As is understood, for objects orbiting a large mass, at the same radial distance, the period of revolution (assuming circular orbits) is the same independent of mass. This means, objects of any mass in the same orbit move at the same velocity. Although, it is always stated that satellites can be struck by space “junk” at very high speeds. I can see this if the space junk is moving in an opposite direction in its orbit but if they are moving in the same direction the relative speeds should be negligible. Is the concern that most objects are in elliptical orbits and that orbits may intersect moving at different velocities? If that is the case, shouldn’t intersecting orbits be quite rare?
I understand your question. Let's look at this question in more detail.
When we state that objects orbiting a large mass at the same radial distance have the same period of revolution, we are referring to Newton's principle of universal gravitation. According to this principle, the gravitational force between two bodies is determined by the mass of the bodies and the distance between them. For objects in circular orbit around a celestial body, the centripetal gravitational force that keeps the object in orbit is balanced by the centrifugal force due to orbital motion, resulting in a stable orbit.
This means that for objects of different masses orbiting in the same orbit, they move with the same orbital velocity. However, as you mentioned, the concern with space "junk" lies in the fact that these objects may be in different orbits and therefore moving at different relative speeds.
Space "junk" can consist of a variety of objects, such as fragments of decommissioned satellites, rocket stages, and other debris left in Earth orbit. Although they may share the same orbit initially, gravitational interactions with other celestial bodies, as well as non-gravitational forces such as solar pressure and atmospheric drag, can cause variations in orbits over time.
These variations can result in cross-orbits or trajectories that get close enough to pose a risk of collision with other orbiting objects. Therefore, even if objects in similar orbits initially move with similar orbital velocities, variations in orbits over time can lead to dangerous encounters in space.
In summary, while objects in similar orbits initially move with the same orbital speed, variations in orbits over time can result in significant relative velocities between these objects, increasing the risk of collisions in space.
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