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May
27, 2009: The Phantom Torso is back, and he has quite
a story to tell.
He's
an armless, legless, human-shaped torso, a mannequin that
looks like he's wrapped in a mummy's bandages. Scientists
at the European Space Agency call him Matroshka, and like
his NASA counterpart Fred, this mannequin is an intrepid space
traveler. Now that he's spent four months on the International
Space Station, scientists are learning about the space radiation
that Matroshka endured.
Right:
The Phantom Torso. [Larger
image]
Lessons
learned from Fred and Matroshka have major implications for
NASA's plans to set up a manned outpost on the Moon and eventually
to send people to Mars. Protecting astronauts from the harmful
effects of space radiation will be a critical challenge for
these extended missions. To design spacesuits, vehicles, and
habitats with enough shielding to keep astronauts safe, mission
scientists need to know how much radiation --and what kinds
--astronauts actually absorb.
Scientists
can estimate this radiation dose using computer models, but
a computer model and real-life can be two wildly different
things. Until now, researchers weren't sure whether their
models accurately predicted the radiation dose astronauts
experience in space.
That's
where the Phantom Torso comes in.
He
provided the real-world test needed to prove that the models
are essentially correct. By analyzing the measurements from
hundreds of radiation sensors embedded throughout Matroshka's
body, Francis Cucinotta of NASA's Johnson Space Center and
his colleagues found that the models are actually quite good:
They're accurate to within 10% of the measured dose. That
means it's "all systems go" for using these models
to plan NASA's return to the Moon or even a trip to Mars.
The
most dangerous kind of radiation astronauts experience is
galactic cosmic rays (GCR). These are bare atomic nuclei,
some as heavy as iron atoms, accelerated to nearly the speed
of light by distant supernovas. Because of their high velocity,
high mass, and positive electric charge, GCR particles can
cause tremendous damage to a person's cells. And traditional
radiation shielding can't stop them.
Understanding
the danger isn't as simple as just knowing how much radiation
is out there.
"What
matters most is how much radiation actually hits a person's
vital organs," says Cucinotta.
And
to reach those organs, particles of radiation must first pass
through the walls of the spacecraft, the person's spacesuit,
and their skin and other body tissues. It's very complex.
Sometimes these barriers will slow down or stop a particle
of radiation. But sometimes the collision between a radiation
particle and a barrier will produce a shower of new radiation
particles called "secondary" radiation. Computer
models must account for all of this.
Space
station astronauts wear sensors on their flight suits to record
total radiation exposure, but there's no practical way to
measure how much radiation actually reaches their vital organs.
Fred has sensors just about everywhere--even on the inside.
Above:
(Left) ISS Science Officer John L. Phillips poses for a picture
beside Matroshka, the ESA's Phantom Torso. (Right) Radiation
sensors are embedded in 35 different slices of the Phantom
Torso. Larger images: #1,
#2.
The
Phantom Torsos are made of a special plastic that closely
mimics the density of the human body, sliced horizontally
into 35 one inch layers. In these layers, researchers embedded
a total of 416 lithium-crystal dosimeters, each of which measures
the accumulated radiation dose at one point in the body over
the course of the experiment. Fred and Matroshka also contain
several "active" dosimeters located where vital
organs such as their brain, thyroid, heart, colon, and stomach
would be. These active sensors keep a record of how the radiation
dose changes moment by moment. Together, these various sensors
thoroughly documented how radiation propagate through their
bodies.
"The geometry and the composition of the torso mimics
the human body very well," Cucinotta says. "I think
it's a very good test."
So
now that these computer models have been verified in the real
world, what do they say about keeping astronauts safe in a
lunar outpost or on Mars?
"Short
lunar missions are fine," Cucinotta says, "but living
in a lunar habitat for 6 months starts to be problematic.
We're going to have to do a really good job with radiation
shielding and perhaps medical countermeasures to have 6-month
missions."
Mars
will be even tougher, these models suggest. Some scenarios
call for missions that would last 18 months or more. "Right
now there's no design solution to stay within safety limits
for such a Mars mission," Cucinotta says. "Putting
enough radiation shielding around a spacecraft would make
it far too heavy to launch, so we need to find better lightweight
shielding materials, and we probably need to develop medical
techniques to counteract damage to cells caused by cosmic
rays." He notes that one of the biggest obstacles to
progress in this area is "uncertainty in the types of
cell damage deep cosmic ray exposure can cause. We still have
a lot to learn."
Right:
"Distant Shores," NASA artwork by Pat Rawlings/SAIC.
[more]
Another
key question: How do solar flares affect astronauts? Fred
and Matroshka have not experienced any intense solar radiation
storms during their time onboard the ISS.
"The
energy spectrum of solar events and how the radiation dose
changes from organ to organ will be very different than what
we have seen so far from cosmic rays," says Cucinotta.
To
find the answer, scientists have recreated the intense radiation
from giant solar flares right here on Earth, and Matroshka
has been chosen as the unlucky volunteer who will experience
the blast. A fake astronaut is about to be subjected to an
artificial solar flare!
Stay
tuned to Science@NASA for the second half of this two-part
article, which will explore these pioneering new experiments
as well as a historical example of an extreme solar flare
that, in 1972, narrowly missed Apollo missions to the Moon.
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Editor: Dr.
Tony Phillips | Credit: Science@NASA
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