Physics of the Future_ How Science Will Shape Human Destiny... Part 17

But colonies on the moon or Mars are quite different. There is no air, liquid water, or fertile soil, so everything would have to be brought by rocket ship, which is prohibitively expensive.

Furthermore, there is little military value to colonizing the moon, at least for the near term. This is because it takes three days on average to reach the moon from the earth or vice versa, but a nuclear war can be fought in just ninety minutes by intercontinental ballistic missiles. A s.p.a.ce cavalry on the moon would not reach the battle on earth in time to make a difference. Hence, the Pentagon has not funded any crash program to weaponize the moon.

This means that if we do initiate large-scale mining operations on other worlds, it will be for the benefit of s.p.a.ce colonies, not for the earth. Colonists will extract the metals and minerals for their own use, since it would cost too much to transport them to earth. Mining operations in the asteroid belt would become economic only when we have self-sustaining colonies that can use these raw materials themselves, which won't happen until late in this century or, more likely, beyond.

s.p.a.cE TOURISM.

But when might the average civilian go into s.p.a.ce? Some visionaries, like the late Gerard O'Neill of Princeton University, dreamed of a s.p.a.ce colony as a gigantic wheel, including living units, water-purification plants, air-recycling units, etc., established to solve overpopulation on earth. But in the twenty-first century, the idea that s.p.a.ce colonies would relieve the population problem is fanciful at best. For the majority of the human race, earth will be our only home for at least a century or more.

However, there is one way in which the average person may realistically go into s.p.a.ce: as a tourist. Some entrepreneurs, who criticize the enormous waste and bureaucracy of NASA, think they can drive down the cost of s.p.a.ce travel using market forces. Already, Burt Rutan and his investors won the $10 million Ansari X Prize on October 4, 2004, by having launched s.p.a.ceShipOne twice within two weeks to just over 62 miles above the earth. s.p.a.ceShipOne is the first rocket-powered s.p.a.cecraft to have successfully completed a privately funded venture into s.p.a.ce. Development costs were about $25 million. Microsoft billionaire Paul Allen helped to underwrite the project.

Now, with s.p.a.ceShipTwo, Rutan expects to begin tests to make commercial s.p.a.ceflight a reality. Billionaire Richard Branson of Virgin Atlantic has created Virgin Galactic, with a s.p.a.ceport in New Mexico and a long list of people who will spend $200,000 to realize their dream of flying into s.p.a.ce. Virgin Galactic, which will be the first major company to offer commercial flights into s.p.a.ce, has already ordered five s.p.a.ceShipTwo rockets. If successful, this might drive down the cost of s.p.a.ce travel by a factor of ten.

s.p.a.ceShipTwo uses several methods to cut costs. Instead of huge booster rockets to carry the payload into s.p.a.ce, Rutan places his s.p.a.ceship atop an airplane, so that it can piggyback on a standard air-breathing plane. This way, you simply consume the oxygen in the atmosphere to reach high alt.i.tudes. Then, at about 10 miles above the earth, the s.p.a.ceship separates from the airplane and turns on its rocket engines. Although the s.p.a.ceship cannot orbit the earth, it has enough fuel to reach almost 70 miles above the earth, above most of the atmosphere, so pa.s.sengers can see the sky turn purple and then black. Its engines are powerful enough to hit Mach 3, or three times the speed of sound (roughly 2,200 miles per hour). This is certainly not fast enough to put a rocket into orbit (you need to hit 18,000 miles per hour for that), but it is enough to take you to the edge of the atmosphere and the threshold of outer s.p.a.ce. In the near future, perhaps a trip to s.p.a.ce may cost no more than a safari in Africa.

(However, to go completely around the earth, you would need to pay considerably more to take a trip aboard the s.p.a.ce station. I once asked Microsoft billionaire Charles Simonyi how much it cost him to get a ticket to the s.p.a.ce Station. Media reports estimated that it cost $20 million. He said he was reluctant to give the precise cost, but he told me that the media reports were not far off. He had such a good time that he actually went into s.p.a.ce twice. So s.p.a.ce travel, even into the near future, will still be the province of the well-off.) s.p.a.ce tourism, however, got a shot in the arm in September 2010, when the Boeing Corporation announced that it, too, was entering the business, with commercial flights for tourists planned as early as 2015. This would bolster President Obama's decision to turn over the manned s.p.a.ceflight program to private industry. Boeing's plan calls for launches to the International s.p.a.ce Station from Cape Canaveral, Florida, each involving four crew members, which would leave free up to three seats for s.p.a.ce tourists. Boeing, however, was blunt about the financing for private ventures into s.p.a.ce: the taxpayer would have to pay most of the bill. "This is an uncertain market," says John Elbon, program manager for Boeing's commerical crew effort. "If we had to do this with Boeing investment only and the risk factors were in there, we wouldn't be able to close the business case."

