[Public domain], via Wikimedia Commons
I’ve been writing a short blog series celebrating Mars Exploration Rover B (MER-B) Opportunity’s 12th anniversary of its landing on Mars. Opportunity has exceeded its original three-month design lifetime by 48 times, and is still operating every day. In previously entries I’ve covered the hazardsOpportunity faces on hostile surface of the red planet as well as some of the advanced materials science technologies that it uses to survive. In this entry, I’ll look at some of themechanical engineer design choices that have allowed the rover to run its own incredibly slow Martian marathon.
Flexible Drive System
The strange-looking 6-wheeled drive system is one of the most recognizable aspects of Martian rover technology. While size, equipment, and power supplies have changed, the same “rocker-bogie” suspension system has been employed on Sojourner 1997, Spirit and Opportunity in 2004 and Curiosity in 2011. It is a simple system that uses the suspension arms as levers to keep the rover upright, instead of springs or complex active machinery that would wear out more quickly in hostile conditions. Furthermore, with a top speed of 5 cm/s,Opportunity doesn’t need any shock absorbers. The rover’s designers kept the center of gravity as low as possible, to the point that Opportunity could tilt sideways 45° without rolling over.[i]
Each wheel has its own dedicated electric motor, a much simpler and more reliable solution than attempting to implement belts or drive shafts off of a single motor. This redundancy allowed Opportunity’s twin rover Spirit survive after losing one of its front wheel motors. The solution? Just drive backwards, dragging the front stuck wheel behind. Each wheel is also independently steerable, which in combination with the dedicated motors means the rover can be configured to spin on the spot, as visible in Figure 1. Finally, in an inspired bit of multipurposing, the front wheels are actually part of the rover’s scientific tool kit: they can be spun independently while the rover is stationary, using their cleats to dig holes or grind Martian soil for analysis.
Opportunity is equipped with autonomous navigation and hazard-avoidance software, which it employs every time it travels anywhere. Due to the distance between Earth and Mars, the 8-24 minute light-delay means that operators at the Jet Propulsion Lab (JPL) in Pasadena can’t control the rover directly (imaging driving your car if it took 20 minutes for the steering wheel to actually turn the front wheels). Opportunitysports two pairs of stereoscopic cameras, one in front and one on back, to analyze the terrain in either direction. This stereo vision allows the navigation software to calculate distances and heights for any obstacle, exactly as our own pair of eyes do, so that it can map the best route over or around those obstacles.
It’s worth noting that Opportunity’s computer uses a grand total of 128MB of RAM, 256 MB of flash memory, and 3MB of non-volatile backup EEPROM (Electrically Erasable Programmable Read-Only Memory). That may not seem like much, but considering what the space shuttle was able to do with 1MB of RAM[ii] and what Apollo capsules did with 4kB of RAM[iii], the space program has always made do with limited resources.
Figure 3: Opportunity caked in dust (left, Jan 2014) and scoured by winds (right, March 2014)
By NASA/JPL-Caltech/Cornell Univ./Arizona State Univ.
The engineers and scientists operating Opportunity have experienced a number of nail-biting moments during the rover’s long stint on Mars. From bouts with computer amnesia to a dust storm blocking 99% of the sunlight (though the winds also cleaned dust off solar panels, as in Figure 3),Opportunity has continued to weather trials and tribulations of the like that permanently shut down Spirit in 2010. For instance, Opportunity found itself mired in a sand dune in a highly-publicized incident in 2005, near the beginning of its journey. JPL created a simulation of the sandy conditions with their backup rover on Earth, and spent two painstaking months planning and moving the rover a centimeter at a time. Opportunity also had a problematic heater plaguing one arm motor for its entire 12-year stay. Operators discovered on the second day of the mission that the heater would not turn off, though redundant interlock equipment initially kept the heater from frying the motor by disconnecting power when temperature rose too high. Over time, however, the motor degraded and finally stalled in 2008. Engineers studied the motor’s readings for a month, determining the time each day when the motor’s electrical resistance was lowest. Finally, they commanded Opportunity to pump as much current as they could into the arm motor at that exact time, and managed to get the arm extended. The rover has spent the remaining eight years with its arm permanently extended forward, necessitating an upgrade to its navigation software to prevent it from accidentally punching a rock with delicate scientific equipment.
Impressive Engineering, Overall
Opportunity represents an incredible engineering success story. The phenomenal work done by the materials scientists, mechanical engineers, electrical engineers, programmers, manufacturers, and everyone else on the project has provided us with not just a fantastic tool for learning about a neighboring planet, but a great Opportunity for analyzing what makes for good engineering. Any device that can exceed its design operational lifetime 48 times over with no maintenance should stand as a pinnacle of engineering achievement, in my opinion.
[i]: NASA. (2003). Mars Exploration Rover Launches [Press release]. Retrieved fromhttp://www.jpl.nasa.gov/news/press_kits/merlaunch.pdf [ii]: Chien, Philip. Space shuttle technology. Retrieved fromhttp://www.atarimagazines.com/compute/issue132/92_Space_shuttle_techno.php [iii]: The Lunar Module Computer. Retrieved fromhttp://www.abc.net.au/science/moon/computer.htm