The Old Way: Chemical Rockets
For decades, satellites have relied on conventional chemical propulsion for maneuvering in orbit. This method involves controlled chemical reactions that produce a powerful, high-thrust burst, similar to how a main rocket engine gets a launch vehicle
off the ground. Once in its initial orbit, a satellite uses smaller chemical thrusters for critical tasks like raising its orbit to the final destination and making small adjustments to maintain its position, a process called station-keeping. While effective, this system has a significant drawback: the propellant is heavy. For a typical large communication satellite, the chemical fuel required for its 15-year lifespan can account for a substantial portion of its total mass at launch. This mass is a major constraint, as heavier satellites are more expensive to launch and leave less room for the valuable scientific or communication payloads that perform the actual mission.
The New Era: What is Electric Propulsion?
Electric propulsion (EP) systems work on a completely different principle. Instead of a forceful chemical explosion, they use electrical power, typically generated by the satellite's solar panels, to accelerate a propellant. One common method involves using electricity to ionize atoms of an inert gas like xenon, turning them into charged particles (ions). These ions are then accelerated by electric or magnetic fields and expelled at extremely high speeds—up to twenty times faster than the exhaust from a chemical thruster. The key difference is in thrust and efficiency. A chemical thruster provides a powerful shove, while an electric thruster provides a gentle but constant push. Although the thrust is much lower, it is far more efficient over time. This incredible efficiency is measured by 'specific impulse,' and EP systems can be six to ten times more efficient than their chemical counterparts.
Lighter Satellites, Longer Journeys
The primary advantage of this high efficiency is a massive reduction in the amount of propellant a satellite needs to carry. ISRO has noted that for a large satellite, the fuel requirement could drop from over 2,000 kilograms of chemical fuel to just a few hundred kilograms of xenon gas. This dramatic weight saving has two major benefits. First, it can reduce the satellite's total launch mass by up to 40%, making launches significantly cheaper and allowing ISRO to use its own launch vehicles for satellites that might have previously required more expensive foreign rockets. Second, the mass saved on fuel can be reallocated to the payload. This means a communication satellite could carry more transponponders, or a scientific satellite could be equipped with more advanced instruments, enhancing the mission's overall capability.
How ISRO is Using This Technology
ISRO's move towards electric propulsion is a strategic and phased one. While an early mission, GSAT-9 in 2017, used an imported electric thruster, the space agency has since focused on developing this critical technology indigenously. A major milestone was the successful long-duration test of a homegrown 300mN Stationary Plasma Thruster (SPT), demonstrating its reliability for space missions. This fully Indian-developed system is set to be validated in space aboard the Technology Demonstration Satellite (TDS-01). These systems will eventually replace chemical propulsion entirely for orbit-raising and station-keeping on future ISRO satellites, particularly for geostationary communication satellites that require long operational lives.
The Future: Mars, Venus, and Beyond
The implications of mastering electric propulsion extend far beyond Earth's orbit. The technology is a critical enabler for ambitious, long-duration interplanetary missions. Journeys to Mars, Venus, or the outer solar system require spacecraft to travel for months or even years. Carrying enough chemical fuel for such a trip is a major engineering challenge. Electric propulsion's fuel efficiency makes these deep-space missions more feasible. By providing a continuous, gentle thrust over long periods, EP systems can enable complex trajectories and extended scientific operations far from Earth. As ISRO sets its sights on future lunar exploration and a potential crewed mission to Mars, the experience gained from this technology will be invaluable.