WILD CARDS.

The punishing cost of s.p.a.ce travel has hindered both commercial and scientific progress, so we need a revolutionary new design. By midcentury, scientists and engineers will be perfecting new booster-rocket technologies to drive down the cost of s.p.a.ce travel.

Physicist Freeman Dyson has narrowed down some experimental technologies that may one day open up the heavens for the average person. These proposals are all high risk, but they might drastically reduce the cost. The first is the laser propulsion engine; this fires a high-power laser beam at the bottom of a rocket, causing a mini-explosion whose shock wave pushes the rocket upward. A steady stream of rapid-fire laser blasts vaporizes water, which propels the rocket into s.p.a.ce. The great advantage of the laser propulsion system is that the energy comes from a ground-based system. The laser rocket contains no fuel whatsoever. (Chemical rockets, by contrast, waste much of their energy lifting the weight of their fuel into s.p.a.ce.) The technology for the laser propulsion system has already been demonstrated, and the first successful test of a model was carried out in 1997. Leik Myrabo of Rensselaer Polytechnic Inst.i.tute in New York has created workable prototypes of this rocket, which he calls the lightcraft technology demonstrator. One early design was six inches in diameter and weighed two ounces. A 10-kilowatt laser generated a series of laser bursts on the bottom of the rocket, creating a machine-gun sound as the air bursts pushed the rocket at an acceleration of 2 g's (twice the earth's gravitational acceleration, or 64 feet per second squared). He has been able to build lightcraft rockets that have risen more than 100 feet into the air (equivalent to the early liquid-fueled rockets of Robert G.o.ddard in the 1930s).

Dyson dreams of the day when laser propulsion systems can place heavy payloads into earth orbit for just $5 per pound, which would truly revolutionize s.p.a.ce travel. He envisions a giant, 1,000-megawatt laser that can boost a two-ton rocket into orbit. (That is the power output of a standard nuclear power plant.) The rocket consists of the payload and a tank of water on the bottom, which slowly leaks water through tiny pores. The payload and the water tank each weigh one ton. As the laser beam strikes the bottom of the rocket, the water instantly vaporizes, creating a series of shock waves that push the rocket toward s.p.a.ce. The rocket attains an acceleration of 3 g's and it leaves the earth's gravitational pull within six minutes.

Because the rocket carries no fuel, there is no danger of a catastrophic booster-rocket explosion. Chemical rockets, even fifty years into the s.p.a.ce age, still have a failure rate of about 1 percent. And these failures are spectacular, with the volatile oxygen and hydrogen fuel creating huge fireb.a.l.l.s and raining down debris all over the launch site. This system, by contrast, is simple, safe, and can be used repeatedly with a very small downtime, using only water and a laser.

Furthermore, the system would eventually pay for itself. If it can launch half a million s.p.a.cecraft per year, the fees from these launches could easily pay for the operating costs as well as its development costs. Dyson, however, realizes that this dream is many decades into the future. The basic research on these huge lasers requires funding far beyond that of a university. Unless the research is underwritten by a large corporation or by the government, the laser propulsion system will never be built.

Here is where the X Prize may help. I once spoke with Peter Diamandis, who created the X Prize back in 1996, and he was well aware of the limitations of chemical rockets. Even s.p.a.ceShipTwo, he admitted to me, faced the problem that chemical rockets are an expensive way to escape the earth's gravity. As a consequence, a future X Prize will be given to someone who can create a rocket propelled by a beam of energy. (But instead of using a laser beam, it would use a similar source of electromagnetic energy, a microwave beam.) The publicity of the X Prize and the lure of a multimillion-dollar prize might be enough to spark interest among entrepreneurs and inventors to create nonchemical rockets, such as the microwave rocket.

