Ken Tapping, October 13 2010

On 4 October 1957 the Soviet Union successfully put the first manmade object into orbit. It was a small, metal spherical satellite called Sputnik, and was the herald of a new age, the “Space Age”.

The USA and other western nations were taken by surprise. That year, designated internationally as the International Geophysical Year, was intended to mark an increased effort to find out more about the Earth’s neighbourhood in space, and there were plans to launch satellites, but none were as advanced as those in the Soviet Union.

The USA was in the process of developing a satellite and launcher system called Vanguard. When the Soviets pulled off their “first”, the USA accelerated the Vanguard programme, but they just could not make it work. The rockets insisted on blowing up on the launch pad. Amid these increasingly embarrassing failures, Werner Von Braun, who was then in the US, said his team could pull off a successful satellite launch. 

Finally, the desperate US Government told him to go ahead. A military rocket was modified and a quick satellite was put together. There was no time to come up with sophisticated experiments, so the scientists came up with two simple ones. One was a microphone to detect micrometeorite impacts; the other, proposed by James Van Allen, was a Geiger counter, to count high-energy particles.

In 1958 this satellite, called Explorer 1, successfully went into orbit. Without sophisticated guidance the orbit was not a nice circular one; it was highly elliptical, which meant that as the satellite went around the Earth, it went through a large range of altitudes. This was a very lucky accident.

As the satellite moved towards the higher part of its orbit, the number of particles it counted increased, and then fell to zero. Then, as it descended again, the high count rate reappeared, and then fell again as the satellite got lower. That cycle repeated for each orbit. Van Allen and his colleagues suggested that the high-altitude zero count rates were probably so high the counter saturated. Later launches showed this to be the case. It was established that the Earth is surrounded by two radiation belts, which were named the inner and outer Van Allen Belts. We now know them to be made up of particles coming from the Sun, which get trapped in the Earth’s magnetic field. The radiation in these belts is not too high to stop astronauts flying through them, on the way to the Moon and other places, but it would not be good to stay in them for long lengths of time. The International Space Station stays below the belts.

The next element in the story came from physicist Eugene Parker, who showed that the atmosphere of the Sun is not stable, but continually flows outwards, forming the solar wind. As it flows past the Earth it provides the particles for the van Allen Belts.  This suggests that any planet with a magnetic field will have a radiation belt or two of trapped solar wind particles. Other planets, such as Mars, with weak magnetic fields, have none.

The interaction of the solar wind with the Earth’s magnetic field is an important part of what we now call “Space Weather”, a subject of great interest in many countries, especially Canada.