NASA EDGE: MSL Mobility
The Mars Science Laboratory
- Jaret Matthews
FRANKLIN: Jaret, you are an engineer with MSL. What kind of engineer are you?
JARET: So, my title on the Mars Science Lab is Mobility Systems Engineer. What that means is I am on the team that has the responsibility for all the parts of the rover involved with driving. That includes the wheels, steering actuators, suspension system as well as the software that helps keep us safe by avoiding obstacles, checking that we’re not going to run off a cliff,etc.
FRANKLIN: There’s a lot of new technology that went into the Mars Science Laboratory, any into the system that you’re working on?
JARET: One of the great things about the fact that the Mars Exploration Rovers that we landed on Mars in 2004 was the fact that they lived so long, and the fact that Opportunity is still going today. The level of sophistication of the software that Spirit and Opportunity landed with was much less than it currently is. And that’s because over the years of operations we’ve been able to continue to write software, improve it, upload it to Mars, test it out on Mars and see it in operations. We’ve benefitted from that kind of continuous development from MMER. And yes, all that code made it into MSL.
FRANKLIN: Does the size of the rover have anything to do with the way that it moves on Mars, because Spirit and Opportunity were substantially smaller than MSL?
JARET: It should, in general, improve our mobility performance. In particular, the size of rock that is a concern to us is larger. A rule of thumb is that the JPL style of rover that we built here is capable of driving over obstacles 1½ times the wheel diameter. So, in the case of MSL, the wheel is ½ a meter, so 1½ times is 75 centimeters. That’s a rock approximately that tall.
FRANKLIN: Pretty nice size.
JARET: That is safe for us to drive over it. In general, we’ll still try to avoid rocks that large but that’s a pretty sizeable rock. In principal, every one of the wheels could be on their own trajectory over an obstacle at the same time and it would still be fine.
FRANKLIN: The style of rim, very sporty, what went in to designing a rim that is so cool?
JARET: As you may have seen there is kind of a hard tire that is machined out of aluminum, and it’s black. That is what we call anodized, so the aluminum has been coated with a special treatment that makes it black. That’s primarily for reducing glint from the sun for our cameras. We don’t want to see reflections coming off the wheels in the camera view. The spokes inside the wheels are actually titanium springs.
FRANKLIN: Yeah, that’s what I’m getting at, right there. Yeah.
JARET: Those are not really for driving. They’re really more for the touchdown event, absorbing that impact on touchdown. Otherwise, they’re really too stiff to be of much help while driving.
FRANKLIN: In the animation, the Sky Crane almost gets Curiosity to the surface but Curiosity could actually be let go a little bit above the surface and drop down.
JARET: Yeah, we could suffer in general about a half a meter free fall. It’s very unlikely that will happen but you can imagine other scenarios where we are driving, maybe over a rock, and the rock rolls or what have you, or maybe we fall off a cliff with one wheel. Those springs will also help us in that event where one wheel smacks the ground after dropping about half a meter.
FRANKLIN: But you probably won’t be falling off of any cliffs because of the avoidance system. Can you tell me a little bit about the cameras that are on Curiosity that help you with maneuvering?
JARET: Sure. We have a couple different types of cameras, mounted low to the ground, what we call the belly pan. Both at the front and at the back are hazard avoidance cameras. Those have big fisheye lenses that allow us to see a wide field of view, including the wheels themselves. They allow us to image the terrain in front of us. And it’s a feature of all of our camera systems that we use two side by side for what we call stereovision. It takes two 2-dimensional images from the cameras and builds up a 3-dimensional map of the world and can determine, hey, this obstacle is too big for me to drive over. I will choose a path that will take me around it. Then, on the top deck of the rover we have what we call our remote science mast. On the top of that mast are what we call our navigation cameras. These are spread farther apart from each other, still a pair. That spread allows us to resolve things in 3-D farther away. That helps us to navigate over bigger distances, whereas the hazard cameras are more for the immediate area around the rover.
FRANKLIN: Will it send back 3-D images that, you know, those of us here on Earth will be able to put on some glasses and actually see?
JARET: Yes, absolutely. It will send back the two images that it used to build up the 3-D map. And exactly as you put it, you can put on 3-D glasses and see Mars in 3-D. The public will be able to do that for sure.
FRANKLIN: So, you’re avoiding obstacles because you’re trying to reach your science objective. And what you said to me a little earlier is that you’re going to be doing a lot of driving. I think I heard a number of 135 meters/hour.
JARET: That’s if you’re continuously running the wheels. It’s about 5 cm/second. But in reality, we don’t really travel that fast even because the software requires us to drive what we call a step, which is usually about ½ meter, then we stop; we reimage the area; we crunch the data; make sure there’s no hazards and then proceed again.
FRANKLIN: Another ½ meter?
JARET: Exactly, yeah. If the terrain looks relatively benign, it will choose to skip steps, maybe skip two or three steps before it reimages again but if you’re in really rocky, undulating terrain, it will likely image every step. That’s where we go from 135 meters that we can drive in an hour, if we’re just going all out…
JARET: …to really only achieving about 150 meters a day because we can’t operate for 24 hours. We have to go to sleep at night, let the batteries recharge and we have to do this stop and think maneuver as we’re advancing.
FRANKLIN: We wish you all the best with your mission and engineering with MSL. We look forward to hearing some follow-up stories about what’s going on.
JARET: Thank you.
FRANKLIN: Thanks, Jaret.