There are other experimental rocket designs, but they involve different risks. One possibility is the gas gun, which fires projectiles out of a huge gun, somewhat similar to the rocket in Jules Verne's novel From the Earth to the Moon. From the Earth to the Moon. Verne's rocket, however, would never fly, because gunpowder cannot shoot a projectile to 25,000 miles per hour, the velocity necessary to escape the earth's gravity. The gas gun, by contrast, uses high-pressure gas in a long tube to blast projectiles at high velocities. The late Abraham Hertzberg at the University of Washington in Seattle built a gun prototype that is four inches in diameter and thirty feet long. The gas inside the gun is a mixture of methane and air pressurized to twenty-five times atmospheric pressure. When the gas is ignited, the payload rides along the explosion at a remarkable 30,000 g's, an acceleration so great that it can flatten most metallic objects. Verne's rocket, however, would never fly, because gunpowder cannot shoot a projectile to 25,000 miles per hour, the velocity necessary to escape the earth's gravity. The gas gun, by contrast, uses high-pressure gas in a long tube to blast projectiles at high velocities. The late Abraham Hertzberg at the University of Washington in Seattle built a gun prototype that is four inches in diameter and thirty feet long. The gas inside the gun is a mixture of methane and air pressurized to twenty-five times atmospheric pressure. When the gas is ignited, the payload rides along the explosion at a remarkable 30,000 g's, an acceleration so great that it can flatten most metallic objects.

Hertzberg has proven that the gas gun can work. But to launch a payload into outer s.p.a.ce, the tube must be much longer, about 750 feet, and must use different gases along the trajectory. Up to five different stages with different gases must be used to propel the payload to escape velocity.

The gas gun's launch costs may be even lower than those of the laser propulsion system. However, it is much too dangerous to launch humans in this way; only solid payloads that can withstand the intense acceleration will be launched.

A third experimental design is the slingatron, which, like a ball on a string, whirls payloads in a circle and then slings them into the air.

A prototype was built by Derek Tidman, who constructed a tabletop model that could hurl an object to 300 feet per second in a few seconds. The slingatron consists of a doughnut-shaped tube three feet in diameter. The tubing itself is one inch in diameter and contains a small steel ball. As the ball rolls around the tube, small motors push the ball so it moves increasingly fast.

A real slingatron that can hurl a payload into outer s.p.a.ce must be significantly larger-hundreds or thousands of feet in diameter, capable of pumping energy into the ball until it reaches a speed of 7 miles per second. The ball would leave the slingatron with an acceleration of 1,000 g's, still enough to flatten most objects. There are many technical questions that have to be solved, the most important being the friction between the ball and the tube, which must be minimal.

All three of these designs will take decades to perfect, but only if funds from government or private industry are provided. Otherwise, these prototypes will always remain on the drawing board.

s.p.a.cE ELEVATOR.

By the end of this century, nanotechnology might even make possible the fabled s.p.a.ce elevator. Like Jack and the beanstalk, we might be able to climb into the clouds and beyond. We would enter an elevator, push the up b.u.t.ton, and then ascend along a carbon nanotube fiber that is thousands of miles long. This could turn the economics of s.p.a.ce travel upside down.

Back in 1895, Russian physicist Konstantin Tsiolkovsky was inspired by the building of the Eiffel Tower, then the tallest structure of its kind in the world. He asked himself a simple question: Why can't you build an Eiffel Tower to outer s.p.a.ce? If it was tall enough, he calculated, then it would never fall down, held up by the laws of physics. He called it a "celestial castle" in the sky.

Think of a ball on a string. By whipping the ball around, centrifugal force is enough to keep the ball from falling. Likewise, if a cable is sufficiently long, then centrifugal force will prevent it from falling back to earth. The spin of the earth would be sufficient to keep the cable in the sky. Once this cable is stretched into the heavens, any elevator cab that rides along this cable could take a ride into s.p.a.ce.

On paper, this trick seems to work. But unfortunately, when using Newton's laws of motion to calculate the tension on the cable, you find that it is greater than the tensile strength of steel: the cable will snap, making a s.p.a.ce elevator impossible.

Over the decades, the idea of a s.p.a.ce elevator was periodically revived, only to be rejected for this reason. In 1957, Russian scientist Yuri Artsutanov proposed an improvement, suggesting that the s.p.a.ce elevator be built top-down instead of bottom-up, that is, a s.p.a.ceship would first be sent into orbit, and then a cable would descend to and be anch.o.r.ed in the earth. Also, science fiction writers popularized the idea of s.p.a.ce elevators in Arthur C. Clarke's 1979 novel The Fountains of Paradise The Fountains of Paradise and Robert Heinlein's 1982 novel and Robert Heinlein's 1982 novel Frida. Frida.

Carbon nanotubes have helped revive this idea. These nanotubes, as we have seen, have some of the greatest tensile strengths of any material. They are stronger than steel, with enough strength to withstand the tension found in a s.p.a.ce elevator.

