Light Propulsion Systems for Spacecraft

Glossary ......................................................................................................................................... 596 References ...................................................................................................................................... 596

e ⃗f Unit vector that is perpendicular to e⃗r and along the projection of f ⃗ in the e⃗t-e⃗h-plane e⃗h Coordinate frame unit vector in orbit normal direction e⃗r Coordinate frame unit vector in radial direction e⃗t Coordinate frame unit vector in transversal direction f ⃗Thrust unit vector F ⃗ Thrust force F⃗f Thrust force along e ⃗f Fn⃗ Thrust force along n ⃗ Fr⃗ Thrust force along e⃗r Ft⃗ Thrust force along t⃗ h Sail „lm’s surface roughness m Spacecraft mass m0 Initial spacecraft mass mf Final spacecraft mass n ⃗Sail normal (unit) vector  Heliocentric coordinate frame P Solar radiation pressure P0 Solar radiation pressure at Earth’s solar distance  Set of optical sail coef„cients r Solar distance r0 Earth’s (mean) solar distance, 1 astronomical unit (1 AU) S Solar radiation Œux s Specular reŒection factor T (Absolute) sail temperature t⃗ Sail tangential (unit) vector v ⃗ Spacecraft velocity vector vesc Solar system escape velocity Ve Propellant exhaust velocity ΔV Required velocity increment that has to be provided by the spacecraft propulsion system α Sail pitch angle β Solar sail lightness number ε Emission coef„cient εb Emission coef„cient of the sail’s back side εf Emission coef„cient of the sail’s front side α ReŒection coef„cient δ Sail clock angle θ Thrust cone angle η Sail ef„ciency parameter ρ ReŒection coef„cient ρb Back reŒection coef„cient ρd Diffuse reŒection coef„cient ρs Specular reŒection coef„cient τ Transmission coef„cient σ Stefan-Boltzmann constant Ψ Non-perfectly reŒecting sail function ϕ Centerline angle μ Gravitational parameter of the Sun Σ Solar radiation dose * Optimal value

While being useful to drive nano-and micro-scale actuators down here on Earth, in space, the radiation pressure of light can be used to propel even macroscopic devices, i.e., spacecraft. Systems that are propelled by sunlight allow-already with current and near-term technology-innovative space missions. In the far future, the radiation pressure of laser light may propel spacecraft even to nearby stars. This chapter focuses on the current to near-term technology of solar sails as light propulsion systems for spacecraft. The more futuristic concept of laser sails and other concepts are also introduced brieŒy.