Sharp rocks on Mars can poke holes into Curiosity's lead wheel. “We’ve had to come up with a whole new way to drive the rover,” said Heverly.
The six-wheeled Curiosity rover exploring Mars’ surface gets its instructions from a lead driver working inside the NASA Jet Propulsion Laboratory in Pasadena, CA.
Earth-to-Mars and Mars-to-Earth communications occur daily, and that’s been the case since Curiosity began performing its various scientific explorations on Mars in August 2012.
“For the first portion of the mission, we actually lived on Mars time. So we’d always show up to work at 5 p.m. Mars time. A Mars day is 40 minutes longer than an Earth day, so sometimes we came into work at 3:00 in the morning because we wanted to be synced up with what the rover was doing,” Matt Heverly, Curiosity’s lead driver, told 2015 SAE World Congress attendees gathered Wednesday afternoon inside the Tech Hub on the main show floor.
The 2000-lb (900-kg) rover gets its navigational and other instructions in a morning uplink, then reports back each evening in a downlink.
“We’ve learned that Mars had a very different past than what it looks like now,” said the 39-year-old who has a bachelor’s degree in mechanical engineering fromCal Poly San Luis Obispo and a master’s degree in robotics from Boston University.
Mars’ history includes a wet environment “with the kind of water that sustains life as we know it. There were lakes. There were rivers. And there were global oceans across Mars,” said an animated Heverly.
The $2.5 billion Curiosity has radiation-hardened onboard computers and 17 stereo cameras supported by metallic structures and components. The wheel’s flexures (springs) are made from titanium, and the wheels are machined from a single, solid piece of aluminum.
“We steer the four corner wheels. The middle wheels aren’t steered, but all six wheels are powered,” said Heverly, noting that each aluminum wheel has a thickness equal to 10 sheets of paper. “We did a ton of testing over very, very sharp rocks, and we didn’t have any damage here on Earth.”
But the terrain on a planet that on average is 140 million mi (225 million km) from Earth has proven to be pretty tough. “The sharp rocks on Mars are part of the embedded bedrock below,” said Heverly, “So the thrust of the other five wheels pushing that front wheel onto that rock is significantly more load than the weight of the vehicle itself.”
Just like Earth’s troublesome potholes can wreck tires and wheels, sharp rocks on the dusty iron planet poked holes into the lead wheel. “We’ve had to come up with a whole new way to drive the rover,” said Heverly. Several changes have resulted, including new control algorithms for controlling the wheels.
Even an experienced rover driver like Heverly admits that it is tricky to avoid what you can’t always see. “We have 25 cm per pixel of resolution from orbit and we’re trying to look for 5 cm tall rocks that are causing this damage.”
The Earth crew uses the vehicle’s stereo camera images to build a 3D map of the terrain around the rover. “We can actually simulate the rover driving through this terrain mesh, so we can avoid the tall rocks that we want to avoid. And we can get it to the science targets that we want to get it to,” Heverly said. In certain situations, autonomous navigation means the rover can decide for itself how to drive safely on Mars.
Curiosity uses a plutonium power source that generates approximately 100 W, which is similar to an incandescent light bulb. But that limited power means a top speed of only 0.1 mph (0.16 km/h).
“We don’t drive fast, but we drive safe,” said Heverly.
The 8.9 ft (2.7 m) long, 8.9 ft wide, and 7.2 ft (2.2 m) tall Curiosity survives 70°C (126°F) temperature swings every Martian day. It frequently navigates large rocks and climbs steep slopes on route to its daily assignments.
“We’ve driven about 10.4 km (6.5 mi) so far on this mission,” said Heverly, who has been a driver for the both the rover Curiosity and the rover Opportunity, which has been active on Mars since 2004.
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