A s.p.a.ce elevator to the heavens may one day vastly reduce the cost of s.p.a.ce travel. The key to the s.p.a.ce elevator may be nanotechnology. (photo credit 6.1)

The problem, however, is creating a pure carbon nanotube cable that is 50,000 miles long. This is a huge hurdle, since so far scientists have been able to create only a few centimeters of pure carbon nanotubes. It is possible to weave together billions of strands of carbon nanotubes to create sheets and cables, but these carbon nanotube fibers are not pure; they are fibers that have been pressed and woven together. The challenge is to create a carbon nanotube in which every atom of carbon is correctly in place.

In 2009, scientists at Rice University announced a breakthrough. Their fibers are not pure but composite (that is, they are not suitable for a s.p.a.ce elevator), but their method is versatile enough to create carbon nanotubes of any length. They discovered, by trial and error, that these carbon nanotubes can be dissolved in a solution of chlorosulphonic acid, and then shot out of a nozzle, similar to a shower head. This method can produce carbon nanotube fibers that are 50 micrometers thick and hundreds of meters long.

One commercial application would be for electrical power lines, since carbon nanotubes conduct electricity better than copper, are lighter, and fail less often. Rice engineering professor Matteo Pasquali says, "For transmission lines you need to make tons, and there are no methods now to do that. We are one miracle away."

Although these cables are not pure enough to qualify for use in a s.p.a.ce elevator, this research points to the day when one might be able to grow pure strands of carbon nanotubes, strong enough to take us into the heavens.

a.s.suming that in the future one will be able to create long strands of pure carbon nanotubes, there are still practical problems. For example, the cable will extend far beyond the orbit of most satellites, meaning that the orbits of satellites, after many pa.s.ses around the earth, will eventually intersect the s.p.a.ce elevator and cause a crash. Since satellites routinely travel at 18,000 miles per hour, an impact could be catastrophic. This means that the elevator has to be equipped with special rockets to move the cable out of the way of pa.s.sing satellites.

Another problem is turbulent weather, such as hurricanes, lightning storms, and high winds. The s.p.a.ce elevator must be anch.o.r.ed to the earth, perhaps on an aircraft carrier or oil platform sitting in the Pacific, but it must be flexible to avoid being damaged by the powerful forces of nature.

There must also be a panic b.u.t.ton and escape pod in case of a break in the cable. If something snaps the cable, the elevator cab must be able to glide or parachute back to the earth's surface in order to save the pa.s.sengers.

To jump-start research in s.p.a.ce elevators, NASA has encouraged several contests. A total of $2 million in prizes is awarded through NASA's s.p.a.ce Elevator Games. According to the rules set down by NASA, to win the Beam Power Challenge, you must create a device weighing no more than 50 kilograms that can climb up a tether at the speed of 2 meters per second for a distance of 1 kilometer. What makes this challenge so difficult is that the device cannot have fuel, batteries, or an electrical cord. The energy must be beamed to the device from the outside.

I had a chance to see firsthand the enthusiasm and energy of engineers working on the s.p.a.ce elevator and dreaming of claiming the prize. I flew to Seattle to meet young, enterprising engineers in a group called LaserMotive. They had heard the siren call of NASA's contest and then began to create prototypes that may one day activate the s.p.a.ce elevator.

I entered a large warehouse that they had rented to test out their ideas. On one side of the warehouse, I saw a powerful laser, capable of firing an intense beam of energy. On the other side of the warehouse, I saw their s.p.a.ce elevator. It was a box about three feet wide, with a large mirror. The laser beam would hit the mirror and be deflected onto a series of solar cells that would convert the laser energy into electricity. This would trigger a motor, and the elevator car would gradually climb a short cable. In this way, you would not need electrical cables dangling from the s.p.a.ce elevator to provide its energy. You would just fire a laser at the elevator from the earth, and the elevator would climb the cable by itself.

The laser was so powerful, we all had to wear special goggles to protect our eyes. It took numerous trial runs, but they finally were able fire the laser and send the device climbing the cable. At least in theory, one aspect of the s.p.a.ce elevator had been solved.

Initially, the task was so difficult that no one won the prize. However, in 2009 LaserMotive claimed the prize. The contest took place at Edwards Air Force Base in the Mojave Desert in California. A helicopter flew over the desert, holding up a long cable. The LaserMotive team was able to make their elevator climb the cable four times in two days, with the best time being 3 minutes and 48 seconds. So all the hard work I had seen finally paid off for these young engineers.

STARSHIPS.

