An unusual space traveller named Fred is orbiting
Earth aboard the International Space Station. His job? To keep astronauts
safe from space radiation.
by Karen Miller
Fred has no arms. He has no legs. His
job is keeping astronauts safe.
Fred is the Phantom Torso, an approximately
95-pound, 3 foot high mock-up of a human upper body. Beneath Fred's
artificial skin are real bones. Fred's organs -- the heart, brain,
thyroid, colon and so on -- are made of a special plastic that matches
as closely as possible the density of human tissue.
Fred, who's spending four months on
board the International Space Station (ISS), will measure the amount
of radiation to which astronauts are exposed. High-energy particles
that pass through the human body can disrupt the way cells function.
Although no astronaut has ever been diagnosed with space radiation
sickness, excessive exposure could lead to health problems.
"We believe the current dose [of radiation
to the crew of the ISS] is too small to be of concern," says Dr.
Gautam Badhwar, the study's principal investigator at the Johnson
Space Centre. "The one possibility for radiation sickness might
be an EVA situation during a solar event, if perhaps a crew member
couldn't be brought back inside safely." But there's still lots
to learn, he added, and that's where Fred can help.
The Phantom Torso is designed to do
three things, explains Badhwar. Firstly, it will determine the distribution
of radiation doses inside the human body at various tissues and
organs. Secondly, it will provide a way to correlate these doses
to measurements made on the skin. "In the past we've typically recorded
doses only on the skin," explains Badhwar, "whereas the risk
to crew members is established by exposure to internal organs. Finally,
the Phantom will help check the accuracy of models that predict
how radiation moves through the body.
Torso, also known as "Fred," is an anatomical model of a human
torso and head. Fred contains hundreds of radiation monitoring
Three types of radiation can endanger
astronauts in space.
The most energetic are Galactic Cosmic
Rays (GCRs) - the nuclei of atoms accelerated by supernova explosions
outside our solar system. Cosmic ray nuclei can be as light as hydrogen,
as heavy as iron, or almost anything in between. Because they lack
their surrounding coat of negatively-charged electrons, GCRs are
positively charged. The heavier nuclei carry the greatest charge,
explains Badhwar. "As the charge increases, the amount of energy
that the particle can deposit in tissue increases as well."
The other forms of particulate radiation
consist mostly of protons. Most high-energy protons in the solar
system come from the Sun. Although their charge is not great and
they are less energetic than GCRs, solar protons can still be dangerous
when they come in intense bursts known as solar flares.
The third kind of radiation, which
surrounds Earth in areas known as Van Allen belts, consist mostly
of decayed products from galactic cosmic ray interactions that have
been trapped by Earth's magnetic field.
Some of this trapped radiation is confined
to a region above the coast of Brazil, known as the South Atlantic
Anomaly. "The Space Station goes through that Anomaly roughly five
times a day," says Badhwar. The passage takes, at most, 22 or 23
minutes. That's good, he says.
"If you go through the trapped radiation
belt in less than twenty minutes or so, then for the next seventy
minutes the body has time to do some repair to the damage done by
the radiation. The radiation from solar flares can actually do more
harm, he says, simply because it comes at a rate that doesn't give
the body time to recover.
n order to measure space radiation
as it propagates through Fred's body, Badhwar and his team have
sliced Fred horizontally into 35 one-inch layers. In each section
they've made holes for radiation detectors called dosimeters. The
torso carries 416 lithium-crystal based passive dosimeters, which
simply record the total radiation dose received throughout the mission.
Fred is also equipped with 5 active detectors. These, placed at
the Phantom's brain, thyroid, heart, colon, and stomach, can track
the times that the radiation exposures took place.
"With the active detectors, we can
correlate the time the radiation was received with the position
of the spacecraft," explains Badhwar. "We can separate out quite
reliably when we were in the Anomaly and when we were in the Galactic
Cosmic Ray region.” This kind of split makes radiation models
derived from such data applicable to interplanetary missions, too.
To assess astronaut exposure on a trip to Mars, for example, "we'll
just switch off the Van Allen Belt particles,” says Badhwar.
like this one accelerate
atomic nuclei to nearly light speed. The resulting "cosmic
rays" pose a potential hazard to astronauts. [More information]
Radiation models devised by Badhwar
and colleagues will be able to estimate how much radiation reaches
an astronaut's internal organs simply by looking at the dose on
his or her skin. That's important, because while the permissible
radiation limits are based on internal exposures, practically speaking,
all that can be measured is what occurs on the skin.
Such models are also scalable. Rather
than giving a blanket risk assessment for all crew members, they
can be customised to each individual in terms of height, weight,
and even personal histories: how the astronaut flies an aircraft,
or what medical tests he or she might have taken. All this contributes,
says Badhwar, to total radiation exposure.
Even our internal bacteria rate a careful
look: If a crew member gets too much radiation, it could kill the
digestive bacteria essential for breaking down food.
Space station crew members will be
sending data from the Phantom's five active dosimeters back to Earth
about every ten days. When the device returns to Earth next fall,
Badhwar and his team will be able to examine results from Fred's
passive detectors as well.
The "South Atlantic Anomaly" is an area of trapped radiation
located over the east coast of Brazil. [More information]
"The thing that we're really going
after is to get as good a handle as we can on what the organ exposures
really are.” he says. The goal is to make sure that the crew
is exposed to the least amount of radiation possible.