How to Drive a Mars Rover, Part 2

Scott Maxwell, using his 3-D simulation software. Courtesy Scott Maxwell

The Mars Exploration Rovers have been traversing the surface of the Red Planet for almost five years now. But how exactly are the two rovers, Spirit and Opportunity, “driven” from Earth, about 150 million km away? Many of us might have visions of joysticks, similar to what are used for remote control toys, but it’s not like that at all. However, being a “Rover Driver” is one job where having experience with video games and simulation software looks good your resume! Scott Maxwell is one of fourteen rover drivers, or planners as they are also called, who last week gave us an update on Spirit and Opportunity’s status. Today, Scott provides the details of how to drive a Mars rover.

“The way we wished it would work,” said Scott in a phone interview from JPL, “is if we could have a joystick where if we pushed forward on the joystick the rover would go forward, or push back and the rover would stop. But, with lag time delays in the radio signals, you would push forward on the joystick and 10 minutes later the rover would get the signal to go. But on Earth, you won’t know if it worked for another 10 minutes after that because of the time it takes for the signal to get back to you.”

This would create a nightmare in logistics, planning and operations, because the drivers can’t “see” what the rover is doing in real time. So instead, the rover drivers work the Martian nightshift.

Recent view from Opportunity's hazard camera.  Credit: NASA/JPL/Cornell
Recent view from Opportunity's hazard camera. Credit: NASA/JPL/Cornell

“We take advantage that our solar powered rovers have to shut down for the night,” said Scott. “So as the sun is going down in the Martian sky, the rover is commanded to take pictures of the world around them and send it before they go to sleep. When we get that data back on Earth, we go to work. We take all the images and put them into a simulation. We have a 3-D simulation world — kind of like a video game — on our computers. Then, we have a simulated rover that we put down in that 3-D world and we drive that rover around instead.”

So in that 3-D world, the rover drivers can test every possibility, make all the mistakes (tip the rover, get stuck, drive off a precipice, crash into a big rock) and perfect the driving sequence while the real rovers are dozing securely and peacefully. This certainly has helped with the long life the rovers have led, as in five years the rover drivers have safely and successfully guided the rovers to drive in and out of craters, climb a challenging hill, and put on more mileage than anyone ever thought possible. The biggest driving calamity has been getting stuck in a sand dune, but now the driving team has a few tricks up their sleeves to avoid that (see Part 1).

So then, when the drivers perfect the commands inside the simulation and hone the exact sequence of movements for the rover, they upload those commands and send it to the real rover. Then as the sun is coming up on Mars, the rover wakes up, receives a communications uplink from Earth, processes the commands and it goes to work while the rover drivers go to sleep. “At the end of the rover’s day, it sends us more pictures and data, and we start the cycle all over again,” Scott said.

Rover test bed.  Credit: JPL
Rover test bed. Credit: JPL

If there’s a particularly difficult situation, such as how much tilt can the rover withstand without tipping over, a test rover can go through the same sequences in a simulated Mars environment out in JPL’s Mars Yard.

Back in 2004 during the “prime mission,” the first three months of the mission (the original length of time the rovers were slated to last) everyone who worked with MER lived on Mars time. Since the two rovers are on opposite sides of the planet, that meant operations going on 24 hours a day. And since a Mars day is 40 minutes longer than Earth’s day, that meant a perpetually shifting sleep/wake cycle, a difficult situation where your body continually feels “jet-lagged.” But now that the mission has been ongoing for such a long time, the planners operate in a more Earth-normal mode and even take some weekends off. But still, a planner’s eight- hour shift can start anywhere from 6:00 a.m. to noon.

So what’s an average drive for the rovers? “It varies widely,” Scott said, “but an average drive is in the neighborhood of a few meters.” Right now Spirit is struggling her way up the side of “Home Plate,” a low plateau, which for a rover is a steep hill. The crumbly soil gives out beneath her wheels as she makes the climb, making it difficult to drive father than a few centimeters in a day. Plus, Spirit is dealing with low power levels from dust-covered solar panels, providing limited energy for any big drives. Just after a recent dust storm, Spirit’s solar panels were producing only 89 watt hours, which is about the energy needed to run a small light bulb for an hour and half.

