Look up at the sky. You might not be able to see them, but there are thousands and thousands of satellites orbiting the Earth. From the International Space Station, to the GPS on your phone, to more accurate weather and climate reporting—none of these would be possible without satellite technology. Now in its 15th year, the United Nations' annual World Space Week (October 4-10) focuses this year on how satellites shape our lives with the theme "Space: Guiding Your Way."
Over 6000 artificial satellites have been launched into orbit, though only half of those are estimated to still be functional. Human-made satellites are often designated as "artificial" to differentiate them from natural satellites, like the Moon. However, natural and artificial satellites both orbit the Earth according to the same rules of physics. The difference is that humans put artificial satellites in space.
But how is a satellite launched into space, and why does it stay in orbit? Artificial satellites were first conceived of by Sir Isaac Newton in his famous 1687 treatise on physics, called the Principia. This thought experiment on satellites is now known as "Newton's cannonball": he imagined a cannon at the top of a very tall mountain, and wondered what would happen if a cannonball were launched at different speeds. Too slow, and the cannonball would simply fall back to Earth; too fast, and it would escape Earth's gravity and launch into space. However, if the cannonball were launched not too fast or too slow, its speed would keep it from falling to the ground, but Earth's gravity would keep it from going to space. A cannonball launched within the right range of speeds would enter Earth's orbit, neither falling to the ground nor entering space. This is the principle behind artificial satellites.
This idea captured people's imaginations for hundreds of years. Science fiction authors like Jules Verne and Arthur C. Clarke wrote about artificial satellites in their work, even before the first artificial satellite, the Soviet Union's Sputnik 1, was launched in 1957. Clarke also suggested the possibility of creating geosynchronous satellites, which are artificial satellites that orbit the Earth at the same rate the Earth spins, which means they stay over a fixed point on Earth. Several decades later, Clarke's vision became a reality.
Today, the thousands of satellites we have in orbit above us provide us with GPS, satellite radio and television, maps and environmental monitoring, and weather and climate data—all thanks to a simple thought experiment and the ideas of science fiction writers. Science NetLinks' lesson The Prometheus Project: The Science Behind Science Fiction explores this very topic. To learn more about artificial satellites, take a look at our Satellite Orbits lesson and The Satellite Site tool. Celebrate World Space Week by attending an affiliated event in your area.