by Michio Kaku
Although Johnson has stellar dreams, he realizes that the reality is much more modest. In 1993, the Russians deployed a sixty-foot Mylar reflector in space from the Mir space station, but it was only to demonstrate deployment. A second attempt failed. In 2004, the Japanese successfully launched two solar sail prototypes, but again it was to test deployment, not propulsion. In 2005, there was an ambitious attempt by the Planetary Society, Cosmos Studios, and the Russian Academy of Sciences to deploy a genuine solar sail called Cosmos 1. It was launched from a Russian submarine. However, the Volna rocket misfired and failed to reach orbit. And in 2008, a team from NASA tried to launch a solar sail called NanoSail-D, but it was lost when the Falcon 1 rocket failed.
But finally, in May 2010, the Japan Aerospace Exploration Agency successfully launched the IKAROS, the first spacecraft to use solar-sail technology in interplanetary space. It has a square-shaped sail, 20 meters (60 feet) on the diagonal, and uses solar-sail propulsion to travel on its way to Venus. The Japanese eventually hope to send another ship to Jupiter using solar-sail propulsion.
NUCLEAR ROCKET
Scientists have also considered using nuclear energy to drive a starship. Beginning in 1953, the Atomic Energy Commission began to look seriously at rockets carrying atomic reactors, beginning with Project Rover. In the 1950s and 1960s, experiments with nuclear rockets ended mainly in failure. They tended to be unstable and too complex to handle properly. Also, an ordinary fission reactor, one can easily show, simply does not produce enough energy to drive a starship. A typical nuclear power plant produces about a billion watts of power, not enough to reach the stars.
But in the 1950s, scientists proposed using atomic and hydrogen bombs, not reactors, to drive a starship. The Orion Project, for example, proposed a rocket propelled by a succession of nuclear blast waves from a stream of atomic bombs. A starship would drop a series of atomic bombs out its back, creating a series of powerful blasts of X-rays. This shock wave would then push the starship forward.
In 1959, physicists at General Atomics estimated that an advanced version of Orion would weigh 8 million tons, with a diameter of 400 meters, and be powered by 1,000 hydrogen bombs.
One enthusiastic proponent of the Orion project was physicist Freeman Dyson. “For me, Orion meant opening up the whole solar system to life. It could have changed history,” he says. It would also have been a convenient way to get rid of atomic bombs. “With one trip, we’d have got rid of 2,000 bombs,” he says.
What killed Project Orion, however, was the Nuclear Test Ban Treaty of 1963, which prohibited aboveground testing of nuclear weapons. Without tests, physicists could not refine the design of the Orion, and the idea died.
RAMJET FUSION
Yet another proposal for a nuclear rocket was made by Robert W. Bussard in 1960; he envisioned a fusion engine similar to an ordinary jet engine. A ramjet engine scoops air in the front and then mixes it with fuel internally. By igniting the mixture of air and fuel, a chemical explosion occurs that creates thrust. He envisioned applying the same basic principle to a fusion engine. Instead of scooping air, the ramjet fusion engine would scoop hydrogen gas, which is found everywhere in interstellar space. The hydrogen gas would be squeezed and heated by electric and magnetic fields until the hydrogen fused into helium, releasing enormous amounts of energy in the process. This would create an explosion, which then creates thrust. Since there is an inexhaustible supply of hydrogen in deep space, the ramjet fusion engine can conceivably run forever.
Designs for the ramjet fusion rocket look like an ice cream cone. The scoop traps hydrogen gas, which is sent into the engine, where it is heated and fused with other hydrogen atoms. Bussard calculated that if a 1,000-ton ramjet engine can maintain the acceleration of 32 feet per second squared (or the gravity felt on the earth), then it will approach 77 percent of the speed of light in just one year. Since the ramjet engine can run forever, it could theoretically leave our galaxy and reach the Andromeda galaxy, 2,000,000 light-years from earth, in just 23 years as measured by the astronauts in the rocket ship. (As stated by Einstein’s theory of relativity, time slows down in a speeding rocket, so millions of years may have passed on earth, but the astronauts will have aged only 23 years.)
