by Michio Kaku
Currently it costs $10,000 or more to send a pound of material into orbit around the Earth (roughly the cost, ounce for ounce, of gold). Each Space Shuttle mission, for example, costs up to $700 million. A space elevator could reduce the cost to as little as $1 per pound. Such a radical reduction in the cost of the space program could revolutionize the way we view space travel. With a simple push of an elevator button, one could in principle take an elevator ride into outer space for the price of a plane ticket.
But formidable practical hurdles have to be solved before we build a space elevator on which we can levitate our way into heaven. At present pure carbon nanotube fibers created in the lab are no more than 15 millimeters long. To create a space elevator, one would have to create cables of carbon nanotubes that are thousands of miles long. Although from a scientific point of view this is just a technical problem, it is a stubborn and difficult problem that must be solved if we are to create a space elevator. Yet, within a few decades, many scientists believe that we should be able to master the technology of creating long cables of carbon nanotubes.
Second, microscopic impurities in the carbon nanotubes could make a long cable problematic. Nicola Pugno of the Polytechnic of Turin, Italy, estimates that if a carbon nanotube has even one atom misaligned, its strength could be reduced by 30 percent. Overall, atomic-scale defects could reduce the strength of the nanotube cable by as much as 70 percent, taking it below the minimum gigapascals of strength necessary to support a space elevator.
To spur entrepreneurial interest in the space elevator, NASA is funding two separate prizes. (The prizes are modeled on the $10 million Ansari X-prize, which successfully spurred enterprising inventors to create commercial rockets capable of taking passengers to the very edge of space. The X-prize was won by Spaceship One in 2004.) The prizes NASA is offering are called the Beam Power Challenge and the Tether Challenge. In the Beam Power Challenge, teams have to send a mechanical device weighing at least 25 kilograms up a tether (suspended from a crane) at the speed of 1 meter per second for a distance of 50 meters. This may sound easy, but the catch is that the device cannot use fuel, batteries, or an electrical cord. Instead, the robot device must be powered by solar arrays, solar reflectors, lasers, or microwaves—energy sources that are more suitable for use in outer space.
In the Tether Challenge, teams must produce 2-meter-long tethers that cannot weigh more than 2 grams and must carry 50 percent more weight than the best tether of the previous year. The challenge is intended to stimulate research in developing lightweight materials strong enough to be strung 100,000 kilometers in space. There are prizes worth $150,000, $40,000, and $10,000. (To highlight the difficulty of mastering this challenge, in 2005, the first year of the competition, no one won a prize.)
Although a successful space elevator could revolutionize the space program, such machines have their own sets of hazards. For example, the trajectory of near-Earth satellites constantly shifts as they orbit the Earth (this is because the Earth rotates beneath them). This means that these satellites would eventually collide with the space elevator at 18,000 miles per hour, sufficient to rupture the tether. To prevent such a catastrophe, in the future either satellites will have to be designed to include small rockets so that they can maneuver around the space elevator, or the tether of the elevator might have to be equipped with small rockets to evade passing satellites.
Also, collisions with micrometeorites are a problem, since the space elevator is far above the atmosphere of the Earth, and our atmosphere usually protects us from meteors. Since micrometeor collisions are unpredictable, the space elevator must be built with added shielding and perhaps even fail-safe redundancy systems. Problems could also emerge from the effects of turbulent weather patterns on the Earth, such as hurricanes, tidal waves, and storms.
THE SLINGSHOT EFFECT
Another novel means of hurling an object near the speed of light is to use the “slingshot” effect. When sending space probes to the outer planets, NASA sometimes whips them around a neighboring planet, so they use the slingshot effect to boost their velocity. NASA saves on valuable rocket fuel in this way. That’s how the Voyager spacecraft was able to reach Neptune, which lies near the very edge of the solar system.
