Satellite Propulsion: Past, Present, and Future

As an Amazon Associate we earn from qualifying purchases.

The Fundamentals of Movement in Space

A satellite, once delivered to orbit by a powerful launch vehicle, is not simply parked in the sky. It is in a perpetual, delicate dance with gravity. Its immense forward velocity, a gift from its rocket, constantly tries to fling it out into space in a straight line. Earth’s gravitational pull constantly tugs it back down. The result of this perfect balance is an orbit—a state of continuous freefall around the planet. From this perspective, it might seem that a satellite, once placed, needs no further assistance. Yet, this is far from the truth. The space environment is not a perfect void, and the demands of modern missions require a level of precision that cannot be left to chance. This is where satellite propulsion comes in. These onboard engine systems are the beating hearts of a spacecraft, providing the critical ability to maneuver, adjust, and ultimately fulfill a mission’s objectives.

Why Satellites Need to Move

The initial orbit a satellite is placed in is rarely its final destination. Propulsion is first needed for orbit raising, a series of engine burns that nudge the satellite into its precise operational altitude and inclination. Once there, the work of the propulsion system has only just begun. A satellite’s orbit is constantly being disturbed by a host of subtle but relentless forces. In low Earth orbit (LEO), which extends up to an altitude of about 2,000 kilometers, there are still trace amounts of atmosphere. This incredibly thin gas creates a tiny amount of drag, which, over weeks and months, will slow the satellite down, causing its orbit to decay. Without periodic boosts from its thrusters, the satellite would eventually fall back to Earth and burn up.

Even at higher altitudes, other forces are at play. The Earth’s gravitational field isn’t perfectly uniform; concentrations of mass in the planet’s crust and mantle create gravitational “lumps” that gently tug on a satellite, perturbing its path. The Sun and Moon also exert their own gravitational influence. Perhaps most surprisingly, the light from the Sun itself exerts a tiny but constant force called solar radiation pressure. Like a steady, imperceptible breeze, this pressure pushes on the satellite over its entire lifetime. To counteract all these disturbances and maintain a precise position—a function known as station-keeping—a satellite must carry its own supply of propellant and fire its thrusters periodically.

Beyond station-keeping, propulsion is essential for attitude control, the ability to orient the spacecraft in a specific direction. While systems like reaction wheels can handle fine pointing adjustments, thrusters are needed to “desaturate” these wheels when they become spun up, and to perform larger, more rapid slewing maneuvers. Furthermore, as the space around Earth becomes increasingly crowded, the ability to perform collision avoidance maneuvers is no longer a luxury but a necessity. Finally, at the end of a satellite’s operational life, international guidelines and responsible stewardship of the orbital environment require it to be moved. For satellites in LEO, propulsion is used to deorbit the spacecraft, ensuring it re-enters and burns up in the atmosphere. For those in high-altitude geostationary orbits, thrusters push the satellite into a final “graveyard” orbit, safely out of the way of active spacecraft. A satellite’s useful life is often not determined by the health of its electronics, but by the moment it fires its thrusters for the last time and runs out of fuel.

The Physics of a Push in a Vacuum

At the heart of every satellite propulsion system is a fundamental principle of physics articulated by Sir Isaac Newton over three centuries ago: his Third Law of Motion. It states that for every action, there is an equal and opposite reaction. This is the simple, elegant law that governs how anything moves in the vacuum of space. With no air to push against, a spacecraft must bring its own “something” to throw. This ejected material is known as reaction mass, or more commonly, propellant.

Imagine an astronaut floating freely in space. To move to the left, she could throw a heavy wrench to the right. As she exerts a force on the wrench, the wrench exerts an equal and opposite force on her, pushing her in the desired direction. A rocket engine does the same thing, but instead of a wrench, it expels a continuous stream of gas particles at high velocity. The combined push of these countless tiny particles creates a steady force in the opposite direction, which is called thrust.

To generate this stream of exhaust, two things are needed: mass and energy. The mass is the propellant itself. The energy is what accelerates that mass. In a chemical rocket, the energy is stored in the chemical bonds of the propellant, released through a combustion or decomposition reaction that creates hot, high-pressure gas. In an electric propulsion system, the energy comes from an external source, like solar panels or a nuclear reactor, and is used to accelerate the propellant using electric or magnetic fields. The total change in velocity a propulsion system can provide to a spacecraft is known as its “delta-v,” and it is the single most important currency for any space mission.

