"Tunnel in the Sky"
Great Heinlein, tome, Bryan!...one of my favorites along with "Stranger in a Strange Land" (of course), and "Time Enough for Love."
And I often think of Heinlein's concept as the ultimate answer for space travel whenever the subject comes up. But now, it seems, it may be more than just conjecture. With the new theories of Dark Matter and Anti-Gravity developing (really Einstein's abandoned theory of the "cosmological constant" revisited!), forces so strong that they bend and consume even light (and may be faster
than the speed of light!), the glint of plausibility of traversing these vast distances begins to gleam on the horizon. (My brain cells begin to rupture whenever I start to contemplate these magnitudes...you know, what's outside the universe? the Big Bang exploded where?...I have to pull back and grab a handful of soil just for reassurance). So, yes, Bryan, in about 1,000 years or so, hold on to your hat! Who knows, with the help of cryogenics you might still be here, there!, wherever!
By the way, there's a good article on this in the current "Time" 6/25.
And I'll never forgive Ronald Reagan for drastically cutting NASA's budget, and rolling back the manned space progam (and indirectly causing the Challenger disaster due to the lack of funding). I firmly believed at one time that by 1990 we would have manned flights to Mars, and colonies on (and maybe even vacation flights to) the Moon. But it wasn't to be...it WILL eventually, but how soon now...who knows?