Curiosity rover meets mission success milestone
BY STEPHEN CLARK
Posted: June 26, 2014
Scientists promised fresh discoveries and engineers redoubled efforts to minimize wear and tear on the Curiosity rover's banged-up wheels as the $2.5 billion mission this week marked one Martian year since landing on the red planet in August 2012.
"This is just another day on Mars, other than the fact that we can now celebrate and tick off the boxes for mission success, including the fact that we did manage to survive for a full Martian year," said Jim Erickson, the rover's project manager at the Jet Propulsion Laboratory in Pasadena, Calif. "It looks like, from all the progress we're making, we're going to be very productive in the future."
Engineers designed the rover from the outset to function for one Martian year in its quest to find the elements essential for life and identify an environment that was once habitable on ancient Mars.
The mission's chief scientist, John Grotzinger, declared last year that Curiosity was a success, at least from a scientific perspective.
Curiosity found the ingredients for life after visiting an ancient riverbed at a site called Yellowknife Bay within months of landing.
The rover's on-board instruments tasted powder extracted from a slab of mudstone and found an array of elements supportive of life.
"You add it all up, and the presence of minerals in various states of oxidation would provide a source of energy for primitive biology," Grotzinger said last year.
Tuesday's milestone satisfied the army of engineers that designed, built and tested the rover to ensure it survived the radiation, wind-blown dust, and other hazards on the Martian surface for one orbit of the sun.
Under the command of ground controllers, the six-wheeled rover is driving across the floor of Gale Crater toward Mount Sharp, a three-mile-high peak scientists believe harbors clay minerals deposited when the red planet was warmer and wetter. Researchers hope to find organic molecules embedded in the rocks, a discovery that has eluded the rover so far.
"We're really moving out to get to the slopes of Mount Sharp," Erickson said in an interview Tuesday. "We're doing very well in that regard. In the last few weeks, we've done about a kilometer and a half toward there, and we've only got about 3.8 kilometers (2.4 miles) to go."
Erickson said Curiosity should arrive at the base of Mount Sharp in two or three months.
The rover paused its journey in May to collect another sample with its hammering drill from a sandstone site called Windjana. The robot's mineral-sniffing instruments, mounted inside the rover's main body, are still analyzing the make-up of the sample.
"Windjana has more magnetite than previous samples we've analyzed," said David Blake, principal investigator for Curiosity's Chemistry and Mineralogy, or CheMin, instrument at NASA's Ames Research Center. "A key question is whether this magnetite is a component of the original basalt or resulted from later processes, such as would happen in water-soaked basaltic sediments. The answer is important to our understanding of habitability and the nature of the early-Mars environment."
According to a NASA press release, scientists say the Windjana sample contains a "more diverse mix of clay minerals than was found in the mission's only previously drilled rocks, the mudstone targets at Yellowknife Bay."
The sandstone also contains the mineral orthoclase, a potassium-rich feldspar abundant on Earth but never before definitively detected on Mars, the press release said. The presence of orthoclase suggests Curiosity's landing site at Gale Crater may have underwent multiple episodes of melting.
"It's too early for conclusions, but we expect the results to help us connect what we learned at Yellowknife Bay to what we'll learn at Mount Sharp," Grotzinger said in a press release. "Windjana is still within an area where a river flowed. We see signs of a complex history of interaction between water and rock."
At the same time, engineers on Earth are testing the same type of wheels on Curiosity to understand how the damage affects their traction. One of the first tests checked the wheel's response to driving across loose granular soil, dense high shear strength soil and a bedrock-like material, according to NASA.
"It's a little complicated because we're picking our paths, so the lifetime depends on how successful we are with that," Erickson said. "We're trying to figure out a way to characterize different terrain types. As an example, on the worst terrain type, what's the wheel life if you drove 100 percent on that? What's the wheel life if you drove 100 percent of the medium version? What's the wheel life if you drove 100 percent on really nice, soft sand that doesn't leave any damage?"
The wheels were designed to drive at least 20 kilometers, or about 12 miles.
