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Did You Know?
In Did You Know? the National Geographic magazine team shares extra information we gathered to expand your knowledge of our featured subjects.

NASA Reinvents The Wheel

NASA's "robot geologists," Spirit and Opportunity, pack an unprecedented tool kit of scientific instruments as they search Mars to uncover evidence of the planet's water-drenched past. But if the vehicles can't move efficiently, the spectrometers, grinders, microscopes, and advanced cameras won't be of much use.

That's why engineers went to such lengths to equip the two rovers with wheels that do much more than simply turn. Paddlewheel-like cleats around the outside provide traction on the rocks, dust, and uneven surfaces of Mars. Spokes that spiral inward toward the hub are able to flex, providing shock absorption for the entire rover. Construction is of aluminum, keeping weight of the six-wheeled, 384-pound (174-kilogram) vehicles to a minimum. As a final touch, the 10-inch-diameter (25-centimeter-diameter) wheels even play a role in science experiments. Controllers can immobilize five of the wheels while the sixth spins, its cleats digging a hole several inches deep to allow scientists to remotely study Martian soil strata.

Bet the all-season radials on your car can't do all that.

—Chris Carroll