Question

List at least two mechanisms by which the accomodation coefficients for a spacecraft material can change due to the vacuum environment. How might these changes be avoided or minimized? Why does the rate of change in the accomodation coefficient decrease with satellite lifetime?

Answer 1

Two mechanisms can be:(1) Sublimation and evaporation

(2)Radiation and particle damage

Radiation and thermal cycling must not degrade thermal-control surfaces or joints in materials possessing different coefficient of expansion.The presence of atomic oxygen in low Earth orbits, a relative newcomer in environmental effects, has been seen to lead to the erosion/corrosion/oxidation of many material surfaces, and more coatings with good atomic oxygen durability need to be developed.The minimum altitude for an Earth-orbiting satellite is 200 km (125 miles), and, once this altitude has been reached, appreciable changes can be produced in common engineering materials, whether they be metals, plastics, or ceramics. The vacuum in space is very high, the pressure falling from 10?6 mm Hg at 200 km to less than 10?12 mm Hg beyond 6500 km. Some polymers will decompose and some metals will tend to sublimate under vacuum. The rate at which the molecules or atoms leave a surface in vacuum will rise rapidly with an increase in temperature.