Space Environment October 1, 2003 H. Kirkici Istanbul Technical University Lecture-6 and 7 Vacuum Environment (in the ionosphere and above range) Effects and Problem Issues Thermal effects Contamination (due to outgassing) UV radiation Mass loss (due to outgassing) Particulate (no kinetic energy)

Space Environment October 1, 2003 H. Kirkici Istanbul Technical University Thermal effectsThermal control of spacecraft Heat transfer processes Convectionnot possible in vacuum Conduction possible but may require additional weight and expense RadiationOnly effective method Heat absorbed from the Sun is: Radiated energy from satellite Lecture-6 and 7

Space Environment October 1, 2003 H. Kirkici Istanbul Technical University Absorbed heat Radiated heat whereis the material’s absorbtance is the surface are normal to the solar flux, in m2 is the solar flux (W/m2) is the material emittance is the Boltzmann’s constant is the total surface are of the object (m2) is the object temperature (K) Lecture-6 and 7

Space Environment October 1, 2003 H. Kirkici Istanbul Technical University If we assume there are not other heat source present To sustain an equilibrium, we have: Absorbed heat = Radiated heat This results in an equilibrium temperature Exercise-1: Drive the equilibrium temperature T for this idea case Absorptance and Emittance of some of the spacecraft materials will be posted as a Table at a later time Lecture-6 and 7

Space Environment October 1, 2003 H. Kirkici Istanbul Technical University Active heat control (thermal radiators) Works good, but adds weight to the spacecraft, may not be an optimum solution) Passive heat control (use materials having low values of absorptance, and high emittance), Works good, but may need to be cautious that the material thermal properties may change over time due to outgassing and UV radiation Solution: Lecture-6 and 7

Space Environment October 1, 2003 H. Kirkici Istanbul Technical University UV Degradation UV light: wavelength less than ~300 nm on orbit, full strength of UV is present Approximate energy of a UV photon at 200 nm is ~6.3 eV at 300 nm is ~3.8 eV Photon Energy: Energy sufficient to breakdown chemical bond between molecules Lecture-6 and 7

Space Environment October 1, 2003 H. Kirkici Istanbul Technical University Contamination (molecular) Outgassing during launch or in orbit Desorption: release of surface molecules, mainly in metals, causes mass reduction Diffusion: random thermal motion, and sufficient thermal energy, mainly seen in organic materials and causes greater mass loss Decomposition: Chemical reaction resulting in smaller molecules, less likely problem than the other two Depend on Activation energy Binding energy And other factors…. Lecture-6 and 7

Space Environment October 1, 2003 H. Kirkici Istanbul Technical University Activation energy Time Dependence Desorption: 1 – 10 kcal/mole Diffusion: 5 – 15 kcal/mole Decomposition: 20 – 80 kcal/molen/a Outgassing characteristics are determined experimentally No definite analytical equations to predict outgassing rates Material dependent (initial contamination and fabrication) Lecture-6 and 7

Space Environment October 1, 2003 H. Kirkici Istanbul Technical University Molecules do not scatter away from the surface Will adhere to the surface and establish thermal equilibrium Will arrive from variety of direction (line-of-sight or non-line-of-sight) May leave the surface when they gain enough energy form the surface Stay longer on cooler surfaces (most difficult on cryogenic surfaces) Approximate resident time is: Lecture-6 and 7

Space Environment October 1, 2003 H. Kirkici Istanbul Technical University Problems due to outgassing Thermal control: solar absorptance change (increases with film thickness) (higher in UV than IR region) Solar cell:output degradation (power conversion efficiency drops) Synergistic Effects: Interactions between the effects can be more problematic than the individual effects totaled absorptance thickness thickness Output power Lecture-6 and 7

Space Environment October 1, 2003 H. Kirkici Istanbul Technical University Particulate contamination: Residual particles on the part during manufacturing and fabrication (not direct space environmental effect) From the spacecraft design and construction point, it is an important factor, many standards available Engineering problem!.... Lecture-6 and 7

Space Environment October 1, 2003 H. Kirkici Istanbul Technical University Questions? Lecture-6 and 7