The Space Environment and Survivability

8.1 The Space Environment

The Solar Cycle; The Gravitational Field and Microgravity; The Upper Atmosphere; Plasmas, the Magnetic Field, and Spacecraft Charging; Radiation and Associated Degradation

8.2 Hardness and Survivability Requirements

The Nuclear Weapons Environment and Its Effect on Space Systems; Other Hostile Environments; Spacecraft Hardening; Strategies for Achieving Survivability

8.1 The Space Environment

Alan C. Tribble, Intellectual Insights D J. Gorney, J.B. Blake, H.C. Koons, M. Schulz, A.L. Vampola, R.L. Walterscheid, The Aerospace Corporation James R. Wertz, Microcosm, Inc.

The near-Earth space and atmospheric environments strongly influence the performance and lifetime of operational space systems by affecting their size, weight, complexity, and cost Some environmental interactions also limit the technical potential of these systems. They can lead to costly malfunctions or even the loss of components or subsystems [Tribble, 1995; Hastings and Garrett, 1996; DeWitt et al., 1993].

By itself, operating under vacuum-like conditions can pose significant problems for many spacecraft systems. When under vacuum, most organic materials will outgas —the generation of spurious molecules which may act as contaminants to other surfaces. Even before reaching orbit, particles from the atmosphere may fall onto optical surfaces and degrade the performance of electro-optical instrumentation. Because there is no practical way to clean spacecraft surfaces once the vehicle reaches orbit, maintaining effective contamination control during design and development is a significant issue for most spacecraft [Tribble et al., 1996].

Once orbit is obtained, the spacecraft is subjected to a very tenuous atmosphere [Tascione, 1994]. At lower orbits a spacecraft will te bombarded by the atmosphere at orbital velocities on the order of ~8 km/s. Interactions between the satellite and the neutral atmosphere can erode satellite surfaces, affect the thermal and electrical properties of the surface, and possibly degrade spacecraft structures.

At shuttle altitudes, -300 km, about 1% of the atmosphere is ionized. This fraction increases to essentially 100% ionization in the geosynchronous environment The presence of these charged particles, called the plasma environment can cause differential charging of satellite components on the surface and interior of the vehicle. If severe, this charging can exceed breakdown electric fields and the resulting electrostatic discharges may be large enough to disrupt electronic components. More energetic space radiation, such as electrons with energies from about 200 keV to 1.5 MeV, can become embedded in dielectric components and produce electrostatic discharges in cable insulation and circuit boards. This bulk charging may disrupt a subsystem's signals or the operation of its devices. Even if mild, the charging may alter the electrical potential of the spacecraft relative to space and affect the operation of scientific instrumentation.

Very energetic (MeV-GeV) charged particles can be found in the trapped radiation belts, solar flare protons, and galactic cosmic rays. The total dose effects of this high-eneigy radiation can degrade microelectronic devices, solar arrays, and sensors. A single energetic particle can also cause single-event phenomena within microelectronic devices which can temporarily disrupt or permanently damage components.

Lastly, orbiting spacecraft are periodically subjected to hypervelocity impacts by 1 Jim or larger sized pieces of dust and debris. If the impacting particles originate in nature they are termed micrometeoroids. If the particles are man-made they are termed orbital debris. A single collision with a large micrometeoroid or piece of orbital debris can terminate a mission. The probability of this occurring will increase significantly with the introduction of large constellations of satellites.

The subject of space environment effects is, by itself, an area of active research. The more critical of the various effects are discussed below.

8.1.1 The Solar Cycle

This subject is of particular interest because of the fact that the solar activity is seen to vary with an 11-year cycle as shown in Fig. 8-1 [NOAA, 1991]. The plot shows the F10.7 index, which is the mean daily flux at 10.7 cm wavelength in units of 10-22 W/m2 • Hz. The peaks in the F10.7 index are called solar maxima, while the valleys are called solar minima. Note that the variations are substantial on a day-to-day basis and that one solar maximum may have levels that vary dramatically from other solar maxima. Consequently, predicting the level at any given future time is highly uncertain. On the other hand, the average over an extended period of time is well known. As will be seen, many space environment effects are strongly dependent on the solar cycle.

