Super Spaceships
The Right Stuff for Super Spaceships
Tomorrow's spacecraft will be built using advanced
materials with mind-boggling properties.
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Sept. 16, 2002: "What I'm really looking for," you say to the salesman, "is a car that goes at least 10,000 miles between fill-ups, repairs itself automatically, cruises at 500 mph, and weighs only a few hundred pounds."
As he stands there wide-eyed, you add, "Oh yeah, and I can only spend about a quarter of what these other cars cost."
Above: A next-generation minivan? Advanced materials will be essential for making dramatically improved spacecraft possible. [more]
A request like this is sure to get you laughed off the new-car
lot. But in many ways, this dream car is a metaphor for the space
vehicles we'll need to expand our exploration of the solar system
in the decades to come. These new spacecraft will need to be
faster, lighter, cheaper, more reliable, more durable, and more
versatile, all at the same time.
Impossible? Before you answer, consider how a rancher from 200
years ago might have reacted if a man had asked to buy a horse
that could run 100 mph for hours on end, carry his entire family
and all their luggage, and sing his favorite songs to him all
the while! Today we call them minivans.
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Such a revolution is happening right now. Three of the fastest-growing sciences of our day--biotech, nanotech, and information technology--are converging to give scientists unprecedented control of matter on the molecular scale. Emerging from this intellectual gold-rush is a new class of materials with astounding properties that sound more at home in a science fiction novel than on the laboratory workbench.
Imagine, for example, a substance with 100 times the strength of steel, yet only 1/6 the weight; materials that instantly heal themselves when punctured; surfaces that can "feel" the forces pressing on them; wires and electronics as tiny as molecules; structural materials that also generate and store electricity; and liquids that can instantly switch to solid and back again at will. All of these materials exist today ... and more are on the way.
With such mind-boggling materials at hand, building the better spacecraft starts to look not so far fetched after all.
Weight equals
money
The challenge of the next-generation spacecraft hinges
on a few primary issues. First and foremost, of course, is cost.
"Even
if all the technical obstacles were solved today, exploring our
solar system still needs to be affordable to be practical,"
says Dr. Neville Marzwell, manager of Revolutionary Aerospace
Technology for NASA's Next Decadal Planning Team.
Lowering the cost of space flight primarily means reducing weight.
Each pound trimmed is a pound that won't need propulsion to escape
from Earth's gravity. Lighter spaceships can have smaller, more
efficient engines and less fuel. This, in turn, saves more weight,
thus creating a beneficial spiral of weight savings and cost
reduction.
Right: This fully-loaded Saturn V moon rocket weighed 6.2 million pounds. It was heavy and expensive to launch. [more]
The challenge is to trim weight while increasing safety,
reliability, and functionality. Just leaving parts out won't
do.
Scientists are exploring a range of new technologies that could
help spacecraft slim down. For example, gossamer materials--which
are ultra-thin films--might be used for antennas or photovoltaic
panels in place of the bulkier components used today, or even
for vast solar sails that provide propulsion while massing only
4 to 6 grams per square meter.
Composite materials, like those used in carbon-fiber tennis rackets
and golf clubs, have already done much to help bring weight down
in aerospace designs without compromising strength. But a new
form of carbon called a "carbon nanotube" holds the
promise of a dramatic improvement over composites: The best composites
have 3 or 4 times the strength of steel by weight--for nanotubes,
it's 600 times!
"This phenomenal strength comes from
the molecular structure of nanotubes," explains Dennis Bushnell,
a chief scientist at Langley Research Center (LaRC), NASA's Center
of Excellence for Structures and Materials. They look a bit like
chicken-wire rolled into a cylinder with carbon atoms sitting
at each of the hexagons' corners. Typically nanotubes are about
1.2 to 1.4 nanometers across (a nanometer is one-billionth of
a meter), which is only about 10 times the radius of the carbon
atoms themselves.
Left: The lattice of carbon atoms in a carbon
nanotube is like a peg-board for hanging other kinds of atoms
and molecules to give the nanotube special chemical, electrical
or thermal properties. Copyright Prof. Vincent H. Crespi Department of Physics
Pennsylvania State University. [more]
Nanotubes were only discovered in 1991, but already the intense
interest in the scientific community has advanced our ability
to create and use nanotubes tremendously. Only 2 to 3 years ago,
the longest nanotubes that had been made were about 1000 nanometers
long (1 micron). Today, scientists are able to grow tubes as
long as 200 million nanometers (20 cm). Bushnell notes that there
are at least 56 labs around the world working to mass produce
these tiny tubes.
