What phenomenon occurs when satellites re-enter the Earth's atmosphere and burn up due to friction?

The correct choice is Orbital decay, which refers to the reduction in altitude of a satellite's orbit due to the effects of atmospheric drag or friction when it encounters the Earth's atmosphere. As a satellite travels at high velocities, it generates friction with the air molecules in the upper atmosphere, resulting in heat. This heat can cause the materials of the satellite to burn up, leading to its eventual disintegration.

This process significantly affects satellites in low Earth orbit, where atmospheric density is sufficient to produce the necessary drag to pull the satellite into a lower orbit. Over time, if the satellite does not have enough propulsion to maintain its altitude, it will experience orbital decay, ultimately leading to re-entry and burning up due to the intense heat generated during this process.

The other choices do not accurately describe this phenomenon. While atmospheric resistance is a factor, it is not a comprehensive term that captures the entire process of orbital decay as it relates to the fireball phenomenon seen upon re-entry. Orbit shift and gravity sink do not relate specifically to the burning up of satellites upon re-entry due to atmospheric drag.