For many Americans, 9/11 changed everything. The United States no longer seems invulnerable to foreign attack, and the world just seems like a more dangerous place.

As we've learned over the past decade, terrorists can strike almost anywhere at any time. Terrorism is effective because it's unpredictable, and therefore terrifying to even think about.

Scientists and homeland security professionals have been busy, though, trying to get a better sense of where terrorists might be likely to strike. James Lein uses data from NASA satellites to track particularly vulnerable areas. Satellites have the ultimate bird's eye view. From its position in orbit, a satellite can see many different details of a landscape, in many different ways.

The satellites that Lein uses have cameras that can zoom in and capture details about an area only a few feet across. This sort of observational power is useful for mapping structures that might be targets for terrorism, such as power lines, chemical and power plants, and public buildings.

Geographers and topographers already know where these structures are, generally, but satellite images give them a much clearer sense of how these buildings and locations are grouped. Plus, satellite data also depicts details of the earth's terrain, potentially revealing places where terrorists might set up operations.

Finally, satellites can see not only what's on the ground, but also what's in the air. Very sensitive detectors can recognize the wavelengths of airborne gases such as chemical weapons.

Satellite data can never guarantee safety from terrorism, of course, but it may help alert us to areas most vulnerable to attack.