Richard Mewaldt of Caltech elaborated: "In 2009, cosmic ray intensities have increased 19 per cent beyond anything we've seen in the past 50 years. The increase is significant, and it could mean we need to rethink how much radiation shielding astronauts take with them on deep-space missions."*

The reason for the livelier cosmic ray levels is the current solar minimum. This lull in the Sun's activity means "flagging" solar winds and, as a result, a reduced heliosphere. Mewaldt explained: "Measurements by the Ulysses spacecraft show that solar wind pressure is at a 50-year low, so the magnetic bubble that protects the solar system is not being inflated as much as usual."

Accordingly, cosmic rays have a "shorter shot into the solar system", and once they enter the heliosphere, face weaker solar winds to impede their progress.

The rays are also less prone to interference from the "heliospheric current sheet" - a complex surface described as resembling a "ballerina's skirt", wrapped round the Sun and extending from its equatorial plane. The current sheet marks the boundary in the Sun's magnetic field where its polarity changes from north to south.

The current sheet's "folds" affect the progress of cosmic rays, but it's now "flattening itself out", allowing the charged particles "more direct access to the inner solar system".

Mewaldt noted: "If the flattening continues as it has in previous solar minima, we could see cosmic ray fluxes jump all the way to 30 percent above previous Space Age highs."

None of this, however, poses a serious threat to Earth. While a single cosmic ray could conceivably, if it hit the right spot, knock out a satellite's electronics, the present cosmic ray high is considerably lower than our planet has experienced in the past.

The proof comes in the form of an isotope of beryllium, 10Be, created by cosmic rays when they impact with Earth's atmosphere, and preserved in polar ice. Tests on ice cores have shown that "hundreds of years ago, cosmic ray fluxes were at least 200 per cent higher than they are now".

Mewaldt concluded: "The space era has so far experienced a time of relatively low cosmic ray activity. We may now be returning to levels typical of past centuries." ®

Bootnote

*Well, the way things are going at the moment, this won't be an issue, since the Sun will have reached solar maximum by the time we get our act together and head off to Mars.