The Great Trade-Off: Thrust vs. Efficiency

When engineers design a propulsion system, they are governed by a fundamental trade-off between two key performance metrics: thrust and specific impulse. Understanding this trade-off is essential to understanding why different satellites use such vastly different types of engines.

Thrust is the raw force, or “push,” that an engine can generate. It’s measured in Newtons (N) or millinewtons (mN). A high-thrust engine can change a spacecraft’s velocity very quickly. This is indispensable for launching from a planet’s surface to overcome gravity, or for performing rapid orbital maneuvers, like an emergency collision avoidance burn or a quick insertion into orbit around another planet. Chemical rockets are the champions of high thrust, capable of producing millions of Newtons of force.

Specific impulse, often abbreviated as Isp, is the measure of an engine’s fuel efficiency. It describes how much thrust is generated from a given amount of propellant over time. It’s typically measured in seconds. A higher specific impulse means the engine is more efficient; it can produce the same amount of push using less propellant, or it can operate for a much longer time on the same amount of fuel. Electric propulsion systems are the masters of high specific impulse, with values ten to twenty times greater than their chemical counterparts.

Herein lies the great trade-off: systems that produce high thrust are almost always low in efficiency, and systems that are highly efficient produce very low thrust. A powerful chemical rocket is like a drag racer: it burns an enormous amount of fuel in a very short time to produce incredible acceleration. An electric ion thruster is like an ultra-efficient hybrid car: it sips fuel, allowing it to travel for an immense distance on a single tank, but its acceleration is so gentle it can take months or even years to achieve a large change in velocity. This inverse relationship dictates every choice in propulsion design. A mission that needs to get to its destination quickly or perform powerful maneuvers will choose a high-thrust chemical system. A mission that has years to travel, like a deep-space probe journeying to the outer solar system, and needs to maximize its delta-v on a limited fuel budget will choose a high-Isp electric system.

The Dawn of In-Space Propulsion: A History

The story of satellite propulsion is intertwined with the history of rocketry itself, a journey of thought and invention that spans centuries. While the dream of space travel is ancient, the scientific and engineering foundations that made it possible were laid down over generations, culminating in the explosive technological advancements of the 20th century.

From Ancient Arrows to Modern Theory

The earliest known application of rocket principles dates back to 13th-century China, where engineers repelled Mongol invaders with “arrows of flying fire.” These were simple tubes filled with gunpowder—a mixture of saltpeter, sulfur, and charcoal—attached to long sticks. When ignited, the burning powder produced gas and sparks that shot out the back, pushing the arrow forward. Over the following centuries, these simple solid-propellant rockets were refined in Europe, with thinkers like Roger Bacon improving gunpowder formulations and Jean Froissart proposing the use of tubes for more accurate launches, a forerunner of the modern bazooka.

The scientific underpinning for all modern rocketry arrived in the latter half of the 17th century with Sir Isaac Newton. His laws of motion provided the mathematical language to describe how rockets work and, crucially, why they could function in the vacuum of space. It was this theoretical foundation that allowed later pioneers to move rocketry from a curiosity into a science. In 1903, a Russian schoolteacher named Konstantin Tsiolkovsky published a report that laid out the core concepts of space exploration by rocket. He was the first to suggest the use of liquid propellants, recognizing that they could provide far greater energy and control than solid gunpowder. In the United States, Robert H. Goddard independently reached similar conclusions and, in 1926, conducted the first successful flight of a liquid-propellant rocket, a feat that earned him the title of the father of modern rocketry.

The Space Age Begins

The development of long-range ballistic missiles during World War II, particularly Germany’s V-2 rocket, provided the technological leap needed to turn the dream of spaceflight into a reality. After the war, both the United States and the Soviet Union leveraged this technology and the expertise of German engineers to create their own missile programs, which directly paved the way for their space programs.

On October 4, 1957, the world changed forever when the Soviet Union launched Sputnik 1, the first artificial satellite, into orbit. This small, polished sphere, broadcasting its simple beep-beep signal, ignited the Space Race. A key point often overlooked is that Sputnik itself had no propulsion system. It was a passive passenger, placed into orbit by its powerful R-7 launch vehicle and left to drift. Its trajectory was determined solely by the velocity and altitude at which it was released. Its mission ended 92 days later when the cumulative effects of atmospheric drag caused its orbit to decay and it burned up on re-entry. Sputnik’s success was a monumental achievement, but its passive nature immediately highlighted the next great challenge: creating satellites that could actively maneuver, control their orientation, and shape their own destiny in space.

