ICESat measures the Earth's polar ice sheets, clouds, mountains and forests with three lasers. Crisscrossing the globe at nearly 17,000 miles per hour, ICESat provides unprecedented accuracy in mapping Earth's vertical characteristics, enabling scientists to see objects on Earth in three dimensions.

The principal mission of ICESat is to measure the surface elevation of the large ice sheets covering Antarctica and Greenland. Measurements of elevation-change over time will show whether the ice sheets are melting or growing as the Earth's climate undergoes natural and human-induced changes.

The instrument on ICESat that makes the claim is the Geoscience Laser Altimeter System (GLAS) instrument. GLAS fired its one billionth shot last Friday morning, November 18.

"One billion shots equal one billion elevation measurements around the world of land, sea clouds, etc." said Waleed Abdalati, head of the Cryospheric Sciences Branch, Hydrospheric and Biospheric Sciences Laboratory, at NASA's Goddard Space Flight Center, Greenbelt, Md.

"That number tops the 6.5 million shots from Mars Orbiter Laser Altimeter, an instrument currently in orbit around Mars on the Mars Global Surveyor spacecraft," he said.

GLAS sends short pulses of green and infrared light though the sky 40 times a second, all over the globe, and collects the reflected laser light with a one-meter telescope.

It also fires a fine laser beam of light that spreads out as it approaches the Earth's surface, to about 65 meters in diameter. On its way to the surface, those photons or particles of light bounce off clouds, aerosols, ice, leaves, ocean, land and more, providing detailed information on the vertical structure of the earth system.

ICESat was launched on January 3, 2003 from the Vandenberg Air Force Base, Calif.

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