As we saw in Chapter 3, the motion of spacecraft in low Earth orbit is affected by the atmosphere. The cause of this is air drag, which you may recall is a tiny force that acts in a direction opposite to the motion of the spacecraft. We saw how this takes energy out of the orbit, causing the spacecraft's height to decrease, with the ultimate prospect of a fiery reentry into the denser, lower atmosphere. Of course, there is no altitude where we can say that the atmosphere stops and the interplanetary medium begins. The density of the atmosphere falls steadily, from the breathable mix of oxygen and nitrogen at Earth's surface, to something approaching a vacuum at high altitude. However, we can measure the effects of air drag at altitudes up to about 1000 km (620 miles). Earlier we discussed the Sun's dominant influence on Earth's environment. The drag effects on a low Earth orbiting spacecraft are also significantly affected by the Sun. Over the 11-year solar cycle, the level of solar activity varies, resulting in peaks and troughs in its electromagnetic and particle radiation output. At times of solar maximum, intense EM radiation from the Sun, in particular in the ultraviolet part of the spectrum, causes the temperature of Earth's upper atmosphere to rise.
It's worth thinking for a moment about what we mean by temperature in this outermost layer of Earth's atmosphere, referred to as the exosphere. Given that the atmosphere is close to a vacuum at these high altitudes, if we tried to use a thermometer to measure the temperature, there would not be enough air around to give any sort of sensible reading. Instead, when we refer to temperature in such tenuous material, we usually talk about something called kinetic temperature. When the Sun's ultraviolet radiation heats up the atmosphere, it essentially ''excites'' the atmospheric particles and causes them to race around at a higher speed, thus increasing their kinetic temperature.
It is important to realize how tenuous the atmosphere is at high altitudes. For example, an atom of oxygen moving around at an altitude of, say, 600 km (370 miles) will not find another atom to bump into for about 300 km (185 miles), and at an altitude of 800 km (500 miles), this increases to over 1000
km (620 miles). The precise values of these numbers actually vary with the level of solar activity, but the main point is how thin the atmosphere is at these heights. Essentially the atoms and molecules that make up the upper atmosphere move around on ballistic trajectories, similar to how cannon-balls fly in a gravity field. This may be simplifying the matter a little, but it does help us understand how solar heating of the atmosphere causes its density to increase.
Putting it all together, then, at times of solar maximum the Sun's ultraviolet output increases, which in turn causes a rise in temperature of Earth's atmosphere. This causes the atmospheric atoms and molecules to rush around more rapidly, allowing them to reach higher altitudes in Earth's gravity field. At a particular height the numbers of atmospheric particles per cubic meter increase, producing a higher density. This effect on the atmosphere is significant; for example, the density of the atmosphere at, say, 600 km at high solar activity can be larger than that at low solar activity by more than a factor of 10. We are not just talking about increases of two or three times, but more than an order of magnitude.
In considering the perturbing effect on a spacecraft orbiting at a 600-km altitude, the drag acting on it is related directly to the atmospheric density, so drag at times of high solar activity can be more than a factor of 10 greater than when the Sun is quiet. This will have a significant effect on spacecraft operations, and the mission analysis team will have to take the solar cycle into account in planning the orbit control activities and in the estimation of how much rocket propellant will be required to compensate for these drag effects.
To show the huge effect that the solar cycle has on atmospheric temperature, Figure 6.6 is a chart of the variation in the average exospheric temperature (the temperature of the atmosphere at orbital altitudes) over the last five solar maxima—since the dawn of the space age. Most surprisingly, the upper atmosphere reaches kinetic temperatures of the order of 1000°C when the Sun's level of activity is high. The 11-year solar activity cycle can also be clearly seen, with the atmospheric temperature at solar maximum being something like 600°C above that at solar minimum. Another striking feature of Figure 6.6 is the spiky nature of the temperature profile at times of solar maximum. This spike is due to temperature variations caused by more short-lived events such as the solar storms mentioned above. During such storms, some of the solar wind particles from the Sun are focused down into the atmosphere at the north and south polar regions by Earth's magnetic field, causing auroral displays. This influx dumps massive amounts of energy into the atmosphere, which causes heating locally at the poles. Within a short period (of the order of hours), this heating propagates toward the
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Figure 6.6: The variation of average exospheric temperature over the last five solar maxima. (Figure compiled from data supplied courtesy of Dr. Hugh Lewis, University of Southampton, UK.)
equator, again causing the atmosphere to expand. This produces an increase in atmospheric densities at orbital altitudes, giving a corresponding increase in drag perturbations on spacecraft orbits.
The atmosphere also has more direct effects on spacecraft and the materials used in their construction. Perhaps the best example of this is atomic oxygen erosion. Down on the Earth's surface we breathe molecular oxygen 02, composed of two oxygen atoms chemically bonded together. We know that the oxygen we find down here gives us life, but it is also aggressive in forming oxides such as rust, which if left untreated over a period of years will have a damaging effect on our machines (bikes, cars, lawn mowers). As you go up to orbital altitudes, the atmosphere is no longer shielded from solar ultraviolet radiation. This causes the 02 bond to be broken, so that at low Earth orbit (LEO) altitudes single oxygen atoms (referred to as atomic oxygen and denoted by the symbol 0) wander around and become the dominant atmospheric constituent.
Atomic oxygen on orbit has a similar erosive character, not only arising from its chemical activity, but also because it hits spacecraft at around 8 km/ sec (5 miles/sec) (due to the vehicle's orbital speed through the atmosphere). The importance of this was first registered when most of the thermal blanket on a camera mounted on the Space Shuttle during its third mission in March 1982 disappeared due to the effects of atomic oxygen erosion. Thermal blankets are used extensively on spacecraft to insulate them from the heating effects of direct solar radiation (see next section), and often give the vehicle its characteristic appearance of being wrapped in gold or silver foil. The blanket is composed of multiple layers of a thin plastic film with a metallic coating, such as aluminium, silver, or gold, similar to the survival blankets handed out at the end of marathons to keep the runners warm (see Chapter 9). In addition to the thermal blanket, various other materials are also particularly prone to atomic oxygen attack, one example being silver, which is commonly used in the construction of solar panels (solar panels are used on spacecraft to convert sunlight into electrical power; see Chapter 9). Clearly, the spacecraft designer needs to be familiar with these environmental effects when choosing appropriate materials in the design.
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