Early Chemical and Electric Steps

In the years immediately following Sputnik, the first satellite propulsion systems emerged, drawing heavily on the mature technology of chemical rocketry. Early satellites were equipped with simple thrusters, often using pressurized cold gas or monopropellants like hydrogen peroxide, primarily for attitude control. These systems were reliable and provided the quick, pulsed bursts needed to keep a satellite properly oriented.

At the same time, a far more futuristic concept was being actively developed: electric propulsion. The idea had been conceived by Tsiolkovsky and Goddard decades earlier, but it wasn’t until the 1960s that it became a practical possibility. In a remarkable coincidence of timing, both the Soviet Union and the United States tested electric thrusters in space for the first time in 1964. The Soviet Zond-2 probe, launched towards Mars, used six pulsed plasma thrusters as actuators for its attitude control system. Just a few months earlier, NASA launched the Space Electric Rocket Test I (SERT-I). This mission carried two ion engines, one of which operated successfully for 31 minutes, proving that the principle of accelerating ions to generate thrust worked in the vacuum of space. This parallel development in the very infancy of the Space Age demonstrated that engineers immediately grasped the fundamental trade-off. Chemical systems were the practical, high-thrust solution for immediate needs, while electric systems, though limited by the meager electrical power available from early solar panels, offered a tantalizing future of unparalleled fuel efficiency. This early divergence set the stage for the next six decades of propulsion development.

NERVA: The Nuclear Rocket Dream

While chemical and electric propulsion developed along their separate paths, a third, audacious program sought to combine the best of both worlds: the Nuclear Engine for Rocket Vehicle Application, or NERVA. A joint effort between NASA and the Atomic Energy Commission, NERVA aimed to build a nuclear thermal rocket, a concept that promised both the high thrust of a chemical engine and a specific impulse more than double that of the best chemical systems.

The working principle was straightforward but immensely challenging. Instead of a chemical reaction, a compact nuclear fission reactor would be used to heat a propellant—liquid hydrogen—to extreme temperatures, over 2,200 K. This superheated hydrogen gas would then be expelled through a nozzle to produce immense thrust with incredible efficiency. The program began as Project Rover in the 1950s and, over the next decade, made astounding progress. A series of ground-based reactor tests, starting with the “Kiwi” series and culminating in the NERVA XE engine tests in the late 1960s, proved the technology was not just feasible but remarkably reliable. The XE engine was started and restarted 28 times and ran for nearly four hours, meeting all the performance requirements for a crewed mission to Mars.

The NERVA program was a stunning technical success. It had strong political backing and was seen as the key to humanity’s expansion into the solar system. its fate was tied to the shifting political and economic landscape. As the Apollo program wound down and the financial pressures of the Vietnam War mounted, the appetite for large, expensive new space initiatives waned. Despite its proven potential, the NERVA program was canceled in 1973. It remains a tantalizing “what if” in the history of space exploration—a powerful technology, proven and ready, that was left on the drawing board.

The Workhorses: Chemical Propulsion Systems

For over sixty years, chemical propulsion has been the bedrock of spaceflight. These systems, which derive their energy from chemical reactions, provide the high thrust necessary for rapid, powerful maneuvers. From tiny attitude control jets to large engines that place satellites in their final orbits, chemical thrusters are the ubiquitous workhorses of the space industry. They are generally categorized by the number of propellants they use.

Cold Gas Thrusters: The Simplest Push

The cold gas thruster is the most elementary form of rocket engine, a perfect illustration of Newton’s Third Law in action. Its design is remarkably simple, often consisting of little more than a tank of pressurized gas, a valve, and a nozzle. To generate thrust, the valve opens, and the compressed gas expands as it escapes through the nozzle. There is no combustion and no complex chemistry, making these systems incredibly reliable, safe, and inexpensive.

The propellants used are typically inert gases like nitrogen or helium, which are easy to handle and pose no contamination risk to sensitive satellite components like camera lenses or solar panels. The performance of a cold gas thruster is modest; it produces very low thrust and has the lowest specific impulse of any chemical propulsion system. As the gas in the tank is used up, the pressure drops, and the available thrust decreases.

Despite these limitations, their simplicity and reliability make them an ideal choice for specific applications. They are frequently used for precision attitude control on small satellites, where their gentle, predictable puffs of gas can finely tune the spacecraft’s orientation. Their inherent safety also made them the perfect choice for astronaut propulsion units. The Manned Maneuvering Unit (MMU), used by astronauts during the Space Shuttle era, was powered by 24 small nitrogen cold gas thrusters, giving them the freedom to fly untethered in the payload bay.