There are several problems facing the ramjet engine. First, since mainly protons exist in interstellar space, the fusion engine must burn pure hydrogen fuel, which does not produce that much energy. (There are many ways in which to fuse hydrogen. The method preferred on earth is to fuse deuterium and tritium, which has a large yield of energy. But in outer space, hydrogen is found as a single proton, and hence ramjet engines can only fuse protons with protons, which does not yield as much energy as deuterium-tritium fusion.) However, Bussard showed that if one modifies the fuel mixture by adding some carbon, the carbon acts as a catalyst to create enormous amounts of power, sufficient to drive a starship.
Second, the scoop would have to be huge—on the order of 160 kilometers—in order to collect enough hydrogen, so it would have to be assembled in space.
A ramjet fusion engine, because it scoops hydrogen from interstellar space, can theoretically run forever. (photo credit 6.2)
There is another problem that is still unresolved. In 1985, engineers Robert Zubrin and Dana Andrews showed that the drag felt by the ramjet engine would be large enough to prevent it from accelerating to near light speed. The drag is caused by the resistance that the starship encounters when it moves in a field of hydrogen atoms. However, their calculation rests heavily on certain assumptions that may not apply to ramjet designs of the future.
At present, until we have a better grasp of the fusion process (and also drag effects from ions in space), the jury is still out on ramjet fusion engines. But if these engineering problems can be solved, then the ramjet fusion rocket will definitely be on the short list.
ANTIMATTER ROCKETS
Another distinct possibility is to use the greatest energy source in the universe, antimatter, to power your spaceship. Antimatter is the opposite of matter, with the opposite charge; for example, an electron has negative charge, but an antimatter electron (the positron) has positive charge. It will also annihilate upon contact with ordinary matter. In fact, a teaspoon of antimatter has enough energy to destroy the entire New York metropolitan area.
Antimatter is so powerful that Dan Brown had the villains in his novel Angels and Demons build a bomb to blow up the Vatican using antimatter stolen from CERN, outside Geneva, Switzerland. Unlike a hydrogen bomb, which is only 1 percent efficient, an antimatter bomb would be 100 percent efficient, converting matter into energy via Einstein’s equation E = mc2.
In principle, antimatter makes the ideal rocket fuel for a starship. Gerald Smith of Pennsylvania State University estimates that 4 milligrams of antimatter will take us to Mars, and perhaps a hundred grams will take us to the nearby stars. Pound for pound, it releases a billion times more energy than rocket fuel. An antimatter engine would look rather simple. You just drop a steady stream of antimatter particles down the rocket chamber, where it combines with ordinary matter and causes a titanic explosion. The explosive gas is then shot out one end of the chamber, creating thrust.
We are still far from that dream. So far, physicists have been able to create antielectrons and antiprotons, as well as antihydrogen atoms, with antielectrons circulating around antiprotons. This was done at CERN and also at the Fermi National Accelerator Laboratory (Fermilab), outside Chicago, in its Tevatron, the second-largest atom smasher, or particle accelerator, in the world (second only to the Large Hadron Collider at CERN). Physicists at both labs slammed a beam of high-energy particles at a target, creating a shower of debris that contained antiprotons. Powerful magnets were used to separate the antimatter from ordinary matter. These antiprotons were then slowed down and antielectrons were allowed to mix with them, creating antihydrogen atoms.
One man who has thought long and hard about the practicalities of antimatter is Dave McGinnis, a physicist at Fermilab. Whi
le standing next to the Tevatron, he explained to me the daunting economics of antimatter. The only known way to produce steady quantities of antimatter, he emphasized to me, is to use an atom smasher like the Tevatron; these machines are extremely expensive and produce only minuscule amounts of antimatter. For example, in 2004, the atom smasher at CERN produced several trillionths of a gram of antimatter at a cost of $20 million. At that rate, it would bankrupt the entire economy of earth to produce enough antimatter to power a starship. Antimatter engines, he stressed to me, are not a far-fetched concept. They are certainly within the laws of physics. But the cost of building one would be prohibitive for the near future.
