Atmospheric gases scatter blue wavelengths of visible light more than other wavelengths, giving the Earth’s visible edge a blue halo. At higher and higher altitudes, the atmosphere becomes so thin that it essentially ceases to exist. Gradually, the atmospheric halo fades into the blackness of space. This astronaut photograph captured on July 20, 2006, shows a nearly translucent moon emerging from behind the halo.
Orbiting the Earth at nearly 17,000 miles per hour, NASA's Ice, Cloud and land Elevation Satellite (ICESat) is collecting spectacular new three-dimensional measurements of the Earth's surface and atmosphere.