Monopropellant Thrusters: The Industry Standard

For decades, the most common type of propulsion system found on satellites and interplanetary probes has been the monopropellant thruster. As the name implies, this system uses a single liquid propellant. The undisputed champion in this category is hydrazine (N2H4).

A monopropellant system works through a process of catalytic decomposition. The liquid hydrazine is stored under pressure in a tank. When thrust is needed, a valve opens, allowing the hydrazine to flow into a reaction chamber filled with a catalyst. This catalyst is typically a porous ceramic material, like alumina granules, coated with the precious metal iridium. Hydrazine is chemically unstable, and upon contact with the iridium catalyst, it decomposes instantly and violently in a highly exothermic reaction. It breaks down into a high-temperature (around 1,000 °C) mixture of nitrogen, hydrogen, and ammonia gas. This rapidly expanding hot gas is then channeled through a nozzle to produce thrust.

The great advantage of this system is its elegant simplicity and reliability. Because the reaction is spontaneous, there is no need for a complex ignition system. The thruster can be fired in extremely short, precise pulses, lasting just milliseconds, which is perfect for the fine adjustments required for satellite attitude control and station-keeping. This reliability has made hydrazine monopropellant thrusters the go-to system for countless missions, from commercial communications satellites to deep-space explorers. They were even used for the powered descent and hovering of the “Sky Crane” that successfully lowered the Curiosity and Perseverance rovers onto the surface of Mars.

hydrazine’s long reign as the industry standard comes with a significant drawback: its toxicity. Hydrazine is a highly toxic and carcinogenic substance. It is also dangerously unstable under certain conditions. This requires extremely strict and expensive safety protocols during manufacturing, transport, and fueling operations. Technicians must wear fully enclosed, self-contained atmospheric protective ensembles, known as SCAPE suits, to avoid any contact with the propellant or its fumes. These hazardous handling requirements add considerable time, cost, and complexity to any mission that uses hydrazine.

Bipropellant Thrusters: Power and Precision

When a mission requires higher performance than a monopropellant system can offer, engineers turn to bipropellant thrusters. These systems use two separate propellants: a fuel and an oxidizer. They are stored in separate tanks and are only mixed within the engine’s combustion chamber, where they react to produce thrust.

This two-propellant approach allows for a much more energetic chemical reaction than the decomposition of a monopropellant. The result is a significantly higher specific impulse, meaning more thrust is generated for each kilogram of propellant burned. This makes bipropellant systems the ideal choice for more demanding maneuvers that require a large change in velocity. These include the final “apogee kick” that places a communications satellite into its geostationary orbit, major trajectory correction maneuvers for interplanetary probes, and orbit insertion burns at other planets.

Many of the most common bipropellant combinations, such as monomethylhydrazine (MMH) as the fuel and nitrogen tetroxide (NTO) as the oxidizer, are hypergolic. This means they ignite spontaneously and immediately upon contact with each other. This hypergolic nature provides the same “igniter-free” reliability that makes monopropellant systems so attractive, which is a critical feature for engines that must fire flawlessly millions of kilometers from Earth. The legendary R-4D engine, originally developed for the Apollo Service Module and Command Module, is a testament to the power and reliability of this technology. Its basic design has been used on countless satellites and probes for over 50 years. While these traditional propellants offer excellent performance, they share the same toxicity and handling hazards as hydrazine, motivating a search for cleaner, safer alternatives.

The Green Revolution: Moving Beyond Hydrazine

Driven by mounting concerns over the safety, environmental impact, and high logistical costs associated with hydrazine and its derivatives, the space industry is in the midst of a major shift toward “green” propellants. This transition is not merely an environmental consideration; it represents a fundamental change in the economics and logistics of building and launching satellites. By eliminating the need for expensive hazardous material handling, green propellants can dramatically reduce the time and cost of preparing a spacecraft for flight.

One of the most promising classes of green propellants is based on high-energy ionic liquids. A leading example is a propellant blend based on Ammonium Dinitramide (ADN), a salt that acts as an oxidizer. The most well-known formulation, LMP-103S, developed in Sweden, dissolves ADN in a mixture of water, methanol, and ammonia. This blend offers superior performance to hydrazine, with a specific impulse that is over 6% higher and a density that is 24% greater. This combination results in a more than 30% increase in the total impulse that can be delivered from a tank of the same volume, enabling longer missions or more extensive maneuvering capabilities. Crucially, LMP-103S is significantly less toxic and safer to handle, classified for transport on passenger aircraft and eliminating the need for SCAPE suits during fueling. NASA has conducted extensive ground testing of thrusters using this propellant, validating its performance and potential.

