As NASA and other agencies refine their plans to return humans to the moon, the continued exploration of the lunar environment seems assured. Practices developed there will prepare future astronauts for the rusted surface of Mars, and assuming a steady progression in technology and in the politics of space exploration, a human presence there can be considered almost a foregone conclusion. Other targets within our solar system, already visited by robotic scouts, may one day also offer explorers the chance to see firsthand the extraordinary beauty of the outer planets and their myriad satellites. If we are to move beyond our solar system, however, future travelers must be equipped with exotic new methods of propulsion capable of propelling their spaceships to speeds vastly in excess of those allowed by chemical rockets. Trips to the outer planets involve staggering distances and, for human astronauts, would require novel drive systems working at extremely high speeds. Travel outside of the solar system — a journey to a nearby star, for example — relies on an entirely different scale, revealing some clue about the actual immensity of the universe.
Besides the sun, the nearest star to Earth is Alpha Centauri, which appears as a single point of light but is actually a system of three individual stars. Of the three, Proxima Centauri is closest overall, at a distance of approximately 4.2 light-years. One light-year is equal to the distance light will travel over the course of a year, slightly less than 6 trillion miles. Even moving at the speed of light, 186,282 miles per second, a traveler anxious to reach Proxima Centauri would have to wait more than four years. The problem, of course, is that traveling at any significant percentage of light-speed is incredibly difficult, and may require some new ideas about physics, spacetime and the structure of the universe.
According to Einstein’s theory of relativity, as an object approaches the speed of light, its mass increases. At light-speed, the mass is infinite, and requires infinite power to move it forward, an obvious problem for any practical astronaut. Beyond approximately 87 percent of the speed of light, the rate of mass-increase becomes unmanageable, so an interstellar vessel may be limited to that velocity. Fortunately, this still equates to almost 163,000 miles per second. Starships launched from Earth will probably operate at much lower speeds, moving across space more quickly only as new technology emerges.
Once limited to the pages of science fiction, these new drive systems are becoming a reality. In laboratories, scientists have managed to create metallic hydrogen, an incredibly dense form of matter created by subjecting hydrogen to unimaginable pressure. The resulting material, if it can be made stable, contains many times the energy contained in liquid or gaseous hydrogen and could, theoretically, propel a 500,000-pound starship to approximately 20 percent of light-speed, almost 40,000 miles per second. The antimatter drive, made famous by Gene Roddenberry’s “Star Trek,” is another highenergy possibility. Based on the idea that every particle has an antimatter counterpart — opposite in properties such as charge and spin — , the antimatter drive would rely on the energy produced by the annihilation that occurs when the two opposite particles meet. The problem is that the gamma rays produced by electrons and positrons, as an example, quickly disperse, moving in all directions. In theory, however, if the energy could be contained, and the resulting stream of charged particles could be channeled into a high-energy exhaust stream, a starship equipped with such a drive could accelerate to speeds similar to those allowed by liquid hydrogen.
The principle behind these ideas is no different from that of a typical chemical rocket. As matter is ejected in one direction, an equal and opposite force must be exerted on the spacecraft ejecting the material. This is true whether the material is an expanding cloud of exploding gas or an immeasurably thin stream of high-energy particles. Another science fiction staple, the ion drive, uses an electric field to accelerate ions and provide thrust. This type of device has been used to propel robotic spacecraft such as Deep Space 1; the recently launched Dawn probe will use a similar thruster to visit the asteroid Vesta and the dwarf planet Ceres.
As early as 1960, physicist Robert Bussard proposed his own novel solution, the interstellar ramjet. Although space is an almost perfect vacuum, every cubic inch of space contains a few atoms of hydrogen. By generating a cone-shaped magnetic field, 2,000 miles wide at its widest point, a starship could collect this errant hydrogen and channel it into a nuclear fusion reaction engine. The combination of virtually unlimited hydrogen and the reduction in onboard fuel supplies means that a spaceship equipped with an interstellar ramjet could accelerate to nearly half the speed of light, more than 93,000 miles per second.
Other ideas, mostly theoretical, are also being examined by scientists around the world. The strange behavior of tachyons, for example, suggest a particle that can travel from one point to another without interacting with the space in between. This “quantum jump” might allow for a way to get around the limits imposed at lightspeed. Other mathematical models suggest the possibility of wormholes, which are “shortcuts” in space. Problematic, of course, is the fact that calculations show a wormhole to be somewhat smaller than a subatomic particle, and to exist for mere fractions of a second.
As humans continue to evolve and as our technological abilities mature, the distance we travel in space will steadily increase. Eventually, our sun will begin to die, growing into an aged red giant, boiling Earth into space. As a practical matter, we must find a way out of this star system, ensuring the survival of our species. At the same time, we will fulfill the human need for exploration, providing our species with new experiences, incredible ideas and, perhaps one day, even a new home.
|26. Science Fiction into Reality : The Next Steps - December 11, 2007||2.14 MB|