Hi Raoul & LJK

Raoul has a good point, though I think closed-loop microworlds can be made stable indefinitely.

The Project reminds of a plan I read about to build an Intergalactic Voyager bound for M31 (of course) and it was of even more immense scale, massing ~ trillions of tons, it was a hollow sphere ~ 1000 km across. Cruising speed of 0.4 c meant about ~ 6 million years flight time. Of grander scale was Olaf Stapledon's description, in "Starmaker", of an attempted integalactic voyage which sought to propel a Sun and its planets across the great Gulf.

I don't take any of them seriously.

Firstly the ~ 100 megaton space colony ignores active rad-shielding which would ameliorate the mass budget or allow more reserve mass for (slow) colony growth. But why send a million people? For the sake of the ultra-slow propulsion assumed, so a stable society could be formed to endure the millennia.

But that's nuts! Surely by 2500 we'd have vehicles able to hit 0.1 c or better. That's 300 years to 30 light years - the probable average habitable planet distance. Not an insane 15,000 years as per the Project. Nothing inherently infeasible with 0.1 c. Sure there are sissies who fret about the power output, but they forget ablative techniques (eg. nuke pulse rockets) and the like - and god knows what else we'll have by 2500.

And my other point about uploads is that they won't be "probes" - a human upload will be a (temporarily) bodiless human. At destination they will be able to either reconstruct their original soma, download into an android or develop an adapted soma suited to the target planet. Or all three. The newly embodied human will be as fully human as his/her peers back home, who can beam their body/minds to the new colony for a visit if they so desire.

Don't get me wrong. I like rockets, but interstellar travel is a big ask for puny rockets. Laser sails, particle beams, or whatever, make more sense.

Adam