Atomic oxygen effects on boron nitride and silicon nitride: A comparison of ground based and space flight data
The effects of atomic oxygen on boron nitride (BN) and silicon nitride (Si3N4) were evaluated in a low Earth orbit (LEO) flight experiment and in a ground based simulation facility. In both the inflight and ground based experiments, these materials were coated on thin (approx. 250A) silver films, and the electrical resistance of the silver was measured in situ to detect any penetration of atomic oxygen through the BN and Si3N4 materials. In the presence of atomic oxygen, silver oxidizes to form silver oxide, which has a much higher electrical resistance than pure silver. Permeation of atomic oxygen through BN, as indicated by an increase in the electrical resistance of the silver underneath, was observed in both the inflight and ground based experiments. In contrast, no permeation of atomic oxygen through Si3N4 was observed in either the inflight or ground based experiments. The ground based results show good qualitative correlation with the LEO flight results, indicating that ground based facilities such as the one at Los Alamos National Lab can reproduce space flight data from LEO.
STS-8 atomic oxygen effects experiment
A flight experiment was performed on the eighth Space Shuttle mission to measure reaction of surfaces with atomic oxygen in the low earth orbital environment. More than 300 individual samples were exposed to ram (normal to surface) conditions for 41.75 hr leading to a total atomic oxygen fluence of 3.5 x 10 to the 20th atoms/sq cm. Reaction rates for surface recession measured primarily by mass change of several organic films were in the range of 3.0 x 10 to the -24th cu cm/atom, and less than 5 x 10 to the -26th cu cm/atom for Teflon. Effects of parameters such as temperature and solar radiation were assessed, as was the importance of atmospheric ionic species on surface recession. In an experiment performed on the fifth Space Shuttle flight, no temperature dependence of reaction rate for the organic films studied was found in the temperature range of 25 to 125 C. Preliminary findings indicate that the reactivity of organic films is not affected by temperature (in the range of 65 to 125 C), solar radiation, or ionic species. Significant surface morphology changes led to a carpet-like appearance also consistent with previous findings.
A global thermospheric model based on mass spectrometer and incoherent scatter data MSIS, 1. N2 density and temperature
Measurements of neutral N2 density from mass spectrometers on five satellites (AE-B, Ogo 6, San Marco 3, Aeros A, and AE-C) and neutral temperatures inferred from incoherent scatter measurements at four ground stations (Arecibo, Jicamarca, Millstone Hill, and St. Santin) have been combined to produce a model of thermospheric neutral temperatures and N2 densities similar to the Ogo 6 empirical model. The satellite- and ground-based measurements provide unique and complementary information on the structure of the thermosphere. Incoherent scatter data have good local time coverage at the station locations and provide data for studies of long-term trends. Measurements from satellites provide information at different altitudes, latitudes, longitudes, solar activities, and seasons. The overall data set covers the time period from the end of 1965 to mid-1975. The global average temperature from the new model for an F10.7 of 150 is 1041°K or 56°K lower than that from the Ogo 6 model. The annual temperature variation is about two-thirds that of the Ogo 6 model, but variations in lower bound density (inferred from low-altitude AE-C data) and lower bound temperature (from incoherent scatter data) result in annual density variations at high altitude very similar to those in the Ogo 6 model. Diurnal and semidiurnal variations in lower bound density and temperature gradient parameters are also introduced. Model diurnal exospheric temperature variations reflect the observed changes with season seen in incoherent scatter measurements. Data used in the model cover a wide range of solar activities (mean F10.7 of 75–180), and the annual and diurnal temperature amplitudes are found to increase with solar activity at twice the rate of the mean temperature. The model fits moderate magnetic activity better than the Ogo 6 model but does not include observed longitude variations. The overall good agreement of the individual data sets with the model confirms the basic consistency of the various measurements taken in different time periods.