Starshine 3 is a 200-pound, 3-foot-diameter glittering ball that was shot into orbit by NASA on Sept. 29th. The satellite is studded with 1,500 student-built mirrors and is designed to study Earth's atmosphere by falling through it.

"We have very few direct measurements of the density of the upper atmosphere. That's why Starshine 3 is so important," says solar physicist Judith Lean of the Naval Research Laboratory in Washington, D.C. "The satellite's well-defined spherical geometry allows us to estimate its ballistic coefficient with a reasonable degree of certainty. As a result, by monitoring Starshine's orbit and studying how that decays we can calculate the density of the gas that's dragging it down."

The satellite is orbiting 292 miles (470 kilometers) above Earth in a region scientists call the thermosphere. That's where the International Space Station (ISS), the space shuttle, and many other satellites orbit.

The air at that altitude is very thin, but it still exerts drag on spacecraft (the ISS has to be periodically boosted).

So the atmosphere is dragging Starshine 3 to Earth, too. It drops a few meters during each 90-minute orbit around Earth.

"Starshine 3 will slowly descend during the next 4 years," said Gil Moore, the director of Project Starshine.

As it sinks into ever-denser layers of the atmosphere, its rate of orbital decay will accelerate. Eventually, when it sinks below the stratosphere, Starshine 3 will burn up completely. If re-entry happens at night, a dazzling fireball would result, casting enough light for onlookers to read a newspaper.

There's no danger to anyone on the ground, Moore said. The craft is made mostly of aluminum, which will vaporize during the fiery decent.

"We designed the satellite so that it will be 100 percent consumed about 80 kilometers up," Moore said. "We had no choice. Otherwise I was going to have to buy a $100 million insurance policy with a $50,000 premium."

Starshine 1, a cousin of Starshine 3, vaporized in Earth's atmosphere last year. Scientists are using data from that mission to predict the decaying trajectories of satellites and bits of orbiting space debris.

"We're hoping that Starshine 3 will improve those predictions even more," Moore said.

Starshine 3 is visible under dark skies. Students, teachers or amateur astronomers are invited to report their observations here.

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