[Adapted from a post on the Kentucky Space blog] Pointing to a paper recently accepted for publication, Paul Gilster at Centauri Dreams explains how the best ideas for fast space travel must cope with another, non-technological, constraint.
For example, while "sundiving" missions designed to place a solar sail-powered craft near the sun for maximum outward acceleration - one study suggests that deployment at .1 AU could send such a craft to 200 AU in only 2.5 years! - might be accomplished, designers would have to account for General Relativistic effects in their craft's navigation or find it potentially millions of kilometers off course by the time it reaches the Oort Cloud. As one commenter points out, the flight calculations for MESSENGER, which has flown past Mercury three times, and which will settle into orbit around the innermost planet in 2011, have also taken General Relativity into account. And yet another points to this Technology Reviewblog entry on "relativistic navigation."
If you're wondering, one "AU" is the distance from our yellow dwarf star to Earth. The Voyager robotic explorers, launched in 1977, have only recently encountered the area where the solar wind meets the void between the stars, called termination shock, and passed into the heliosheath on their way to intersteller space. Voyager 1 was approximately 100 AU from the sun in 2006. Both craft, by the way, are still functioning and returning data.