8.1.2 The Gravitational Field and Microgravity*

Microgravity, also called weightlessness, free fall, or zero-g, is the nearly complete absence of any of the effects of gravity. In the microgravity environment of-a satellite, objects don't fall, particles don't settle out of solution, bubbles don't rise, and convection currents don't occur. Yet in low-Earth orbit, where all of these phenomena occur, the gravitational force is about 90% of its value at the Earth's surface. Indeed, it is the gravitational field that holds the satellite in its orbit.

Year

Fig. 8-1. Observed Dally Radio Flux at 10.7 cm Adjusted to 1 AU.

Year

Fig. 8-1. Observed Dally Radio Flux at 10.7 cm Adjusted to 1 AU.

In Earth orbit microgravity comes about because the satellite is in free fall—i.e., it is continuously falling through space and all of the parts of the satellite are falling together. In a circular orbit the forward velocity of die spacecraft (tangential to the direction to the Earth) is just enough that the continual falling of the spacecraft toward the Earth keeps the satellite at the same distance from the Earth's center. Moving in a circular orbit requires a continuous acceleration toward the center.

The term microgravity is used in the space environment because, in practice, zero gravity cannot actually be achieved. Two objects traveling very near each other in orbit will not travel in quite the same path due to differences in the gravitational forces or external nongravitational forces acting on them. Two objects held side by side and dropped from a tall building will both accelerate toward the center of the Earth and, as they fall, will converge slightly toward each other. From the point of view of the objects, there is a small component of Earth gravity that pulls them toward each other. An orbiting spacecraft under the influence of atmospheric drag or solar radiation pressure will feel a very small force due to this external pressure. This force can mimic the effect of gravity, causing heavy particles in solution to settle toward the front end of a moving spacecraft Similarly, a rotating spacecraft produces "artificial gravity" due to centrifugal force. Finally, tidalforces, sometimes called gravity-gradient forces, come about because of very small differences in the force of gravity over an extended object For a spherical bubble drifting in orbit, the force of gravity on the lower edge of the bubble will be stronger than at the center of mass and weaker at the far edge of the bubble. This very small difference in forces results in "tides" which will distort the shape of the bubble and elongate it toward and away from the direction to the Earth.

For most practical applications, microgravity effects in low-Earth orbit can be reduced to the level of 1(H g (= 1 Jig). A level of 10-7 g can be achieved over a very small region near the center of mass of the spacecraft Table 8-1 provides formulas for the most common forces in the microgravity environment In this table, z is the direction toward nadir and z is the acceleration in the nadir direction. «Bis the angular velocity of the satellite in its orbit Note that for a nadir-pointing spacecraft, the spacecraft rotates in inertial space at a rate of one rotation per orbit and thus will add to the acceleration environment due to the centrifugal force of this rotation. In low-Earth orbit, the lowest microgravity level can be achieved in an inerti ally-oriented spacecraft rather than a nadir-oriented spacecraft The last row in Table 8-1 is the coriolis force which is an apparent sidewise force that occurs whenever objects move (e.g., fluid flow in a chemical process) in a rotating reference frame, such as a gravity-gradient stabilized spacecraft

TABLE 8-1. Equations for Microgravity Level. (0 is the orbital angular velocity. 0)^ = {¡i/a3)1® = 0.00106 rad/s. x, y, and z are the distances from the spacecraft center of mass.

Source

x direction (velocity)

y direction (orbit normal)

z direction (nadir)

Aerodynamic Drag

x = 0.5 (GqA / m)pa2io2

y = 0

z = 0

Gravity Gradient

0 0

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