Understanding the Propulsion Systems of Satellites in Orbit

Once launched into orbit, satellites do not require continuous propulsion to maintain their trajectory, unlike aircraft. The principles of orbital mechanics, as described by Sir Isaac Newton, ensure their continued motion once in orbit, provided there are no significant external forces disrupting their path. In this guide, we will explore the various types of propulsion systems used by satellites, particularly in low Earth orbit (LEO), medium Earth orbit (MEO), and geostationary orbit (GEO).

Orbital Mechanics and Satellite Propulsion

The key concept here is inertia. As per Newton's first law of motion, an object in motion will stay in that motion unless acted upon by an external force. Satellites in orbit behave much like a ball rolling down a hill, continuously falling towards the Earth but perpetually missing it due to the curvature of the planet. In the case of satellites, there is a very small drag force due to the thin atmosphere, which can cause the orbit to degrade over time. To counteract this, many satellites are equipped with small thrusters that provide occasional, gentle nudges to maintain a precise orbital track.

Types of Propulsion Systems

There are several types of thrusters used for satellite propulsion, each suited to different operational needs. These can be broadly categorized as chemical, electric, and experimental systems.

Chemical Thrusters

Chemical thrusters are the most conventional and widely used type. They operate by decomposing a propellant, typically hydrazine, in the presence of a catalyst or through the combustion of a fuel and oxidizer. These systems provide a strong, instantaneous burst of thrust, making them ideal for significant orbital maneuvers. For example, satellites like those in the Starlink constellation use chemical thrusters to reach their final orbit and maintain it for several years. Once the fuel supply is exhausted, the operators may use the remaining propellant to deliberately de-orbit the satellite for disposal.

Electric Thrusters

Electric thrusters, on the other hand, use electric fields to accelerate a material and provide thrust. These thrusters are highly efficient, capable of operating over long durations with minimal fuel consumption. They are particularly useful for maintaining orbit and providing station-keeping maneuvers. Examples of electric thrusters include those that use iodine or xenon as the propellant. Although less powerful than chemical thrusters, electric thrusters can help extend the lifespan of satellites by reducing the need for frequent refueling or repositioning.

Experimental Propulsion Systems

One experimental method involves using the Earth’s magnetic field for propulsion. By running a current through a long conductor, a small but controllable thrust can be generated. This technique is still in the experimental phase and has not been widely adopted yet.

Big Gyroscopes for Precision Control

While propulsion is not always required, especially for passive satellites, gyroscopes play a crucial role in precise orbital maintenance and alignment. Gyroscopes, powered by solar energy, can provide a stable reference for the satellite’s orientation. By rotating the gyroscope, a satellite can achieve precise station-keeping or reorient itself for imaging or communications purposes.

Conclusion

Understanding the types of propulsion systems used by satellites is essential for grasping how they operate in space. While some classes of satellites, like those in low Earth orbit, may require regular maintenance through thruster firings, others rely on the principles of orbital mechanics to maintain their positions. Whether through chemical, electric, or experimental methods, each system plays a vital role in the operational success and longevity of satellites in orbit.

Keywords: satellite propulsion, orbital mechanics, electric thrusters