Spirit's dusty solar panels.  Credit:  NASA/JP
Spirit's dusty solar panels. Credit: NASA/JP

But Opportunity’s power levels are much better, and she recently had drives as long as 216 meters, as she puts the pedal to the metal in an attempt to reach Endeavour Crater, about 12 km away.

Some of the rover drivers work mainly with one rover (Steve Squyres has said it’s easy to get attached to one rover or the other, depending which one you’re working with) but Scott goes back and forth between the two. “That’s in part because I’m a team lead, and part because I’m the kind of person who wants to run around and be part of everything all the time!” he said. When we talked with Scott last week, he was working with Spirit, and thought that this week he will probably do a drive or two with Opportunity.

Currently Spirit’s total odometry is at about 7,530 meters (over 4.6 miles), while Opportunity’s odometer reads almost 14,000 meters (about 9 miles).

JPL has some wonderful videos of the rover’s treks, travails and progress, and you can keep track of the rovers’ progress by checking for regular updates on the MER website.

Tomorrow: What have you been doing the past five years? Scott Maxwell shares what five years of driving the Mars rovers has been like.

How to Drive a Mars Rover, Part 1

How to Drive a Mars Rover, Part 3

How to Drive the Mars Rovers, Part 1: Rover Updates

Rover Driver Scott Maxwell with a model of MER. Photo courtesy Scott Maxwell

In January of 2004, NASA’s twin robot geologists, the Mars Exploration Rovers Spirit and Opportunity, landed on the Red Planet. During those nearly five years, the rovers have returned hundreds of thousands of images and enough data to keep scientists busy for decades. But almost nine years ago, Scott Maxwell started working on developing software and techniques for driving the rovers around on Mars surface. Today he’s the Mars Rover Driver Team Lead for MER at JPL, and he says that every day of working on this mission has been incredible. “It’s been an amazing experience,” he said, “and I like to say it’s the best job on two planets.” To celebrate the upcoming fifth anniversary of the rovers on Mars, Universe Today caught up with Scott to get an update on the current status of the two rovers, to find out what the five-year MER mission has been like for a rover driver, and to ask the pressing question, just how do you drive a rover from 150 million kilometers away?

Both rovers have been inactive recently because of solar conjunction, where the sun is between Earth and Mars, which makes communications difficult because the amount of radio noise generated by the Sun. So, when I talked to Scott on Wednesday of this week he was just working on the commands that would be sent to Spirit for the first drive she has taken since several weeks ago. So how is Spirit doing these days?

“Spirit is struggling valiantly to climb up the north face of Home Plate,” Scott said. “As you know, we’ve just come out of solar conjunction, and so we’re picking up where we left off on Spirit’s climb up the face. Her solar array energy levels are not as good as they were before the mini-dust storm we had before the conjunction, so that’s obviously a cause for concern. It’s unfortunate because that means we have less energy for driving. But she’s still alive and that’s a lot better than what we thought she’d be five years into the mission.”

Home Plate is the raised plateau.  Spirit is the dark spot at the 1 o'clock position.  Image: NASA/JPL/University of Arizona
Home Plate is the raised plateau. Spirit is the dark spot at the 1 o'clock position. Image: NASA/JPL/University of Arizona

Home Plate is a low plateau about 80 meters (260 feet) in diameter. Spirit spent the Martian winter parked on the north side of the plateau with her solar panels slanted towards the low sun in order to stay alive. But Spirit’s solar arrays are severely dust-covered, decreasing the amount of power available for science activities and driving. But the scientists and engineers haven’t given up on Spirit, and still have big plans for her.

“Our longer term goal is to head south from Home Plate to a pair of features called ‘Goddard’ and ‘Von Braun’,” said Scott. “Von Braun is a hill and Goddard is a crater-like feature next to it, and that’s the next area we’d like to explore. As you know, the area around home plate appears to be a region of past hot-springs or volcanic fumarole activity, the kind of place where life might have formed on Earth, so it makes it a particularly exciting place to explore on Mars, as we try to find out more about what was going on here.”

