Most people know that satellites in orbit do useful things such as collect images of the Earth's surface. At the National Air and Space Museum I use satellite images in my job to understand changes in the Earth's land surface. Today millions of people are acquainted with satellite imagery on internet map services. People sometimes ask me if it's possible to see even more detail from space. In this post I'll explore what is really visible in different types of satellite data. There are dozens of orbiting remote sensing satellites, and the level of detail they see depends on its precise mission. "Remote sensing" just means looking at something from a distance. In this case we're talking about viewing the Earth from at least hundreds of miles above the atmosphere. The images on internet map servers are provided by a recent generation of satellites that collect detailed images, including the GeoEye and WorldView satellites. Objects smaller than automobiles are visible in some of these images. In the past, only military and reconnaissance satellite were capable of this kind of detail.
Other satellites observe large areas and discern things the size of agricultural fields. These spacecraft, including the Landsat satellites, are useful for mapping cities or regional changes in land cover.
Another class of satellites orbit thousands of miles out in space. These spacecraft, including the GOES satellites, are designed to observe changing weather over an entire hemisphere of the Earth. They cannot discern small details.
In the past, it was often incorrectly stated that the Great Wall of China was the only man-made thing visible from space. Although an astronaut would probably not be able to see it with unaided eyes, the Great Wall is visible using orbiting sensors. However, plenty of other things made by humans are also visible. It was sometimes even stated that the Great Wall is visible from the Moon, but that's definitely not possible. If you stood on the Moon, the entire Earth would appear to be about the size of a quarter held at arm's length. In some Hollywood films, satellites provide moving images from space. The hero immediately targets a satellite to search for evildoers. While this type of real-time imagery looks very cool, it's not really how satellites work. Orbiting satellites pass over a particular point only every couple of weeks, and they cannot be immediately moved or collect moving images. There is a way to get imagery like that, but it's from unmanned airplanes. Drone aircraft can provide real-time imagery and even be equipped with weapons to attack targets. In reality, satellite imagery is used for "before" and "after" images. These can be used for research purposes and for responses to emergencies. Recently media outlets widely used imagery from the GeoEye-1 satellite to show tsunami devastation in Japan. Sometimes a satellite passes overhead at just the right time to capture a rapid change. The Indian Ocean tsunami on December 26, 2004 was one of those times. The QuickBird satellite just happened to pass over Sri Lanka when the wave of water crashed ashore, providing an amazing (and scary) image. In 2005 the same satellite provided images of New Orleans immediately after Hurricane Katrina. I had an opportunity to closely examine those images at the time, and I remember making a sobering calculation of how much of the city remained submerged. So the detail visible in a satellite image all depends on the mission of each satellite and the scale of its observations. A few non-military satellites can see objects down to about half the size of a car. Some military satellites can still see even smaller things. But that does not tell us the whole story. For most applications we need to see larger areas, which requires other satellites that observe at a different scale. For each satellite imaging project, we need to choose between seeing small details or seeing a large area. You can't usually have both. But increased computing power has made it possible to combine highly detailed images to cover very large areas. The seemless imagery on internet map servers actually consists of many thousands of individual images that have been combined. Scientists use the same kind of approach to view fine scale vegetation changes across continents. Methods of combining small images will continue to be valuable for making detailed observations of the Earth in the future.