"Great strides are being made, so making bulk materials
using nanotubes will probably happen," Bushnell says. "What
we don't know is how much of this 600 times the strength of steel
by weight will be manifest in a bulk material. Still, nanotubes
are our best bet."
Right: The tensile strength of carbon
nanotubes greatly exceeds that of other high-strength materials.
Note that each increment on the vertical axis is a power of 10.
[more]
Beyond merely being strong, nanotubes will likely be important
for another part of the spacecraft weight-loss plan: materials
that can serve more than just one function.
"We used to build structures that were just dumb, dead-weight
holders for active parts, such as sensors, processors, and instruments,"
Marzwell explains. "Now we don't need that. The holder can
be an integral, active part of the system."
Imagine that the body of a spacecraft could also store power,
removing the need for heavy batteries. Or that surfaces could
bend themselves, doing away with separate actuators. Or that
circuitry could be embedded directly into the body of the spacecraft.
When materials can be designed on the molecular scale such holistic
structures become possible.
Spacecraft skins
Humans can feel even the slightest pinprick anywhere on their bodies. It's an amazing bit of self-monitoring--possible because your skin contains millions of microscopic nerve endings as well as nerves to carry those signals to your brain.
Likewise, materials that make up critical systems in a spaceship
could be embedded with nanometer-scale sensors that constantly
monitor the materials' condition. If some part is starting to
fail--that is, it "feels bad"--these sensors could
alert the central computer before tragedy strikes.
Molecular wires could carry the signals
from all of these in-woven sensors to the central computer, avoiding
the impractical bulk of millions and millions of today's wires.
Again, nanotubes may be able to serve this role. Conveniently,
nanotubes can act as either conductors or semi-conductors, depending
on how they're made. Scientists have made molecular wires of
other elongated molecules, some of which even naturally self-assemble
into useful configurations.
Above: This piezoelectric material, developed at
NASA's Langley Research Center (LaRC), can "feel" deformations
such as bending or surface pressure, producing a small voltage
in response that can act as a signal for a central computer.
Image courtesy NASA's Morphing Project at LaRC.
Your skin is also able to heal itself. Believe it or not, some
advanced materials can do the same thing. Self-healing materials
made of long-chain molecules called ionomers react to a penetrating
object such as a bullet by closing behind it. Spaceships could
use such skins because space is full of tiny projectiles--fast-moving
bits of debris from comets and asteroids. Should one of these
sand- to pebble-sized objects puncture the ship's armor, a layer
of self-healing material would keep the cabin airtight.
Meteoroids aren't the only hazard; space is filled with radiation,
too. Spaceships in low-Earth orbit are substantially protected
by our planet's magnetic field, which forms a safe bubble about
50,000 km wide centered on Earth. Beyond that distance, however,
solar flares and cosmic rays pose a threat to space travelers.
Right:
A solar flare blasts energetic radiation into space. [more]
Scientists are still searching for a good solution. The trick is to provide adequate shielding without adding lots of extra weight to the spacecraft. Some lightweight radiation-shielding materials are currently being tested in an experiment called MISSE onboard the International Space Station. But these alone won't be enough.
The real bad guy is Galactic Cosmic Radiation (GCR) produced
in distant supernova explosions. It consists, in part, of very
heavy positive ions--such as iron nuclei--zipping along at great
speed. The combination of high mass and high speed makes these
little atomic "cannon balls" very destructive. When
they pierce through the cells in people's bodies, they can smash
apart DNA, leading to illness and even cancer.
"It turns out that the worst materials you can use for shielding
against GCR are metals," Bushnell notes. When a galactic
comic ray hits a metallic atom, it can shatter the atom's nucleus--a
process akin to the fission that occurs in nuclear power plants.
The secondary radiation produced by these collisions can be worse
than the GCR that the metal was meant to shield.
Ironically, light elements like hydrogen and helium are the best defense against these GCR brutes, because collisions with them produce little secondary radiation. Some people have suggested surrounding the living quarters of the ship with a tank of liquid hydrogen. According to Bushnell, a layer of liquid hydrogen 50 to 100 cm thick would provide adequate shielding. But the tank and the cryogenic system is likely to be heavy and awkward.