"Once they tested it to 20 kilometers, they said, 'OK, we're done. We can meet that requirement,'" Erickson said. "Unfortunately, nobody planned on finding this really rough embedded rocky material that tore of the wheels so well."
The results of the first test showed the wheels should last 25 kilometers, or about 15.5 miles, according to Erickson.
"We think it's still driveable, and it still works fine, but looking at what it looks like after that 25 kilometers, I wouldn't count on it going much farther than that," Erickson said. "That was our first guess at looking at a conservative bad terrain. We think we've got more than that, but we've got a bunch of other tests we're doing right now, including ones where we try to do the worst, middle and best [terrain] so we can come up with a formula for what it really means based on what we're actually going to have to drive on on Mars. It really does depend on what terrain you're on as to how much you can drive."
Curiosity has logged about 7.9 kilometers, or 4.9 miles, of driving since landing. With the advent of software to help autonomously guide the rover, Curiosity has extended its daily range.
Last week, the rover drove 143 meters, or 469 feet. It was the longest drive in a single day since Curiosity's landing.
While the wheels bear close watching, Erickson said the rover's ground team must remain vigilant against other risks as Curiosity enters an extended mission.
"We're focused on keeping the vehicle safe," Erickson said. "That's the bottom line. We spend a lot of time and effort not just resting on our laurels but finding ways to make it even safer, so we'll eliminate the possibility of more mistakes."
NASA will soon decide whether to approve the Curiosity mission's request for another two years of operations on Mars. Money currently runs out Oct. 1, Erickson said.
"The wheels, for example, there are scenarios where it got so bad you could lose mobility and essentially the mission," Erickson said. "However, the speed at which that happens is very slow, so if you're beginning to do something wrong, you can continue mitigations along the way. The high-consequence command error is one that you don't have that luxury. If you make a really bad mistake, it's likely to be all at once and the consequence happens rapidly. There's no chance of saying, 'Oh well, we made that mistake but it only destroyed the vehicle a little bit and we go from there.'"
Officials monitor the rover for mechanical failures, where moving parts introduce the inherent risk of failure. Many of Curiosity's suite of 10 research instruments also have finite lifetimes, some driven by the decay of radioisotopes used to help make measurements.
"It will wear out, and the question is exactly when and what you can do to postpone it and get the most science you can," Erickson said. "The bottom line is we always try and take an asset on the surface and use it up, getting the best science we can. The fact that eventually things break is, in a way, a good thing. It shows you used the vehicle and you didn't leave anything on the table. It would be a crime to go to Mars and end up with a vehicle that was completely pristine, but you didn't get much science. That's just the wrong way to do it."
At more than 2,000 pounds, Curiosity is the heaviest rover ever put on Mars, weighing about five times more than the Spirit and Opportunity rovers landed in 2004.
Spirit and Opportunity never suffered wheel damage, but they did become trapped in sand dunes. In Spirit's case, the clutch of a sand trap led to the rover's demise.
"We were really focused on making sure we were not getting into sand and getting stuck," Erickson said. "When the wheels started deteriorating rapidly, that was just out of left field. Nobody expected that to be any kind of an issue."
Officials say NASA's next Mars rover, scheduled for launch in 2020 and based on Curiosity's design, will likely switch to a different wheel design to avoid the damage encountered by Curiosity.
Engineers working on the European Space Agency's ExoMars rover, set for liftoff in 2018, are also watching Curiosity's wheel woes.
"We don't want that to happen to us," said Paul Meacham, ExoMars systems engineer at Airbus Defence and Space, builder of the ExoMars rover.
MDA Corp. is supplying wheels for the ExoMars mission. The wheels will be made of bendable steel or titanium, not aluminum like Curiosity, Meacham said.
The ExoMars rover is bigger than NASA's Spirit and Opportunity rovers, but much lighter than Curiosity, weighing about 300 kilograms, or 660 pounds.
"We don't put the same force through the wheels," Meacham said in an interview in February. "Also, because they're a bigger vehicle with bigger wheels, they might attempt something where they drop farther off a rock, whereas we just wouldn't attempt to go over anything that big."
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