Van Allen Belts
The Van Allen belts are two areas of the Earth's magnetosphere where large amounts of high-energy charged particles are concentrated, mostly caused by the solar wind captured by the Earth's magnetic field. They are named after their discoverer, James Van Allen. They were discovered thanks to the launch of the US Explorer 1 satellite, which was initially unsuccessful due to its elongated shape, which, together with a poorly designed control system, hampered adjustment to orbit.
Structure
These belts are ring-shaped areas with a toroidal surface where protons and electrons spiral in large numbers between the planet's magnetic poles.
There are two Van Allen belts:
- The belt extends from about 500 km above the Earth's surface to beyond 5000.
- The outer belt, which extends from about 15 000 km to about 58 000 km, which affects high/medium orbit satellites, such as geostationary ones, located at about 58 000 km 36 000 km altitude.
With low orbit (LEO) satellites, a compromise must be sought between the convenience of a considerable altitude to avoid the residual resistance of the upper atmosphere, which shortens the useful life of the satellite, and the need to be below of 1000 km so as not to suffer long stays in the radiation belts or cross areas of high intensity, which are very harmful for these satellites.
A region of the inner belt, known as the South Atlantic Anomaly (SAA), extends to low orbits and is dangerous for spacecraft and artificial satellites that pass through it, as both electronic equipment and humans can be harmed by radiation. The South Atlantic Anomaly is a region where the Van Allen radiation belts lie a few hundred kilometers from the Earth's surface. As a result in that region the radiation intensity is higher than in other regions. The AAS (South Atlantic Anomaly) or SAA (acronym in English) is produced by a "depression" in the Earth's magnetic field in that area, caused by the fact that the center of the Earth's magnetic field is deviated from its geographic center by 450 km. Some think that said anomaly is a side effect of a Geomagnetic reversal.
These radiation belts are caused by the strong magnetic field of the Earth that is a product of its rotation. This field traps charged particles (plasma) coming from the Sun (solar wind), as well as charged particles that are generated by the interaction of the Earth's atmosphere with cosmic radiation and high-energy solar radiation.
These highly radioactive belts contain antiprotons, antiparticles with enormous electromagnetic force.
Antiprotons in the Van Allen belts
In 2011, an article was published reporting the detection by the PAMELA satellite of a significant natural flow of antiprotons, of greater density in the area of the South Atlantic Anomaly.
Implications for space travel
Spacecraft traveling beyond low-Earth orbit enter the radiation zone of the Van Allen belts. Beyond the belts, they face additional dangers from cosmic rays and solar particle events. A region between the inner and outer Van Allen belts lies between 2 and 4 Earth radii and is sometimes referred to as the "safe zone".
Solar cells, integrated circuits and sensors can be damaged by radiation. Geomagnetic storms occasionally damage the electronic components of spacecraft. Miniaturization and digitization of electronic components and logic circuitry have made satellites more vulnerable to radiation, as the total electrical charge on these circuits is now small enough to be comparable to the charge on incoming ions. Satellite electronics must be radiation hardened to function reliably. The Hubble Space Telescope, among other satellites, often has its sensors turned off when passing through regions of intense radiation. A 3 mm aluminum armored satellite in an elliptical orbit (200 by 20,000 miles (321.9 by 32,186, 8 km)) passing through the radiation belts will receive about 2,500 rem (25 Sv) per year. (For comparison, a whole-body dose of 5 Sv is lethal.) Almost all of the radiation will be received by passing through the inner belt.
The Apollo missions were the first in which humans traveled through the Van Allen belts, which was one of several radiation hazards known to mission planners. The astronauts had low exposure in the Van Allen belts due to the short amount of time spent flying through them.
The astronauts' total exposure was dominated by solar particles once they left Earth's magnetic field. The total radiation received by the astronauts varied from mission to mission, but was measured between 0.16 and 1.14 rads (1.6 and 11.4 mGy), much less than the 5 rem (50 mSv) norm (For beta, gamma, and X-rays the absorbed dose in rads is equal to the dose in rem) per year established by the United States Atomic Energy Commission for people working with radioactivity.