Orbital decay is a process by which the altitude of a satellite gradually decreases over time due to various factors. Satellites in low Earth orbit (LEO) are particularly susceptible to orbital decay.

There are several factors that contribute to orbital decay:

1. Atmospheric drag: Even though the atmosphere becomes thinner at higher altitudes, it is not completely absent. Atmospheric molecules can cause drag on the satellite, which results in a gradual decrease in altitude. This effect is more pronounced for satellites in lower altitudes where the atmospheric density is higher.

2. Solar radiation pressure: Solar radiation exerts pressure on the satellite's surface due to the momentum carried by photons. This pressure can cause a small but continuous force that pushes the satellite in the opposite direction of the Sun. Over time, this force can contribute to orbital decay.

3. Gravitational perturbations: The gravitational pull from the Earth, as well as other celestial bodies such as the Moon and the Sun, can cause variations in the satellite's orbit. These perturbations can gradually decrease the altitude of the satellite if they are not compensated for.

To counteract orbital decay and maintain a satellite's desired orbit, various measures can be taken:

1. Propellant usage: Satellites equipped with thrusters can periodically fire them to counteract the effects of orbital decay. By boosting their altitude, satellites can compensate for the altitude loss caused by atmospheric drag and other factors.

2. Aerodynamic design: Satellites can be designed with shapes that minimize atmospheric drag, reducing the rate of orbital decay.

3. Altitude adjustment: Satellites can be intentionally moved to higher orbits to mitigate the effects of atmospheric drag. This requires careful planning and fuel management.

If a satellite's altitude decreases significantly due to orbital decay and it reaches a critical point, known as the "debris mitigation altitude," it may become uncontrolled and reenter Earth's atmosphere, burning up upon reentry. This is done to prevent the accumulation of space debris and potential collisions with other satellites or space objects.