The Radiation Belts!

Since the beginning of the manned-space era, humans have been affected by Belt particles, though not as severely as some people might believe. Science fiction stories, and movies, have been written about the van Allen Belts and their deadly cargoes of lethal particles. Some people even believe that Apollo astronauts could not have safely traveled to the Moon because of these Belts, and so the Apollo moon landings were a 'hoax'. In order to separate fact from fiction, we need to learn a little about how to measure the biological effects of radiation exposure. Scientists measure radiation exposure by using a unit called the Rad which stands for 'RAdiation-equivalent Dose' not 'radical'. If a piece of biological tissue is exposed to 1 Rad of radiation, this means a specific amount of energy as been delivered to the tissue. This energy can either do nothing, or it can cause damage. The damage it does is worse if it happens to be particles like electrons and protons, than if it is just electromagnetic radiation. So, to take the kind of radiation into account, a new unit is used called the Rem. When your parents visit their doctor and have to take a chest X-ray, this is worth about 0.020 Rem's - a unit we can also write as '20 milli-Rems' or 20 mRem. The place where you live has different kinds of naturally-occuring radiation from the ground and air, and in one year you get about 350 mRem of dosage. There is little you can do about this, but some places are worse to live than others.  If you live in the Rocky Mountains, your exposure might be 380 mRem per year.  If you live near the sea coast, it might be only 300 mRem per year.  In areas India with soils rich in uranium, dosages are over 700 mRem per year! Space Shuttle and Space Station astronauts inside their crafts receive about 2 mRems of additional dosage each time they pass through the South Atlantic Anomaly. In one day they can accumulate 30 mRem of dosage. Over the course of a week, this adds up to 7 x 30 = 210 mRems which is just below the dosage you get at ground-level in a single year (about 350 mRem). But what about the Apollo astronauts who traveled the most intense regions of the Belts in their journey to the Moon? Fortunately, the travel time through the Belts was only about 30 minutes. Their actual radiation exposures inside the Apollo space capsule were not much more than the total dose received by Space Shuttle astronauts in a one-week stay in orbit. This fact counters some popular speculations that the moon landings were a hoax because astronauts would have instantly died as they made the travel through the belts. In reality, the Apollo astronauts might have experienced minor radiation sickness if they had been in their spacesuits on a spacewalk, but no spacewalk was ever scheduled for this very reason. The metal shielding provided by the Apollo space capsule walls was more than enough to protect the astronauts from all but the most energetic, and rare, particles. Still, the astronauts reported seeing 'shooting stars'. These were caused by very energetic particles streaking through the fluid in their eyes and colliding with retinal cells, leaving behind the appearance of a luminous, but fleeting, trail of light. Similar streaks have been reported by astronauts in the Space Shuttle and other near-earth missions during the most intense solar storms. It is not known what the long-term consequences of these kinds of brief exposures are upon astronauts, but prospective travelers to Mars will no doubt see many more of them! Humans have also made a significant impact upon the Belts through older programs of nuclear testing. In fact, during tests in the early 1960's before they were banned, humans temporarily created a new radiation belt in a 'notch' region between the Inner and Outer Belts. This temporary belt eventually dissipated, but its traces could still be detected during Space Shuttle missions in the1980's.