Princeton physicist Freeman Dyson proposed that in the far future we might find two neutron stars that are revolving around each other at great speed. By traveling extremely close to one of these neutron stars, we could whip around it and then be hurled into space at speeds approaching a third the speed of light. In effect, we would be using gravity to give us an additional boost to nearly the speed of light. On paper this just might work.
Others have proposed that we whip around our own sun in order to accelerate to near the speed of light. This method, in fact, was used in Star Trek IV: The Voyage Home, when the crew of the Enterprise hijacked a Klingon ship and then sped close to the Sun in order to break the light barrier and go back in time. In the movie When Worlds Collide, when Earth is threatened by a collision with an asteroid, scientists flee the Earth by creating a gigantic roller coaster. A rocket ship descends the roller coaster, gaining great velocity, and then whips around the bottom of the roller coaster to blast off into space.
In fact, however, neither of these methods of using gravity to boost our way into space will work. (Because of the conservation of energy, in going down a roller coaster and coming back up, we wind up with the same velocity as that with which we started, so there is no gain in energy whatsoever. Likewise, by whipping around the stationary sun, we wind up with the same velocity as that with which we originally started.) The reason Dyson’s method of using two neutron stars might work is because the neutron stars are revolving so fast. A spacecraft using the slingshot effect gains its energy from the motion of a planet or star. If they are stationary, there is no slingshot effect at all.
Although Dyson’s proposal could work, it does not help today’s Earth-bound scientists, because we would need a starship just to visit rotating neutron stars.
RAIL GUNS TO THE HEAVENS
Yet another ingenious method for flinging objects into space at fantastic velocities is the rail gun, which Arthur C. Clarke and others have featured in their science fiction tales, and which is also being seriously examined as part of the Star Wars missile shield.
Instead of using rocket fuel or gunpowder to boost a projectile to high velocity, a rail gun uses the power of electromagnetism.
In its simplest form, a rail gun consists of two parallel wires or rails, with a projectile that straddles both wires, forming a U-shaped configuration. Even Michael Faraday knew that a current of electricity will experience a force when placed in a magnetic field. (This, in fact, is the basis of all electrical motors.) By sending millions of amperes of electrical power down these wires and through the projectile, a huge magnetic field is created around the rails. This magnetic field then propels the projectile down the rails at enormous velocities.
Rail guns have successfully fired metal objects at enormous velocities over extremely short distances. Remarkably, in theory, a simple rail gun should be able to fire a metal projectile at 18,000 miles per hour, so that it would go into orbit around the Earth. In principle, NASA’s entire rocket fleet could be replaced by rail guns that could blast payloads into orbit from the Earth.
The rail gun enjoys a significant advantage over chemical rockets and guns. In a rifle the ultimate velocity at which expanding gases can push a bullet is limited by the speed of shock waves. Although Jules Verne used gunpowder to blast astronauts to the moon in his classic tale From the Earth to the Moon, one can compute that the ultimate velocity that one can attain with gunpowder is only a fraction of the velocity necessary to send someone to the moon. Rail guns, however, are not limited by the speed of shock waves.
But there are problems with the rail gun. It accelerates objects so fast that they usually flatten upon impact with the air. Payloads have been severely deformed in the process of being fired out of the barrel of a ra
il gun because when the projectile hits the air it’s like hitting a wall of bricks. In addition, the huge acceleration of the payload along the rails is enough to deform them. The tracks have to be replaced regularly because of the damage caused by the projectile. Furthermore, the g-forces on an astronaut would be enough to kill him, easily crushing all the bones in his body.
One proposal is to install a rail gun on the moon. Outside the Earth’s atmosphere, a rail gun’s projectile could speed effortlessly through the vacuum of outer space. But even then the enormous accelerations generated by a rail gun might damage the payload. Rail guns in some sense are the opposite of laser sails, which build up their ultimate speed gently over a long period of time. Rail guns are limited because they pack so much energy into such a small space.
