Voyager 1 is now leaving solar system, making it the first manmade probe to enter interstellar space. That's quite an achievement, and it only took 30+ years. But if we're going to get serious about boldly going where no man has gone before, and send humans beyond the solar system, we're gonna need a cheap and plentiful energy source to help us get there.
Exactly how much energy are we talking about? Well, back in January, a paper appeared on the arXiv by Marc Millis, a former head of NASA's Breakthrough Propulsion Physics Project, calculating the costs -- in terms of energy -- of a truly interstellar manned space mission. And it wasn't good news.
For one scenario, he assumed a 500-person space ship on a one-way trip to establish a human colony on some distant exoplanet. That would require an exajoule of energy, or 1018 J, i.e., just about the same amount of energy consumed by everyone on Earth in one year.
An unmanned mission to Alpha Centauri would be even more of an energy hog, because of the need for more complicated maneuvers (deceleration, etc). That would require 1018 J. Millis figures we won't have the ability to generate that kind of energy until 2200 for the passenger ship, and 2500 for the unmanned probe.
Scientists are mulling all kinds of options, of course, including harnessing the power of the stars, i.e., nuclear fusion (as opposed to the nuclear fission underlying nuclear power plants). As Carl Sagan once noted, "Every time you look up at the sky, every one of those points of light is a reminder that fusion power is extractable from hydrogen and other light elements, and it is an everyday reality throughout the Milky Way Galaxy."