Another leading alternative is a hydroxylammonium nitrate (HAN) based propellant, officially known as AF-M315E and marketed as ASCENT. Developed by the U.S. Air Force Research Laboratory, this fuel was successfully demonstrated in space on NASA’s Green Propellant Infusion Mission (GPIM), which launched in 2019. Like the ADN-based blends, ASCENT offers higher density and performance than hydrazine with greatly reduced toxicity, further proving the viability of green monopropellants.

A different approach to green propulsion is being pursued with bipropellant systems. Companies like Dawn Aerospace are pioneering thrusters that use a combination of nitrous oxide (laughing gas) and propene (a common hydrocarbon). These propellants are non-toxic, inexpensive, and widely available from domestic suppliers, avoiding the complex supply chains of traditional propellants. A key innovation of this system is that the propellants are self-pressurizing; they are stored as liquids and naturally create their own tank pressure as they vaporize. This elegantly simple design eliminates the need for a separate high-pressure helium tank and its associated regulators and plumbing, which are common points of failure in traditional systems. This reduction in component count not only lowers the mass and cost of the propulsion system but also significantly increases its overall reliability.

The Marathon Runners: Electric Propulsion Systems

While chemical rockets provide the powerful sprints needed for launch and major orbital changes, a different class of engine excels at the marathon. Electric propulsion (EP) systems have revolutionized long-duration space missions by trading raw power for incredible fuel efficiency. They are the gentle, persistent engines that have enabled spacecraft to journey to the farthest reaches of the solar system on a mere whisper of propellant.

The fundamental principle of electric propulsion is entirely different from its chemical counterpart. Instead of releasing energy stored within the propellant’s chemical bonds, EP systems use an external source of electrical power—almost always generated by the spacecraft’s solar panels—to accelerate a propellant. This process ejects a small amount of reaction mass at extremely high velocities, often ten to twenty times faster than the exhaust from a chemical rocket. According to the principles of rocketry, a higher exhaust velocity translates directly to a higher specific impulse. This means EP systems are vastly more fuel-efficient.

The trade-off for this remarkable efficiency is extremely low thrust. The force produced by a typical electric thruster is often measured in millinewtons, a force comparable to the weight of a single sheet of paper resting on your hand. This gentle push is far too weak to overcome a planet’s gravity, making EP systems unsuitable for launch. in the frictionless environment of space, this tiny thrust, applied continuously for months or even years, can produce a massive change in velocity. This makes electric propulsion the ideal choice for missions that are not time-critical, such as station-keeping for geostationary satellites, orbit raising for large satellite constellations, and interplanetary travel for robotic science probes.

Electrothermal Thrusters: Heating Things Up

The simplest category of electric propulsion, electrothermal thrusters, acts as a bridge between chemical and purely electric systems. They use electricity for the sole purpose of heating a propellant to increase its energy before it is expelled through a conventional nozzle.

The most basic of these is the resistojet. It operates much like a simple cold gas thruster, but with one key addition: an electric heating element, essentially a super-hot resistor filament, placed in the propellant flow path. As the gas passes over this element, it is heated to high temperatures. The heated gas molecules have more thermal energy, which is converted into kinetic energy as they expand through the nozzle. This simple act of heating can increase the specific impulse by 40% to 100% compared to an unheated cold gas system using the same propellant.

Taking this concept a step further is the arcjet. Instead of relying on a solid heating element, which is limited by its melting point, an arcjet strikes a high-voltage electric arc directly through the flow of propellant. This arc, similar to a continuous lightning bolt or the arc in a welder, heats the gas to plasma temperatures, reaching thousands of degrees Celsius. This intense heating allows arcjets to achieve a specific impulse significantly higher than resistojets. When using hydrazine as a propellant, for example, an arcjet can first benefit from the energy of catalytic decomposition and then add electrical energy via the arc, pushing its specific impulse to nearly 600 seconds.

Electrostatic Thrusters: The Power of Ions

The most prevalent and high-performance electric propulsion systems in use today fall into the category of electrostatic thrusters. These devices don’t just heat a gas; they use electric fields to directly accelerate charged particles, or ions, to incredible speeds.