Rail guns that can fire objects to nearby stars would be quite expensive. In one proposal the rail gun would be built in outer space, extending two-thirds of the distance from Earth to the sun. It would store solar energy from the sun and then abruptly discharge that energy into the rail gun, sending a 10-ton payload at one-third the speed of light, with an acceleration of 5000 g’s. Not surprisingly, only the sturdiest robotic payloads would be able to survive such huge accelerations.
THE DANGERS OF SPACE TRAVEL
Of course, space travel is no Sunday picnic. Enormous dangers await manned flights traveling to Mars, or beyond. Life on Earth has been sheltered for millions of years: The planet’s ozone layer protects the Earth from ultraviolet rays, its magnetic field protects against solar flares and cosmic rays, and its thick atmosphere protects against meteors that burn up on entry. We take for granted the mild temperatures and air pressures found on the Earth. But in deep space, we must face the reality that most of the universe is in turmoil, with lethal radiation belts and swarms of deadly meteors.
The first problem to solve in extended space travel is that of weightlessness. Long-term studies of weightlessness by the Russians have shown that the body loses precious minerals and chemicals in space much faster than expected. Even with a rigorous exercise program, after a year on the space station, the bones and muscles of Russian cosmonauts are so atrophied that they can barely crawl like babies when they first return to Earth. Muscle atrophy, deterioration of the skeletal system, lower production of red blood cells, lower immune response, and a reduced functioning of the cardiovascular system seem to be the inevitable consequences of prolonged weightlessness in space.
Missions to Mars, which may take several months to a year, will push the very limits of the endurance of our astronauts. For long-term missions to the nearby stars, this problem could be fatal. The starships of the future may have to spin, creating an artificial gravity via centrifugal forces in order to sustain human life. This adjustment would greatly increase the cost and complexity of future spaceships.
Second, the presence of micrometeorites in space traveling at many tens of thousands of miles per hour may require that spaceships be equipped with extra shielding. Close examination of the hull of the Space Shuttle has revealed evidence of several tiny but potentially deadly impacts from tiny meteorites. In the future, spaceships may have to contain a special doubly reinforced chamber for the crew.
Radiation levels in deep space are much higher than previously thought. During the eleven-year sunspot cycle, for example, solar flares can send enormous quantities of deadly plasma racing toward Earth. In the past, this phenomenon has forced the astronauts on the space station to seek special protection against the potentially lethal barrage of subatomic particles. Space walks during such solar eruptions would be fatal. (Even taking a simple transatlantic trip from L.A. to New York, for example, exposes us to about a millirem of radiation per hour of flight. Over the course of our trip we are exposed to almost a dental X-ray of radiation.) In deep space, where the atmosphere and magnetic field of the Earth no longer protect us, radiation exposure could be a serious problem.
SUSPENDED ANIMATION
One consistent criticism of the rocket designs I have presented so far is that even if we could build such starships, it would take decades to centuries to reach nearby stars. Such a mission would need to involve a multigenerational crew whose descendants would arrive at the final destination.
One solution, proposed in such movies as Alien and Planet of the Apes, is for space travelers to undergo suspended animation; that is, their body temperature would be carefully lowered until bodily functions almost cease. Animals that hibernate do this every year during the winter. Certain fish and frogs can be frozen solid in a block of ice and yet thaw out when the temperature rises.
Biologists who have studied this curious phenomenon believe that these animals have the ability to create a natural “antifreeze” that lowers the freezing point of water. This natural antifreeze consists of certain proteins in fish, and glucose in frogs. By flooding their blood with these proteins, fish can survive in the Arctic at about-2°C. Frogs have evolved the ability to maintain high glucose levels, thereby preventing the formation of ice crystals. Although their bodies might be frozen solid on the outside, they are not frozen on the inside, allowing their bodily organs to continue to operate, albeit at a reduced rate.
