Building a test planet takes serious engineering. Researchers at the University of Maryland have constructed a 30-ton sphere that spins at more than 90 mph to generate magnetic fields. The 10-ft.-dia. sphere is filled with 13.5 tons of liquid sodium to mimic the Earth’s liquid-iron center core. A 3.3-ft.- dia. stainless-steel sphere inside the larger one counterrotates to approximate the motion of the planet’s solid iron inner core. The action of Earth’s inner liquid produces a magnetic field that makes compasses work, deflects harmful cosmic rays and protects the planet from solar wind. The field reverses every couple of hundred thousand years. By using a model instead of a computer simulation, scientists hope to determine how these reversals occur and predict the next one.