The Mars Exploration Rover Opportunity has dipped its wheels into Endurance Crater as controllers follow a cautious approach to ensure the robot doesn't become marooned inside the stadium-sized crater.

The rover put all six wheels over the crater rim Tuesday, collecting data on the slope and wheel traction before reversing course and backing out.

"This is a small, what we call a toe-dip, drive into the crater just to be able to take some images from a different vantage point and to check out our models on the slope slippage of the rover as it drives into the crater," flight director George Chen said.

"After that six-wheel drive into the crater, we backed back out and spent the night outside the crater."

Wednesday's plan called for Opportunity to roll into the crater again and remain there overnight.

"After that little bit of confidence drive into the crater, we drove a total of four meters into the crater and we are going to back up just about 1.4 meters back out -- but not all the way out of the crater, and in fact spend the night in the crater for the first time," Chen said.

Engineers used a rover mock-up to perform extensive testing on Earth to determine how steep of a slope Opportunity could navigate. That testing, coupled with the estimated slope of Endurance Crater, allowed officials to decide it was safe to send the rover into the crater.

"It looks like the slope information is pretty much what the model predicted -- about an 18-degree slope going down into the crater and steeper as it goes further in," Chen said after Tuesday's first drive.

Opportunity is taking the risk of going into Endurance so its instrument-laden arm can examine a layered rock formation along the crater's inner wall that will give scientists more clues about the past environment on the Red Planet.

"If you think of the layered rocks as history books that tell you about the Martian past, we have read the top layer. Now, we want to learn even further back in the history of Mars. So we are ready to turn the page," said Firouz Naderi, manager of NASA's Mars Exploration Program. "For that, we need access to the layers beneath. We have access to those layers because they are exposed in Endurance Crater."

The rock layers that Opportunity hopes to probe are lower in the ground than those studied at the much smaller Eagle Crater in which the rover landed four-and-a-half months ago. The Eagle Crater layer of sulfur-rich rock told scientists that water once flowed at the site. The Endurance layer is older and presents a "compelling" need to examine.

"This unit will tell us what came before the salty water environment the Eagle Crater unit told us about. We want to get to the contact between the two units to see how the environment changed. Is it gradual? Is it abrupt? Even if the lower layers formed under dry conditions, they may have been exposed to water later. The water's effect on them could have left telltale evidence of that interaction," said lead rover scientist Steve Squyres.

"The science is very compelling, but there is some possibility that once we go in may not be able to get out," Naderi warned. "If I stay with the analogy of history books, if you want to learn about history there are worst places to get stuck than the library, which this (crater) practically would be."

The rover is expected to reach the rocky area in question sometime early next week.

"As we evaluate the data and understand a little bit about how the rover is able to safely traverse, we will get more confidence driving inside the crater and we will able to approach some of the very interesting targets of opportunity," Chen said.

Additional coverage for subscribers:
VIDEO: OPPORTUNITY'S DRIVE PATH AND PLANS DESCRIBED QT
VIDEO: EARTH TESTING OF ROVER MOCK-UP FOR CRATER DRIVE QT
VIDEO: LEAD SCIENTIST EXPLAINS OBJECTIVES INSIDE CRATER QT
VIDEO: OPPORTUNITY'S TARGET ROCK SHOWN WITH NARRATION QT
VIDEO: NEW COLUMBIA HILLS PANORAMA FROM SPIRIT ROVER QT
VIDEO: WATCH TUESDAY'S MARS ROVER NEWS CONFERENCE QT
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