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.

During the preparation of the observing programme of the TYCHO project on board the HIPPARCOS mission we started thinking about the large number of new variable stars that would be discovered. And since the TYCHO experiment yields only a scanty number of scattered measurements of each star during the life time of the satellite, it is immediately evident that one will encounter the problem of recognising the type or class of variability to which the variable star belongs. Such classification is - even with abundant data - not a trivial task, since many variable stars have light curves which, at first sight, look very similar. In addition, proper classification needs much more than a good-looking light curve, since luminosity and effective-temperature photometric indices also play a role, as well as miscellaneous data obtained with apparatus that are complementary to photometric instruments.

We thought to get some help by looking for standard light curves of typical variable stars that would be used as a template during the process of classification. We discovered then, with some surprise, that a compilation of typical photoelectric light curves of variable stars has never been published, nor does there exist a concise compendium of photometric properties of groups and classes of variables. What can be found, instead, is a large number of detailed morphological descriptions and numerous photometrically-incompatible photographic and visual light curves, scattered over many books and journals.

So, we decided to fill this gap and we started the compilation of typical light curves in a format that enables quick recognition of the pattern of variability.

Algol type eclipsing binaries

The Algol type eclipsing variables (EA) are a subgroup of the eclipsing binaries segregated according to light curve shape. The light remains rather constant between the eclipses, i.e., variability due to the ellipticity effect and/or the reflection effect is relatively insignificant. Consequently, the moments of the beginning and the end of the eclipses can be determined from the light curve.Eclipses can range from very shallow (0m01) if partial, to very deep (several magnitudes) if total. The two eclipses can be comparable in depth or can be unequal. In a few cases the secondary eclipse is too shallow to be measurable (when one star is very cool), or absent altogether (highly eccentric orbit).

Light curves of this shape are produced by an eclipsing binary in which both components are nearly spherical, or only slightly ellipsoidal in shape. Though not explained in the GCVS, one component may be highly distorted, even filling its Roche lobe, provided it contributes relatively little to the system's total light. This is, in fact, the case for at least half of the known EA variables.

Among the EAs one may find binaries of very different evolutionary status:

1. (i) binaries containing two main-sequence stars of any spectral type from O to M, with CM Lac an example

2. (ii) binaries in which one or both components are evolved but have not yet overflowed their Roche lobes, with AR Lac an example

3. (iii) binaries in which one star unevolved and the other overflowing its Roche lobe and causing mass transfer, with RZ Cas an example

4. […]

Ap and roAp stars

The existence of stars whose surface is severely depleted in He with, at the same time, overabundance of Fe, Si and Cr in spots, has been known since the early days of spectral classification, when the phenomenon was first detected in Ap stars (for details, see Jaschek & Jaschek 1987, Morgan 1933).

Chemically-peculiar (CP) stars, in general, are stars of spectral type B2- F of which the spectra reveal signatures of chemical peculiarities such as, for example, strongly-enhanced spectral lines of Fe and rare-earth elements. In this group, there is a magnetic sequence - referring to, as Hensberge (1994) puts it, ‘ those stars that show a magnetic field that is strong and global (a large dipolar contribution to the field), so that it is detectable with the present [observing] techniques. It does not imply that HgMn stars, or metallic-line (Am) stars, etc. would have no magnetic field at all. Stars in the non-magnetic sequence may be either without field, with a significantly weaker global field, or with a strong field of complicated structure, such that the measurable effect, averaged-out over the visible disc, is insignificant’. Ap stars have global surface magnetic fields of the order of 0.3 to 30 kG (thousands of times stronger than that of the sun), and the effective magnetic-field strength varies with rotation, a situation that led to interpretation in terms of the oblique-rotator model in which the magnetic axis is oblique to the rotation axis (this model was first suggested by Stibbs in 1950). The time scales of light variations seen in Ap stars range from minutes to decades.

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.