Measurements of radiation around Earth marked early days of Space Age

Earth's radiation belts

The Space Age began on Oct. 4, 1957, when the Soviet Union launched the Earth's first artificial satellite.

At the time, the Cold War was in full swing, with the world's power blocs competing for power and prestige. With the launch of that first satellite, called Sputnik (Russian for "little traveller"), the then-USSR scored a major goal in the prestige competition. At that time, the only other country actively working on launching spacecraft into orbit was the United States, with its Project Vanguard.

The fact a Russian satellite was passing overhead several times each day, transmitting its "bleep, bleep" signals on a frequency almost all radio amateurs could hear, pushed Project Vanguard into high gear.

The Soviet satellite was launched using an inter-continental ballistic missile (ICBM), sending the not-so-subtle message that if the missile could put a satellite into orbit, it could be used to attack anywhere on Earth. However, Project Vanguard was completely different. The launcher rocket was being specifically developed for launching satellites. The program for developing ballistic missile weapons was entirely separate.

An attempted launch of the Vanguard launcher with a satellite in December that year failed spectacularly. A backup plan was quickly put into action. An intermediate range ballistic missile (IRBM) was modified for launching a satellite, and a satellite, known as Explorer1, was hastily put together.

With time and weight constraints, the number of onboard instruments was limited to four, the main one being a Geiger counter. This device, invented by Hans Geiger and Walther Müller, is a device for detecting high-energy particles, such as those produced by radioactive materials. It was known there were high-energy particles in space, coming from the Sun and other sources.

Explorer 1 was successfully launched on Feb. 1, 1958. The speed with which it was achieved is a real indicator of what can be done if it is wanted enough. Unlike Sputnik, which was in a near-circular orbit, Explorer's orbit was highly elliptical, ranging between 358 and 2,550 kilometres from the Earth.

As the spacecraft moved around its orbit, the scientists noted something odd. When it was close to the Earth, the Geiger counter was detecting the expected 30 or so counts per second due to cosmic radiation particles. However, as Explorer moved towards the highest part of its orbit, the count rate increased and then suddenly dropped to zero. This caused some puzzlement, until it was realized that the Geiger counter was encountering such high count rates it was overloading and reporting zero. This was not expected.

As the spacecraft moved to and fro between the lowest and highest points in its orbit (referred to in astronomy and space science as the perigee and apogee respectively), the Geiger counter data showed the Earth was surrounded by two radiation belts. These were named the inner and outer van Allen belts, after a magnetospheric physicist who pioneered putting scientific instruments on satellites. These radiation belts are made up of mainly high-energy particles from the Sun that had become trapped in the Earth's magnetic field. They are not strong enough to endanger astronauts.

Spacecraft flying past Jupiter and exploring the Jupiter system showed the giant planet has an intense magnetic field filled with enough high-energy particles to damage spacecraft electronics and pose a very severe hazard for human astronauts. Saturn, Uranus and Neptune have radiation belts too.

Most planets probably have them, with the stronger the magnetic field, the higher the radiation hazard.


• Venus and Mars lie extremely low in the dawn glow.

• Jupiter shines low in the west after sunset.

• The Moon will be new on April 8.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.

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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]

The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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