The classic example is the gridded ion engine. Its operation is a multi-step process. First, a neutral propellant gas, typically the noble gas xenon, is fed into a discharge chamber. Inside the chamber, a stream of electrons bombards the xenon atoms, knocking their own electrons loose. This process, called ionization, leaves behind a cloud of positively charged xenon ions. At the rear of the chamber is a set of two or three finely perforated metal grids. A very high positive voltage is applied to the inner grid, while a high negative voltage is applied to the outer grid. This creates a powerful electrostatic field between them. The positive ions are drawn toward the grids and are then accelerated to tremendous speeds—up to 100 kilometers per second—as they pass through the holes. This high-velocity beam of ions produces the thrust. A final, crucial component is the neutralizer, a separate cathode outside the engine that emits a stream of electrons into the ion beam. This neutralizes the exhaust, preventing the spacecraft from building up a negative electrical charge that would otherwise attract the positive ions back to it, canceling out the thrust. NASA’s groundbreaking Deep Space 1 and Dawn missions used gridded ion engines to successfully travel to asteroids and dwarf planets, demonstrating the technology’s remarkable endurance and efficiency over years of continuous operation.

A more recent and widely adopted innovation is the Hall-effect thruster (HET). Hall thrusters also use an electric field to accelerate ions, but they achieve this without the complex and potentially life-limiting grid system. In a Hall thruster, an annular channel is surrounded by electromagnets that create a strong radial magnetic field. An anode at the back of the channel injects the neutral xenon propellant, while a cathode outside the thruster emits electrons. These electrons are drawn toward the positive anode but are trapped by the magnetic field, forcing them into a swirling, high-speed circular current within the channel. This trapped cloud of electrons effectively forms a “virtual cathode.” As the neutral xenon atoms drift into this energetic electron cloud, they are efficiently ionized. The newly created positive xenon ions are not massive enough to be trapped by the magnetic field and are instead powerfully accelerated out of the channel by the strong electric field between the anode and the trapped electron cloud. Hall thrusters generally offer more thrust than gridded ion engines for a given amount of power, though at a slightly lower specific impulse. This balance of characteristics makes them exceptionally well-suited for commercial applications like raising large telecommunications satellites to geostationary orbit and for maintaining the orbits of large satellite constellations like Starlink.

Novel Propellants for an Electric Future

The performance and viability of electric propulsion systems are inextricably linked to the propellants they use. For decades, one element has dominated the field, but its limitations are now driving a wave of innovation toward new and more sustainable alternatives.

Xenon: The Noble Standard

For years, xenon has been the propellant of choice for high-performance electric propulsion. As a noble gas, it is chemically inert, meaning it won’t corrode engine components over long-duration missions. It has a high atomic mass, which is desirable for generating thrust, and a relatively low ionization energy, making it efficient to create the ions needed for acceleration. The primary drawback of xenon is its scarcity and cost. It is a rare byproduct of industrial air separation plants, and its price can fluctuate dramatically, making it a significant cost driver for missions that require large amounts, such as satellite constellations. Furthermore, it must be stored as a high-pressure gas, which requires heavy and bulky tanks, a significant disadvantage for small, volume-constrained satellites.

Iodine: The Dense Challenger

Iodine is rapidly emerging as a revolutionary alternative to xenon. Its performance as a propellant in a Hall thruster is remarkably similar to that of xenon, but it possesses one transformative advantage: it can be stored as a solid. Solid iodine is nearly three times denser than xenon stored as a compressed gas. This means a spacecraft can carry significantly more propellant in the same amount of space, or use a much smaller and lighter tank for the same amount of fuel. This dramatic increase in storage density is a game-changer for small satellites, potentially giving a CubeSat enough delta-v to perform an interplanetary mission. To be used, the solid iodine is gently heated, causing it to sublimate directly into a gas, which is then fed to the thruster. The first successful in-space demonstration of an iodine-fueled propulsion system occurred in 2019, marking a major milestone for this promising technology.

Water: The Ubiquitous Option

Looking toward a future of sustainable, long-term space operations, water is being seriously investigated as the ultimate green propellant. It is abundant, incredibly cheap, and perfectly safe to handle. In its simplest application, water can be vaporized and used in an electrothermal thruster like a resistojet. A more advanced and powerful concept involves using an onboard electrolyzer, powered by solar panels, to split water into its constituent hydrogen and oxygen. These gases can then be used as high-performance propellants in either a chemical bipropellant thruster or a highly efficient electric thruster. The development of water propulsion is not just about finding a safer propellant for satellites launched from Earth; it is a foundational technology for a future of in-situ resource utilization (ISRU). Water ice is known to exist on the Moon, on Mars, and on many asteroids. A spacecraft capable of mining this ice and turning it into propellant could refuel itself in deep space, enabling an era of nearly unlimited exploration of the solar system.

