SCIENCE

"But it was too round," McFadden said in a telephone interview. If it had been homogeneous, she said, centrifugal forces would have collapsed the light-density material into a more "oblate" shape -- flatter on top, swollen in the middle.

"There's something that's keeping it from collapsing on itself," McFadden said, and the likeliest suspect is water ice, which expands when it freezes. If that is so, Ceres -- only 580 miles in diameter -- may have more fresh water on it than Earth.

-- Guy Gugliotta

Backpack Makes Its Own Power

Anyone who has hiked with a backpack full of loosely packed metal gear knows from the resulting din that the contents tend to move up and down with each step.

Now a team of researchers has invented a pack that takes advantage of that annoying reality. Free to slide up and down on its frame as a person walks, it transforms the energy from that vertical motion into an electrical current strong enough to power a cell phone, an MP3 player and other devices.

The power-generating pack may someday help soldiers, campers and field researchers lighten their loads by cutting down on batteries, say Lawrence Rome and colleagues from the University of Pennsylvania and the Marine Biological Laboratory in Massachusetts.

Scientists have tried for years to capture power from the energy of walking. Incentives have only grown as the number of helpful gadgets has increased, including global positioning systems and scientific and military instruments. But the best systems until now have generated only a few hundredths of a watt.

Portable electronics are increasingly lightweight and energy efficient, but most still require a watt or so. For device-heavy hikers, that means batteries can add up to several pounds -- in some cases accounting for 25 percent of their total pack weight.

Rome's group realized from treadmill studies that a person's hips -- and any backpack along for the ride -- move up and down about 2 to 3 inches with each step. They installed a so-called pinion-gear generator that, like some "self-winding" wrist watches, can transform the energy of oscillating motion into electricity. Depending on weight, the device generates up to 7.4 watts of power -- enough to run a half-dozen or more modern gadgets, the team reported in Friday's issue of the journal Science.

-- Rick Weiss