There are problems with adapting this ability to mammals, however. When human tissue is frozen, ice crystals begin to form inside the cells. As these ice crystals grow, they can penetrate and destroy cell walls. (Celebrities who want to have their heads and bodies frozen in liquid nitrogen after death may want to think twice.)
Nevertheless, there has been recent progress in limited suspended animation in mammals that do not naturally hibernate, such as mice and dogs. In 2005 scientists at the University of Pittsburgh were able to bring dogs back to life after their blood had been drained and replaced by a special ice-cold solution. Clinically dead for three hours, the dogs were brought back to life after their hearts were restarted. (Although most of the dogs were healthy after this procedure, a few suffered some brain damage.)
That same year scientists were able to place mice in a chamber containing hydrogen sulfide and successfully reduce their body temperature to 13°C for six hours. The metabolism rate of the mice dropped by a factor of ten. In 2006 doctors at Massachusetts General Hospital in Boston placed pigs and mice in a state of suspended animation using hydrogen sulfide.
In the future such procedures may be lifesaving for people involved in severe accidents or who suffer heart attacks during which every second counts. Suspended animation might allow doctors to “freeze time” until patients can be treated. But it could be decades or more before such techniques can be applied to human astronauts, who may need to be in suspended animation for centuries.
NANOSHIPS
There are several other ways in which we might be able to reach the stars via more advanced, unproven technologies that border on science fiction. One promising proposal is to use unmanned probes based on nanotechnology. Throughout this discussion I have assumed that starships need to be monstrous devices consuming vast amounts of energy, capable of taking a large crew of human beings to the stars, similar to the starship Enterprise on Star Trek.
But a more likely avenue might be initially to send miniature unmanned probes to the distant stars at near the speed of light. As we mentioned earlier, in the future, with nanotechnology, it should be possible to create tiny spacecraft that exploit the power of atomic and molecular-sized machines. For example, ions, because they are so light, can easily be accelerated to near the speed of light with ordinary voltages found in the laboratory. Instead of requiring huge booster rockets, they might be sent into space at near the speed of light using powerful electromagnetic fields. This means that if a nanobot were ionized and placed within an electric field, it could effortlessly be boosted to near light speed. The nanobot would then coast its way to the stars, since there is no friction in space. In this way, many of the problems plaguing large starships are immediately solved. Unmanned intelligent nanobot spaceships might be able to reach nearby star systems at a mere fraction o
f the cost of building and launching a huge starship carrying a human crew.
Such nanoships could be used to reach nearby stars or, as Gerald Nordley, a retired Air Force astronautical engineer, has suggested, to push against a solar sail in order to propel it through space. Nordley says, “With a constellation of pinhead-sized spacecraft flying in formation and communicating with themselves, you could practically push them with a flashlight.”
But there are challenges with nano starships. They might be deflected by passing electric and magnetic fields in outer space. To counteract these forces, one would need to accelerate the nanoships to very high voltages on the Earth so they would not be easily deflected. Second, we might have to send a swarm of millions of these nanobot starships to guarantee that a handful would actually make it to their destination. Sending a swarm of starships to explore the nearest stars might seem extravagant, but such starships would be cheap and could be mass-produced by the billions, so that only a tiny fraction of them would have to reach their target.
What might these nanoships look like? Dan Goldin, former head of NASA, envisioned a fleet of “Coke-can sized” spacecraft. Others have talked about starships the size of needles. The Pentagon has been looking into the possibility of developing “smart dust,” dust-sized particles that have tiny sensors inside that can be sprayed over a battlefield to give commanders real-time information. In the future it is conceivable that “smart dust” might be sent to the nearby stars.
Dust-sized nanobots would have their circuitry made by the same etching techniques used in the semiconductor industry, which can create components as small as 30 nm, or roughly 150 atoms across. These nanobots could be launched from the moon by rail guns or even by particle accelerators, which regularly send subatomic particles to near light speed. These devices would be so cheap to make that millions of them could be launched into space.
