The biggest solar proton storm in
15 years erupted on the 20th January 2005. We discuss what it might
have done to someone on the Moon.
by Dr Tony Phillips
NASA is returning to the Moon - not
just robots, but people. In the decades ahead we can expect to see
habitats, greenhouses and power stations up there. Astronauts will
be out among the moondust and craters, exploring, prospecting, building.
Last week, though,
there were no humans walking around on the Moon.
On January 20th, 2005,
a giant sunspot named "NOAA 720" exploded. The blast sparked
an X-class solar flare, the most powerful kind, and hurled a billion-ton
cloud of electrified gas (a "coronal mass ejection") into
space. Solar protons accelerated to nearly light speed by the explosion
reached the Earth-Moon system minutes after the flare - the beginning
of a days-long "proton storm."
Here on Earth, no one
suffered. Our planet's thick atmosphere and magnetic field protects
us from protons and other forms of solar radiation. In fact, the
storm was good. When the plodding coronal mass ejection arrived
36 hours later and hit Earth's magnetic field, sky watchers in Europe
saw the brightest and prettiest auroras in years: gallery.
The Moon is a different
"The Moon is totally
exposed to solar flares," explains solar physicist David Hathaway
of the Marshall Space Flight Center. "It has no atmosphere
or magnetic field to deflect radiation." Protons rushing at
the Moon simply hit the ground - or whoever might be walking around
The Jan. 20th proton
storm was by some measures the biggest since 1989. It was particularly
rich in high-speed protons packing more than 100 million electron
volts (100 MeV) of energy. Such protons can burrow through 11 centimeters
of water. A thin-skinned spacesuit would have offered little resistance.
caught outside when the storm hit would've gotten sick," says
Francis Cucinotta, NASA's radiation health officer at the Johnson
Space Center. At first, he'd feel fine, but a few days later symptoms
of radiation sickness would appear: vomiting, fatigue, low blood
counts. These symptoms might persist for days.
Astronauts on the International
Space Station (ISS), by the way, were safe. The ISS is heavily shielded,
plus the station orbits Earth inside our planet's protective magnetic
field. "The crew probably absorbed no more than 1 rem,"
One rem, short for
Roentgen Equivalent Man, is the radiation dose that causes the same
injury to human tissue as 1 roentgen of x-rays. A typical diagnostic
CAT scan, the kind you might get to check for tumors, delivers about
1 rem [ref].
So for the crew of the ISS, the Jan. 20th proton storm was no worse
than a trip to the doctor on Earth.
sunspot 720 (and a passing airplane) photographed by amateur
astronomer Jan Koeman of the Netherlands on Jan. 15, 2005
On the Moon, Cucinotta
estimates, an astronaut protected by no more than a space suit would
have absorbed about 50 rem of ionizing radiation. That's enough
to cause radiation sickness. "But it would not have been fatal,"
To die, you'd need
to absorb, suddenly, 300 rem or more.
The key word is suddenly.
You can get 300 rem spread out over a number of days or weeks with
little effect. Spreading the dose gives the body time to repair
and replace its own damaged cells. But if that 300 rem comes all
at once ... "we estimate that 50% of people exposed would die
within 60 days without medical care," says Cucinotta.
Such doses from a solar
flare are possible. To wit: the legendary solar storm of August
It's legendary (at
NASA) because it happened during the Apollo program when astronauts
were going back and forth to the Moon regularly. At the time, the
crew of Apollo 16 had just returned to Earth in April while the
crew of Apollo 17 was preparing for a moon-landing in December.
Luckily, everyone was safely on Earth when the sun went haywire.
"A large sunspot
appeared on August 2nd, 1972, and for the next 10 days it erupted
again and again," recalls Hathaway. The spate of explosions
caused, "a proton storm much worse than the one we've just
experienced," adds Cucinotta. Researchers have been studying
it ever since.
of the August 1972 solar flares. Click to view a 2-MB
mpeg movie of the explosion, which solar physicists
call "the seahorse flare."
that a moonwalker caught in the August 1972 storm might have absorbed
400 rem. Deadly? "Not necessarily," he says. A quick trip
back to Earth for medical care could have saved the hypothetical
Surely, though, no
astronaut is going to walk around on the Moon when there's a giant
sunspot threatening to explode. "They're going to stay inside
their spaceship (or habitat)," says Cucinotta. An Apollo command
module with its aluminum hull would have attenuated the 1972 storm
from 400 rem to less than 35 rem at the astronaut's blood-forming
organs. That's the difference between needing a bone marrow transplant
… or just a headache pill.
Modern spaceships are
even safer. "We measure the shielding of our ships in units
of areal density - or grams per centimeter-squared," says Cucinotta.
Big numbers, which represent thick hulls, are better:
The hull of an Apollo
command module rated 7 to 8 g/cm2.
A modern space shuttle
has 10 to 11 g/cm2.
The hull of the ISS,
in its most heavily shielded areas, has 15 g/cm2.
Future moonbases will
have storm shelters made of polyethelene and aluminum possibly exceeding
A typical space suit,
meanwhile, has only 0.25 g/cm2, offering little protection.
"That's why you want to be indoors when the proton storm hits,"
But the Moon beckons
and when explorers get there they're not going to want to stay indoors.
A simple precaution: Like explorers on Earth, they can check the
weather forecast - the space weather forecast. Are there
any big 'spots on the sun? What's the chance of a proton storm?
Is a coronal mass ejection coming?
All clear? It's time
to step out to the moon.