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.
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.
Solar minimum
"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.
Extra shielding
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.
Down time
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.