01/04/2018 10:42:53

A short history of Earth's radiation belts

For communication and navigation, we rely on more than 800 satellites operating in the zones around the Earth known as Van Allen belts, or radiation belts. There are two such regions around the planet: the inner belt and the outer belt. Both harbor energetic charged particles, making long-term flights in near-Earth space dangerous. Streams of low-energy protons can disable solar panels and harm thin optical coatings. In case of a prolonged stay in the inner belt, living organisms in a spaceship can suffer radiation damage caused by high-energy protons. During the periods of intense space weather, the density and energy of the particles in the radiation belts can increase, posing a danger to astronauts, spacecraft, and even ground-based equipment. That said, the more we understand about what happens in the Van Allen belts, the better we can protect our satellites and astronauts.

The radiation belts are located in the inner regions of the magnetosphere, where charged particles — protons, electrons, and alpha particles — are trapped by the Earth’s magnetic field. These particles possess energies between 10,000 and 100 billion electron volts, depending on the belt and the specific region in it. The Lorentz force causes them to spiral along the magnetic field lines, bouncing back and forth between the North and South poles. Particles also drift along the longitudes around the Earth, though at lower speeds. Electrons, which have a negative charge, move eastward, while the positively charged ions move westward. Depending on its energy, a particle circles the Earth in several minutes or days.

The inner and outer belts are confined to different altitudes and dominated by different particles. The inner belt mainly contains protons and lies 3,000-12,000 kilometers above the surface of the planet. Sometimes this region of harmful radiation expands to include the orbits of the International Space Station and many other satellites. The outer belt mostly contains electrons and extends from 18,000 to 57,000 kilometers above the Earth. However, the division into the inner and outer belts is largely nominal, because the entire near-Earth space is filled with charged particles driven by the planet’s magnetic field.

The earliest data proving the existence of the Van Allen belts were collected by Explorer 1, the first U.S. satellite. Its observations confirmed the existence of the inner belt. The outer belt was discovered by Luna 1, which was launched Jan. 2, 1959, by the Soviet Union and became the first spacecraft to reach the vicinity of the Moon. Luna 1, also known as Mechta, which means “a dream” in Russian, was the first lunar rover. It was meant to crash into the moon, yet an error in the calculation of the burn time — how long the rocket’s engines were supposed to be on — led to the probe missing its target by about 6,000 kilometers. Despite this, Luna 1 successfully collected vital information using a magnetometer, a Geiger counter, a scintillation counter, and a micrometeorite detector carried on board. On its way to the moon, it picked up new data on the Earth’s mysterious Van Allen radiation belts.

As for the objective of hitting the surface of the moon, it was achieved by Luna 2 on Sept. 13, 1959. But that is a different story that must be told some other time.

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