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The Right Stuff for Super Spaceships
for Space Science News
Huntsville - 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."
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.
Revolutions in technology--like the Industrial Revolution that replaced horses with cars--can make what seems impossible today commonplace tomorrow.
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
"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.
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.
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."
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.
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.
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.
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.
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
"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.
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
"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."
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.
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Cruising About In The 22nd Century
Huntsville - Sep 16, 2002
Every parent has heard the cry of a child from the back seat of the car "are we there yet" It usually begins about 15 minutes after the start of any family trip. Good thing we rarely travel more than a few hundred or a few thousand miles from home.
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