Solar activity can be surprisingly good for astronauts.
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October 7, 2005: Last month, the sun went haywire.
Almost every day for two weeks in early September, solar flares
issued from a giant sunspot named "active region 798/808."
X-rays ionized Earth’s upper atmosphere. Solar protons peppered
the Moon. It was not a good time to be in space.
Or
was it?
During
the storms, something strange happened onboard the International
Space Station (ISS): radiation levels dropped.
"The crew of the ISS absorbed about 30% fewer cosmic
rays than usual," says Frank Cucinotta, NASA's chief
radiation health officer at the Johnson Space Center. "The
storms actually improved the radiation environment inside
the station."
Right:
The International Space Station (ISS). [More]
Scientists
have long known about this phenomenon. It's called a "Forbush
decrease," after American physicist Scott E. Forbush,
who studied cosmic rays in the 1930s and 40s. When cosmic
rays hit Earth's upper atmosphere, they produce a shower of
secondary particles that can reach the ground. By monitoring
these showers he noticed, contrary to intuition, that cosmic
ray doses dropped when solar activity was high.
The
reason is simple: When sunspots explode, they often hurl massive
clouds of hot gas away from the sun. These clouds, called
CMEs (coronal mass ejections), contain not only gas but also
magnetic force fields, knots of magnetism ripped away from
the sun by the explosion. Magnetic fields deflect charged
particles, so when a CME sweeps past Earth, it also sweeps
away many of the electrically-charged cosmic rays that would
otherwise strike our planet. This is the "Forbush decrease."
Wherever
CMEs go, cosmic rays are deflected. Forbush decreases have
been observed on Earth and in Earth orbit onboard Mir and
the ISS. The Pioneer 10 and 11 and Voyager 1 and 2 spacecraft
have experienced them, too, beyond the orbit of Neptune.
A
single CME can suppress cosmic rays for a few weeks. Sustained
solar activity can suppress them for a much longer time: "2005
has been a surprisingly active year on the sun," notes
Cucinotta. Since January, astronomers have counted 14 powerful
X-class solar flares and an even greater number of CMEs. As
a result, "the crew of the ISS has absorbed fewer cosmic
rays all year long."
Right:
Scott E. Forbush discovered the surprising inverse relationship
between solar activity and cosmic rays. [More]
This
whole story is ironic because flares and CMEs are themselves
sources of deadly radiation. CMEs, in particular, cause "proton
storms." En route to Earth, CMEs race through the sun's
outer atmosphere, plowing through the hot gas at speeds exceeding
a million miles per hour. Protons caught in the path of a
CME can be accelerated to dangerous energies.
No
astronaut wants to encounter a swarm of high-energy solar
protons. Severe storms are literally sickening; exposure causes
vomiting, fatigue and low blood counts. Without medical attention,
an astronaut suffering from radiation sickness could die.
Now for the good news: few solar protons are able to penetrate
the hulls of NASA spaceships. As long as astronauts stay inside,
they're safe.
Cosmic
rays are different—and worse. Cosmic rays are super-charged
subatomic particles coming mainly from outside our solar system.
Sources include exploding stars, black holes and other characters
that dwarf the sun in violence. Unlike solar protons, which
are relatively easy to stop with materials such as aluminum
or plastic, cosmic rays cannot be completely stopped by any
known shielding technology.
Even
inside their ships, astronauts are exposed to a slow drizzle
of cosmic rays coming right through the hull. The particles
penetrate flesh, damaging tissue at the microscopic level.
One possible side-effect is broken DNA, which can, over the
course of time, cause cancer, cataracts and other maladies.
![see caption](images/afraid/moscow_fd_sqashed2.jpg)
Above:
Neutron counts from a cosmic ray monitoring station in Moscow.
Radiation levels dropped in early Sept. during a period of
intense solar activity. [More]
No
one knows all the things cosmic rays might do to humans. "We
haven't been in space long enough," says Cucinotta.
Except
during brief trips to the Moon forty years ago, he explains,
astronauts have never been fully exposed to galactic cosmic
rays. Close to Earth where the ISS orbits, crews are protected
not only by their ship's hull, but also by Earth's magnetic
field and the gigantic solid body of Earth itself. A 6-month
trip to Mars, far from these natural shields, is something
new. What are the long-term risks? How much shielding is needed
to keep astronauts safe? NASA researchers are grappling with
these questions.
One
thing is clear. "Reducing exposure is a good thing,"
he says.
The
sun can help. Every 11 years, solar activity reaches a fever
pitch called Solar Max. It last happened in 2000; future episodes
are due around 2011 and 2022. During Solar Max, CMEs are produced
daily, and the solar wind blows knotty magnetic fields through
the inner solar system as a matter of routine. These fields
provide a measure of extra protection for trips to the Moon
and Mars, dropping cosmic ray fluxes in the biologically dangerous
energy range 100 MeV to 1000 MeV by 30% or more. Mission planners
of the future might actually schedule long trips through the
solar system to coincide, roughly, with Solar Max, thus taking
advantage of this decline in cosmic rays.
Maybe
solar flares aren't so bad after all.
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Author: Dr. Tony
Phillips | Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
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Sickening
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