Solar propulsion

Deutsch: Solarer Antrieb / Español: Propulsión solar / Português: Propulsão solar / Français: Propulsion solaire / Italiano: Propulsione solare

Solar propulsion in the space industry refers to propulsion systems that utilise solar energy to generate thrust or power spacecraft. This can involve direct conversion of solar radiation into kinetic energy through solar sails or indirect use by converting solar energy into electricity to power electric propulsion systems. Solar propulsion is a sustainable and efficient method for deep space missions and long-duration operations, relying on the abundant energy of the sun.

Description

Solar propulsion represents an innovative and sustainable approach to spacecraft propulsion in the space industry, harnessing solar energy to enable long-duration, fuel-efficient space travel. It contrasts with traditional chemical propulsion by using solar radiation either to directly produce thrust or to generate electrical power for propulsion systems.

There are two primary types of solar propulsion systems commonly used or researched in the space industry:

• Solar Sail Propulsion: This method uses large, reflective sails made of lightweight materials such as Mylar or Kapton to capture momentum from photons emitted by the sun. Although photons have no mass, they carry momentum, and their continuous impact on the sail generates a small but constant thrust. Over time, this thrust can accelerate a spacecraft to significant speeds. Solar sails are fuel-free and ideal for long-distance, low-thrust missions.

• Solar Electric Propulsion (SEP): In SEP systems, solar panels convert sunlight into electrical energy, which powers ion thrusters or Hall-effect thrusters. These engines expel ionised particles at extremely high velocities, generating thrust. Although SEP provides low thrust compared to chemical propulsion, it is highly efficient and suitable for deep space missions, satellite station-keeping, and orbit transfers.

Solar propulsion systems offer several advantages:

• Efficiency: Especially with SEP, solar propulsion provides a much higher specific impulse (a measure of efficiency) than chemical rockets.
• Sustainability: Solar energy is abundant and renewable within the inner solar system, eliminating the need for large quantities of onboard propellant in the case of solar sails.
• Longevity: Continuous and steady thrust from solar propulsion enables extended missions and trajectory corrections over time.

Historically, solar propulsion concepts date back to the early 20th century. The first practical demonstration was IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun), launched by Japan's JAXA in 2010. IKAROS was the first spacecraft to successfully demonstrate solar sail technology in interplanetary space.

Solar Electric Propulsion has already been applied in missions such as NASA’s Dawn spacecraft, which used ion propulsion powered by solar panels to explore the asteroids Vesta and Ceres. SEP is also a critical component of the European Space Agency’s (ESA) BepiColombo mission to Mercury.

As solar power diminishes with distance from the sun, solar propulsion is generally limited to missions within the inner solar system (out to the orbit of Jupiter). Beyond this range, the efficiency of solar panels and photon pressure decreases significantly, requiring alternative propulsion methods such as nuclear power.

Special Considerations in Solar Propulsion Design

Special Challenges in Solar Sail Deployment and SEP Power Generation

Solar sail missions face technical challenges with deployment mechanisms—sails need to be extremely large yet lightweight, requiring precise unfolding in space. The sails must be made of materials that can withstand micrometeoroid impacts, UV radiation, and thermal variations.

In Solar Electric Propulsion, the design of high-efficiency solar arrays is crucial. Arrays must be optimised for maximum power generation while maintaining lightweight structures and minimising degradation over time due to radiation exposure.

Thermal control systems are also essential to prevent overheating of solar panels and thruster components in proximity to the sun.

Application Areas

• Deep Space Exploration: Enabling long-duration missions to the asteroids, inner planets, and even beyond the solar system (conceptually).
• Satellite Station-Keeping: Maintaining satellite positions in geostationary orbits with low-thrust adjustments.
• Orbit Transfers: Efficiently moving satellites between different orbital planes or altitudes.
• Asteroid Missions: Powering spacecraft like Dawn for multiple target exploration in the asteroid belt.
• Interstellar Precursor Missions: Conceptual missions proposing solar sails as a means of achieving high speeds for leaving the solar system.

Well-Known Examples

• IKAROS (JAXA): The first spacecraft to successfully deploy and navigate with a solar sail in interplanetary space.
• LightSail 2 (The Planetary Society): Demonstrated controlled solar sailing in Earth orbit, using sunlight for propulsion.
• NASA Dawn Mission: Employed solar electric propulsion to study Vesta and Ceres, demonstrating the feasibility of SEP in deep space.
• BepiColombo (ESA and JAXA): Utilises SEP to travel from Earth to Mercury, leveraging the efficiency of electric propulsion powered by solar energy.
• NEA Scout (NASA): A solar sail mission planned to explore near-Earth asteroids using a deployable solar sail.

Risks and Challenges

• Reduced Efficiency with Distance: Solar energy availability decreases with distance from the sun, limiting mission effectiveness in the outer solar system.
• Structural and Material Challenges: Solar sails require extremely thin yet durable materials, and deployment mechanisms are prone to failure.
• Low Thrust Levels: Both solar sails and SEP provide continuous but low acceleration, making them unsuitable for rapid manoeuvres or high-thrust applications.
• Thermal Management: Maintaining optimal operating temperatures for solar arrays and propulsion systems in varying solar intensities is complex.
• Radiation and Degradation: Prolonged exposure to solar radiation can degrade materials, reducing efficiency over time.

Similar Terms

• Electric Propulsion: Encompasses propulsion systems like ion thrusters and Hall-effect thrusters, often powered by solar energy in SEP configurations.
• Photon Propulsion: A concept where propulsion relies on momentum from photons, applicable in solar sails.
• Nuclear Electric Propulsion (NEP): An alternative to SEP that uses nuclear reactors instead of solar panels to generate electricity for propulsion.
• Laser-Pushed Sails: An advanced concept using ground-based or orbital lasers to push solar sails, potentially enabling interstellar travel.

Summary

Solar propulsion in the space industry provides a sustainable and efficient method of powering spacecraft, enabling long-duration missions with minimal propellant use. Through solar sails and solar electric propulsion systems, this technology supports a wide range of applications, from satellite maintenance to deep space exploration, advancing the future of space travel with environmentally friendly and cost-effective solutions.

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