BUILDING A TEST PLANET

30-Ton Metal Sphere Spins for Magnetic Fields

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.

Stats of Spin

Little Earth: Both spheres are driven by 350-hp electric motors. At full speed the outer sphere spins at 240 rpm; the inner sphere reaches 960 rpm.

Dangerous Metal: Sodium is highly conductive and melts more easily than iron, but it can explode if touched by water. (The lab in Maryland has no sprinklers.)

Fuel Metal Jacket: The corrugated half-pipes around the sphere are filled with oil, not water or steam, that heats the sodium inside to its melting point, 207.9 F.