I copied this:
Here are four examples of what it would take to send a canister about the size of a Shuttle payload (or a school bus) past our nearest neighboring star...and allowing 900 years for it to make this journey.
Well....If you use chemical engines like those that are on the Shuttle, well..., sorry, there isn’t enough mass in the universe to supply the rocket propellant you’d need.
So let’s step up to next possibilities, nuclear rockets with a predicted performance that’s 10 to 20 times better!
Well...it’s still not looking all that good. For a fission rocket you would need a BILLION SUPERTANKER size propellant tanks to get you there, and even with fusion rockets you would still need a THOUSAND SUPERTANKERS!
Even if we look at the best conceivable performance that we could engineer based on today’s knowledge, say an Ion engine or an antimatter rocket whose performance was 100 times better that the shuttle engines, we would need about ten railway tanker sized propellant tanks.
That doesn’t sound too bad, until you consider that we didn’t bring along any propellant to let us stop when we get to the other star system...or if we want to get there quicker than 9 centuries.
Once you add the desire to actually stop at your destination, or if you want to get there sooner, you’re back at the incredible supertanker situation again, even for our best conceivable rockets. It takes as much energy thrust to stop as it does to start.
So... we develop ion motors and in space build a 2000 foot long space ship. And were off to the nearest star to see what's around it. 30 generations later - a period equating from the 1100 AD until now - humans finally get there. Darn, nothing there. Now everyone dies. No fuel to return.
But it was a damn good thought!