Physics of the Future: How Science Will Shape Human Destiny and Our Daily Lives by the Year 2100 (48 page)

(However, to go completely around the earth, you would need to pay considerably more to take a trip aboard the space station. I once asked Microsoft billionaire Charles Simonyi how much it cost him to get a ticket to the Space 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 space twice. So space travel, even into the near future, will still be the province of the well-off.)

Space 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 spaceflight program to private industry. Boeing’s plan calls for launches to the International Space Station from Cape Canaveral, Florida, each involving four crew members, which would leave free up to three seats for space tourists. Boeing, however, was blunt about the financing for private ventures into space: 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.”

The punishing cost of space 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 space 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 space. 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 space.)

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 Institute 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 Goddard 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 space 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 space. 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 space age, still have a failure rate of about 1 percent. And these failures are spectacular, with the volatile oxygen and hydrogen fuel creating huge fireballs 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 spacecraft 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 SpaceShipTwo, 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.
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 space, 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 space 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.

By the end of this century, nanotechnology might even make possible the fabled space 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 button, and then ascend along a carbon nanotube fiber that is thousands of miles long. This could turn the economics of space 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 space? 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 space.

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 space elevator impossible.

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

A space elevator to the heavens may one day vastly reduce the cost of space travel. The key to the space 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 space 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 space 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.

Assuming 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 passes around the earth, will eventually intersect the space 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 passing satellites.