Meet the Rovers

Spirit and Opportunity are squat and sturdy, as large as golf carts and almost as tall as an adult (plate 3). Each weighs 400 pounds.18 They have six wheels, each with its own, independent motor. Four – wheel steering allows them to make tight turns and swerve or turn on a dime. The design is based on the “rocker-bogie” system of the previous Sojourner rover.19 The wheels can swivel in pairs and pivot vertically to maintain overall balance. The saddest thing that could happen to a very expensive rover millions of miles from home would be for it to tip over onto its side or back, wheels spin­ning uselessly. So Spirit and Opportunity have a suspension system that balances the load any time a wheel goes up or down. As a result, the bodies of the rovers experience half the range of motion of the wheels and legs. The rovers can tilt up to 45 degrees without overturning, but software is designed to sound the alarm any time the tilt exceeds 30 degrees. The wheels have cleats for gripping in soft sand and scrambling over rocks.

In case you had a mental image of a NASA engineer, cap turned sideways, slamming a joystick from left to right as the rover ca­reens over the Martian dunes, the truth is a bit more sedate. The rovers have a top speed over smooth ground of 2 inches per sec­ond, or 1/10 of a mile per hour. But even at that speed, there’s no danger of recklessness; hazard avoidance software is used that makes each rover stop and evaluate its progress every couple of seconds, reducing the true speed to a glacial 1/50 mph. On Earth, the rover might get overtaken by a snail.

The body of the rovers is the warm electronics box, a tough outer layer designed to protect the computer, electronics, and batteries—the rover’s brains and heart. These vital organs must be buffered from the worst extremes of the Martian climate. Tem­peratures at the landing sites can plunge from a daytime high of 70°F to a nighttime low of -150°F, a range many times larger than we’re accustomed to on Earth. Engineers use gold paint, heaters, and insulating aerogel to keep the rovers in a comfort zone. Within the warm electronic box, the “brain” of each rover is a computer that receives data from the scientific instruments and relays it to one of the Mars orbiters and then on to Earth. Spirit and Oppor­tunity are exquisite machines, but they’re far from state of the art in terms of processing power and data transmission rate. Moore’s law—the near doubling of computational power every eighteen months—marches on, but the hardware on the rovers had to be tested for the severe conditions of space and “frozen” long before launch. So each rover’s brain has only 128 Mb of RAM, and the data rate to the orbiters is 128 kb per second. That’s the equiva­lent of a very low-end netbook and only twice as fast as a dial-up modem. If you have an iPhone, it easily eclipses the computational capabilities of both rovers.

The beating heart of the rovers is their solar panels. Mars re­ceives half the intensity of sunlight that the Earth does, and it’s a real challenge to gather enough energy to power the rover and its suit of instruments. Each rover can function on 140 Watts, the equivalent of a standard light bulb. The deck of each rover is tiled with solar panels which charge batteries within the warm electronic box. As the Martian winter approaches, there’s insuf­ficient power for driving, so each rover navigates to a north-facing slope and hunkers down for the months in which the tempera­ture remains below freezing. This imperative has always been greater for Spirit, which is much farther from the equator than Opportunity.