Here
again, nanotubes might be useful. A lattice of carbon nanotubes
can store hydrogen at high densities, and without the need for
extreme cold. So if our spacecraft of the future already uses
nanotubes as an ultra-lightweight structural material, could
those tubes also be loaded up with hydrogen to serve as radiation
shielding? Scientists are looking into the possibility.
Above: When high-energy cosmic rays crash into
astronauts' DNA, it can cause damage leading to cancers or other
radiation-induced illnesses. Images courtesy NASA's Office
of Biological and Physical Research.
Going one step further, layers of this structural material could
be laced with atoms of other elements that are good at filtering
out other forms of radiation: boron and lithium to handle the
neutrons, and aluminum to sop up electrons, for example.
Camping out in
the cosmos
Earth's surface is mostly safe from cosmic radiation, but other
planets are not so lucky. Mars, for example, doesn't have a strong
global magnetic field to deflect radiation particles, and its
atmospheric blanket is 140 times thinner than Earth's. These
two differences make the radiation dose on the Martian surface
about one-third as intense as in unprotected open space. Future
Mars explorers will need radiation shielding.
"We can't take most of the materials
with us for a long-term shelter because of the weight consideration.
So one thing we're working on is how to make radiation-shielding
materials from the elements that we find there," says Sheila
Thibeault, a scientist at LaRC who specializes in radiation shielding.
Right: Astronauts setting up camp on Mars will need protection from space radiation. Image credit: Frassanito and Associates, Inc.
One possible solution is "Mars bricks." Thibeault
explains: "Astronauts could produce radiation-resistant
bricks from materials available locally on Mars, and use them
to build shelters." They might, for example, combine the
sand-like "regolith" that covers the Martian surface
with a polymer made on-site from carbon dioxide and water, both
abundant on the red planet. Zapping this mixture with microwaves
creates plastic-looking bricks that double as good radiation
shielding.
"By using microwaves, we can make these bricks quickly using
very little energy or equipment," she explains. "And
the polymer we would use adds to the radiation-shielding properties
of the regolith."
Mars shelters would need the reliability of self-sensing materials,
the durability of self-healing materials, and the weight savings
of multi-functional materials. In other words, a house on Mars
and a good spacecraft need many of the same things. All of these
are being considered by researchers, Thibeault says.
The folks back
home
Mind-boggling advanced materials will come in handy
on Earth, too.
"NASA's
research is certainly focused on aerospace vehicles," notes
Anna McGowan, manager of NASA's Morphing Project (an advanced
materials research effort at the Langley Research Center). "However,
the basic science could be used in many other areas. There could
be millions of spin-offs."
Left: Crafted from smart materials, tomorrow's airplanes
could have self-bending wings that operate without flaps--thus
reducing drag and lowering fuel costs. [more]
But not yet. Most advanced materials lack the engineering refinement
needed for a polished, robust product. They're not ready for
primetime. Even so, say researchers, it's only a matter of time:
Eventually that car salesman will stop laughing ... and
start selling your space-age dream machine.
Buck Rogers, Watch Out! --Science@NASA article: NASA researchers are studying insects and birds, and using "smart" materials with uncanny properties to develop new and mindboggling aircraft designs.
Samples of the Future --Science@NASA article: The advanced space ships of tomorrow will be crafted from far-out materials with extraordinary resistance to the harsh environment of space. The Materials International Space Station Experiment (MISSE) aims to find out which materials work best.
Right:
Backdropped by the rising Sun, MISSE juts into space outside
the International Space Station. [more]
Digging in and taking cover --Science@NASA article: Lunar and Martian dirt could provide radiation shielding for crews on future missions. See also "Making Mars Bricks"
Center for Nanotechnology (CNT) -- at NASA's Ames Research Center
Needs of future missions -- listing of technologies needed for future space exploration and some possible solutions, from the CNT
Nanotube Links: Nanotubes & Buckyballs(Nanotechnology Now); Carbon nanotubes(Penn State University); Johnson Space Center Nanotube Project(NASA)
Research in molecular electronics: a nano-scale transistor from IBM; a simple logic gate made from nanowires; a customizable nanotube for wires or structures from Purdue University
Space Weather on Mars --Science@NASA article: Future human explorers of Mars can leave their umbrellas back on Earth, but perhaps they shouldn't forget their Geiger counters! A NASA experiment en route to the Red Planet aims to find out
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