I was recently asked about orbital decay. I wasn’t sure how to explain it to my friend. So I did some research. What I found is that orbital decay is really quite fascinating. The definition of orbital decay is the process of prolonged reduction in the altitude of a satellite's orbit. So basically, when you hear about objects entering the earths atmosphere (such as satellites), and burning up. Orbital decay can be seen in three different ways.
The first way is atmospheric drag. Atmospheric drag is the most common form of orbital decay of satellites. When a satellite is low in earths orbit it increases orbital speed, due to the earths gravitational pull. Due to the dense atmosphere it causes increased drag, therefore increased friction. Most satellites are small enough that they burn up in the earths atmosphere, before ever reaching the earths surface. The best example of this would be when we see meteor showers. There are also controlled and uncontrolled examples of orbital decay. An example of controlled orbital decay is when, the Mir space station was pulled from orbit, to destroy it safely. An example of uncontrolled decay would be when the Skylab space station was brought down.
Another example of orbital decay is by tidal effects. This happens when the orbiting body is large enough in size for it to raise a "tidal bulge" on the body that it is orbiting. The result of the interaction causes a loss of momentum from the orbiting body. This momentum is then transferred to the orbited body. When that happens it causes the object in orbit to lose altitude, resulting in increased friction. Examples of tidal effects, are mars’ moon Phobos, and Neptune’s moon Triton.
The third and final form of orbital decay is called gravitational radiation. This is when two masses orbit one another. This then causes gravitational waves of orbital energy. When the masses are very large such as black holes or neutron stars, the energy can move fast enough to cause the orbits to merge together over time. Gravitational radiation is orbital decay on the largest scale. Gravitational radiation is undetectable unless seen in astrophysical sources like supernovae.
Those are the three forms of orbital decay. In reality here on earth we don’t have much to worry about unless a large asteroid or the earths moon enter our orbit. If you have a further interest in studying orbital decay, I would recommend researching the physics part of orbital decay. I hope I was able to help your understanding of orbital decay.