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updated 9/10/2010 3:50:48 PM ET 2010-09-10T19:50:48

Two years after setting out for a big Martian crater that could hold clues about the Red Planet's potential to support life, NASA's rover Opportunity has hit a milestone: It's more than halfway there.

Opportunity turned its wheels toward the Endeavour crater in August 2008, after exploring another crater, Victoria, for about two years. But Endeavour is a different beast; at 14 miles (22 km) wide, it will be the biggest crater a NASA rover has ever seen up close.

The trip from Victoria to Endeavour is about 12 miles (19 km) long. The Opportunity rover has covered about 6 miles (10 km) of that distance at this point, NASA officials confirmed Wednesday. [ Latest photos from Opportunity rover.]

"We are actually beyond halfway," said Matt Golombek, chairman of the Mars rover science operations working group at NASA's Jet Propulsion Laboratory.

Get to the clay

Opportunity landed on Mars with its twin rover, Spirit, in January 2004. Their missions were supposed to last only about three months, but both far surpassed that lifetime. Spirit got bogged down in soft sand in 2009 and stopped communicating with Earth in spring of this year. Opportunity, though, is still going strong.

NASA scientists are keen for the rover to reach Endeavour. Last year, the agency's Mars Reconnaissance Orbiter spacecraft detected clay deposits on the crater's rim. Clay-bearing rocks are a strong indication of the past presence of water, which is necessary for life as we know it.

"They're a kind of mineralogical marker bed for places with biological potential," Golombek told SPACE.com. "So that's our ultimate goal, to get to them."

In fact, NASA's next Mars rover, the car-sized Curiosity, will prioritize looking at clays. The rover is scheduled to launch in November 2011 and land on Mars in August 2012. All of its potential landing sites sport clays similar to those found on Endeavour's rim, Golombek said. 

Another intriguing aspect of Endeavour's clay is its age. The terrain surrounding Endeavour dates to about four billion years ago, Golombek said around the time when life likely started on Earth. Mars may have been more conducive to life back then, and such old rocks could tell us more.

Scientists also have a general interest in Martian craters, clay-bearing or not. Big holes in the ground allow researchers to peer beneath the top layers of dirt much deeper than rovers' robotic arms can dig.

"You can use craters as a poor-man's probe into the subsurface," Golombek said. "You can often see actual strata or outcrop on their upper rims."

Still chugging along

It's taken Opportunity about two years to go 6 miles (10 km), so the rover won't be reaching Endeavour for a while. Its travels are of the slow-but-steady variety.

"On a really good day, we can go about 100 meters [330 feet]," Golombek said. "A short day is maybe half of that."

Still, there's no reason to think Opportunity won't make it, he said. The rover is in pretty good shape, besides a broken right front wheel that doesn't turn. For that reason, engineers are driving it backward, but that shouldn't pose a problem.

"We're not seeing anything that's going to stop us," Golombek said.

If Opportunity doesn't reach to Endeavour, it should still gather some useful data along the way.

"Even if we don't make it, we're going through some tremendously interesting landscapes," Golombek said.

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