What goes up must come down, except for spacecraft. Some tend to keep going up, or at least stay up for a long time. However, all of the spacecraft, satellites, and junk in earth orbit still comes down. In fact, they have, are, and will always be on their way down. Everything in orbit is falling, because everything in orbit feels the gravitational pull of earth.
Based on our cultural knowledge we accumulate from media here on earth, it appears as if there is no gravity in orbit. The incredible pictures and videos we are given by NASA and other space travel organizations paint a picture the of harsh, yet serene silence and stillness of space, where everything is so isolated and untouchable that not even gravity can displace it. However, despite the fact that everything in space appears to hover and not fall to gravity, that is not the case. Everything in orbit is under the influence of gravity, and it’s all falling.
Why is reality different from perception? Wikipedia put it best: “objects in orbit are in a continuous state of free fall, resulting in an apparent state of weightlessness.” Everything in the International Space Station is falling just as quickly as the International Space Station itself is falling. Just as you feel weightlessness in a falling roller coaster, so do astronauts and cosmonauts in the falling space station.
So why has the space station been falling from an altitude of 250 miles for 20 years and never hit the ground?
Because it’s going insanely, ridiculously, crazy fast. Specifically, the space station is floating along at about 7.67 kilometers per second, or 17,200 miles per hour. This is over 22 times the speed of sound and fast enough to orbit the earth every hour and a half. This means that falling, in the traditional and logical sense, does not apply here.
https://youtu.be/Xjs6fnpPWy4?t=1m25s
a real-time video demonstrating the International Space Station's one and a half hour orbit period
To illustrate this, compare the International Space Station to a ball tied to a string.
a rather colorful, round space station (source)
When the ball is spinning around, there is a constant pulling force toward the center of the circular path. However, because the ball is constantly moving forward, the force only shifts the direction in which the ball is travelling. The physics students among you would recognize this as centripetal acceleration; those of you who haven’t taken physics would recognize this as really boring.
What’s not boring is that the International Space Station undergoes the exact same process on an almost unimaginably ginormous scale. It experiences 90% as much of the pull of gravity as we feel down on the surface of earth, but it is just going so fast that gravity does not pull it down to the ground, but just around, leading to a circular path around the planet that we simply call orbit.