A thick extraterrestrial sand dune was no match for an army of engineers and scientists who worked for over a month to free NASA's Opportunity rover from its clenching grasp on the surface of Mars.

"We're out!...all six wheels are on top of the soil," Steve Squyres wrote in an online status report Saturday. Squyres is the principal investigator for the Opportunity and Spirit rovers.

Views from Opportunity taken on Sols 446, 471 and 483 show the rover's progress to exit the sandy dune. Credit: NASA/JPL

Now over 16 months into a mission originally planned to last for 90 days, the Opportunity rover has been exploring a geologically-rich region of Mars known as Meridiani Planum, where the six-wheeled robot found conclusive evidence last year of the presence of great amounts of liquid water long ago.

On its way to visit its third impact crater in late April, the durable rover entered a treacherous region known as the "etched terrain" fraught with obstacles such as thick sand dunes. Opportunity became stuck in one such ripple on April 26, and engineers controlling the mission at the Jet Propulsion Laboratory opted to take their time to complete studies of the situation before making any subsequent action.

The ground team spent several weeks putting together a "recipe" to create a similar type of soil in an Earth laboratory to model the situation Opportunity found itself in on Mars. Mock-up rovers were stripped of two-thirds of their weight to simulate the weaker gravity on the Red Planet. The testbeds were then placed in the soil to determine the best way to exit the trap.

After straightening the wheels, the control team began commanding Opportunity to slowly spin its wheels each day to begin the process of extracting the bogged-down rover in a forward direction with a slight turn to the left. From mid-May until Friday, the craft had spun its wheels enough to normally travel 581 feet, but the actual movement detected was just about three feet.

But that deliberate movement proved worth it as Opportunity crested the foot-high dune and its ten-inch wheels emerged on the surface during Friday's scheduled operation.

"We've been confident all along that this would happen, but still...what a relief," Squyres said. "It's been an arduous process, and it feels very good to be free."

During its time stuck, the rover conducted a number of remote atmospheric science observations and used its high-resolution panoramic cameras to take pictures of the surrounding area where other potentially dangerous dunes are found.

Opportunity has driven 3.32 miles since exiting its lander in January 2004, vastly further than any pre-launch estimate could have predicted. Its twin Spirit on the opposite side of the planet has traveled almost three miles.

The robot is now about 1,300 feet from its next probable target known as Erebus crater - a formation quite a bit larger than earlier impact sites visited.

Normal operations will resume on Monday, but controllers will bide their time before sending orders to tell Opportunity to begin moving again.

"Clearly we're going to have to put some additional safeguards in place when we drive in this kind of terrain, and those safeguards certainly will reduce our driving speed somewhat," Squyres told Spaceflight Now. "But we feel very good about being able to continue southward at a reasonable rate."

"When we're actually going to drive away from this place remains to be seen, but we're in a position right now to begin studying the dune that we ran into, and we're going to start that immediately."