The Heavy Burden of Space Travel
Traditionally, satellites are launched with a large reserve of chemical propellants. This fuel is essential for two main jobs once in space: making the final journey to the correct operational orbit (orbit raising) and making tiny, periodic adjustments
to maintain that orbit against gravitational pulls and other disturbances (station-keeping). However, this chemical fuel is incredibly heavy. For a typical communication satellite, the propellant can account for more than half of its total launch mass. This has a huge knock-on effect. A heavier satellite requires a more powerful, and therefore more expensive, launch vehicle. The more fuel you carry, the less room and weight capacity you have for the actual payload—the transponders, sensors, and cameras that do the important work.
Enter Electric Propulsion: A Lighter Touch
This is where electric propulsion, specifically plasma thrusters, comes in. Instead of a powerful, short-lived chemical burn, electric thrusters use electrical power, often generated by the satellite's solar panels, to create a very gentle but continuous thrust. While the force they produce at any given moment is tiny—often described as being equivalent to the weight of a sheet of paper—they are incredibly efficient. By firing continuously for long periods, they can achieve the same overall change in velocity as a chemical rocket but by using a tiny fraction of the propellant mass. The most common type, a Hall-effect thruster, uses an inert gas like xenon.
How Plasma Thrusters Work
Imagine a plasma thruster as an incredibly efficient particle accelerator. First, a small amount of xenon gas is fed into a ceramic chamber. Here, a magnetic field traps electrons, which are generated by a component called a cathode. These trapped, high-energy electrons collide with the neutral xenon atoms, knocking their electrons off and turning the gas into a plasma—a superheated mix of positive ions and free electrons. An electric field then powerfully accelerates these newly created positive xenon ions, shooting them out of the back of the thruster at extremely high speeds. This high-speed ejection of ions creates a small but steady thrust in the opposite direction, pushing the satellite forward. Because the exhaust velocity is so high, you get a lot of push for very little fuel.
The Twin Benefits: Less Weight, More Life
The primary advantage is a dramatic reduction in satellite weight. ISRO officials have stated that using electric propulsion can slash propellant needs significantly. For instance, a satellite that would normally need around two tonnes of chemical fuel might only require 200 kilograms of xenon gas for the same mission. This mass saving is transformative. A 3,500 kg satellite using electric propulsion could perform the same job as a conventional 5,000 kg satellite. This means ISRO can either pack more transponders and instruments onto a satellite of the same size or use smaller, cheaper rockets to launch them. The second major benefit is maximising orbital endurance. A satellite's operational life is often determined by how long its station-keeping fuel lasts. Since plasma thrusters are five to six times more fuel-efficient than chemical systems, a satellite can maintain its correct orbit for many more years, extending its useful life and return on investment.
ISRO's Path to Plasma Power
ISRO has been strategically developing this capability. The agency first tested an electric propulsion system on the GSAT-9 satellite in 2017, which used an imported thruster. Since then, significant progress has been made on developing a fully indigenous system. Recently, ISRO's Liquid Propulsion Systems Centre (LPSC) successfully completed a 1,000-hour life test of its own 300 millinewton (mN) Stationary Plasma Thruster, a major milestone proving its reliability. This home-grown technology is slated to be validated on the upcoming Technology Demonstration Satellite (TDS-01), which will use the system for orbit raising. The success of this mission will pave the way for future ISRO communication satellites to rely entirely on electric propulsion, cementing India's place among the nations with advanced space capabilities.
