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Built to last Well beyond their ‘best by’ dates, Spirit and Opportunity continue to collect data from the surface of Mars. By Graham Pitcher.

hen NASA sent the Spirit and put together and flown in Pathfinder. Opportunity Rovers to Mars in There were similar sensing elements, a W2003, the plan was for the camera and in situ investigation vehicles to operate for 90 days; anything equipment. It was just a matter of more would be a bonus. But six years putting it all together on a new platform.” later, the Rovers are still working; even if NASA had wanted to include sample one of them is currently bogged down in storage, but that element dropped from the equivalent of a Martian sand trap. the mission because there wasn’t Bearing in mind that Mars is not the enough funding. friendliest of environments for such Matijevic said the Rovers ‘decide’ devices to work in, what was so special what to look at using input from their about their design? Why are the Rovers remote sensing instruments. “They work still in action? out what looks like an interesting target. Jake Matijevic, chief engineer for the Once in position, local instruments – Mars Rovers with NASA’s Jet Propulsion such as a spectrometer and an alpha Laboratory (JPL), said there is no particle analyser – are deployed and difference between the two vehicles. work together.” “They’re identical in that they were built These operations are controlled by a using the same environmental central computer. The computer in each guidelines. They have the same payload Rover features a 32bit Rad 6000 and were built together.” microprocessor – a radiation hardened What was the top level design version of the PowerPC processor Matijevic: for; protected from the environment.” contstraint? “Mobility,” said Matijevic. running at 20MIPS. Onboard memory This was done by building an “Basically, we created a drive system, includes 128byte of ram, as well as “we almost insulated core box with heating ability, suspension and control system, then built 256Mbyte of flash memory and smaller never have a creating an environment – an acceptable the vehicles based on what can effectively amounts of other non volatile memory, temperature range – within which the be done on the surface of Mars.” which allows the system to retain data chance to electronics can function. “This required a Beyond that are a number of other even without power. lot of ‘give and take’ with the initial constraints, led by how much each Rover Once the target has been identified, design from sizing,” Matijevic recalled, “to allow what can weigh – both weigh 185kg – and the the way in which it is explored has to be scratch. you think will be the end requirement.” physical volume that is available in the organised. “Decisions are made by a Other design issues included how to launch platform. Finally, and just as central sequencer,” Matijevic explained. Instead, we wire up individual components and important, the payload is determined by “Some instruments can work in parallel use tried and where to put the support electronics. “As the way in which the Rovers would be but, to a large extent, it’s a matter of we went through the design phase,” he landed on Mars – both devices were doing one thing and then another.” tested continued, “we kept checking to see if landed using an airbag structure. Working out what electronics could components.” we met not only mass and volume The mission required vehicles that be fitted within the Rovers was constraints, but also those of power. could explore a small area around their determined by what Matijevic called the Once you get through the critical design landing site and to perform some local ‘overriding’ volume constraint. “The evaluation, you have an idea of what can analysis. “In a way,” Matijevic continued, electronics have to fit amongst the other be built and where it can fit.” “the mission was a sequel to what was constraints and they have to be cared The Rovers were created in three

20 12 January 2010 www.newelectronics.co.uk P020_NELE_JAN12.qxp:Layout 1 6/1/10 12:27 Page 21

Special Report: Space Electronics

precise configuration is specialised and the system wouldn’t be producing there’s quite a bit of building support enough energy after six months.” equipment,” he noted. “It’s a leap of faith Power is stored in lithium ion in some senses, but we’re tweaking batteries: a crucial improvement over designs that have been used in previous previous approaches. Matijevic said: “It systems. Flying VME cages had been was a new technology at the time and done, so the design was available and had shown good charge/discharge adaptable.” characteristics. It has proved very robust It’s also interesting to discover that and is one of the reasons why the redundancy is not always an issue. “Most mission has lasted. The batteries are still of the Rover’s systems are, surprisingly, capable of charging and discharging.” single string,” Matijevic said. “We had a Communications are another element lengthy debate about where redundancy picked up from Pathfinder. The Rovers would be acceptable, but the decision was can ‘talk’ directly with Earth using X band that, for a three month mission, saving communications. “We put the antenna on mass was more important.” a kind of paddle,” Matijevic continued, The Rovers are solar powered. Both “allowing a full range of coverage, have strings of GaAs panels, producing at regardless of the vehicle’s tilt and best, said Matijevic, 1kWhr from a day’s orientation.” This is supplemented by a exposure. “We went through a process in uhf system that relays data to satellites which we tried to understand what the in Mars orbit, which then transmit to mission performance would be during Earth. “We didn’t have an opportunity to the different Martian seasons. We try out the uhf system before launch,” he thought there was a reasonable chance said. “But everything worked fine when that the Rovers would have enough they got to Mars.” energy to get through their 90 day “We’re taking advantage of the mission and be good for another three heritage of other systems with similar months after that. But we didn’t know functions and similar parts. We’re what it would be like to go through a full somewhat driven by time, which means Martian year. Over time, dust we almost never have a chance to accummulates on the solar panels, which design from scratch. Instead, we use reduces energy production. Even using tried and tested components,” Matijevic Photographs: David Butow conservative assumptions, we thought concluded. Jake Matijevic, chief years. “That’s a very intensive period,” engineer for the Mars Matijevic noted, “where all the pieces Rovers at NASA’s Jet Propulsion Laboratory, have to be put together.” with a model of the Interestingly, the Rovers featured a craft in one of the Rover lot of previously designed electronics. control rooms. “We often select from components that have already been built and used,” he observed. “For example, the computer was built for the Pathfinder mission in the 1990s.” Then there’s the question of the general support electronics. Surprisingly, this comes in the format of VME boards housed in a card cage. With the high levels of vibration involved in a rocket launch and a controlled, but hard, landing on Mars, one might think that board might come loose. And Matijevic pointed out that, as far as possible, it’s a COTS operation. “The