In 1978, Donald J. Kessler proposed a scenario: since the spent rockets, satellites and other space trash have accumulated in orbit increasing the likelihood of collision with other debris, these collisions create more debris creating a runaway chain reaction of collisions and even more debris. This scenario is now known as known as the Kessler Syndrome.
This domino effect and feedback runaway can create a physically impassable barrier in the Low Earth Orbit (LEO) to space exploration that occurs in higher orbits.
Donald J. Kessler, then an astrophysicist working at NASA, demonstrated that once the amount of debris in a particular orbit reaches critical mass, collision cascading begins even if no more objects are launched into the orbit. Once collisional cascading begins, the risk to satellites and spacecraft increases until the orbit is no longer usable.
Kessler proposed it would take 30 to 40 years for such a threshold to be reached. Today, interestingly exactly 40 years later Kessler demonstrated his theory, some experts think we are already at critical mass in low-Earth orbit (LEO).
So far, about 200 explosions and at least 5 collisions in space have occurred. Further explosions and collisions are very likely. The explosions are mainly caused by onboard energy sources, either due to pressure build-up in propellant tanks, battery explosions, or the ignition of hypergolic fuels. Each explosion creates thousands of small debris objects.