Spirit and Opportunity: A Pair of Really 'Way Out' EVs
By Bill Moore
NASA's twin Mars Exploration Rovers or MERs are the largest and the smartest robot geologists yet sent to the distant planet. The size of earth-bound golf carts, the 384 pound machines -- which only weigh the equivalent of 144 pounds on Mars -- are designed to search for evidence of water on our now desiccated solar neighbor, and hence the possibility of life.
EV World's interest in the mission stems from the MER's electric drive system, which has to be the most "way-out" EV in the solar system. It is propelled by six electric hub motors and powered by solar-generated electricity stored in the rover's lithium-ion battery pack.
We wanted to learn more about these two historic EVs, so we arranged to talk to one of the engineers who helped design and build both machines. Our 40 minute-long discussion is available also in a pair of MP3 audio files, available to EV World Premium subscribers. This is part one of that interview.
It turns out that Kobie T. Boykins is an Omaha native who attended Northwest High School and graduated Cum Laude from Rensselaer Polytechnic Institute in Troy, New York. He works in Pasadena at the Jet Propulsion Laboratory where his official title is Cognizant Engineer for the Solar Array Mechanisms and Structures on the Mars Exploration Rovers Assembly, Test and Launch Operations Engineer (ATLO).
I began our dialog by asking him what it must feel like to have a machine you helped build land on another planet.
"I am still sort of standing on cloud nine," he responded. "It's been an exciting first couple of days here on Sol Eleven that finished last evening. We're really excited [and] looking forward to getting down on the surface here Sol Twelve, which will be this evening, and getting off the lander and staring our roving geologist... Spirit... moving on and starting to do some field geology on the surface of Mars."
The first of two MERs, "Spirit" -- a name suggest by a 10 year girl -- landed safely on January 4, 2004 in the middle of the Gusev Grater, thought by NASA scientists to have once been a giant lake bed the size of the state of Connecticut. Just before our interview, JPL announced that it had located the "exact" position of what NASA calls "Columbia Memorial Station" in honor of the astronauts lost last year in the break-up of the Columbia Space Shuttle. Boykins noted that "exact" is an interesting statement.
"We have a really good idea, plus or minus 15 to 20 meters. That's pretty good in terms of GPS navigation, I guess." From 125,000,000 miles away... yeah, I'd say that's pretty good.
Because of concerns over an air bag that had not fully retracted and could possibly prevent the safe roll-off of the rover, JPL engineers turned the rover around approximately 120 degrees. This not only enabled it to avoid the air bag obstacle, but also pointed it in the direction of its first assignment, a small crater 200 meters or so away. From there, it will then turn roughly east and head for a small range of hills, which have now been identified from images taken on previous missions, about five kilometers away.
In Boykins's terms, that distance is about "a factor of ten" further than JPL designed the rover to travel, so Spirit will probably not reach them. Boykins explained that although the solar panels on the rover can produce enough power to drive the vehicle, its on board research equipment and communications package, the days on Mars are getting shorter and a slow build up of electrostatically charged dust particles will gradually erode the power output of the panels. He added that had the solar panels, which he had a hand in designing, not successfully unfolded and deployed, the rover's batteries would provided only enough power for three days of operation.
Follow the Water
"We may not get there [the distant hills], but it is definitely a goal," Boykins commented. "That's really where the scientists are going, and they are really trying to follow the water and look for signs that would say that water existed there. The reason water is so important is that everywhere we find water, at least terrestrially here on earth, we find life."
In this quest for water and life, NASA's second MER, named "Opportunity" by the same young lady, is scheduled to land on the opposite side of Mars in a region known to have high level of hematite, a mineral formed in the presence of water. Boykins said that region, called Meridani Planum, has a darker albedo, meaning the soil will look darker than that found in the 150 mile-wide Gusev Crater. The Meridani landing will be the first mission to investigate this type of area.
Some Subtle Experimentation
Both rovers are equipped with state of the art investigative tools and instrumentation. The binocular vision of the rover's twin video cameras have the same visual acuity of a human, but with numerous rotatable color filters to investigate different portions of the light spectrum. The vehicle is a rolling laboratory as good as anything on earth, according to one mission scientist.
