What are Debris Clouds?
Any concentration of debris particles or fragments in a well-defined region of space is referred to as a debris cloud. Debris clouds are formed whenever debris is being created by a single source. For example, discarded upper stages generally are surrounded by a cloud of particulates that are released over time by degradation of various materials such as paint and multilayer insulation.
Whenever an orbital breakup occurs, a debris cloud is instantly formed. Such debris clouds first take on the form of an expanding three-dimensional ellipsoid. The center of the debris cloud moves along a well-defined orbit, which for explosions is identical to the orbit of the original object. The debris cloud gradually spreads around this orbit in a spiral pattern. As time passes, the debris cloud eventually envelopes the entire orbit and any other satellites in the nearby vicinity.
Due to the laws of orbital motion and to physical processes involved in an explosion or collision, fragments are not spread uniformly throughout a debris cloud. At some locations, spatial density of fragments is much greater than at others. When spatial fragment density is high, the collision risk posed to satellites that fly through the cloud is greatly increased.
Certain regions of the debris cloud are constricted to nearly one or two dimensions. There are three types of debris cloud constrictions: pinch points, pinch lines, and pinch sheets. Spatial fragment density is, relatively, very high at these constrictions.
Pinch points and pinch lines are particularly important to satellite constellations. Neither of these constrictions moves with the debris cloud around its orbit. They remain nearly fixed in inertial space while the debris cloud repeatedly circulates through them. In many satellite constellations there are multiple satellites in each orbital ring. If one of these satellites breaks up, the remaining satellites in the ring will all repeatedly fly through the pinch point and pinch line. If many fragments are produced by the breakup, the risk of damaging another satellite in the ring may be significant.
Animations of debris particles can look very alarming and can give the impression that space is enormously crowded. Indeed, the pinch points can appear extremely hazardous due to the relative concentration. But space is very large and the distances are vast. The smallest pixel on a typical monitor used to represent a debris particle would be many miles across if it were drawn to the same scale as the Earth. The risks from these “clouds” of debris may be much higher than the risk of flying through other parts of the orbit, but in an absolute sense, the risks are still low.
If satellites from two orbital rings collide, two debris clouds will be formed, one in each ring. The constrictions of each cloud then pose a hazard to the remaining satellites in both rings.