Our Earth is the only planet in our solar system that is highly hospitable to life. So if we want to find any planet like our own, we must dream of traveling to planets revolving around other stars. The Kepler Space Telescope has discovered more than 100 planets and more than 2000 planet candidates. Some of these are Earth-sized planets in the habitable zone, meaning they might bear life. Scientists estimate our galaxy of more than 100 billion stars could easily contain more than a billion life bearing planets.
But while the stars offer the most enticing imaginable opportunity for exploration, the task of being able to travel to the stars is one of the most difficult imaginable technical tasks. The main problem is the incredibly vast distance between stars. The nearest star is Proxima Centauri, part of the Alpha Centauri star system. It is about 40 trillion kilometers away. This is such a great distance that light takes more than 4 years to traverse it.
Now you might think: no problem, we'll just keep building rocket engines more and more powerful, until we can get there in a flash. Unfortunately, nature has told us: you can't do that. According to Einstein, nothing can travel faster than the speed of light.
That means that even if we could build the most powerful spaceship possible, it would still take four years to get to the nearest star. If you wanted to take a grand tour of ten or twenty stars (similar to a voyage of the starship Enterprise), that would apparently take a lifetime or longer.
In fact, there are practical engineering reasons why it seems unlikely that any interstellar starship (basing on conventional rocket principles) could reach a speed of more than about half of the speed of light. One can imagine a spaceship that used antimatter as the rocket fuel (which would release incredible amounts of energy). One can also imagine a spaceship that scoops up matter between the stars, and uses that as rocket fuel (a type of design called a Bussard Ramjet). But given various engineering limitations, it would seem to be all but impossible to build any starship (using conventional rocket principles) capable of traveling faster than half the speed of light.
Would this rule out the possibility of humans traveling to the nearest star? No. We are used to space missions that last a few days or months (and have imagined space missions to Mars lasting a few years). But even without imagining an extension of the human life span, it is easy to imagine a twenty or thirty year interstellar voyage. A spaceship can easily be designed with artificial gravity. It would simply need to have a large spinning centrifuge similar to the one depicted in the movie 2001: A Space Odyssey. If the spaceship crew lived inside the rim of a large spinning wheel (preferably one of at least 50 meters in width), the crew would experience artificial gravity similar to the gravity on Earth. If such a spinning habitat were made large enough, it could provide a comfortable home that a crew could live in for a voyage of twenty or thirty years. Then when the spaceship arrived at the distant star, the crew could colonize or explore the planets revolving around the star. For such an approach to work, the crew might have to be quite young when the spaceship left Earth, presumably no older than 18 or 20.
Under this scenario (assuming no extension of the human lifespan), it would probably not be practical to imagine the spaceship returning to Earth after an interstellar journey. But that would not be much of a problem, because the crew of a starship could use radio and television transmissions to beam back to Earth all of its discoveries. There would be a gap of at least four years before the television transmissions reached the Earth.
What if we wanted to explore other more distant stars, using these types of spaceships traveling at no more than half of the speed of light? It could be done, but it would be a long slow affair. After reaching a planet revolving around another star, a starship could colonize that planet. Eventually a new expedition to another star could be launched, using young crew members from the colony. If the colony became really advanced, it might build its own starships and send them out to colonize or explore nearby stars. In such a manner the human race might spread out very slowly among the stars. But the rate of expansion would be very slow if we could not build ships faster than the speed of light.
The discussion above assumes that we could build a spaceship capable of traveling at a decent fraction of the speed of light (perhaps 25% of the speed of light, or 50% of it). But what it proves too difficult to travel at such speeds? What if we can't make any ship traveling faster than 5% or 10% of the speed of light?
In such a case there are still two ways we could explore the stars. The first method is simply robotized exploration. We would have little difficulty creating robots that could endure a long interstellar journey of a hundred years or more. The second method is the approach of creating a multigenerational starship.
A multigenerational starship would be a kind of traveling miniature slice of the planet Earth. The idea would be that the original passengers would bear children on the starship, and live out the rest of their lives on the starship before it reached its destination. Their children would do the same thing. Finally, after the passage of many generations, the starship would arrive at a planet revolving around another star. The passengers at that time would be people who had never lived on Earth.
Such an approach seems perfectly feasible, and requires no technical breakthroughs. The ship would have to be a large one that provided artificial gravity to its passengers. But we already know exactly how to do this. You simply build a large torus-shaped structure (the same shape as an inflatable ring), and have the starship passengers live on the inner rim of the huge structure. As soon as you start rotating the structure, it would produce artificial gravity through centrifugal force.
Still another possibility is that of having a starship crew spend years in a kind of deep freeze, through some kind of hibernation or suspended animation. That would require a major breakthrough, which is not required to launch the multigeneration starship.
I have discussed here relatively slow methods of interstellar travel, which require no huge technical breakthroughs. There are other possibilities for faster interstellar flight, involving exotic (and perhaps improbable) breakthroughs such as warp drives. I will discuss those in another post.