Unimpressed by Michael Jackson's iconic moonwalk? How about a "Marswalk"? NASA's Curiosity rover showed off its fancy footwork on the Red Planet this week by driving backward. The Mars Science Laboratory robot just finished its longest drive in three months, and it did so in reverse, riding 329 feet, reaching a total of 3.24 miles since its 2012 touchdown in Gale Crater.
But Curiosity's handlers aren't just testing the rover's skills for fun; they're doing it to save the rover's thin aluminum wheels from further, more serious damage.
"We wanted to make sure we had a tool in the toolbox that we could use in the right circumstances," Jim Erickson, Curiosity's project manager at Jet Propulsion Laboratory, said in an interview.
Late last year, the JPL team in La Cañada Flintridge noticed an alarming number of holes and scratches scarring the wheels' surface.
"That gave us concern," Curiosity's lead scientist and Caltech geologist John Grotzinger said at the time. "We expect to get damage to the wheels, but we were surprised at the rate."
As it turned out, Gale Crater appears to host strangely fierce winds that are very good at sharpening rocks to little pointy pyramids, Grotzinger said. That power of erosion is potentially useful for exposing organic matter for the rover to examine, but it's not so good for the wheels.
The researchers also think Curiosity might be more vulnerable to the rocks than its 2004 predecessors, twin rovers Spirit and Opportunity, because it's so much bigger, and all that weight coming down on a pointy rock can do a lot more damage.
The team noticed that as the rover moved, the front and middle wheels were taking more of a beating than the rear wheels, Erickson said.
They decided to drive the rover backward, which would presumably shift the burden to the rear wheels and give the front wheels a rest. Although they thought it wouldn't decrease the overall damage, it would even it out a bit. They tested this theory on their Mars Yard rover.
But to their surprise, the engineers also found an added bonus: Driving backward was actually reducing the total amount of damage, not just shifting the damage from the front to the back wheels.
That's because the "rocker-bogie" system that the rover uses to drive up and over rocks uses the front wheels to put extra pressure on the rocks, sort of like getting a good toehold before you climb, to avoid slipping. But that extra pressure might not be a good idea if it's being applied to sharpened points, over and over again.
"We realized that some of the rocker-bogie characteristics that we like in terms of mobility and our ability to go over rocks – those things work against us," Erickson said.
But since the rocker-bogie system works only in the forward position, driving the rover backward eliminates that damaging pressure, he said.
Of course, this means there's potentially a little more risk of it slipping on certain rocks without that pressure grip, he noted, so the trick is to know when to drive forward and when to turn it backward.
This "Marswalk" was just a test; the rover is currently driving on a soft, smooth sandy route, so there's little need for more backward driving. The rover detoured to this path after the team realized how damaging its previous rocky route was, but it had to brave its longest dune yet, at a spot called 'Dingo Gap,' without getting stuck in the sand. That's what happened to Opportunity at a place called Purgatory and what led to Spirit's ultimate demise in a sand trap at a spot named Troy.
Now, the rover is heading to its next target, about half a mile away: a spot called KMS-9, now named Kimberley after an area in northwestern Australia with ancient rocks. (There's a Down-Under theme to these names, in case "Dingo Gap" wasn't a tip-off.) At Kimberley, the scientists hope to wield the rover's drill if they find the right kind of rock that could yield signs of habitable environments.