It may be just a matter of time before the Earth becomes uninhabitable. As astrophysicists and avid science fiction fans, we naturally find the prospect of interstellar colonization intriguing and exciting. But is it practical, or even possible? Or is there a better solution? Science fiction has painted a certain picture of space travel in popular culture. Drawing on stories of exploration from an age of tall ships, with a good helping of anachronisms and fantastical science, space exploration is often depicted in a romantic style: a crew of human travelers in high-tech ships wandering the Galaxy, making discoveries and reporting back home. Perhaps they even find habitable words, some teeming with life (typically humans with different-colored skin), and they trade, colonize, conquer or are conquered. Pretty much, they do as humans have always done since the dawn of their time on Earth. How close do these ideas resemble what we may be able to achieve in the next few hundred years? The laws of physics and the principles of engineering will go a long way to helping us answer this question. Nature's speed limit Nature has given us a speed limit. We call it the speed of light – about 186,000 miles per second – because we first noticed this phenomenon by studying the properties of light, but it is a hard upper limit on all relative speeds. So, if it takes light one year to get somewhere, we can't possibly get there sooner than one year. There is also the fact that the universe is big, really big. It takes light about eight minutes to get to our Sun, three years to get to the next-nearest star, 27,000 years to get to the center of our own Galaxy and more than 2,000,000 years to get to the next galaxy. The amazing thing about these distances is that, as far as the universe is concerned, this is all in the neighborhood. The vast distances between solar systems combined with the speed-of-light limit puts severe constraints on the realities of space travel. Every space-based science fiction writer has to decide early on how to deal with this white elephant standing proudly in the room. Much of the more recent science fiction employs some form of "worm hole" or "warping space": bending the four-dimensional structure of space and time to create shortcuts between two spatial locations in the universe. Such possibilities have been analyzed with some mathematical rigor, and although the studies are tantalizing, they show that these methods cannot work unless we discover a form of matter that behaves very differently than anything we have ever seen. Limits of propulsion Practical space propulsion systems available today and for the foreseeable future are based on Newton's laws. Read more via IO9.com.