracking this mission for a while. On this particular day, some of Sam's guts had been removed. What I could see was a box about the size of a microwave oven. Various pipes and cables were sticking out. Paul Mahaffy is the chief scientist for Sam.

This big cover you see here is where the 74 cups of Sam and the sample manipulation system go.

Mahaffy explained that pulverized rock from the robotic arm will be deposited into one of the cups.

It gets pushed up into an oven. We heat it up in the oven to 1,000 degrees and as the gases are coming off, we measure their composition with an aspectrometer.

One of the elements Sam will be able to measure is carbon. Carbon is essential for life. But Mahaffy and everyone else associated with this mission says finding carbon compounds will not be proof that there is or was life on Mars. It will just be another piece of evidence pointing in that direction.

MAHAFFY: We fully don't expect we're going to go to Mars and get a definitive answer; yes, there was life or no, there wasn't life unless we happen absolutely to hit a homerun and land in exactly the right spot and conditions were exactly right.

PALCA: If everything had gone according to plan, Sam and the rest of MSL would already be on Mars. The mission was supposed to launch in 2009 but delays in building hardware forced a two-year postponement. When I spoke with Mahaffy last week he sounded relieved that Sam was about to leave Earth.

We've been anxiously awaiting the launch for a long time and even more anxiously awaiting August 6th of 2012 when we land in Gale crater and start exploring.

Gale crater is MSL's target. It's a giant crater with an enormous mountain in the middle of it. There is lots of interesting geology, and maybe more evidence that Mars was once habitable. Joe Palca, NPR News.

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