Outer space is big. Really, really, really big.
And that's why NASA has no plans at present to send a spacecraft to any of the 1,876 known planets (as of January 9, 2015) beyond our solar system.
Alpha Centauri is the nearest star system to our sun at 4.3 light-years away. Can't we even go that far?
The answer is … not easily.
A distance of 4.3 light-years equals trillions of miles away from Earth - nearly 300,000 times the distance from the Earth to the sun.
How might we travel to Alpha Centauri, the next-nearest star?
And how long would it take to get there?
Would a conventional rocket work?
Consider the Space Shuttle, which traveled only a few hundred kilometers into space. If Earth were the size of a sand grain, this would be about the width of a hair in contrast to a 10-kilometer distance to Alpha Centauri.
You'd need about 10,000 shuttle main engines in sequence just to build up a decent speed (say, 1/100th light speed).
The Space Shuttles weren't starships. At a maximum speed of about 17,600 mph (about 28,300 kph), it would have taken a Space Shuttle about 165,000 years to reach Alpha Centauri.
How about the Voyager spacecraft? These two unmanned space probes - Voyager 1 and Voyager 2 - were launched in 1977. They're now heading out of our solar system. The Voyagers aren't aimed toward Alpha Centauri, but if they were, they'd take tens of thousands of years to get there.
On the other hand, eventually, the Voyagers will pass other stars. In about 40,000 years, Voyager 1 will drift within 1.6 light-years (9.3 trillion miles) of AC+79 3888, a star in the constellation of Camelopardalis. In some 296,000 years, Voyager 2 will pass 4.3 light-years from Sirius, the brightest star in the sky. Hmm, 4.3 light-years. That's the distance between us and Alpha Centauri.
The problem with conventional rockets is that, if you're carrying fuel, you need more fuel in order to carry your fuel to accomplish star-to-star travel.
Using current technology, a trip to Alpha Centauri would take tens to hundreds of thousands of years.
But what if we would travel faster than light? Sound impossible?
A couple of years ago, Dr. Harold "Sonny" White - who leads NASA's Advanced Propulsion Team at Johnson Space Center - claimed to have made a discovery which made plausible the idea of faster-than-light travel, via a concept known as the Alcubierre warp drive.
This concept is based on ideas put forward by Mexican physicist Miguel Alcubierre in 1994. He suggested that faster-than-light travel might be achieved by distorting spacetime, as shown in the illustration above.
Harold "Sonny" White has been working to investigate these ideas further, and, in June of 2014, he unveiled images of what a faster-than-light ship might look like. Artist Mark Rademaker based these designs on White's theoretical ideas. He said creating them took more than 1,600 hours, and they are very cool. See the 2014 faster-than-light spacecraft designs on this Flickr page.
The video below presents Harold White's talk at the SpaceVision 2013 Space Conference in November, 2013 in Phoenix. He talks about the concepts and progress in warp-drive development over recent decades.
Is it faster-than-light travel possible, via the Alcubierre warp drive? As with conventional propulsion systems, the problem is energy. In this case, it's the type of energy the warp drive would need. Daily Kos said:
In order to form the warp field/bubble, a region of space-time with negative energy density (i.e. repulsing space-time) is necessary. Scientific models predict exotic matter with a negative energy may exist, but it has never been observed. All forms of matter and light have a positive energy density, and create an attractive gravitational field.
So faster-than-light travel via the Alcubierre warp drive is highly speculative, to say the least. With current technologies, it's not possible.
However, if it could be accomplished, it would reduce the travel time to Alpha Centauri from thousands of years to just days.
Other alternate propulsion methods have been discussed, for example, antimatter engines. They work on the principle that, when antimatter and matter meet, they annihilate each other, releasing vast amounts of energy.
Scientists have observed bits of antimatter in particle accelerators. But no one knows how to create enough antimatter, or how to store it, for a trip to the stars.
How about light sails? This very romantic notion for travel among the stars would rely on thin, lightweight reflective sails, powered by the sun, other stars, or even lasers fired from Earth. You start slow, but accelerate up to light speeds. However, no one imagines a light sail could enable us to travel to Alpha Centauri within a human lifetime.
But the propulsion issues are just part of the problem. The real problem might be when to decide to go.
The real problem with traveling to Alpha Centauri. Let's suppose that faster-than-light travel isn't going to become a reality. Suppose we have to chose another method of travel - a conventionally powered space arc of some kind, or even an antimatter drive, or a solar sail. Now suppose we set out for a trip among the stars.
Suppose that, generations from now, our descendants arrive at a planet in the Alpha Centauri system.
They might be greeted by brass bands and crowds of earthlings - who left later, but traveled via a more efficient process - and so made the trip in a shorter time. All aboard!
Bottom line: At 4.3 light-years away, the Alpha Centauri system is the nearest star system to our Earth and sun, but getting there would be extremely difficult.