By the end of the century, even despite recent setbacks in funding for manned s.p.a.ce missions, scientists will likely have set up outposts on Mars and perhaps in the asteroid belt. Next, they will set their sights on an actual star. Although an interstellar probe is hopelessly beyond reach today, within 100 years it might become a reality.

The first challenge is to find a new propulsion system. For a conventional chemical rocket, it would take about 70,000 years to reach the nearest star. For example, the two Voyager Voyager s.p.a.cecrafts, launched in 1977, have set a world record for an object sent into deep s.p.a.ce. They are currently about 10 billion miles into s.p.a.ce but only a tiny fraction of the way to the stars. s.p.a.cecrafts, launched in 1977, have set a world record for an object sent into deep s.p.a.ce. They are currently about 10 billion miles into s.p.a.ce but only a tiny fraction of the way to the stars.

Several designs and propulsions systems have been proposed for an interstellar craft: *solar sail *nuclear rocket *ramjet fusion *nanoships

I had a chance to meet one of the visionaries of the solar sail when I visited the NASA Plum Brook Station in Cleveland, Ohio. There, engineers have built the world's largest vacuum chamber for testing s.p.a.ce satellites. The chamber is truly cavernous: it is 100 feet across and 122 feet tall, large enough to contain several multistory apartment buildings and big enough to test satellite and rocket parts in the vacuum of s.p.a.ce. Walking into the chamber, I felt overwhelmed by the enormity of the project. But I also felt privileged to be walking in the very same chamber where many of the United States' landmark satellites, probes, and rockets have been tested.

There, I met one of the leading proponents of the solar sail, NASA scientist Les Johnson. He told me that ever since he was a kid reading science fiction, he dreamed of building rockets that could reach the stars. Johnson has even written the basic textbook on solar sails. Although he thinks it might be accomplished within a few decades, he is resigned to the fact that an actual starship may not be built until long after he has pa.s.sed away. Like the masons who built the great cathedrals of the Middle Ages, Johnson realizes that it may take several human life spans to build a ship that can reach the stars.

The solar sail takes advantage of the fact that, although light has no ma.s.s, it has momentum, and hence can exert pressure. Although light pressure from the sun is extremely tiny, too small to be felt by our hands, it is enough to drive a starship if the sail is big enough and we wait long enough. (Sunlight is eight times more intense in s.p.a.ce than on the earth.) Johnson told me his goal is to create a gigantic solar sail, made of very thin but resilient plastic. The sail would be several miles across and built in outer s.p.a.ce. Once a.s.sembled, it would slowly revolve around the sun, gaining more and more momentum as it moves. After several years...o...b..ting the sun, the sail would spiral out of the solar system and on to the stars. Such a solar sail, he told me, could send a probe to 0.1 percent the speed of light and perhaps reach the nearest star in four hundred years.

In order to cut down the time necessary to reach the stars, Johnson has looked into ways to add an extra boost to the solar sail. One possibility is to put a huge battery of lasers on the moon. The laser beams would hit the sail and give it added momentum as it sailed to the stars.

One problem with a solar saildriven s.p.a.ceship is that it is difficult to stop and reverse, since light moves outward from the sun. One possibility is to reverse the direction of the sail and use the destination star's light pressure to slow down the s.p.a.cecraft. Another possibility is to sail around the distant star, using the star's gravity to create a slingshot effect for the return voyage. And yet another possibility is to land on a moon, build laser batteries, and then sail back on the star's light and the laser beams from that moon.

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 s.p.a.ce from the Mir s.p.a.ce station, but it was only to demonstrate deployment. A second attempt failed. In 2004, the j.a.panese 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 j.a.pan Aeros.p.a.ce Exploration Agency successfully launched the IKAROS, the first s.p.a.cecraft to use solar-sail technology in interplanetary s.p.a.ce. 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 j.a.panese 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 s.p.a.ce. 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 s.p.a.ce, 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 pa.s.sed 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 s.p.a.ce, 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 s.p.a.ce, 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 a.s.sembled in s.p.a.ce.

A ramjet fusion engine, because it scoops hydrogen from interstellar s.p.a.ce, 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 a.s.sumptions 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 s.p.a.ce), 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 s.p.a.ceship. 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 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 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 E = = mc 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 t.i.tanic 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. While standing next to the Tevatron, he explained to me the daunting economics of antimatter. The only known way to produce steady quant.i.ties 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 ma.s.s-producing these machines, it might be possible to create sizable quant.i.ties 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 s.p.a.ce. 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 s.p.a.ce.

If large quant.i.ties of antimatter are found in s.p.a.ce, 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.