Metals: The Solid State Solution

Some of the most advanced electric propulsion concepts eliminate the need for propellant tanks, pipes, and valves entirely by using solid metal as the propellant. In a type of thruster known as a vacuum arc thruster, a high-current electric arc is struck directly on the surface of a solid, conductive cathode. The intense energy of the arc vaporizes and ionizes a small amount of the cathode material itself, creating a plasma plume that is then accelerated away to produce thrust. The cathode, in this elegant design, serves as both a structural component of the engine and the entire propellant supply. Metals like molybdenum, titanium, and even recycled aluminum alloys can be used. This approach offers a radical simplification in propulsion system design, promising high reliability and an extremely compact form factor, making it an intriguing option for future small spacecraft.

The Horizon: Next-Generation Propulsion

While chemical and electric propulsion systems have enabled the exploration of our solar system, the grander ambitions of humanity—rapid interplanetary travel, interstellar probes, and a sustainable in-space economy—will require new technologies that push the boundaries of physics and engineering. On the horizon are several next-generation concepts that promise to redefine the limits of what is possible in space.

Propulsion Without Propellant

The ultimate dream of space propulsion is to move without the need to carry and expend reaction mass. This would free spacecraft from the constraints of the Tsiolkovsky rocket equation, allowing for nearly unlimited delta-v. Two such concepts are moving from science fiction to reality.

Solar Sails

A solar sail is a technology that achieves propulsion by harnessing the most abundant resource in the solar system: sunlight. It is a large, lightweight, and highly reflective membrane, akin to a giant mirror in space. Light, though massless, carries momentum. When photons from the Sun strike the surface of the sail and bounce off, they transfer a tiny amount of their momentum to it, resulting in a gentle but continuous push. This force, known as solar radiation pressure, is incredibly small—on a sail the size of a boxing ring, the total force at Earth’s distance from the Sun is equivalent to the weight of a paperclip.

in the vacuum of space, this minuscule force is applied constantly. Day after day, year after year, the sail continues to accelerate. Over long periods, a solar sail-propelled spacecraft can reach speeds far greater than what is achievable with conventional chemical rockets. By changing the angle of the sail relative to the Sun, a spacecraft can “tack” through the solar system, spiraling inwards or outwards, much like a sailboat on the ocean. This technology was successfully demonstrated by The Planetary Society’s LightSail 2 mission. Launched in 2019, this small CubeSat deployed a 32-square-meter sail and, over several months, used only the pressure of sunlight to measurably raise its orbit, proving the viability of controlled solar sailing.

Electrodynamic Tethers

Another propellantless concept is the electrodynamic tether. This system is designed to work in orbit around a planet with a significant magnetic field and an ionosphere, such as Earth. It involves deploying a very long, electrically conductive wire, potentially many kilometers in length, from the spacecraft. As the conductive tether sweeps through the planet’s magnetic field at orbital velocity, a voltage is induced along its length through Faraday’s law of induction.

By driving a current through the tether using power from solar panels, the system can generate a Lorentz force. This is the same principle that makes an electric motor turn. This force pushes against the planet’s magnetic field, which in turn exerts a force on the tether and the attached spacecraft, causing it to accelerate and raise its orbit. The process can also be run in reverse. By drawing current from the induced voltage and running it through a resistor, the tether can generate drag against the magnetic field, slowing the spacecraft down and lowering its orbit, while simultaneously generating electrical power. This makes electrodynamic tethers a unique, propellant-free technology for both propulsion and deorbiting.

The Return of the Atom

For missions that require a combination of high thrust and high efficiency—particularly rapid human transit across the solar system—engineers are once again turning to the immense power locked within the atomic nucleus. Building on the legacy of the NERVA program, nuclear propulsion is experiencing a major resurgence.

