Space debris pose a significant threat to spacecraft in Earth orbit. Varying in size from tiny collision fragments to large non-operational satellites, and travelling at high velocities, a collision even with a small piece of debris can knock a satellite out of commission. The number of debris keeps growing as space gets more crowded and collisions between them cause pieces of debris to further fragment. If this trend continues unabated and a critical debris density is reached, a run-away effect may happen where collisions between objects cause a cascade, a sort of space debris chain reaction. This scenario is called the Kessler syndrome and can make low Earth orbit exceedingly difficult to use.

Despite mitigation measures being put in place, space debris will remain a significant risk to spacecraft at least for the near future. In addition to astronauts on the ISS and other planned space stations, a rise in private human spaceflight and space tourism is expected, potentially putting more humans in space than ever. A scenario is possible, as depicted e.g. in the film Gravity, where humans are put in danger by space debris. In November 2021, a Russian anti-satellite missile test created 1,500 pieces of debris, sending the 7 astronauts aboard the ISS to their capsules for safety.