Seeing Earth from satellites

Imagine looking down at the whole world from space.

Satellites can do that. They take pictures of land, sea, clouds and ice. The pictures help us learn about our planet.

A satellite is anything that orbits a planet or star, NASA's Space Place explains, but the word usually means a machine launched into space to circle Earth. Thousands of these satellites orbit our planet. Some take pictures of Earth that help weather forecasters predict the weather and track hurricanes.

The U.S. Geological Survey (USGS) points out the big advantage: cameras on satellites and airplanes take images of large areas, so we can see much more than we can from the ground.

Studying Earth from a distance is called remote sensing. The USGS defines it as detecting and monitoring the physical characteristics of an area by measuring its reflected and emitted radiation from a distance, usually from a satellite or aircraft. Special cameras collect these images, which help researchers "sense" things about Earth.

NASA's Space Place explains why satellites are so valuable: their view lets them see large areas of Earth at one time, so they can collect more data, more quickly, than instruments on the ground. Satellites look outward too, above the clouds and dust that block telescopes on the ground, but this topic is about the ones that look down.

Satellite pictures are made of tiny squares.

Each square is one color. Put lots of squares together and you see the picture.

Satellites send their data to the ground by radio. There, computers turn it into a picture made of tiny squares called pixels, short for picture elements. Each pixel shows how much light came from one small patch of Earth.

The smaller the patch each pixel covers, the smaller the things you can see. NASA's Earth Observatory gives an example: an instrument on Landsat 7 could see details down to 15 meters, so each pixel stood for a square 15 meters on each side.

Satellite data are transmitted by radio to ground stations and assembled into digital images made of pixels, each recording the relative reflected light energy from one part of the scene. NASA's Earth Observatory explains that pixel size measures a sensor's ability to resolve objects: Landsat 7's Enhanced Thematic Mapper had a maximum resolution of 15 meters, so each pixel represented 15 m × 15 m, or 225 square meters.

Pixels are also data. Counting the pixels of one kind in an image gives an area; Earth Observatory's example is counting green pixels in a false-color image to calculate the area covered by vegetation. Higher resolution means smaller pixels and smaller objects that can be told apart.

Satellites can see light our eyes cannot see.

Computers turn that light into colors. Then we can see things like where plants grow.

Many satellite instruments measure sunlight bouncing off Earth, just as your eyes do, but they also measure kinds of light people cannot see. NASA explains that computers then show each kind of light as red, green or blue on a screen.

The result is called a false-color image. The colors are not what you would see out of a window, but they make differences easy to spot, such as which areas are covered with plants. To read one, you need to know which kind of light each color stands for.

NASA's Earth Observatory divides remote sensing instruments into passive and active. Passive instruments detect natural energy reflected or emitted from the scene; reflected sunlight is the most common source. Active instruments, such as radar, send out their own radiation and measure what comes back; because it travels at the speed of light, the timing gives the distance.

Satellites record brightness in several bands of the spectrum, including wavelengths invisible to us. Displaying three bands as blue, green and red produces a "false color" image. Our eyes can distinguish only about 20 to 30 shades of gray but 20,000 or more color tints, which is why color helps. Earth Observatory warns that without knowing which band went into each color, we cannot be sure why the colors are what they are.

Some satellites stay over one place.

They watch the clouds all day. Others fly over the poles and see the whole Earth.

Some weather satellites, called GOES, orbit 22,236 miles (about 35,800 kilometers) above the Equator, moving at the same speed Earth spins. NOAA explains that this keeps them over the same region, so they can watch it all the time.

Other satellites fly over the poles. NOAA's JPSS satellites circle Earth from pole to pole 14 times a day and see the whole planet twice daily. Landsat 8 and Landsat 9 fly much lower, at 705 kilometers, and pass over the same place every 16 days.

Orbit choice sets what a satellite can see. NOAA's GOES satellites orbit 22,236 miles above the Equator at a speed matching Earth's rotation, holding position over one region for continuous coverage of weather, lightning and space weather. NOAA's polar-orbiting JPSS satellites circle pole to pole 14 times a day, giving full global coverage twice daily, and supply most of the data used in U.S. numerical weather forecasts.

Landsat 8 and 9 orbit at 705 km in a sun-synchronous orbit, crossing the sunlit side of Earth from north to south. According to the USGS, each satellite images a 185-km-wide swath, circles Earth every 99 minutes, completes about 14 orbits a day, and passes over the same location every 16 days.

Satellite pictures help people in many ways.

They show where clouds and storms are. They show forest fires. They show how towns grow.

The USGS lists many uses for satellite images. Large forest fires can be mapped from space, showing rangers far more than they could see from the ground. Satellites track clouds to help predict the weather and watch erupting volcanoes and dust storms. They can even make images of temperature changes in the oceans.

Because Landsat has been taking pictures since 1972, scientists can compare old and new images to see how cities have grown, coastlines have shifted and forests have changed.

The USGS lists uses that range from the urgent to the slow: mapping large forest fires, tracking clouds to predict weather, watching erupting volcanoes and dust storms, imaging ocean temperature changes, and tracking the growth of cities and changes in farmland or forests over years and decades.

That last kind of work depends on long, consistent records. NASA's Landsat program describes its carefully calibrated archive, gap-free since 1972, as what lets researchers study how cities have grown, coastlines shifted, crop cycles changed and forests transformed since the 1970s. The USGS notes that Landsat imagery is available at no cost, and a USGS study estimated its annual benefit to users at $2.19 billion in 2011.