Spacecraft Interaction with Atmospheric Species in Low Earth Orbit

Abstract

Spacecraft interaction with ambient atmospheric species (O, NI, O + , and electrons) in low Earth orbit involves collisions at high energies, giving rise to such complex phenomena as the shuttle glow, plume-atmosphere interactions, and plasma modifications. A survey of these and other effects is presented, and conclusions are drawn about possible ambiguities in interpretation of present data. For example, a suggestion is made that ions may play a role in the Shuttle glow phenomenon. Lack of data on the accommodation (in terms of energy and in terms of adsorption and subsequent reactions) of atmospheric species, such as O and NI, on amorphous surfaces at hyperthermal energies is a distinct handicap in the development of appropriate numerical codes for predicting the effects of spacecraft interactions. Introduction T HE atmosphere in low Earth orbit (LEO, 200-700-km attitude) consists primarily of neutral O and N 2 and of O + and electrons. At an altitude of 300 km the densities of the atmospheric constituents are 1 [O] ~ 10 8 cm~3, [N 2 ] -2 x 10 7 cnr 3 , [O + ] -[e] -10 5 cnr 3 .Although the neutral density is considerably greater than the plasma density, it should be kept in mind that ion-neutral reactions have rate coefficients that are 100-1000 times greater than neutral-neutral reactions. Thus, under some circumstances ion-neutral reactions may become as important as the neutral-neutral reactions. Likewise, ionsurface reactions may play an important role in surface effects, even though the densities are low.These atmospheric constituents collide with the spacecraft and its local gaseous atmosphere at orbital velocity, leading to two major types of interaction: gas-phase reactions and gas-surface reactions. Figure 1 provides an illustration of these interactions. The gas-phase reactions may be further broken down into a neutral component and a plasma component, and the gas-surface reactions may be broken into several components: surface-catalyzed reactions, surface neutralization of ions, chemical reactions with surfaces, and inelastic collisions. Because of their strong connection with surface charging phenomena, 2 we will not discuss ion-surface collisions; reference is made to two recent comprehensive reviews. 3 ' 4 Likewise, inelastic collisions between the atmosphere and surfaces represent a very specialized topic intimately connected with atmospheric density measurements. Readers are referred to well-established reviews. 5 ' 9 The aim of this review is to present a concise summary of work on spacecraft-atmosphere interactions, to present, when possible, ideas about the implications of the results, and to suggest work that needs to be done still. Gas-phase reactions will be considered first, followed by gas-surface interactions. Gas-Phase InteractionsA dramatic indication of the transition from the dense local atmosphere of a spacecraft to the ambient atmosphere is shown in Fig. 2, which presents pressure-gauge data obtained on STS-39. The pressure gauge was located on a free-flying satellite, which was ...

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