Two somewhat more suitable experiments will involve the first-of-its-kind attempt at visual contact between the rover and two orbiting satellites. While the Mars Odyssey satellite passes overhead, looking down on the Gusev Crater, Spirit will attempt look up at it. A similar experiment will be conducted with the European Space Agency's Mars Express. The object is to find ways to better understand what is the make-up of Mars' atmosphere.
A second experiment will use the rover's hub motors to lock five of the six wheels, while spinning the remaining free wheel to better understand the soil properties in the Gusev Crater. Ever since NASA discovered what appears to be a soil anomaly created when the lander's air bags retracted, scientists have been theorizing how the "magic carpet" was created. On earth, we'd assume quickly thawed and then refrozen mud that breaks free of the permafrost just below the warm surface. But that presumes liquid water, so the wheel spin experiment will attempt to better understand what is the make up of the soil at this particular landing site.
An Inhospitable Place for An EV
I asked Boykins about some of the challenges of designing an electric drive vehicle to operate on Mars. In his view the first major challenge is dealing with the extreme cold and wild temperature swings of 100 degrees centigrade that daily occur on the Red Planet.
The rover's batteries and research equipment are kept warm in an insulated box beneath the solar array. From Boykins' perspective, the extreme cold poses a challenge in finding a way to mount the solar panels so they will stay in place, while also being able to fold out on landing, what he refers to at "reverse origami".
The lander is a tetrahedron and the 5 foot-long rover had to fit inside it, as did the solar panels.
"The first day that we landed, there was a whole series of things that had to occur perfectly for us to get the first images that evening, and it did." This other-worldly wonder amazed everyone in mission control, Boykins said.
"Almost everybody here was sitting in the room talking to each other saying, I couldn't have written this out on a piece of paper and said that this happened."
For Boykins, he too hadn't expected the post-landing script to play out as flawlessly as it did. "As soon as we got the first beeps back that we were going to get the Odyssey path, which meant they were going to get a transfer through Odyssey back to earth, I started to do a little happy dance, saying the solar arrays are open, the PanCam mast was up and that we were going to get images. It was very exciting. Everything worked just perfectly nominally."
A Bigger "Bug"
Looking down on the rover from the PanCam mast, the vehicle resembles a beetle and I asked Boykins if that was deliberate or just the way the engineering and "origami" worked out. He chuckled and replied that it was a bit of both.
"The six wheel rover starts out because that's a really good mobility platform. It's something that we built on from the Sojourner rover which landed in '97. So we sort of knew that basic functionality was going to be there.
"Then when I started to work the solar array, it was how do we make it fit and still have enough surface area to produce enough power for the vehicle," Boykins explained, adding that the team looked at many different configurations, but the one that seemed to work the best was what he called the "swept wing" version.
"It stowed really well, which was a good think for me and the team I was working with in terms of deploying it. And it sort of gave it a neat look."
So, it turns out the insect-like appearance of the rover -- which Boykins likened to a combination of a preying mantise and a lady bug -- was purely coincidental, though he acknowledges that the team could have been unconsciously mimicking nature.
With the exception of some minor differences in the wiring configuration, the twin MERs are essentially identical, having been built at the same time using both custom and off-the-shelf parts, the latter including the brushed electric hub motors.
Compared to the first successful Mars rover, Sojourner, Spirt and Opportunity are giants. Sojourner was roughly the size of a microwave, he noted, while the twin MERs are "five foot by Shaquille O'Neal," over seven feet wide when the solar "wings" are fully extended. NASA compares them to golf carts.
Unlike Sojourner which was just an extension of the Pathfinder lander in which most of the computer brains were housed, the MERs are autonomous vehicles that carry their own communications system and intelligence, which Boykins likens to the brain-power of a flea.
Still, the MERs are no mean achievement. Whereas Sojourner was an order of magnitude more complex than anything NASA had ever previously attempted to send to another planet or moon, the new twin rovers are yet another order of magnitude greater in their complexity.
Part Two Continued Next Week....
To learn more about the Spirt and Opportunity missions visit: http://marsrovers.jpl.nasa.gov.