Ablation of materials in the low-earth orbital environment

Abstract

The ablation of materials exposed on external ram surfaces of a spacecraft in orbit by gas particles in the atmosphere is investigated. A spacecraft in low-earth orbit moves through the atmosphere at about 8 km sec , allowing gas particles to impinge upon spacecraft surfaces with kinetic energies comparable to the cohesive energies of elementary materials. Experiments flown on Space Shuttle missions indicate that certain materials can undergo significant mass loss due to exposure to the atmospheric particle flux. These exposure experiments are reviewed. A physical sputtering model is presented for use in predicting the behaviors of materials in the low-earth orbital environment. The physical sputtering rates of a number of elementary materials are estimated as a function of orbital altitude in the 100–1000 km range. It is shown that the physical sputtering rates of certain materials may result in significant mass loss after only a few months or years of exposure time. For many materials the effect of physical sputtering in the low-earth orbital environment is not observable even after many tens of years of exposure time. Chemical sputtering rates by atomic oxygen in the low-earth orbital environment are compared with calculated physical sputtering rates, and it is noted that the model presented here is a lower bound calculation since chemical sputtering generally causes mass loss at a higher rate. The need for physical sputtering data on polymer materials for comparison with ablation rates measured in Shuttle exposure experiments is also discussed. This work has implications for the Tethered Satellite System, Space Station, and Long-Duration Exposure Facilities to be deployed in the low-earth orbital environment, and may be extended to orbiting spacecraft in non-terrestrial atmospheres as well.