Donald J. Kessler is an American astrophysicist known for his work in space debris research. He gained prominence for developing the Kessler Syndrome concept, which describes a scenario where the density of objects in low Earth orbit is high enough that collisions between objects could cause a cascade effect, leading to an exponential increase in space debris. Kessler has contributed significantly to the understanding of the risks posed by space debris to satellites and other spacecraft. His research has influenced policies and practices regarding spacecraft design and debris mitigation, aiming to ensure the long-term sustainability of space activities.