For spacecraft in LEO and PEO, the dominant environment is the ambient neutral atmosphere. The neutral gases that make up the atmosphere in this environment form a distinctive structure around the spacecraft and give rise to drag, surface erosion, and spacecraft glow. The neutral gases emitted by the spacecraft itself give rise to contamination on other parts of the spacecraft. In this chapter, these interactions are systematically evaluated. Primary emphasis is on the physics of the flows associated with the interactions.

Neutral Gas Flow Around a Spacecraft

For a spacecraft in a LEO or PEO, the ambient mean free path for momentum exchange is given by Eq. (2.38). With a typical elastic scattering cross section of O (10−20 m2) and with mean densities around the orbit from Table 3.4, the ambient mean free path is of the order of many kilometers. This is illustrated in Figure 4.1 for profiles of the number density, collision frequency, mean free path, and particle speed from the surface to 700 km for the U.S. Standard Atmosphere. Where the Knudsen number [see Eq. (2.39)] satisfies Kn ≫ 1, the flow of ambient neutral gas around the spacecraft is collisionless. Since from Table 3.4, the gas temperature is typically hundreds to thousands of degrees Kelvin, the thermal velocity is of the order of 700 m/s. For an orbital velocity of 8 km/s, the speed ratio (Section 2.3.2) satisfies S ≫ 1. Therefore, the ambient neutral gas flow around spacecraft in a near Earth orbit will be collisionless and supersonic.

Luminous Blue Variables/S Dor stars

The observed H-R diagram has an upper luminosity limit of which the contour line is temperature dependent (Humphreys & Davidson 1987). Some of the most massive and luminous (106Lo) stars near that line (P Cyg, AG Car, HR Car, η Car, ...) sporadically show dramatic mass-ejections (seen as ‘eruptions’) followed by periods of quiescence. Such stars are called hypergiants, some of them are Luminous Blue Variables (LBVs), though LBVs, do not necessarily need to be blue, since the phenomenon is not restricted to early-type stars (de Jager & van Genderen 1989). The above-mentioned LBVs, together with 164GSco = HD 160529 (Fig. 2.1) and WRA751 are notorious galactic LBVs. de Koter (1993) estimates the number of LBVs in our galaxy at no more than 60, but the number of LBVs that possibly can be considered for observation, obviously, is much less. In the LMC, the well-known LBVs (also called S Dor stars) are S Dor (Fig. 2.2), R71 (Figs. 2.3, 2.4) and R127 (Fig. 2.5), with R66, R81 (Fig. 2.6) and R110 as additional candidates. Finally there are the ‘ Hubble-Sandage variables’, discovered by Hubble & Sandage (1953) in M31 and M33, which are identifiable with the S Dor variables, that complete the group that is commonly designated as LBVs. During outburst these stars are - apart from supernovae - the visually brightest stars in the universe, and thus potentially belong to the most powerful extragalactic distance-indicators (Wolf 1989). Today, only a few dozen LBVs are known.

Introduction

Before the space age began, it was realized that space was not empty. Comet tails, meteors, and other extraterrestrial phenomena demonstrated the presence of a “space environment.” Much as an aircraft operates in and interacts with the atmosphere (indeed the air is necessary for lift), so a spacecraft operates in and interacts with this space environment. The environment can, however, limit the operation of the spacecraft and in extreme circumstances lead to its loss. Concern over these adverse environmental effects has created a new technical discipline – spacecraft–environment interactions. The purpose of this text is to describe this new field and introduce the reader to its many different aspects.

Historically, the field of spacecraft–environment interactions has developed primarily as a series of specific engineering responses to each interaction as it was identified. Consider the discovery of the radiation belts and their effects on electronics. This led to the development of radiation shielding and microelectronic-hardening technology. Similarly, in the early seventies, the loss of a spacecraft apparently due to spacecraft charging from the magnetospheric plasma led to intense efforts to understand charge accumulation on surfaces in space and to methods for mitigating the effects. Ultimately, these efforts culminated in the 1979 launch of a dedicated spacecraft, SCATHA (Spacecraft Charging at High Altitudes), into a near geosynchronous orbit for studying this interaction. Likewise, in the eighties, certain materials were found to erode rapidly in the low-Earth space environment because of chemical interactions with atomic oxygen.

