Radiation belts

Ken Tapping, 12thJanuary, 2016

In the sky this week…

  • Jupiter rises in the late evening and is high in the southern sky before sunrise.
  • Mars is fainter, lower and left of Jupiter.
  • Venus and Saturn lie close together low in the southeast before sunrise.
  • The Moon will reach First Quarter on the 16th.

This took the Western World by surprise. The USA was working on a project to launch a satellite. It was called Vanguard, but there were persistent problems with the launch vehicle, which had been specifically designed for the purpose. Determined not to be left too far behind in what became to be called “The Space Race”, and with no prospect of the Vanguard launch vehicle being ready any time soon, the US Government accepted Werner Von Braun’s proposal to make a launch vehicle out of existing military missile technology. A pencil-shaped satellite called Explorer 1 was put together. The main instrument on board the satellite was a Geiger counter, to measure the number of high-energy particles it encountered in space, above our protective atmosphere.

Explorer 1 was launched successfully on 1 February, 1958, just a few months after getting the “Go”. Since the priority was to get a satellite launched as soon as possible, not a lot of attention was paid to choosing the precise orbit for the satellite, so it ended up in an elliptical orbit taking it between heights of 360 and 2600 km. Sputnik 1 was in a nice circular orbit at a height of 580 km. This resulted in Explorer 1 detecting something important that Sputnik 1 could not. The data from the Geiger counter on Explorer 1 was odd. Scientists expected the radiation count to increase a bit as the satellite moved towards the highest point in its orbit, and indeed that happened, but the counts rose far more rapidly than expected, and then went to zero. Then, on the way back down a high count suddenly appeared, which then slowly decreased. The satellite was moving through a zone where the radiation count was so high the Geiger counter saturated. Scientist James Van Allen suggested that the Earth is surrounded by belts of high-energy particles from the Sun that had got trapped in the Earth’s magnetic field. These belts became known as the Van Allen Radiation Belts. The intensity of the radiation is such that long-term exposure would be bad for astronauts, but astronauts passing through them on the way to the Moon or other planets would not be particularly endangered. The International Space Station, orbiting at heights between 409 and 416 km, lies well below the Van Allen Belts.

Since the solar wind flows out past all the planets, we would expect all planets with magnetic fields to have radiation belts of some kind. Jupiter, the largest planet in the Solar System, and fifth from the Sun, has a strong magnetic field, and has radiation belts that are far more intense than those around the Earth, and extend some 300,000 km out from the planet. Measurements by the Cassini spacecraft indicate the radiation is sufficiently intense to damage or destroy spacecraft electronics and be a hazard for astronauts.

If the Earth had no magnetic field, there would be no radiation belts. However, the magnetic field holds the solar wind away from our atmosphere. Otherwise the atmosphere would be slowly scoured away. Mars has no magnetic field, and most of the planet’s atmosphere has gone.

Since 4 October, 1957, we have come to appreciate the complex interactions that take place between planets and the solar wind, and the consequences of the Sun’s behaviour. Now that we have found well over a thousand planets orbiting other stars, many of which differ in brightness and temperature from the Sun, we have the opportunity to learn even more about the relationships between planets and their "suns".

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

Telephone: 250-497-2300
Fax: 250-497-2355

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