Is it impossible to travel to the Moon, because of the Van Allen Belt?
From: Peter Wingerter
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Message ID Number: 958408099.As
Is there any truth to the rumor? Is it impossible to travel to the moon, because of the Van Allen Belt?
This is an especially interesting question, though maybe more about psychology and epistemology than about astronomy or physics. Nevertheless, the same question comes up again and again, in one form or another, so it really is very important.
It has a number of possible answers:
This is the straightforward, scientific answer. It is correct, to the best of my knowledge and belief.
This is a simple common-sense answer. Also correct, I think.
I know for a fact this one is false -- but how can you know that?!
I am as sure as I think one can reasonably be about anything that this one is false, but of course how could I possibly be absolutely certain, in principle?
There is a lot of truth in this one, especially in principle. In practice, we can usually do quite a bit better, especially in the sciences; but the issue is not silly or unimportant, even so. The head of the government of South Africa, for example, is in serious doubt about whether the human immunodeficiency virus, HIV, causes AIDS, because he is (probably sincerely, I guess) in doubt about whom to trust; although there seems to be no serious scientific controversy about the issue. Millions of lives could be at stake as a result.
Now let's take a little more substantial look at my first answer. The idea is to outline the basic facts of the case, and give you the materials you need to verify my statements, to whatever level of detail you wish. This is the traditional scientific way of answering a question. There are three basic issues.
Regarding the Van Allen belts, and the nature of the radiation in them, they 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, never mind the Moon! 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. Unfortunately I have not located quantitative information in the time available, but 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. You have perhaps heard that to escape from Earth requires a speed of about 7 miles per second, which is about 11.2 km per sec. At that speed, it would require less than an hour to pass outside the main part of the belts at around 38,000 km altitude. However it is a little more complicated than that, because as soon as the rocket motor stops burning, the spacecraft immediately begins to slow down due to the attraction 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.
Unfortunately calculating the average radiation dose received by an astronaut in the belts is quite intricate in practice, though not too hard in principle. One must add up the effects of all kinds of particles, of all energies. 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:
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.
Much of this material can be found in the 1999 "Review of Particle Properties", (see below) in the sections on "Atomic and nuclear properties of materials", on "Radioactivity and radiation protection", and on "Passage of particles through matter".
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! Nevertheless, I have barely scratched the surface, and waved my hands a bit, to make it seem likely that I'm not full of baloney. But 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. I hope you will try to find the possible but not perfect in-between path by learning some science. It is hard, but it is fun and interesting, and it gives you your own power to think and evaluate for yourself, albeit in a limited and imperfect way.
Health Physics Society, professional society concerned with radiation effects and radiation protection.
University of Michigan Radiation and Health Physics page. Good general reference on radiation in the environment, including many links about radiation in space.
"Radiation Hazards to Crews of Interplanetary Missions: Biological Issues and Research Strategies", by the Task Group on the Biological Effects of Space Radiation, Space Studies Board, Commission on Physical Sciences, Mathematics, and Applications of the National Research Council; National Academy Press, 1997. About radiation hazards of possible long-term future missions in space.
"Health Effects of Ionizing Radiation in Manned Space Activities" http://radefx.bcm.tmc.edu/ionizing/publications/space.htm containing an extensive bibliography on the subject.
Standard reference for the Van Allen Belts is AP8 & AE8 Models for the Trapped Radiation Environment, NSSDC, GSFC.
"The Radiation Environment", by Janet Barth of GSFC, 1999; available at http://radhome.gsfc.nasa.gov/radhome/papers/apl_922.pdf.
"An Annotated Bibliography of the Apollo Program" Compiled by Roger D. Launius and J.D. Hunley Published as Monographs in Aerospace History, Number 2, July 1994. http://www.hq.nasa.gov/office/pao/History/Apollobib/contents.html
Berry, C.A. "Summary of Medical Experience in the Apollo 7 Through 11 Manned Spaceflights." Aerospace Medicine. 41 (May 1970): 500-19. Described as, "This is a sophisticated scientific paper describing the results of biomedical experiments during the early history of Apollo. It is especially helpful in discussing the problem of radiation and other effect on the astronauts during the missions to the Moon of Apollo 8 and 11."
1999 Edition of the "Review of Particle Properties" compiled by the Particle Data Group at Lawrence Berkeley Laboratory, and collaborators. "http://www-pdg.lbl.gov/1999/contents_sports.html"
Summary of paper AIAA-1969-19, RADIATION PLAN FOR THE APOLLO LUNAR MISSION (1969); complete paper available from the AIAA.
Note: some links and references updated 6/4/2007.