In this chapter, the principal natural (unperturbed) environments responsible for spacecraft interactions are introduced. These are the solar environment, the neutral atmosphere, the geomagnetic field, the plasma environment, the geostationary environment, energetic particle radiation, electromagnetic and optical radiation, and particulates (debris and meteoroids). The ambient space environment defined by these components has been the subject of numerous books and review papers [e.g., Jursa (1985)] or the excellent short descriptions of the environment in MIL-STD-1809 (1991). Unlike most of these sources, which deal primarily with the details of the space environment, the intent here is to provide the reader with sufficient background to evaluate the potential impact of the environment – both natural and man-made – on a spacecraft.

The relationships between the orbit classes and the natural environment are summarized in Figure 3.1. Table 3.1 is used to indicate which environments must be considered for a given class of orbits.

Influence of the Sun

The dominant energy source for the space environment in the solar system is the Sun. The chief solar influence on the space environment is through its electromagnetic flux (see Section 3.4.2) and the charged particles that it emits. The solar particle flux is composed basically of two components: the very sporadic, high-energy (E > 1 MeV) plasma bursts associated with solar events (flares, coronal mass ejections, proton events, and so forth) and the variable, low-energy (E ≈ tens of eV) background plasma referred to as the solar wind.

Introduction

As electronic components have grown smaller in size and power and have increased in complexity, their enhanced sensitivity to the space radiation environment and its effects has become a major source of concern for the spacecraft engineer. The three primary considerations in the design of spacecraft are the description of the sources of space radiation, the determination of how that radiation propagates through material, and, thirdly, how radiation affects specific circuit components. As the natural and man-made space radiation environments were introduced in Chapter 3, the objective of this chapter is to address the latter two aspects of the radiation problem. In particular, because the “ambient” environment is typically only relevant to the outer surface of a space vehicle, it is necessary to treat the propagation of the external environment through the complex surrounding structures to the point inside the spacecraft where knowledge of the internal radiation environment is required. Although it is not possible to treat in detail all aspects of the problem of the radiation environment within a spacecraft, by dividing the problem into three parts – external environment, propagation, and internal environment – a basis for understanding the process of protecting a spacecraft from radiation will be established that can be applied to a wide range of radiation problems.

Radiation Interactions with Matter

From the standpoint of radiation interactions with matter, three particle families need to be considered:

1. photons (primarily EUV, X rays, and gamma rays),

2. charged particles (protons, electrons, and heavy ions),

3. neutrons.

Particle Impacts on Spacecraft

In this chapter, the effects of space particulates – hypervelocity impacts and scattering – are considered. Hypervelocity impacts, the primary effect of meteoroids and space debris, can be roughly divided into effects on single surfaces (namely, cratering or penetration of single surfaces), spall formation, and double-wall (or Whipple) shield penetration. A major consideration for each of these is the target. For example, if the target is an optical surface, then the damage induced on single surfaces is important. If the target is a tank, then penetration and/or failure of the tank is important and the characteristics of its contents become crucial. For electronic components inside a box, the size and distribution of the spall or spall/impactor products coming off the wall of the box are important. Examples of these factors are discussed below with emphasis on the practical considerations that must be taken into account in arriving at an effective and economical (in mass) protection system. A particularly important issue for meteoroid shielding design has arisen in recent years because of the need to have interplanetary spacecraft carrying nuclear RTGs use the Earth for gravitational assists. To provide adequate safety margins for these missions, which often require hypervelocity flybys of the Earth at distances of 300 km, mission planners must target the vehicle so that it will be extremely unlikely for a meteoroid impact on the spacecraft to lead to an Earth-impact trajectory.