A "designer material" derived from plastic could
help protect astronauts on their way to Mars
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August 25, 2005: After reading this article, you
might never look at trash bags the same way again.
We
all use plastic trash bags; they're so common that we hardly
give them a second thought. So who would have guessed that
a lowly trash bag might hold the key to sending humans to
Mars?
Most
household trash bags are made of a polymer called polyethylene.
Variants of that molecule turn out to be excellent at shielding
the most dangerous forms of space radiation. Scientists have
long known this. The trouble has been trying to build a spaceship
out of the flimsy stuff.
Right:
Humans set off on a journey to Mars, an artist's concept.
[More]
But
now NASA scientists have invented a groundbreaking, polyethylene-based
material called RXF1 that's even stronger and lighter than
aluminum. "This new material is a first in the sense
that it combines superior structural properties with superior
shielding properties," says Nasser Barghouty, Project
Scientist for NASA's Space Radiation Shielding Project at
the Marshall Space Flight Center.
To
Mars in a plastic spaceship? As daft as it may sound, it could
be the safest way to go.
Less
is more
Protecting
astronauts from deep-space radiation is a major unsolved problem.
Consider a manned mission to Mars: The round-trip could last
as long as 30 months, and would require leaving the protective
bubble of Earth's magnetic field. 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 the trip to Mars.
Barghouty
is one of the skeptics: "Going to Mars now with an aluminum
spaceship is undoable," he believes.
Plastic
is an appealing alternative: Compared to aluminum, polyethylene
is 50% better at shielding solar flares and 15% better for
cosmic rays.
Left:
Cosmic rays crash into matter, producing secondary particles.
[More]
The
advantage of plastic-like materials is that they produce far
less "secondary radiation" than heavier materials
like aluminum or lead. Secondary radiation comes from the
shielding material itself. When particles of space radiation
smash into atoms within the shield, they trigger tiny nuclear
reactions. Those reactions produce a shower of nuclear byproducts
-- neutrons and other particles -- that enter the spacecraft.
It's a bit like trying to protect yourself from a flying bowling
ball by erecting a wall of pins. You avoid the ball but get
pelted by pins. "Secondaries" can be worse for astronauts'
health than the original space radiation!
Ironically,
heavier elements like lead, which people often assume to be
the best radiation shielding, produce much more secondary
radiation than lighter elements like carbon and hydrogen.
That's why polyethylene makes good shielding: it is composed
entirely of lightweight carbon and hydrogen atoms, which minimizes
secondaries.
Right:
Ethylene, the building block of polyethylene, is rich in hydrogen
and carbon. [More]
These
lighter elements can't completely stop space radiation. But
they can fragment the incoming radiation particles, greatly
reducing the harmful effects. Imagine hiding behind a chain-link
fence to protect yourself in a snowball fight: You'll still
get some snow on you as tiny bits of snowball burst through
the fence, but you won't feel the sting of a direct hit from
a hard-packed whopper. Polyethylene is like that chain link
fence.
"That's
what we can do. Fragmenting -- without producing a lot of
secondary radiation -- is actually where the battle is won
or lost," Barghouty says.
Made
to order
Despite
their shielding power, ordinary trash bags obviously won't
do for building a spaceship. So Barghouty and his colleagues
have been trying to beef-up polyethylene for aerospace work.
That's
how Shielding Project researcher Raj Kaul, working together
with Barghouty, came to invent RXF1. RXF1 is remarkably strong
and light: it has 3 times the tensile strength of aluminum,
yet is 2.6 times lighter -- impressive even by aerospace standards.
"Since
it is a ballistic shield, it also deflects micrometeorites,"
says Kaul, who had previously worked with similar materials
in developing helicopter armor. "Since it's a fabric,
it can be draped around molds and shaped into specific spacecraft
components." And because it's derived from polyethylene,
it's an excellent radiation shield as well.
Left:
Raj Kaul, co-inventor of RXF1, holding a brick of the material.
[More]
The
specifics of how RXF1 is made are secret because a patent
on the material is pending.
Strength
is only one of the traits that the walls of a spaceship must
have, Barghouty notes. Flammability and temperature tolerance
are also important: It doesn't matter how strong a spaceship's
walls are if they melt in direct sunlight or catch fire easily.
Pure polyethylene is very flammable. More work is needed to
customize RXF1 even further to make it flame and temperature
resistant as well, Barghouty says.
The
Bottom Line
The
big question, of course, is the bottom line: Can RXF1 carry
humans safely to Mars? At this point, no one knows for sure.
Some
"galactic cosmic rays are so energetic that no reasonable
amount of shielding can stop them," cautions Frank Cucinotta,
NASA's Chief Radiation Health Officer. "All materials
have this problem, including polyethylene."
Cucinotta
and colleagues have done computer simulations to compare the
cancer risk of going to Mars in an aluminum ship vs. a polyethylene
ship. Surprisingly, "there was no significant difference,"
he says. This conclusion depends on a biological model which
estimates how human tissue is affected by space radiation--and
therein lies the rub. After decades of spaceflight, scientists
still don't fully understand how the human body reacts to
cosmic rays. If their model is correct, however, there could
be little practical benefit to the extra shielding polyethylene
provides. This is a matter of ongoing research.
Because
of the many uncertainties, dose limits for astronauts on a
Mars mission have not been set, notes Barghouty. But assuming
that those dose limits are similar to limits set for Shuttle
and Space Station flights, he believes RXF1 could hypothetically
provide adequate shielding for a 30 month mission to Mars.
Today,
to the dump. Tomorrow, to the stars? Polyethylene might take
you farther than you ever imagined.
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Author: Patrick
L. Barry | Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
|