Nuclear Thermal Propulsion (NTP)

There is renewed and serious investment from NASA and other agencies in developing Nuclear Thermal Propulsion for crewed missions to Mars. The fundamental advantage of NTP remains unchanged from the NERVA era: it offers the potential for a specific impulse of 900 seconds or more, double that of the best chemical rockets, while still producing a high level of thrust. This unique combination could drastically reduce the transit time to Mars, potentially cutting a one-way trip from 9 months down to 6 or even less. This is not just a matter of convenience; a shorter trip time significantly reduces the crew’s exposure to the dangers of deep-space radiation and the physiological effects of prolonged weightlessness. Current research is focused on developing new reactor designs and advanced fuel materials that can operate at the required high temperatures while using high-assay low-enriched uranium (HALEU), which is safer and more proliferation-resistant than the highly enriched uranium used in the past.

Nuclear Electric Propulsion (NEP)

For missions where high power and extreme efficiency are more important than travel time, Nuclear Electric Propulsion is the leading future candidate. NEP uses a space-rated nuclear fission reactor not to heat propellant directly, but to generate vast amounts of electricity—from hundreds of kilowatts to several megawatts—over a mission lifetime of a decade or more. This electricity would then be used to power a bank of advanced, high-performance electric thrusters, such as Hall thrusters or ion engines.

An NEP system would enable ambitious robotic science missions that are simply not possible today. A probe sent to the outer planets could carry a much larger and more capable suite of scientific instruments. Upon arrival, it would have enough power and propellant to spend years maneuvering between moons, entering and leaving orbits at will, rather than being limited to brief flybys. For human exploration, NEP is seen as a key technology for transporting heavy cargo to Mars in advance of a crew’s arrival, efficiently pre-positioning habitats, supplies, and return vehicles.

A Comparative Look at Propulsion Technologies

The vast array of propulsion technologies, from the simplest cold gas thruster to the most advanced nuclear concepts, highlights a central theme: there is no single “best” engine. The choice of a propulsion system is always a complex trade-off between thrust, efficiency, power requirements, cost, and mission objectives. A satellite designed for Earth observation in a crowded LEO environment needs the responsiveness of chemical thrusters for rapid deployment and collision avoidance, but also the high efficiency of electric thrusters for long-term drag compensation. A scientific probe destined for Jupiter may use a powerful chemical upper stage to escape Earth’s gravity, followed by years of gentle, continuous thrust from a solar-powered ion engine. A crewed mission to Mars demands the unique blend of high thrust and high specific impulse that only nuclear thermal propulsion can provide.

The future of space exploration will likely not be dominated by a single technology but by a diverse and integrated ecosystem of specialized systems. We may see the rise of dedicated Orbital Transfer Vehicles (OTVs), essentially space tugs equipped with powerful and efficient engines, designed to move satellites from their initial drop-off orbit to their final operational positions. A satellite could be delivered to LEO by a launch vehicle, picked up by a green chemical OTV for a rapid transfer to geostationary orbit, and then released to perform its own station-keeping for 15 years using a highly efficient iodine-fueled Hall thruster. This modular, multi-stage approach to in-space transportation is the logical evolution of the diverse and increasingly capable propulsion technologies being developed today.

Summary

The journey of satellite propulsion mirrors the evolution of our ambitions in space. It began with the silent, passive drift of Sputnik, a marvel simply for reaching orbit. This quickly gave way to the necessity of control, ushering in the era of chemical propulsion. For decades, reliable but hazardous propellants like hydrazine powered the expansion of our satellite infrastructure, enabling everything from global communications to planetary exploration. These high-thrust systems remain the workhorses for rapid, powerful maneuvers.

The present is defined by two parallel revolutions. The first is the rise of electric propulsion. With efficiencies that dwarf their chemical cousins, ion and Hall thrusters have unlocked the ability to conduct long-duration missions with finite fuel, transforming deep-space science and enabling the mega-constellations that now populate low Earth orbit. The second revolution is the move toward sustainability. Driven by a need for safer and more cost-effective operations, “green” chemical propellants and novel electric propellants like iodine and water are rapidly maturing. These innovations are not just incremental improvements; they are fundamentally changing the logistics and economics of spaceflight, making it more accessible and sustainable.

Looking to the horizon, the next generation of propulsion technologies promises to open the solar system in ways previously confined to science fiction. Propellantless systems like solar sails and electrodynamic tethers offer the prospect of nearly limitless travel by harnessing the natural forces of the cosmos. And the return of nuclear propulsion, in both its thermal and electric forms, holds the key to the rapid human exploration of Mars and the comprehensive robotic exploration of the outer planets. From a simple puff of cold gas to the controlled fury of a nuclear reactor, the engines of the cosmos will continue to evolve, carrying our dreams and discoveries ever further into the void.