Mars Exploration Rovers Update: Opportunity Sprains ‘Ankle’ but Perseveres on Walkabout

The autumn skies over Endeavour Crater remained hazy as dust from the summer storms continued to rain down, but Opportunity encountered some unexpected and serious June gloom when her right front steering wheel jammed during the walkabout atop Perseverance Valley. Although the rover’s “sprained ankle” slowed the pace of the Mars Explorations Rovers (MER) mission for a couple of weeks, the veteran robot field geologist astonished her human colleagues on Earth and roved on.

More than 12-and-a-half years ago, the steering actuator for Opportunity’s right front wheel jammed and stuck in a toed-in position of about 8 degrees and she has driven with it like that ever since. But when the rover was making a basic arc back maneuver to turn on June 4th and the steering actuator for her left front wheel stalled and stopped with the wheel toed-out 33 degrees, it cast a pall on the mission team at the Jet Propulsion Laboratory (JPL), the birthplace of all NASA’s Mars rovers, as well as team members around the world.

“Little Miss Perfect,” as this rover came to be known way back when, showed her MER mettle yet again. After a series of tests and straightening attempts – with, of course, a lot of help from her engineer colleagues on Earth – Opportunity succeeded in straightening that wheel on June 17th.

“It was miraculous,” said MER Principal Investigator Steve Squyres, of Cornell University. “We could have operated the vehicle with the wheel cocked off at that crazy angle. But it’s terrific news, just wonderful news and the vehicle will be much easier to operate with that wheel pointed straight.”

When the telemetry confirming Opportunity’s wheels were all straight arrived on Earth, it was Father’s Day and a gift like no other for the thousands of “fathers” who helped put this rover and her twin, Spirit, on the Red Planet and roving along on NASA’s first overland expedition of Mars. It seemed to prove, too, just how much this rover really loves to rove.

Still, it’s not all good. No one, even the best Mars rover engineers in the world who tend to Opportunity know exactly why the actuator stalled or how they managed to get it straight again. They formed a tiger team of current and past MER ops engineers to further investigate. Until then, they are driving the rover in a cautionary mode, concerned that this incident could be an omen, a heads-up that the rover’s wheel actuators are suffering the strains of aging.

So while Opportunity roves on, things have changed. "For at least the immediate future, we don't plan to use either front wheel for steering," announced MER Project Manager John Callas, of JPL, on June 23rd.

“Whatever went wrong with it and whatever we did to straighten it, the steering wheel actuator is still not ‘healthy’ and the problem is only mitigated,” said JPL’s Chief of MER Engineering, Bill Nelson, “The wheel is straight now and we can steer the vehicle, but we’re not willing to steer the wheel itself. We’re not going to mess with it.”

Those unfamiliar with the rocker bogie and wheel systems of Spirit and Opportunity might view this as a rather significant conundrum. But, as reported in these pages many times over the years, these rovers were built to last and with a significant amount of redundancy.

Of the MERs six wheels, four – the two on the front and the two on the rear – were designed with both steering and drive capabilities and so each of those wheels boasts two actuators, one for steering and one for driving. The steering actuators turn the wheels back and forth, and the drive actuators turn the wheels round and round.