But ‘Goddard’ and ‘Von Braun’ are on the south side of Home Plate and Spirit is on the north side. The easiest route would be to “climb back up on the top of Home Plate and kind of skate across it where the driving is good” Scott said, but if Spirit isn’t able to make the climb, they will drive down the north slope and go around Home Plate the long way. But that might take more time, and time might be getting limited for Spirit.

Bonestell panorama, taken by Spirit during her winter stay on the north side of Home Plate.  Credit:  NASA/JPL/Cornell
Bonestell panorama, taken by Spirit during her winter stay on the north side of Home Plate. Credit: NASA/JPL/Cornell

So, the shortest way is up and over Home Plate. But Spirit has a bum right front wheel, and is trying to climb up some difficult terrain. “Imagine you’re in the desert, climbing up a sand dune, but every step you take the sand crumbles out from beneath you,” said Scott. “That’s what Spirit is experiencing. So even though we’re commanding the wheels to go several meters, she might only make a few centimeters of progress in a sol (Martian Day).”

But the driving team will keep trying, as ‘Von Braun’ and ‘Goddard’ are of interest to the science team.

Opportunity, on the other hand, is in very different driving conditions. “Right now she’s basically on a parking lot, with only a couple of speed bumps every once in awhile,” Scott said. “Opportunity can drive 100 meters a sol, like the length of a football field every day, without breaking a sweat. We recently had a nearly record-setting drive, with Opportunity where we drove nearly 216 meters in one day,” Scott said proudly. “So that’s our silver medal drive, our second longest drive ever with either of the rovers.” (The longest drive was 220 meters in one day.)

One thing Opportunity does have to watch out for is sand dunes in the region. In 2005, Opportunity became stuck in one of those dunes, and it took the rover driving team over a month to figure out how to maneuver Opportunity out of the sand trap, called Purgatory Dune. In honor of the difficulties and lessons learned from getting stuck, all the potential sand traps in the region are called “Purgatoids.”
The "Purgatory" dunes around Opportunity.  Credit: NASA/JPL/Cornell
“Opportunity is in a region where Purgatiods are all around her.” Scott said. “But the good news is that we have better data now, than we did when we first encountered these features.” The MER team now has the benefit of the Mars Reconnaissance Orbiter’s HiRISE Camera in orbit around Mars, looking down at — if not watching over – the rovers and their activities. “So we have the data and images from HiRISE, and we think we have identified a way to pick out these Purgatoids from orbit.” Scott said. “So we take the images from MRO, and use them as part of our path planning for Opportunity every day, and also for our longer scale path planning. On top of that we have other measures we have adopted after that first Purgatory incident, where the rover stops every once in awhile and ‘checks’ itself, gauging whether it is actually moving or if it is stuck and the wheels are just spinning. So even if we get into a Purgatoid, we’ll be able to catch it before too long and have the chance to get ourselves out before we dig in too far.”

But so far, with the new technique of being able to identify Purgatoids from orbit, Opportunity hasn’t run into a single one.

Opportunity's traverse map through Sol 1716 As of sol 1707 (Nov. 11, 2008), Opportunity's total odometry was 13,493.85 meters (8.38 miles).
Opportunity's traverse map through Sol 1716 As of sol 1707 (Nov. 11, 2008), Opportunity's total odometry was 13,493.85 meters (8.38 miles).

“It makes us happy to put the pedal to the metal and just drive,” Scott said, “It’s a lot of fun.”

Opportunity is “putting the hammer down” to reach a crater about 12 kilometers (7 miles) away called Endeavour. The huge crater is 22 kilometers (13.7 miles) across, and scientists expect to see a much deeper stack of rock layers than Opportunity saw while she was in Victoria Crater the past two years. The 12 km driving distance would match the total distance it has traveled from 2004 to mid-2008. Even at the 100-meter plus pace each sol, the journey could take two years.

But Scott Maxwell and the 13 other rover drivers working on the MER mission are up for the challenge.

Tomorrow: Part 2: Just how do you drive a rover on another planet?
How to Drive a Mars Rover, Part 3