Project Scientist, Space Radiation Shielding
Project, NASA’s Marshall Space Flight Center,
Some scientists believe that materials such as aluminum, which
provide adequate shielding in Earth orbit or for short trips
to the Moon, would be inadequate for longer-duration space missions,
such as a manned trip to Mars. NASA scientists recently have
invented a polyethylene-based material called RXF1 that's even
stronger and lighter than aluminum -- compared to aluminum,
polyethylene is 50% better at shielding solar flares and 15%
better for cosmic rays. Dr. Barghouty is a co-inventor of RXF1.
He has been serving as the project scientist for the Space Radiation
Shielding Project (SRSP) at Marshall since May of 2002.
NASA Tech Briefs: What is the mission of NASA’s
Space Radiation Shielding Project?
Dr. Nasser Barghouty: NASA needs the capability
to gauge how effective a certain material is for shielding astronauts
and systems in general against space radiation. To be able to
do that, there are some basic developments of transport codes
that are needed. As inputs to these transport codes, you need
some basic physics information – essentially nuclear physics
information because the radiation issue is related to the nuclear
issue (basic nuclear interactions of radiation and matter).
Matter in this case could be the skin of the spacecraft or tissue
(i.e., liver) of an astronaut.
We help support measurements of these nuclear physics cross-sections.
These are probabilities of these nuclear processes to take place.
We also fund efforts to develop these radiation transport codes.
These are the two important things we do in addition to funding
work to develop materials that are lighter and stronger than
aluminum that can meet the shielding requirements. So there
is the analysis part and the development part of novel materials
for radiation shielding.
NTB: What is deep-space radiation?
Dr. Barghouty: On Earth, the atmosphere protects
us, for the most part. We also are protected to a lesser degree
by the Earth’s magnetic field. In our area of work, we
are dealing with very high-energy and highly charged particulate
radiation, which means that they [the particulates] can actually
zip through a path unaltered; you can barely do anything to
them as they travel through matter. So these are deeply penetrating,
high-ionizing types of radiation – the worst kind.
NTB: What is it about polyethylene that makes
it an appealing alternative to shield against space radiation?
Dr. Barghouty: First, there is very little
that can be done about deep-space radiation, unless you use
1-meter-thick steel, which in reality makes things worse. What
we can do is alter the particulate radiation ionization damage
by making them fragment as they go through a shielding material,
and, as we do this, not produce secondary products as a result
of this interaction. In other words, minimize the waste. Aluminum
does not do this. Aluminum, to a certain degree, can actually
make things worse because while it fragments the particulate
radiation, it produces a lot of secondary products. These are
bad and essentially add to the dose of radiation.
What you want is something that is effective at fragmenting
but does not produce a lot of secondary products. It turns out
materials that are rich in hydrogen and carbon – like
polymers – can accomplish this. Polymer-based composites
have theoretically been thought of as good candidates because
they not only meet the requirements for shielding, but also
because they are composites and therefore can be designed as
strong or as flexible as desired (designer materials). RXF1
typifies this polymer-based composite; it is certainly not the
only one, but so far it is the only one that has been tested
NTB: What is RXF1? What are its benefits?
Dr. Barghouty: Theoretically, these polymer
composites were already known to be good shielding materials.
RXF1 is different in the sense that it actually was the first
to prove this as a real material. We lab tested RXF1 at three
different accelerators and exposed the material. As we predicted,
the results of those test were very promising. The mechanical
tests that the materials have passed with flying colors. We
are currently conducting environmental tests and are addressing
issues of flammability, toxicity, and other structural matters
that are required by structural engineers and we are near to
closing these gaps. This is where we believe that some work
A patent on the material is pending, so the specifics of how
RXF1 is made are secret. However, generally speaking, RXF1 begins
with a polyethylene matrix, the backbone of which is graphite
fiber giving it strength. These are the types of materials that
are good for radiation shielding and are the basic building
blocks of the material.
What’s novel is that the fibers could be designed any
way you want. You also could design the shape as well as how
to put the fiber into that shape. There is a beautiful synergy
between what you want and what you can do, a synergy between
function and property. This is what makes composites attractive.
What we have added is the idea of radiation, which is new to
the composites industry. It turned out to be easy because polymers
are naturally rich in hydrogen and that’s all we needed.
Additionally,for the nuclear industry, it turns out that materials
with a high concentration of hydrogen also address their issues,
which are neutrons. Nuclear reactors produces a lot of neutrons
and materials rich in hydrogen tend to slow down or “thermalize”
NTB: How will NASA utilize RXF1?
Dr. Barghouty: NASA has plans, for example,
to go to the Moon first before we go to Mars. These missions
[to the Moon] typically do not last more than a few days to
two to three weeks. For these types of missions, the issue of
deep-space radiation is not that critical. The issue of radiation
shielding becomes critical the longer you stay, because the
risk of exposure is too high. Keep in mind that NASA does not
yet have limits on what is considered dangerous and what is
not. We have a lot of limits and information from the International
Space Station and the shuttle, but nothing from staying on the
Moon for a month. For anyone to actually put the limits down
on paper you need some real data on Earth, which is also very
limited – how many people have actually gone out into
space and come back with radiation sickness? We really do not
know what is safe and what isn’t, so these limits are
going to have to be put forward first. We do, however, have
a rough idea of what the limits should be.
For these long-duration missions, on the Moon for example, one
of the most immediate dangers is that while there a solar flair
erupts. For some of these solar particle events, the doses received
on the surface of the Moon or in transit to the Moon can be
quite high. Protection from solar activities is an immediate
need for mission designers. With a material like RXF1, a storm
shelter can be built either on the surface of the Moon or in
the vehicle that can serve as a good shield against solar particle
events. Some of these events can actually be quite nasty, they
are very unpredictable, and from our early warning system we
have only about two hours to do something about them.
NTB: Do commercial applications exist for RXF1?
Dr. Barghouty: A good portion (60-70%) of
the newest Airbus A380 is made of composites – not necessarily
polymer composites, but they are carbon composites. Wherever
carbon composites are applicable, polymer composites can actually
perform similar functions. If carbon composites are being used
in a specific application, one may want to look at polymer composites
once the issues of flammability and toxicity are resolved. Europeans
are making a lot of progress on this front. In terms of the
Airbus, there was a need for them to make it lighter, and aluminum
obviously would not work. So they utilized carbon, and I believe
some polymer composites.
We know it can be strong enough for structural applications
and the chemical tests are very promising. When it comes to
properties needed by structural engineers and designers, if
the gaps are closed then you have a material that can be designed
according to your specific application. It is light and it is
strong, so it could replace almost anything. Carbon composites
have been made really strong, but they are still much to heavy
for what we need at NASA.
They also could be used in the radiation shielding business
for terrestrial applications, such as the nuclear industry,
nuclear medicine, and nuclear power generation. The issues of
radiation are somewhat different in these industries, but the
superior properties of these composite materials for your own
radiation issues can be custom tailored.
These are just two general examples of structural and radiation
uses of the material; between the two, I believe the commercialization
opportunities are very promising.
For more information, contact Dr. Nasser Barghouty at Abdulnasser.F.Barghouty@nasa.gov.