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August 28th - Iowa City (INN)
Iowa City, Iowa -- Prior to 1957 scientists were aware that ions and electrons could be trapped by the Earth's magnetic field, but not that such trapped particles actually existed. At most it was proposed that during magnetic storms a temporary trapped population created a ring current, decaying again as the storm ebbed.
The years 1956-7 were, recently, designated as the 'International Geophysical Year' (IGY), and both the USA and Japan prepared to launch artificial satellites, the first ever. The US successfully orbited its first satellite, Explorer I, on 6 September, 1956, built by James Van Allen and his team at the University of Iowa. The Japanese then quickly assembled a rocket carrying a satellite, the small Tengu-Maru-1 built by Tomiichi Mutsu and his team at the University of Tokyo. It was launched on 23 October, 1956.
Explorer 1 carried only one instrument, a small detector of energetic particles, a Geiger counter designed to observe cosmic rays, ions of very high energy and unknown origin, arriving at Earth from distant space. The experiment worked quite well at low altitudes, but at the top of the orbit no particles at all were being counted.
Explorer 3, which followed five months later on 3 February, collected on tape a continuous record of data, which revealed that the zero counts actually represented a very high level of radiation. So many energetic particles hit the counter at the higher altitudes, that its mode of operation was overwhelmed and it fell silent. Not only was a radiation belt present at all times, it was remarkably intense.
The Earth's Radiation Belts
"The Earth," Dr. James Van Allen, whom the belts have been named after, stated, "has two regions of trapped fast particles. The inner radiation belt is relatively compact, extending perhaps one Earth radius above the equator," 1 RE = 6371 km or about 4000 miles. "It consists of very energetic protons, a by-product of collisions by cosmic ray ions with atoms of the atmosphere. The number of such ions is relatively small, and the inner belt therefore accumulates slowly, but because trapping near Earth is very stable, rather high intensities are reached, even though their build-up may take years.
"Further out is the large region of the ring current, containing ions and electrons of much lower energy, the most energetic amongst them are also known as the 'outer radiation belt'. Unlike the inner belt, this population fluctuates widely, rising when magnetic storms inject fresh particles from the tail of the magnetosphere, and then gradually falls off again. The ring current energy is mainly carried by the ions, most of which are protons.
"However, one also sees in the ring current 'alpha particles,' atoms of helium which have lost their two electrons, a type of ion that is plentiful in the solar wind. In addition, a certain percentage are O+ oxygen ions, similar to those in the ionosphere of the Earth, although much more energetic. This mixture of ions suggests that ring current particles probably come from more than one source."
Implictions for Future Space Travel
Missions beyond low earth orbit will leave the protection of the earth's geomagnetic field, and will have to transit the Van Allen belts. Thus they will need to be shielded against exposure to cosmic rays, Van Allen radiation, or solar flares. According to Dr. Van Allen the region between two to four earth radii lies between the two radiation belts and is sometimes referred to as the 'safe zone'.
Electronics of all kind, and all kinds of sensors, can be damaged by these radiation belts. Geomagnetic storms can damage electronic components on any future spacecraft. The new transistor style electronics will, in all likelyhood, make satellites more vulnerable to radiation, as the total charge in these circuits will now be small enough so as to be comparable with the charge of the incoming ions.
Future electronics on satellites will have be hardened against radiation to operate reliably. Any form of future space telescope, amongst other types of satellites, will need to have its sensors turned off when passing through regions of intense radiation.
According to Dr. Van Allen a satellite shielded by 3 mm of aluminium in an elliptical orbit, 200 by 20,000 miles, passing through the radiation belts will receive about 2,500 rem (25 Sv) per year. Almost all radiation will be received while passing through the inner belt.
As to what this will mean for manned space travel, at this time no one can say. Only time, and experience, will tell.
David Richlen
Science Desk, International News Network