In the summer of 1961 the upper stage of the rocket carrying the Transit 4A satellite blew up about two hours after launch. It was the first known human-made object to unintentionally explode in space, and it created hundreds of fragments of useless space junk. Some of these pieces were pulled into the atmosphere where they burned up but around 200 of them are still up and orbiting today.
At the time, people were not all that concerned about a few bits of metal floating around in the vastness of space. But like the ocean and other frontiers, space isn’t endless as it first appears.
The universe may be infinite but orbital space is finite and the amount that we use regularly is even more limited. Most satellites end up in a few particular orbits (one can think of them as space freeways). Low Earth orbit, a region of space that extends up to an altitude of about 2000 kilometers, is particularly congested. When a satellite stops working in low Earth orbit it can sometimes stay in that region for hundreds of years.
In the late 1960s a scientist at NASA named Donald Kessler began to look into this phenomena. At the time, Kessler was studying what happens when two meteoroids collide and break apart into smaller pieces. He started doing similar research on space debris and found that a breakup increases the probability of another collision (as parts multiply). And the population of space junk increases exponentially over time. This concept became known as the Kessler Syndrome.
This cascading phenomenon over the course of a hundred years could get out of control, creating an environment that becomes extremely hazardous to spacecraft.