Whether human beings will one day travel to other stars has remained, for hundreds of years, a question that has captured the imagination of scientists, writers, and dreamers alike. Closer to the era of space exploration and more cognizant of the universe, the promise of interstellar travel remains teasingly beyond reach. There are technological barriers, huge distances between stars, and restrictive human life in space-a very big dream, with many obstacles. Yet, the hope lingers that perhaps someday this dream could be overcome by humanity on journeys beyond our solar system.
The Distance Challenge
Simply great distance between the stars stands as the most important obstacle to interstellar travel. The nearest star system to Earth is Proxima Centauri, which is about 4.24 light-years from Earth. To put that into context, a light-year is roughly about 9.461 trillion kilometers [5.88 trillion miles]. And even at speeds comparable to today's spacecraft-say, NASA's Parker Solar Probe, whose speed reaches up to about 700,000 kilometers per hour-it would take tens of thousands of years to reach Proxima Centauri.
First of all, space trips to other star systems demand the spacecraft to move with a sizeable percent of the speed of light that is approximately 300,000 kilometers per second. How to achieve such relativistic velocities is one of the most profound unsolved problems in astronautics.
Speed Limit
As pointedly expressed by Mike Gruntman, a professor of astronautics: "The biggest unsolved mystery in astronautics is reaching exceptionally high speeds of space vehicles, constituting a significant fraction of the speed of light." Current propulsion technologies, such as chemical rockets, are simply incapable of accelerating spacecraft to the kinds of high speeds that would be required for interstellar travel. The most powerful rockets today can reach speeds that are useful for travel within our solar system but fall very far short of what is needed for interstellar voyages.
It could be that the key to surmounting this speed barrier depends upon new, ground-breaking technologies that as yet remain to be developed. Nuclear propulsion, antimatter propulsion, or even fusion-based drives-all of which promise speeds that might be appropriate for interstellar travel-are being researched by some scientists and engineers. However, such technologies are in their very early days and could be decades, if not centuries, into the future.
Learning from UFOs?
Most people assume that UFOs-if they were of extraterrestrial origin-might provide at least some clues to propulsion technologies far beyond our current ones, capable of interstellar travel. Indeed, for an extraterrestrial spacecraft, it would have to span enormous distances of space in ways that far exceed our present capability to study the universe. Could UFO sightings offer clues on how it is accomplished?
Gruntman thinks it unlikely. He said most of the past UFO sightings, in fact, had been of secret aircraft: U.S. Air Force's U-2 and A-12 spy planes, otherwise known as the SR-71, from space craft of extra-terrestrial visitors. While UFOs capture the imagination of the public, Gruntman is more skeptical of the idea that they offer any real insight into the future of space travel. "Learning from unidentified flying objects does not help," he says, knowing full well this will be disappointing to those who might have been hoping for a technological breakthrough from such sightings.
The Physics of Interstellar Travel
It is not only technological, but also physical laws, which present a big problem for interstellar travel. According to the relativity theory of Albert Einstein, as the speed of the object approaches the speed of light, its mass increases and the energy required to further accelerate the object begins to surge upwards extremely fast. It means that the faster a spacecraft travels, the more energy it requires; hence, the great difficulty in reaching or even approaching the speed of light.
This can, in turn, be dodged by using space-time as the method of conveyance. Concepts like wormholes, which are short cuts through space-time, or the Alcubierre drive-which, in theory, would allow a spacecraft to travel faster than light by distorting space-time around it-have been made. Still, these concepts remain highly speculative, with no experimental evidence that these concepts actually stand even the slightest chance of working in nature. The amount of energies involved is also beyond our reach at present.
Engineering versus Social Engineering
Gruntman stresses that it will be engineering—and not wishful thinking or social policies—that will take humanity to the flight deck of a starship. "What we do know, however, is that the required breakthrough in physics could only be achieved by promoting excellence in a merit-based environment," he says. He claims that history has demonstrated that social engineering-or the solution of technical problems by non-technical means-is "sure road to mediocrity." In other words, human ingenuity and technical expertise are what will carry us through, not short cuts or societal changes.
