Abstract

At altitudes in excess of 70 km, dissociation of the atmospheric gases occurs because of solar radiation. Above 120 km, atomic oxygen is a major constituent of the atmosphere, and in the exosphere and in interplanetary space the atmosphere is largely atomic hydrogen. The drag on satellites and other objects in the upper atmosphere is necessarily more complicated than in the lower atmosphere, which is composed of chemically stable systems, for in addition to thermal accommodation and the angular distribution of particles reflected by vehicle skins, it is necessary to consider (1) the reassociation of atoms into molecules at the surface, (2) the manner in which energy of reassociation is divided between kinetic energy of the newly formed molecules and surface heating and (3) the chemical reactions between the atomic atmosphere and the surface material of any vehicle. Under very-high-altitude flight conditions, the pressure is sufficiently low that the free molecular flow regime is realized, and the speed of the gas relative to the vehicle is of the order of 10⁶ cm/sec.

For investigations of atom-surface interactions under these conditions, the use of beams of hydrogen and oxygen atoms is particularly appealing. However, a basic difficulty resides in tracing an atomic beam of number density of the order of 10⁸ atoms/cm³ through the residual gas of number density of the order of 10¹⁰ molecules/cm³ (corresponding to a pressure of 2.5 × 10^-7 mm Hg) in a laboratory vacuum system. This difficulty is overcome by modulating the atomic beam; in the present case, this is done by mechanically interrupting it at a frequency of 100 c/s. Under these conditions, any effect arising from the beam may be identified by its occurring at the modulation frequency and in a specified phase. Using this modulation technique, it becomes possible to use mass-spectrometric detection of the beam, since the electron-impact ionization cross-sections for hydrogen and oxygen atoms are known.

Two types of experiments will be discussed. In the first, an incident beam impinges on a surface and the particles leaving the surface are examined by means of a mass spectrometer. By varying the incident beam and/or the surface temperature, thermal accommodation coefficients are determined for a variety of surfaces and gases. Using an incident atomic beam, the probability that an atom will rebound from the surface as a free atom is measured, and from examination of the reflected molecular signal, information on the probability of reassociation at the surface is obtained.

In the second type of experiment, the atomic beam, which is monitored mass-spectrometrically, is allowed to strike a surface placed on a torsion balance, and a momentum transfer is measured directly. In these experiments, the atomic-beam apparatus is used as an atomic wind tunnel; when a hydrogen-atom beam from a furnace source operating at 3000°K is used, the case of a high-altitude satellite is fairly closely duplicated.

Results of a number of experiments using these techniques are presented.

*1 This research was supported by the United States Air Force through the Air Force Office of Scientific Research of the Air Research and Development Command under Contract No. AF49(638)-356. Reproduction in whole or in part is permitted for any purpose of the United States Government.

Present address: The Budd Company, Instruments Division, Phoenixville, Pennsylvania.