Trapped Particle Radiation

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As we saw earlier, some of the solar wind particles emanating from the Sun, generally charged particles such as electrons, protons, and atomic nuclei stripped of their attendant electrons, penetrate the protective shield of Earth's magnetic field. Some are focused by the field into the atmosphere above the north and south polar regions, causing auroral displays. Others are trapped by the magnetic field, producing radiation belts that pose a hazard to people and spacecraft alike. These belts are called the Van Allen belts, after their discoverer James Van Allen, who was the first to confirm their existence using data from the Explorer 1 and 3 satellites in 1958.

These charged particles, once trapped by Earth's magnetic field, move rapidly in a particular way governed by the field, to give the Van Allen belts their characteristic shape of giant doughnuts stretched around Earth's equator (see Figure 6.8).

To understand how this structure comes about, we need to consider how charged particles move in a magnetic field. Put simply, they tend to gyrate about the magnetic field lines; their paths through space echo a shape a bit like a corkscrew, as shown in Figure 6.7a. If we also recall the shape of Earth's magnetic field, which is similar to that of a bar magnet (see Figure 6.3a), then the particles travel paths like that shown in Figure 6.7b. A trapped

Charged Particle Magnetic Field
Figure 6.7: (a) Charged particles gyrate around the magnetic field lines, traveling paths that resemble a corkscrew, (b) Charged particle radiation is trapped in Earth's magnetic field, bouncing rapidly between mirror points 2 and 3, producing the Van Allen radiation belts,

Electron belt Proton belt

Figure 6.8: A slice through Earth's trapped radiation belts reveal a shape that echoes the magnetic field. The regions of maximum intensity of the proton and electron belts are shown.

Electron belt Proton belt

Figure 6.8: A slice through Earth's trapped radiation belts reveal a shape that echoes the magnetic field. The regions of maximum intensity of the proton and electron belts are shown.

particle moving north across the equator at point 1 will corkscrew about the field line toward point 2. As it approaches Earth at point 2, the magnetic field strength increases and the field lines converge, producing a mirror point, which causes the particle to bounce back along the field line toward point 1 again. At the other end of its journey, point 3 also acts as a mirror point, which reflects the particle back toward its starting point again. Thus our trapped particle is destined to bounce back and forth between points 2 and 3 indefinitely, unless it leaks away into space or is captured by Earth's tenuous upper atmosphere. This gives the characteristic shape of the Van Allen belts as shown in Figure 6.8. These are regions where spacecraft and people should not linger, due to the high density of energetic particle radiation, composed mainly of high-energy electrons and protons. The maximum intensity of electron radiation occurs at an altitude of approximately 27,500 km (17,000 miles), and the greatest intensity of the more damaging proton radiation occurs at around a height of 4500 km (2800 miles).

We would not contemplate orbiting a manned space station at these sorts of heights, as the consequences for the crew would be serious. However, we do know that exposure of humans to the radiation belts for a short spell, although not desirable, is tolerable; for example, the Apollo astronauts had to fly through the belts on their way to the moon and on their return.

Long-term exposure of unmanned spacecraft to the radiation belts is also damaging, mainly causing degradation of electronic and electrical power systems. Deploying a satellite in a circular orbit at the heart of the Van Allen proton belt would be foolhardy, but as mentioned in Chapter 2, many spacecraft occupy large elliptical orbits and fly through the radiation belts on each orbit revolution. The main problem for these vehicles is that their solar panels suffer radiation damage, which causes the amount of power they produce from sunlight to decrease with time. A spacecraft in this type of orbit for many years may suffer a power loss up to 50% of the solar panel's original output. However, the power subsystem engineer is able to predict the likely deterioration for the particular type of orbit flown, and make due allowance in the spacecraft's design.

Other electronic components onboard are also subject to radiation damage, but unlike solar panels, they can be shielded to some degree from the energetic particles by increasing the thickness of the walls of the metal boxes (typically made of aluminium) in which they are usually mounted. However, this needs to be done carefully as it will increase the spacecraft's mass, and as we have seen in Chapter 5, an increase in mass means a larger, more expensive launch vehicle. Another way of providing radiation protection, which goes some way toward solving this mass-growth problem, is to place radiation-sensitive components sensibly within the spacecraft so that they are shielded by less sensitive adjacent equipment.

As well as these total dose effects, the operation of onboard electronic equipment is also disrupted by single-event upsets (SEUs). These are temporary effects caused by the passage of a single high-speed particle through a computer processor, for example, producing random bit flips in onboard software, that is, switching a 0 bit to a 1 bit in a computer program, which can have undesirable and unpredictable results onboard! To help overcome this, spacecraft computer programs include error correction codes, which continuously and routinely check the onboard computer memory. Another type of problem caused by particle radiation, a single event burnout (SEB), is much more serious as it can cause permanent damage to the spacecraft's electronic systems. In this case a single energetic particle can kick off a runaway current in an electronic component, causing the device to burn out. To overcome this, the designer can build in more shielding, or choose to protect the device with current-sensing and -limiting circuitry, but again there is a balance to be struck to prevent mass growth in the spacecraft design.

In this brief tour of how environmental effects influence spacecraft design, the final topic involves impacting particles again, but this time somewhat larger ones than the subatomic particles we find in the Van Allen belts.

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Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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