Spacecraft propulsion

Spacecraft propulsion comprises the range of technologies and methods used to accelerate spacecraft and artificial satellites once they have entered space. Unlike launch propulsion, which must overcome atmospheric drag and Earth’s gravity, in-space propulsion operates exclusively within the vacuum of space and is fundamental to orbital manoeuvring, trajectory shaping, station-keeping, and interplanetary travel. A wide array of established and hypothetical propulsion systems have been developed to support missions involving satellites in Earth orbit, robotic probes exploring the outer planets, and proposals for future human expeditions to Mars and beyond.

Background and Conceptual Foundations

In-space propulsion begins where the upper stage of the launch vehicle ceases to operate. The basic purpose of any propulsion system in space is to change a spacecraft’s velocity (Δv). Even very small impulses can meaningfully alter an orbit, making precise manoeuvrability both possible and essential.
The fundamental physics of spacecraft propulsion are captured in the Tsiolkovsky rocket equation, which applies the conservation of momentum to rocket motion. To accelerate in one direction, a spacecraft must expel mass—known as propellant or reaction mass—in the opposite direction. The expelled mass carries kinetic energy, the source of which must be supplied either through chemical combustion or through an external energy source such as solar or nuclear power.
The escape velocity from Earth, approximately 11.2 km/s, illustrates the depth of Earth’s gravity well and underscores the high energy requirements for interplanetary missions. Other celestial bodies possess lower escape velocities, simplifying ascent, but the requirement for efficient propulsion remains.
As humans evolved in a 1 g environment, long-duration free fall causes physiological issues including nausea, muscular atrophy and bone demineralisation. Continuous low-level acceleration—if available—could theoretically mitigate these effects, although no current propulsion system provides sustained 1 g acceleration for interplanetary travel.

Established Chemical Propulsion Technologies

Chemical propulsion remains the most commonly used method for spacecraft manoeuvring. It includes monopropellant, bipropellant, solid, and hybrid propulsion systems, many of which are time-tested and offer high thrust suitable for rapid orbital adjustments.
Key characteristics include:

• High thrust: suitable for quick orbital changes, capture burns, and descent/ascent operations.
• Low specific impulse: typically around 300 seconds, meaning they consume more propellant per unit of thrust compared with electric alternatives.
• Reliability: chemical thrusters are simple, well-understood, and easily integrated into satellite systems.

Most satellites use small chemical thrusters for station-keeping and attitude control. For example:

• Monopropellant hydrazine thrusters provide reliable, moderate-thrust manoeuvring.
• Resistojet systems, which heat a propellant electrically before expulsion, offer improved efficiency over basic monopropellant systems.
• Reaction control systems (RCS) such as those used on the Apollo Lunar Module provide fine attitude and translation control.

Chemical propulsion also dominates interplanetary missions due to its high thrust output, which is necessary for major trajectory-altering burns, gravity-assist corrections and landing sequences.

Electric Propulsion Technologies

Electric propulsion systems use electrical power—typically from solar arrays or nuclear sources—to accelerate ionised propellant to extremely high exhaust velocities. These systems have been used extensively by Russian and Soviet-era satellites and are increasingly being adopted by Western geostationary spacecraft for efficient station-keeping and orbit raising.
Common electric propulsion technologies include:

• Ion thrusters: which accelerate ions using electrostatic fields and can achieve specific impulses around 3000 seconds.
• Hall-effect thrusters: which use magnetic fields to trap and accelerate electrons, offering a balance of efficiency and thrust.
• Electrodynamic (or electromagnetic) thrusters: which use electric and magnetic fields to accelerate charged particles to very high velocities.

Advantages include:

• Extremely high efficiency: far greater momentum change per unit of propellant mass.
• Reduced propellant requirements: allowing a much larger fraction of spacecraft mass to be allocated to instruments, payload, or shielding.
• Suitability for long-duration missions: where small but continuous thrust accumulates to achieve significant Δv.

Limitations include:

• Low thrust: making electric propulsion unsuitable for launch vehicles or for manoeuvres requiring rapid impulse.
• Power constraints: spacecraft must carry substantial power generation capability.

Even with these limitations, electric propulsion enables efficient deep-space missions, and many modern spacecraft trade higher transit times for substantial mass savings.

Propulsion for Orbital Manoeuvring and Station-Keeping

In-space propulsion systems play a crucial role in maintaining stable orbits and controlling spacecraft attitude. The two primary categories of manoeuvres are:

• Prograde/retrograde burns: which increase or decrease orbital altitude by altering the tangential velocity.
• Inclination changes: achieved by firing thrusters perpendicular to the orbital plane.

Specific operations supported by in-space propulsion include:

• Geostationary orbit insertion
• Hohmann transfer manoeuvres
• Attitude control
• Station-keeping against gravitational perturbations
• Orbit maintenance for low Earth orbit satellites

Additional micro-impulses can be provided by reaction wheels and control moment gyroscopes, though these devices store momentum rather than expelling mass and thus are not classified as propulsion systems.

Efficiency and Specific Impulse

The efficiency of a propulsion system is most commonly measured using specific impulse (Iₛₚ), defined as the impulse delivered per unit weight of propellant. In practical terms, it represents how effectively a system uses its reaction mass.
Two equivalent formulations of specific impulse are used:

• Impulse per unit weight on Earth (seconds)
• Effective exhaust velocity (m/s)

Electric propulsion systems demonstrate the advantages of high specific impulse. Although they deliver low thrust, they require far less propellant to achieve the same Δv. For instance:

• A traditional chemical propulsion spacecraft may deliver only 2% of its initial mass to a deep-space destination.
• An electric-propelled spacecraft may deliver over 70% of its initial mass.

However, achieving these efficiencies requires substantial electrical power, and therefore electric systems are not viable for high-thrust applications such as launch or rapid braking during orbit insertion.

Hypothetical and Advanced Propulsion Concepts

Proposed future propulsion methods aim to enable high-speed travel throughout the Solar System, reduced transit times for crewed missions and improved mission reliability. Concepts include:

• Advanced nuclear propulsion: such as nuclear-thermal and nuclear-electric systems.
• Beamed propulsion: using directed energy such as lasers to push light sails or provide power to distant spacecraft.
• Magnetic sails: which interact with solar wind or planetary magnetic fields.
• Fusion-based systems: offering extremely high specific impulse if technological hurdles are overcome.

These technologies, though not yet operational, are considered potential enablers for long-range exploration missions that could launch at flexible times and reach destinations more rapidly.

Mission-Level Implications and Considerations

Propulsion systems influence every major aspect of spacecraft mission design, including:

• Payload capacity: since propellant mass must be balanced against scientific instruments or crew life-support.
• Transit time: a critical factor for human missions where prolonged exposure to radiation and microgravity presents physiological challenges.
• Cost efficiency: systems with higher specific impulse can reduce overall mission mass and associated launch costs.
• Reliability and risk: redundant or hybrid propulsion architectures may be preferred for long-duration missions, where failures could jeopardise mission success.

In high-gravity environments, such as Earth, only high-thrust propulsion can provide positive net acceleration. In contrast, once in orbit, even very small accelerations accumulate to enable precise manoeuvres.
As mission requirements diversify, expert consensus indicates that no single propulsion system can meet all future needs. Instead, a portfolio of complementary technologies—from high-thrust chemical rockets to high-efficiency electric thrusters and advanced experimental systems—is expected to serve the broad spectrum of near-Earth, lunar, and deep-space missions.