Our solar system's natural shields are faltering, letting in a flood of cosmic rays. The sun's recent listlessness is resulting in record-high radiation levels hazardous to human and robotic space missions.
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Galactic cosmic rays are speeding charged particles that include protons and heavier atomic nuclei. They come from outside the solar system, though their exact sources are still being debated.
Earth dwellers are protected from cosmic rays by the planet's magnetic field and atmosphere. But outside Earth's protective influence, cosmic rays can play havoc with spacecraft electronics – they may be responsible for some recent computer glitches on NASA's Kepler spacecraft, which temporarily halted its planet-hunting observations. They can also damage astronaut DNA, which can lead to cancer.
Now, the influx of galactic cosmic rays into our solar system has reached a record high. Measurements by NASA's Advanced Composition Explorer (ACE) spacecraft indicate that cosmic rays are 19 per cent more abundant than any previous level seen since space flight began a half century ago.
"The space era has so far experienced a time of relatively low cosmic ray activity," says Richard Mewaldt of Caltech, who is a member of the ACE team. "We may now be returning to levels typical of past centuries."
The sun's magnetic field normally blocks some of the cosmic rays, preventing them from entering the solar system. But that protection has weakened of late. The solar wind, which helps project the sun's magnetic field out into space, has dropped in pressure to a 50-year low. And the strength of the magnetic field in interplanetary space is down to just 4 nanoTesla, compared to the more typical 6 to 8 nanoTesla.
The recent weakening of the shield is due to cycles in solar activity. The sun is at a minimum in its 11-year cycle of magnetic activity, and this particular dip is deeper than any other seen in nearly a century.
That may be a sign that the unusually active sun of the past 100 years or so is returning to the historical norm of lower activity, or even entering a so-called grand minimum of exceptionally low activity that could last centuries.
Scientists can infer variations in the sun's magnetic activity over the past 10,000 years from the abundance of rare isotopes in Greenland ice cores.
If the increase in cosmic rays is here to stay, it could make long-duration human missions in space more challenging. Astronauts aboard the International Space Station are still close enough to enjoy protection from Earth's magnetic field, but any sent in future to the moon or beyond will be outside that field.
"The increase is significant, and it could mean we need to re-think how much radiation shielding astronauts take with them on deep-space missions," Mewaldt says.
If there's a long-term increase, it might also make sense to design future robotic missions for extra robustness against radiation, says Roger Hunter of NASA's Ames Research Center in Moffett Field, California, who manages NASA's planet-hunting Kepler mission.
It is not clear whether Kepler's temporary computer glitches were due to cosmic-ray hits, he says. But the spacecraft is designed to be able to recover from such events, going into a safe mode while mission controllers work to restore it to normal operation, he adds.
"Our only concern is will we see more events as a result of the cosmic-ray increase," he says. Since its launch in March, Kepler has lost 3.5 days of observing time due to glitches that put it into safe mode. However, the mission team always planned for occasional days lost to glitches, and considers up to 12 lost days per year to be acceptable.