Engage the x drive: Ten ways to traverse deep space, New Scientist

"Apart from the mundane problems of budgets and political will, the major roadblock is that our dominant space-flight technology - chemically fuelled rockets - just isn't up to the distances involved. We can send robot probes to the outer planets, but they take years to get there. And as for visiting other stars, forget it. As an example of why, the Apollo 10 moon probe is currently listed as the fastest manned vehicle in history, having reached a maximum speed of 39,895 kilometres per hour. At this speed, it would take 120,000 years to cover the 4 light years to Alpha Centauri, the nearest star system."