Consequently, breakthroughs related to interstellar travel will be realized by a combination of new physics, advanced engineering, and the most unparalleled global effort. That means that long-term commitment to space research, education, and innovation will be needed the moment we have traveled to other stars.
The Human Element
Even assuming that all of the technical problems of interstellar travel were somehow overcome, there remains a host of unknowns about human considerations involved in such journeys. Space travel is ruinous to human physiology: prolonged exposure to cosmic radiation, the psychological effects of long-term isolation, and the logistics of providing food, water, and air for journeys that could take decades-all these things need to be addressed before it will be safe for humans to take a trip to the other stars.
Generation ships-some sort of spacecraft that would be able to support several generations of people; the original crew would hand over the mission to their offspring-are one possible solution to problems of time and health. Another approach would be cryogenic sleep or hibernation: astronauts could be "frozen" for the duration of the trip and revived at their destination. Both these ideas have captured science fiction, but neither is anywhere close to becoming real.
The Role of AI and Robotics
Given the truly enormous difficulties posed by interstellar travel for humans, the first to reach other stars may well be robotic explorers or even artificial intelligence. Indeed, robotic space probes have already made the first steps beyond our solar system. The two Voyager spacecraft launched in 1977 now travel through interstellar space-the region outside of the heliosphere, or the bubble of space that our Sun dominates. But they are doing so at a fraction of the velocity needed to conduct meaningful interstellar travel. Each will take tens of thousands of years to reach even nearby star systems.
Another exotic might be the dispatch of self-replicating probes-machines that would launch copies of themselves into space, using materials available in that environment. Telemetrically they could spread across the galaxy in millions of years, collecting information and transmitting it back to Earth. Though the idea is largely speculative, it points out how an autonomous system may become an important contributor in the early stages of interstellar exploration.
The Search for Habitable Planets
Another critical question of interstellar travel is also where we would go. Though Earth's is the only known habitat which has been found to support life, over several thousands of exoplanets have been discovered by astronomers in orbit around distant stars. Some of those planets-which are now located in what is called the habitable zone, where conditions might be right for liquid water and, potentially, life-constitute the prime targets for future exploration.
The Kepler Space Telescope, along with other missions, has discovered several Earth-like planets that conceivably can harbor human life. But these worlds are generally tens or hundreds of light-years away, placing them well out of our reach with today's technology. Now being a subject of scientific investigation, in time they can inspire and motivate future generations to go further in space travel than what is currently possible.
What's Next?
So, will humans travel to other stars? The answer, for now, is unknown. The technological, physical, and biological problems are very serious. Still, human curiosity and the drive to explore have always made us breach those apparently insurmountable obstacles. After all, we once thought flight, space travel, and landing on the Moon were impossible. In the same way, someday interstellar travel may move from the realm of science fiction to reality.
This could involve centuries of research and development to travel to other stars. It may require breakthroughs in physics, propulsion methods, and designs of spacecraft that we cannot yet envision. It may even take completely new ways of envisioning life and consciousness-such as downloading human consciousness into machines that can travel to the stars unhindered by a fragile biological form.
While this may not occur within our lifetime, this quest to see the stars will go on; literally, an urge from within ourselves to make the discovery of the unknown. Whether through robotic probes, AI-driven missions, or human explorers, the first steps toward interstellar travel have already been undertaken.
Conclusion
One of the gigantic visions of humanity is to make trips to other stars. However, there is a long way to go ahead with main critical obstacles. Answers about interstellar travel are to be found in critical breakthroughs in technology, engineering, and merit-based scientific inquiry. Yet, even though the answers remain to be discovered, continued knowledge and exploration will lead one day to a time when humans finally will embark on journeys to the stars.
As Gruntman correctly reminds us, it is through excellence in engineering and scientific advance—not speculation or social policy—that someday we might aspire to overcome the difficulties of interstellar travel and perhaps to settle among the stars.
