Types of Perturbation

As many assume satellites orbit Earth in a perfect trajectory, it is a common misconception. In reality, due to Earth’s rotation the poles are slightly squished at the top, making it an oblate spheroid. This means that gravity is not evenly distributed across the planet, therefore, affecting satellite’s orbits. The J2 parameter is one of the parameters used to compensate for this uneven gravitational field, to adjust and keep the satellites in their proper orbit.  The following picture illustrates how the J2 parameter acts to account for the Oblateness of the Earth: it pulls away from the Earth as, around the poles, the satellite is further from any mass that would attract it. Therefore, the closer to the equator, the smaller the J2 perturbation (the smaller arrows on the picture), and the closer to the poles, the bigger the perturbation (the bigger arrows).

In LEO (Low Earth Orbit), satellites encounter an additional challenge. Satellites’ velocity faces some resistance from the thickness of the atmosphere. This is known as atmospheric drag, and over time satellites lose altitude. The impact of drag’s effects depends on the satellite’s mass and cross-sectional area. The heavier the satellite is, the less likely it is to be affected by acceleration (as explained by the Law of Inertia) and the larger it is, the more drag it experiences. Some satellites operators/space agencies take advantage of some perturbations in their orbits. You can read more about this topic on “Space Curiosity – Types of Orbits”! 

In addition, the gravitational forces of the Sun and the Moon also get to influence the satellites’ orbits. Since their bodies are massive and close enough to have noticeable influence, their effects need to be considered into long-term calculations. Moreover, the Sun constantly sends out particles of light that have an influence on the satellite’s trajectory – solar radiation pressure.