Last year a read a couple of books by the accomplished science fiction writer Charles Stross. If you haven’t read anything by him I suggest that you do. On his blog he has a new post on the feasibility of starships entitled “The myth of the starship”.
“Actually, I tell a lie. There are five starships that we know of; Pioneer 10,Pioneer 11, Voyager 1, Voyager 2, and New Horizons. But they’re a far cry from the gleaming interstellar transports of science fiction. New Horizons is the most recent of them. Launched in late 2006, it is the fastest human-launched vehicle so far. It raced past Lunar orbit within nine hours of take-off: nevertheless, it will take around 10 years to reach Pluto (its proximate target — for a three-hour flyby). It weighs around 478 Kg, and is currently travelling outwards from the sun at around 17km/sec — about fifty times as fast as a rifle bullet.
We are 4.37 light years, or 140 million light-seconds, from Alpha Centauri, give or take. One light second is 300,000 km; it takes New Horizons about five hours to travel one light second. So: in very roughly 30 million days, or on the order of 300,000 years (if it was going in the right direction, which it isn’t), New Horizons could reach Alpha Centauri.
And that’s the best we’ve done to date, admittedly without really trying …” -Charles Stross at Charlie’s Diary