From last time: Laboratory samples of extraterrestrial matter

• Solar Wind sample collection by Apollo 11 astronauts (Aldrin)
• interplanetary dust (Stardust-NExT)

Fundamental Quantities

• intensity, flux
• inverse square law, care and feeding
• energy density, pressure
• integrated quantities
• broadband filters: transmission curves for the home team

Transfer

• conservation of surface brightness
• sources and sinks
• transfer equation

Scattering

• cross section
• mean free path
• optical depth