MadSci Network: Physics |
Kathleen, The answer to this lies in what you are trying to shield. In the space environment, radiation is omnipresent and falls into several distinct categories. There are low energy trapped electrons and protons (Van Allen Belts), Solar protons and other heavier ions (Solar Wind), and Galactic Cosmic Rays (GCRs). As you have no doubt been told, the best way to shield something is to put a material with a high Z number facing the source to block the high energy component and put a low Z material behind it to absorb the lower energy secondary particles. In the case of the ISS or the Shuttle, you are trying to shield a biological system (i.e. humans), so you put polyethylene inside the primary shield to absorb the secondaries created by interactions with the skin of the spacecraft. This reduces the amount of radiation reaching the sensitive region (the person) to a reasonable minimum. With the shielding on a manned mission, the exposure inside the cabin is minimized the best as is possible for crew safety. Most of what gets in are reactions from high energy solar particles and GCRs. Keep in mind that cost is a factor here. It costs several thousand dollars per pound to put something into orbit. Since people are the most important cargo, the expense is justified in putting more shielding in to protect them. For a satellite, you can't usually justify the expense, thus the shielding concerns are a little different. In a satellite, there is the metal skin of the space craft which blocks out (hopefully) all of the trapped electrons and protons, and a reasonable fraction of the solar particles. In the event of a massive solar flare, all bets are off, however. The end result is that there is a low energy spectrum of secondary particles present inside the spacecraft all the time. There is NO shield for GCRs, they are simply too energetic and are omnidirectional so there is no practical way to shield them. (You'd need feet of shielding as opposed to the 50 to 200 mils of shielding you typically get on a spacecraft.) For electronics, the key things are size and weight. Polyethylene shielding would take up lots of room and weigh more than the standard ceramic package that most space flight parts are packaged in. Also, the spectrum of energies will be higher inside the spacecraft than in a manned vehicle. Electronics are quite sensitive to energetic particles dumping their energy into the sensitive region of the part die, so the idea is to block the most low energy stuff you can to prevent this from happening. The ceramic tends to be a mid Z material so it is well suited to this specialized task. Figuring out how radiation will screw up a part is what I do for NASA. I work in the Radiation Effects and Analysis Group of the Office for System Safety and Mission Assurance, NASA Goddard Space Flight Center. If you are really curious about just how involved this is, pick up the IEEE Transactions on Nuclear Science for any year and look at #6 in that volume. These are the proceeding of the Nuclear and Space Radiation Effects Conference (NSREC), held each year. As it happens, it's next week in Vancouver, BC. We have 6 posters and 2 papers this year, business is good. P.S. Don't let radiation intimidate you, it really isn't as scary as you've been led to believe. Scott Kniffin Senior Engineer, Orbital Sciences Corporation NASA Goddard Space Flight Center Office of Systems Safety and Mission Assurance Radiation Effects and Analysis Group, Code 562
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