In Chapter 1, the four basic environmental interactions were introduced. To help in understanding the physics of these interactions with a spacecraft, the nondimensional physical parameters that determine the interactions are described in Chapter 2. An understanding of the magnitudes of these nondimensional parameters simplifies the complex physics describing the interactions. This is analogous to the idea that, for a fluid, the definition of a Reynolds number allows the physics of the fluid behavior to be divided into two regimes. When the Reynolds number is small compared to unity, the physics of the fluid is dominated by viscous effects. When the Reynolds number is large compared to unity, the flow is inviscid. Likewise, for a compressible gas, the Mach number allows the physics of the flow to be divided into subsonic physics and supersonic physics. This idea of fundamental scales, like the Reynolds and Mach numbers, is exploited in this and subsequent chapters as a means of characterizing the effects and importance of the fundamental interactions under varying environmental constraints.

The classification of X-ray binaries is somewhat ambiguous. Some authors consider X-ray binaries to be any kind of interacting close binary with a compact degenerate object - that is, a white dwarf, a neutron star, or a black hole. A more specific definition is that X-ray binaries are only those interacting close binary systems which contain a neutron star or a black hole. In this chapter we shall restrict ourselves to the latter definition; interacting close binaries with a white dwarf are usually called cataclysmic variables which are described in Chapter 5 of this book.

The main (empirical) difference between the cataclysmic variable and the X-ray binaries as defined above is the X-ray luminosity: whereas X-ray binaries have X-ray luminosities of 1O∧35 - 10 ∧38 erg s which corresponds to 25 to 25000 times the total solar luminosity, cataclysmic variables have Lx ≦ 1034 erg s. Hence, X-ray binaries are discovered on the basis of their strong X-ray emission. The basic model of X-ray binaries is a close binary system with a ‘normal’ star (main sequence or giant, in exceptional cases a degenerate star too) filling its Roche lobe and transferring matter to the compact object, a neutron star or a black hole. Such a system is called a ‘semi-detached’ system. Due to the orbital angular momentum the matter cannot directly fall onto the compact object, and it forms an accretion disc around the latter (see also Section 5.4). Due to internal friction in the accretion disc (also called viscosity) the matter spirals inward until it eventually falls onto the compact object.

Supernovae

A supernova explosion is a rare type of stellar explosion which dramatically changes the structure of a star in an irreversible way. Large amounts of matter (one to several solar masses) are expelled at high velocities (several tens of thousands km s-1). The light curve in the declining part is powered by thermalized quanta, released by the radioactive decay of elements produced in the stellar collapse, mainly 56Co and 56Ni. The ejected shell interacts with the interstellar medium and forms a SN remnant, which can be observed long after the explosion in the radio, optical and X-ray regions.

Supernovae can be divided into two classes (and several subclasses), viz. SN I and SN II.

SN I have fairly similar light curves (see, for example, Fig. 5.1) and display a small spread in absolute magnitudes. Spectra around maximum show absorption lines of Ca II, Si II and He I, but lack lines of hydrogen. They occur in intermediate and old stellar populations. Their progenitor stars are not clearly identified, but massive white dwarfs (WDs) that accrete matter from a close companion and are pushed over the Chandrasekhar limit are good candidates. Another possibility is the hypothesis of the fusion of a binary consisting of two WDs. The collapse of the white dwarf leads in both cases to an explosive burning of its carbon, and the released energy is sufficient to trigger a disintegration of the complete object.

Spacecraft–Plasma Interactions

The plasma environment that a spacecraft will encounter as a function of orbit is described in Section 3.3. Although the plasma environment is not necessarily the dominant environment in a particular case, it can nevertheless have a profound and destructive effect on a spacecraft or its payload. In particular, major plasma effects follow from the slow accumulation of charge on surfaces. This accumulation of charged particles from the surrounding space plasma on spacecraft surfaces, termed surface charging, produces electrostatic fields that extend from surfaces into space and can result in a number of adverse interactions:

• surface arc discharges that generate electromagnetic interference cause surface damage, induce currents in electronic systems, produce optical emissions, and enhance the local plasma density;

• enhanced contamination leading to changes in surface, thermal, and optical properties;

• a shift of the spacecraft electrical ground, leading to problems with detectors collecting charged particles from the environment; and

• coulomb forces on the spacecraft components and materials as well as modifications of the drag coefficient and electromagnetic torques on the spacecraft.

In addition to these concerns, there are some less obvious effects. Of particular concern to the manned spacecraft community, differential charge accumulation between two spacecraft that come into contact (the Shuttle and the station or an astronaut during extra-vehicular activity and the Shuttle) may result in damaging current flows between the spacecraft. These can cause arc discharges, electronic burnout, and other safety hazards.