Snowballs from space.

Heads up earthlings! Thousands of house-size snowballs are bombarding our planet every day from outer space.

That's the theory posed by Louis Frank, a physicist at the University of Iowa. He estimates that these cometlike objects have been vaporizing, or changing into water vapor, in Earth's upper atmosphere for 4.5 billion years. They shower the planet with water that eventually becomes lakes, seas, and oceans!

Frank first detected the cosmic snowballs on satellite images taken more than a decade ago. At first, he thought the specks were just "visual static." But recent images from NASA's new Polar satellite orbiting Earth captured similar specks "streaking" toward the planet. The tails of these tiny comets (about 12 meters, or 40 feet, in diameter) seemed to show traces of oxygen and hydrogen--the elements that make water. Frank has calculated that about 43,000 "snowballs" enter Earths atmosphere each day.

But don't worry about being pelted by a cosmic snowball. The comets break apart at 372 to 9,300 km (600 to 15,000 miles) above Earth. Heated by sunlight, "they dissipate into huge water clouds that get mixed up with our atmosphere." Frank says. Eventually, the water rains down and collects in lakes and oceans--about an inch of new water every 10,000 to 20,000 years.

But many scientists think Frank's theory just doesn't hold water. First, they say, the solar system is very dry. Where would space snowballs form? And why haven't scientists detected the tiny comets pelting the Moon, or nearby planets like Mars and Venus?

In addition, Frank's snowball theory challenges conventional earth science, which holds that a fixed amount of water continually circulates between Earth and its atmosphere. By suggesting that water from space is filling the oceans, Frank would force scientists to rethink the water cycle.

But that's the way science goes, comments William Bottke, a planetary scientist at the California Institute of Technology. "Someone puts out an idea and the rest of the scientific community debates whether its valid." As for Louis Frank, he believes his theory is rock--uh, ice--solid.

RELATED ARTICLE: Space Snowball Theory

1. Every day, thousands of icy snowballs as big as houses come flying toward Earth from space.

2. When the snowballs are about 600 to 15,000 miles above Earth's surface, they begin breaking into small pieces.

3. In Earth's atmosphere, sunlight melts the pieces into drops of water. They mix with clouds that produce rain.