When the Curiosity rover set out last July on its much-anticipated drive to the Mars mission’s ultimate destination — the three-mile-high science prize called Mount Sharp — everyone knew the going might get rough. The terrain ahead was more rugged than anything experienced before, and the winter nighttime temperatures regularly plunged to 120 degrees below zero.

Keeping all the sophisticated instruments onboard safe while guiding the one-ton rover over rises and through sand dunes would be an unprecedented challenge. Though it was only a six-mile road trip, team leaders predicted it might take as long as a year to finish.

Mars, however, had other plans for the rover, and they weren’t cordial. The allotted year is almost over, and Curiosity is but halfway to Mount Sharp.

Mount Sharp is unlike anything extraterrestrial ever explored. The rocks and minerals there “will tell us so much about the geological history of the region, and maybe more of the globe,” says Curiosity geologist Ralph Milliken of Brown University. Given the scientific treasures known to be present, the base of Mount Sharp is often described by the Curiosity team as “the Promised Land.”

The delay in getting to Mount Sharp is largely the result of one big, worrisome and time-consuming problem: damage to the rover’s wheels from their contact with sharp Martian rocks embedded in unyielding sandstone. Some gradual deterioration had been anticipated, but not the punctures and tears that began showing up late last year.

A puncture in the left front wheel of Curiosity was discovered in 2013, most likely the result of passing over a sharp rock. (JPL/NASA)

“What was happening to the wheels was a really big surprise to the team, and not a good one,” said Curiosity project manager James Erickson. “We had done extensive testing on those wheels, but we didn’t do testing on extremely sharp and pointy rocks embedded into the ground. But it turns out that Mars has many, many of them.”

Project scientist John Grotzinger said the wheel issue “quickly became an epic-scale problem for the mission. . . . It’s a little like being told you’re critically ill. You don’t know how much longer you have, but you know it will be a rough road.”

A rapid deterioration

The rover had been on Mars for roughly 400 “sols” (that’s 411 Earth days) when images of the wheels began to reveal some wear, according to deputy project scientist Ashwin Vasavada. That was some five months after Curiosity left Yellowknife Bay, where a low-lying area in Gale Crater yielded the mission’s greatest findings so far: that long-ago conditions at the once-watery site had been conducive to the existence of life.

Made of milled aluminum, each wheel has raised and reinforced treads that support a tire, which is only 0.03 inches thick. These are the first wheels of their kind to be used on Mars, designed to be light, to be flexible enough for the vehicle to land on them, and to have the traction needed to climb Mount Sharp.

“As the days went on, what we saw was alarming,” Vasavada said. “Not alarming in the sense that the wheels were in serious danger, but alarming in that the rate of deterioration appeared to be picking up.”

The first priority was to stop the damage, and that meant parking the rover. It sat for two weeks in the dead of Martian winter as the team worked feverishly to understand two problems: Which terrains were tearing up the wheels, and how could those damaging areas be avoided?

Finding answers required long hours of matching landscape pictures taken earlier by Curiosity with images of the wheels taken around the same time. Images from the orbiting HiRISE (High Resolution Imaging Science Experiment) camera were also blown up to their maximum to identify rocks along the path and to get overviews to help identify areas that appeared problematic.

Marc Kaufman, author of “Mars Landing 2012: Inside NASA’s Curiosity Mission,” explains the importance of the Curiosity mission, which is being hailed “the mission of the decade” by NASA’s chief scientist. (Marc Kaufman and JulieAnn McKellogg/The Washington Post)

At NASA’s Jet Propulsion Laboratory in Pasadena, Calif., the six wheels of a Curiosity double were given similar damage, and it was put through its paces in the Mars Yard, a simulated Martian landscape at the lab. Gradually, the team leaders became convinced it was safe to resume driving.

The drives were short: 10 to 30 meters. That began to change only when permission was granted to drive the vehicle backward, a maneuver that limits wear and tear on the most damaged middle and front wheels.

But it was an awkward way to travel. Most of the cameras that Curiosity’s drivers had used to plot their paths are on the front of the rover, so there was only limited camera coverage to show what lay ahead when driving backward.

Only recently has the wheel problem been deemed manageable, though with a significant change in how the traverse — and the upcoming climb of Mount Sharp — would proceed.

“We’ll be driving on sand whenever we can, and avoiding the bedrock,” Grotzinger said. “The wheels will no doubt continue to take on some damage, but we know much better now how to limit that.”

Making progress

As a welcome sign of that return to near normal, the rover used its autonomous navigation in late May for the first time since the wheel problem arose. The “autonav” program – with its ability to independently identify risks and avoid them — allows the rover to drive farther than rover drivers initially program for the day. But the rover is still generally being driven in reverse, and the autonav works only when the rover is going forward. That means a day’s drive now usually includes a U-turn after the driver-controlled part of the trek is finished and the autonav takes over.

Despite the progress, the wheel drama was a sobering reminder that, as NASA officials often put it, “Mars is hard.” An expedition that had been highly productive and relatively problem-free for more than a year suddenly had a threatening, and seemingly worsening, problem.

All during that Yellowknife campaign, there had been voices calling for a speedier wrap-up so the traverse to Mount Sharp could begin.

The scientists didn’t dispute the value of what was being investigated and discovered, but they did worry that the rigors of working on Mars could have unexpected consequences on the rover and so it would be better to head for the main target as soon as possible. On Mars, time always equals risk.

But the discoveries kept coming, so the planned short detour to the Yellowknife area lasted for more than eight months. Top scientists and many others advocated staying. Studying Mount Sharp is a key goal of the expedition, but Curiosity is also officially on a “mission of discovery” that allows for detours.

While the wheel drama has dominated the traverse, Curiosity has continued to return some exciting results. The team, for instance, found evidence of the long-ago presence of water across the now parched landscape. And Curiosity team science papers presented in the spring reported the strongest evidence yet that the Martian surface holds simple carbon-based organic compounds — the building blocks of life that have so far eluded clear detection.

Curiosity is now in full drive mode again, Grotzinger said. He was unable to predict when the rover might reach Mount Sharp, but he did say that the nuclear generator powering the rover will probably wear out before the wheels do — and that neither would happen for quite a few years.

“This was a huge bonding experience for [the] mission,” Grotzinger said. “Success is always great, but there’s nothing like impending doom to bring people together. Now we can say, ‘We licked that one.’ ”

Kaufman’s book “Mars Up Close: Inside the Curiosity Mission” will be published in August.