OVERVIEW

The neutral portion of the Earth's atmosphere between about 90km and 600km above the Earth's surface is known as the thermosphere, while that portion above 600km is known as the exosphere. The neutral gases of the thermosphere tend to stratify based on their molecular weight. Atomic Oxygen(AO) is the dominant constituent in the lower thermosphere with helium and hydrogen dominating the higher regions. The neutral gas temperature in the lower thermosphere increases rapidly with increasing altitude from a minimum at 90km until it becomes altitude independent and approaches an asymptotic temperature known as the exospheric temperature at about 600km. The temperature as well as the density and composition of the thermosphere and the exosphere are very sensitive to the level of solar activity because of heating by absorption of the solar extreme ultraviolet (EUV) radiation. The effects of changing solar EUV radiation have been effectively modeled using a proxy parameter for the EUV radiation, the 10.7cm solar radio flux that is transmitted by the Sun.

Spacecraft Effects

Density of the neutral gas is the primary atmospheric property that affects a spacecraft's orbital altitude, lifetime, and motion. Even though space is thought of as a vacuum, there is enough matter to impart a substantial drag force on orbiting spacecraft. Unless this drag force is compensated for by the vehicle's propulsion system, the altitude will decay until reentry occurs. Density effects also directly contribute to the torques experienced by the spacecraft due to the aerodynamic interaction between the spacecraft and the atmosphere, and thus, must be considered in the design of the spacecraft attitude control systems.

Many materials used on spacecraft surfaces are susceptible to attack by AO, a major constituent of the low-Earth orbit (LEO) thermosphere region. Due to photodissociation, oxygen exists predominantly in the atomic form. The density of AO varies with altitude and solar activity and is the predominant neutral species at altitudes of about 200 to 400 km during low solar activity. Simultaneous exposure to the solar ultraviolet radiation, micrometeoroid impact damage, sputtering, or contamination effects can aggravate the AO effects, leading to serious deterioration of mechanical, optical, and thermal properties of some material surfaces. A related phenomenon which may be of concern for optically sensitive experiments is spacecraft glow. Optical emissions are generated from metastable molecules which have been excited by impact on the surface of the spacecraft. Investigations show that the surface acts as a catalyst, thus the intensity is dependent on the type of surface material.
 

back to Ionosphere and Thermosphere TWG