So how do we see them? With telescopes of course. These were invented around the time of Galileo, and were about as powerful as a pair of binoculars you can get at the store. But over time, these instruments became more powerful, with larger and larger mirrors to collect and focus light, so that we could see objects that are more and more distant, until these instruments became as big as a house. But there was always one huge limitation. From earth’s surface, we can only see a certain percentage of the space that surrounds us, and we have to peer through the atmosphere to see it, so our view had always been obscured and hindered to some degree, especially for instruments operating with wavelengths of light that tend to be absorbed by particles in the atmosphere. If only we could put a telescope in space! Well in fact, we eventually ended up doing exactly that. But that wasn’t the first thing we put up into orbit. There are currently over a thousand active artificial satellites in orbit around the earth. We say artificial to distinguish them from natural objects, like the moon, which is earth’s natural satellite, and the artificial ones orbit much more closely than the moon does. So what are these things? How did we get them up there, and what do they do? 

The first satellite launched into orbit was the Russian Sputnik 1, in 1957, and it was pretty much just a small metal sphere with some radio antennas. This triggered the Space Race, which was part of the Cold War between Russia and the United States, based on a fear that Russia would put weapons in space to attack America. Born out of this fear was a frenzy of aeronautical innovation, which led to the moon landing, and thousands of satellites, both for spying and for doing science. So how far away are these things? Well satellites can orbit at a variety of distances from the Earth.There is low-earth orbit. This means around two thousand kilometers above the surface. Objects here go around the earth very fast, in about two hours or a little less. This is where the international space station is, so except for the humans that have been to the moon, all the manned spaceflight that has happened so far has been in low-earth orbit. So how do these objects stay up there instead of falling to the earth? It’s not because there is no gravity. In truth, gravity is almost as strong in low-earth orbit as it is on the surface of the earth. 

The reality is that these objects are, in fact, falling to the earth. It’s simply that they are moving so incredibly fast that they fall to the earth at the same rate that earth’s curvature is produced, always falling but never landing. This is why astronauts experience weightlessness, because they are literally in freefall. Objects that are very close to the earth have to orbit very fast, at least about eight kilometers per second. But the farther away objects get from earth, the more slowly they can go and maintain orbit, so orbital velocity and orbital radius are inversely proportional. If we go a little higher, there is medium-earth orbit. This contains objects between two thousand and about thirty-six thousand kilometers above the earth. Objects right in the middle of this region go around the earth in about twelve hours, a little more slowly than their low-earth orbit counterparts, closer to five or so kilometers per second. And lastly, many of the satellites are very far away, in what we call geostationary orbit. These objects are about thirty-six thousand kilometers above sea level, traveling around three kilometers per second, and they go around the earth once a day. That’s why we call it a geostationary orbit. Objects go around the earth at the same rate that the earth rotates, so they always point to the same spot on earth. So how are each of these orbital distances useful? Low-earth orbits are cheapest, and provide high bandwith for communication with very little time lag, since light can bounce there and back in a tiny fraction of a second. Also, since they are so close, they can image earth’s surface very effectively, so we can get some great pictures. But they go so fast that they are always over a different part of the earth. For this reason, it is usually the case that a network of satellites will be deployed to complete any given function. The low-earth orbit environment has become rather congested with objects, which has resulted in a lot of space debris. This can be very dangerous, as the probability of collisions becomes much greater, which given the incredible orbital velocities, can be destructive to any structures and would certainly be deadly to any humans in the way. Medium-earth orbit satellites are typically used for navigation, communication, and observations regarding geodynamical phenomena that can be seen best from a little ways a way.And lastly, the advantages of a geostationary orbit are very clear, because it allows for a satellite to be directly above the same part of the earth at all times.