One reason antimatter is so prohibitively expensive is because the atom smashers necessary to produce it are notoriously expensive. However, these atom smashers are all-purpose machines, designed mainly to produce exotic subatomic particles, not the more common antimatter particles. They are research tools, not commercial machines. It is conceivable that costs could be brought down considerably if one designs a new type of atom smasher specifically to produce copious amounts of antimatter. Then, by mass-producing these machines, it might be possible to create sizable quantities of antimatter. Harold Gerrish of NASA believes that the cost of antimatter might eventually go down to $5,000 per microgram.
Another possibility lies in finding an antimatter meteorite in outer space. If such an object were found, it could supply enough energy to power a starship. In fact, the European satellite PAMELA (Payload for Antimatter Matter Exploration and Light-Nuclei Astrophysics) was launched in 2006 specifically to look for naturally occurring antimatter in outer space.
If large quantities of antimatter are found in space, one can envision using large electromagnetic nets to collect it.
So although antimatter interstellar rockets are certainly within the laws of physics, it may take until the end of the century to drive down the cost. But if this can be done, then antimatter rockets would be on everyone’s short list of starships.
NANOSHIPS
When we are dazzled by the special effects in Star Wars or Star Trek, we immediately envision a huge, futuristic starship bristling with all the latest high-tech gadgets. Yet another possibility lies in using nanotechnology to create tiny starships, perhaps no larger than a thimble, a needle, or even smaller. We have this prejudice that a starship must be huge, like the Enterprise, and capable of supporting a crew of astronauts. But the essential functions of a starship may be miniaturized by nanotechnology so that perhaps millions of tiny nanoships might be launched to the nearby stars, only a fraction of which actually make it. Once they arrive on a nearby moon, they might create a factory to make unlimited copies of themselves.
Vint Cerf, one of the original creators of the Internet, envisions tiny nanoships that can explore not just the solar system but eventually the stars themselves. He says, “The exploration of the solar system will be made more effective through the construction of small but powerful nano-scale devices that will be easy to transport and deliver to the surface, below the surface, and into the atmospheres of our neighboring planets and satellites …. One can even extrapolate these possibilities to interstellar exploration.”
In nature, mammals produce just a few offspring and make sure that all survive. Insects produce large quantities of offspring, only a tiny fraction of which survive. Both strategies can keep the species alive for millions of years. Likewise, instead of sending a single, expensive starship to the stars, one can send millions of tiny starships, each costing a penny and requiring very little rocket fuel.
This concept is patterned after a very successful strategy found in nature: swarming. Birds, bees, and other flying animals fly in flocks or swarms. Not only is there safety in numbers but the swarm also acts as an early warning system. If a dangerous disturbance happens in one part of the swarm, such as an attack by a predator, the message is quickly relayed to the rest of the swarm. They are also quite efficient in energy. When birds fly in a characteristic V pattern, the wake and turbulence created by this formation reduce the energy necessary for each bird to fly.
Scientists characterize a swarm as a “superorganism,” one that appears to have an intelligence of its own, independent of the abilities of any single individual. Ants, for example, have a very simple nervous system and a tiny brain, but together they can create complex anthills. Scientists hope to incorporate some of these lessons from nature by designing swarm-bots that might one day journey to other planets and stars.
This is similar to the hypothetical concept of smart dust being pursued by the Pentagon: billions of particles sent into the air, each one with tiny sensors to do reconnaissance. Each sensor is not very intelligent, but collectively they can relay back mountains of information. The Pentagon’s DARPA has funded this research for possible military applications, such as monitoring enemy positions on the battlefield. In 2007 and 2009, the Air Force released position papers detailing plans for the coming decades, outlining everything from advanced versions of the Predator (which today cost $4.5 million apiece) to swarms of tiny sensors smaller than a moth costing pennies.
