Physics of the Future: How Science Will Shape Human Destiny and Our Daily Lives by the Year 2100 (49 page)

Another problem is turbulent weather, such as hurricanes, lightning storms, and high winds. The space elevator must be anchored 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 button 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 passengers.

To jump-start research in space elevators, NASA has encouraged several contests. A total of $2 million in prizes is awarded through NASA’s Space 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 space 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 space 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 space 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 space 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 space 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.

By the end of the century, even despite recent setbacks in funding for manned space 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
spacecrafts, launched in 1977, have set a world record for an object sent into deep space. They are currently about 10 billion miles into space 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 space 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 space. 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 passed 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 mass, 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 space 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 space. Once assembled, it would slowly revolve around the sun, gaining more and more momentum as it moves. After several years orbiting 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 sail–driven spaceship 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 spacecraft. 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 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.

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.

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.

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.