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In Reply to: thanks you're the best (NT) posted by lydia on October 13, 2002 at 21:29:44:
I said I didn't want to get into it, but here is some more info. Gee, I already spent more time on this than I should.
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How did their spacesuits help the astronauts survive in the heat of the Moon's day? Objects that are heated cannot be cooled by space.
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1. Heat is released through radiation:
All objects above absolute zero radiate heat, even spacesuits. Therefore, some of the heat energy received from the Sun was radiated back into space as infrared rays. Some layers in the spacesuits were specially designed incorporating material which radiated heat (outwards), not too different from the radiators found in houses.
2. Utilizing reflection:
Much of the Sun's radiant energy can be reflected away. The astronaut's spacesuits were white because this color reflects the most radiation, thereby minimizing the amount absorbed.
3. Water circulation - evaporation system
It's all thanks to natural laws of physics. The spacesuits where equipped with a cooling/heating system (a mesh located close to the body and not the out layers of the suit) that utilized water as a medium to carry away access heat or distribute heat (when it was cold). When it was hot, water was sprayed into a vacuum. When water is sprayed into a vacuum, it experiences a very rapid drop in pressure and, consequently, temperature. Hence, when a small amount of water was sprayed onto a cooling element on the rear of the spacesuit, its temperature dropped so much that it would immediately freeze onto the element. The cooling water of the spacesuit was then pumped through this element. The heat of the cooling water melted the ice, which then rapidly boiled off (due to being in a vaccum) and carry into space the unwanted heat. This evaporation also further freezes the element.
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How could any astronauts travel through the Van Allen Radiation Belts? They would receive lethal doses of radiation.
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This is simply not supported by the data. I'd suggest befriending people who own commercial satellites. They'd give you lots of info about the Van Allen Belts.
Radiation was a definite concern for NASA before the first space flights, but they invested a great deal of research into it and determined the hazard was minimal. It took Apollo about an hour to pass through the radiation belts - once on the outbound trip and once again on the return trip. The total radiation dose received by the astronauts was about 2 rem. A person will experience radiation sickness with a dose of 100-200 rem, and death with a dose of 300+ rem. Clearly the doses received fall well below anything that could be considered a significant risk. Despite claims that "shielding meters thick would have been needed", NASA found it unnecessary to provide any special radiation shielding.
Here is more on the the technical specifics of the Van Allen belts.
The Van Allen belts are doughnut-shaped regions where charged particles, both protons and electrons, are trapped in the Earth's magnetic field. The number of particles encountered (flux is the technical jargon, to impress your friends!) depends on the energy of the particles. In general, the flux of high-energy particles is less, and the flux of low-energy particles is more. Very low energy particles cannot penetrate the skin of a spacecraft, nor even the skin of an astronaut. Very roughly speaking, electrons below about 1 million electron volts (MeV) are unlikely to be dangerous, and protons below 10 MeV are also not sufficiently penetrating to be a concern.
The actual fluxes encountered in the Van Allen belts is a matter of great commercial importance, as communications satellites operate in the outer region, and their electronics, and hence lifetimes, are strongly affected by the radiation environment. Thus billions of dollars are at stake. The standard database on the fluxes in the belt are the models for the trapped radiation environment, AP8 for protons, and AE8 for electrons, maintained by the National Space Sciences Data Center at NASA's Goddard Spaceflight Center. Barth (1999) gives a summary which indicates that electrons with energies over 1 MeV have a flux above a million per square centimeter per second from 1-6 earth radii (about 6,300 - 38,000 km), and protons over 10 MeV have a flux above one hundred thousand per square centimeter per second from about 1.5-2.5 Earth radii (9,500 km - 16,000 km).
Then what would be the radiation dose due to such fluxes, for the amount of time an astronaut crew would be exposed? This was in fact a serious concern at the time that the Apollo program was first proposed. My recollection is that the dose was roughly 2 rem (= 20 mSv, milli-Sievert).
The time the astronauts would be exposed is fairly easy to calculate from basic orbital mechanics, though probably not something most students below college level could easily verify. Escaping from Earth normally requires a speed of about 7 miles/sec.or 11.2km/sec. At that speed, it would require less than an hour to pass outside the main part of the belts at around 38,000km altitude. However as soon as the rocket stops firing (which it does at this altitude), the spacecraft immediately begins to slow down due to the pull back of gravity. At 38,000 km altitude it would actually be moving only about 4.6 km per sec, not 11.2. If we just take the geometric average of these two, 7.2 km per sec, we will not be too far off, and get about 1.5 hours for the time to pass beyond 38,000 km.
To calculate the average radiation dose received by an astronaut in the belts, the effects of all particles and energies must be tallied. For each kind of particle (electrons and protons in this situation) you have to take account of the shielding due to the Apollo spacecraft and the astronaut space suits. Here are some approximate values for the ranges of protons and electrons in aluminum:
(the figures below might bunch up together in this post, but they are meant to be 3 columns, for Energy[MeV], Electrons and Protons)
Range in Aluminum [cm]
Energy[MeV] Electrons Protons
1 0.15 ~ nil
3 0.56 ~ nil
10 1.85 0.06
30 no flux 0.37
100 no flux 3.7
For electrons, the AE8 electron data shows negligible flux (< 1 electron per square cm per sec) over E=7 MeV at any altitude. The AP8 proton compilations indicates peak fluxes outside the spacecraft up to about 20,000 protons per square cm per sec above 100 MeV in a region around 1.7 Earth radii, but because the region is narrow, passage takes only about 5 min. Nevertheless, these appear to be the principal hazard.
These numbers seem generally consistent with the ~2 rem doses I recall. If every gram of a person's body absorbed 600,000 protons with energy 100 MeV, completely stopping them, the dose would be about 50 mSv. Assuming a typical thickness of 10 cm for a human and no shielding by the spacecraft gives a dose of something like 50 mSv in 300 sec due to protons in the most intense part of the belt.
For comparison, the US recommended limit of exposure for radiation workers is 50 mSv per year, based on the danger of causing cancer. The corresponding recommended limits in Britain and Cern are 15 mSv. For acute doses, the whole-body exposure lethal within 30 days to 50% of untreated cases is about 2.5-3.0 Gy (Gray) or 250-300 rad; in such circumstances, 1 rad is equivalent to 1 rem.
So the effect of such a dose, in the end, would not be enough to make the astronauts even noticeably ill. The low-level exposure could possibly cause cancer in the long term. I do not know exactly what the odds on that would be, I believe on the order of 1 in 1000 per astronaut exposed, probably some years after the trip. Of course, with nine trips, and a total of 3 X 9 = 27 astronauts (except for a few, like Jim Lovell, who went more than once) you would expect probably 5 or 10 cancers eventually in any case, even without any exposure, so it is not possible to know which if any might have been caused by the trips.
By this point I have no doubt told you more than you really wanted to know about the Van Allen belt and the Apollo radiation problem! In the end you always have to either do it all yourself, or trust a stranger completely, or try to find some path in between: which means understanding a little science, so you can judge for yourself if my arguments make any sense at all, check a little, think about it, maybe do a bit of research on your own from the references if you are interested. The only alternative is to trust no one and do everything, which is simply impossible for anyone; or really give up all your judgements to other people, who may be saints or crooks, wise or insane. Learning some science is fun and interesting, and it gives you your own power to think and evaluate it for yourself, albeit in a limited and imperfect way.