Scientists are also interested in this concept. They might want to spray smart dust to instantly monitor thousands of locations during hurricanes, thunderstorms, volcanic eruptions, earthquakes, floods, forest fires, and other natural phenomena. In the movie Twister, for example, we see a band of hardy storm chasers risking life and limb to place sensors around a tornado. This is not very efficient. Instead of having a handful of scientists placing a few sensors during a volcanic eruption or tornado to measure temperature, humidity, and wind velocity, smart dust can give you data from thousands of different positions at once over hundreds of miles. When fed into a computer, this data can instantly give you real-time information about the evolution of a hurricane or volcano in three dimensions. Commercial ventures have already been set up to market these tiny sensors, some no larger than the head of a pin.
Another advantage of nanoships is that they require very little fuel to send them into space. Instead of using huge booster rockets that can reach only 25,000 miles per hour, it is relatively easy to send tiny objects into space at incredible velocities. In fact, it is easy to send subatomic particles near the speed of light using ordinary electric fields. These nanoparticles carry a small electric charge and can be easily accelerated by electric fields.
Instead of using enormous resources to send a probe to another moon or planet, a single probe might have the ability to self-replicate, and thus create an entire factory or even moon base. These self-replicating probes can then blast off to explore other worlds. (The problem is to create the first self-replicating nanoprobe, which is still in the distant future.)
In 1980, NASA took the idea of self-replicating robot probes seriously enough to convene a special study, called Advanced Automation for Space Missions, which was conducted at the University of Santa Clara and looked into several possibilities. One explored by NASA scientists was to send small, self-replicating robots to the moon. There, the robot would use the soil and create unlimited copies of itself.
Most of the report was devoted to the details of constructing a chemical factory to process moon rocks (called regoliths). The robot, for example, might land on the moon, disassemble itself, then rearrange its parts to create a new factory, much like a toy transformer robot. For example, the robot might create large parabolic mirrors to focus sunlight and begin melting the regoliths. It would then use hydrofluoric acid leeching to begin processing the regoliths to extract usable minerals and metals. The metals could then be fabricated into the moon base. Eventually, the robot would construct a small moon factory to reproduce itself.
Building on this report, in 2002, NASA’s Institute for Advanced Concepts began funding a series of projects based on these self-replicating robots. One scientist who has taken seriously the proposal of a starship on a chip is Mason Peck of Cornell University.
I had a chance to visit Peck in his laboratory, where you cou
ld see his workbench filled with components that may eventually be sent into orbit. Next to his workbench was a small, clean room, with walls draped in plastic, where delicate satellite components are assembled.
His vision of space exploration is quite different from the one given to us by Hollywood movies. He envisions a microchip, one centimeter in size and weighing one gram, that could be accelerated to 1 percent to 10 percent of the speed of light. He takes advantage of the slingshot effect that NASA uses to hurl spacecraft to enormous velocities. This gravity-assist maneuver involves sending a spacecraft around a planet, like a rock from a slingshot, thereby using the planet’s gravity to increase the spacecraft’s speed.
But instead of gravity, Peck wants to use magnetic forces. His idea is to send a microchip spaceship whipping around Jupiter’s magnetic field, which is 20,000 times greater than the earth’s field. He plans to accelerate his nanostarship with the magnetic force that is used to hurl subatomic particles to trillions of electron volts in our atom smashers.
He showed me a sample chip that he thought one day might be hurled around Jupiter. It was a tiny square, smaller than your fingertip, crammed with scientific circuitry. His starship would be simple. On one side of the chip, there is a solar cell to provide energy for communication. On the other side, there is a radio transmitter, camera, and other sensors. The device has no engine, since it is propelled using only Jupiter’s magnetic field. (NASA’s Institute for Advanced Concepts, which funded this and other innovative proposals for the space program since 1998, was unfortunately closed in 2007 due to budget cuts.)