NASA has created an animation to give a unique, and almost mesmerizing, look at its fleet of Earth-observing satellites that spend their time looking down on our planet.
The entire fleet and their paths around us as they gather data from low-Earth orbits 400 miles above our heads have been captured in a visualization prepared by the space agency's Goddard Space Flight Center in Maryland.
The animation shows only NASA's own satellites, leaving out those of other space agencies or commercial operators.
The animation covers roughly 12 hours of February 16, so it shows about 8 of the satellites' orbits, each of which takes about an hour and a half.
Different satellites have different tasks, including measuring rainfall, clouds, ocean surface height, ocean salinity, solar heat radiating back from the Earth's surface and other qualities of the global environment.
"Together, [ground--observing satellites) provide a picture of the Earth as a system," NASA says.
The majority of the satellites are in polar orbits, meaning they can, after sufficient orbits, cover the entire Earth as it rotates beneath them.
Some are in a special kind of polar orbit known as a solar-synchronous orbit, which allows them to gather data from the same spot on the Earth at the same local time each day.
One group of four satellites in such an orbit -- dubbed the "A-train" and consisting of the satellites Aqua, Aura, CALIPSO and CloudSat -- fly in close proximity, with CloudStat and CALIPSO orbiting just 58 miles apart.
Although close, that's still a safe distance, NASA says, and following each other so closely provides a particular benefit.
The four "A-train" satellites are equipped with a total of 15 instrument among them, so the data gathered -- from the same places on Earth always at the same times -- is easily compared and can be combined for detailed observation of the Earth below.
"It's acting like one satellite with 15 instruments onboard," says Goddard animator Ernie Wright.
Although the video suggests the satellites pass each other closely, there is little or no chance of their colliding, as their orbits are carefully and specifically calculated and the amount of "elbow room" in the space above the Earth is vast.
Not to say collisions don't happen; in 2009 a decommissioned Russian satellite collided with a U.S. Iridium communications satellite, creating a shower of debris that is still orbiting the Earth.
NASA's Scientific Visualization Studio maintains a website where the public can view a full list of the agency's satellites and their missions.
