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.
No comments:
Post a Comment