Does it have to be attached to sungrazer ? If else you can do the journey in 250 years, 10% speed of light, using a form of fusion or anti- matter drive. Maybe even faster if people are gonna be reconstructed, and you dont have to worry about g's.

Technically to attach it to a comet, you first have to catch up to said comet ... which means you don't need the comet in the first place XD (Because once you've reached a certain speed, even if you turn off thrusters you remain at that speed in space).

@Chinu in this world, fusion and anti-matter are still a long way off from being applicated. unless you're talking about fusion bombing? would that work in space?

@KingofSnakes the plan is to get into the path of the comet and let it pick us up on the way (great big smash but our replicators can fix that)

@ChrisJ You are looking for Nuclear Pulse Propulsion, pretty crude but still works.

1 year of 1g acceleration gets you to 0.8c and uses 1kg/kg of matter antimatter mix fuel. So 6 years to alpha cen is doable in g forces if you have antimatter by the ton. or on the order of 30 years for your target.

@DonaldHobson "At present, antimatter costs $62.5 trillion per gram" - I'm not sure that it's believable to have a tonne of the stuff

Can you define "conventional means" a bit better? Are we allowing fission drives? Fusion drives? Anti-matter drives? None of those exist in usable form today, so they may all be considered to be unconventional.

@MikeScott conventional means are anything that has been proved to work as at this point in time, fission/fusion/anti-matter are all off the plate. I'll update to reflect this.

@ChrisJ Nothing has been proved to work for interstellar distances, so the answer becomes pretty simple. We can't achieve interstellar travel by conventional means.

My doubt is whether it would be possible, or believable, that a civilisation that is able to "reconstruct" people from raw materials won't have solved problems such as controlled fission.

@MikeScott We got voyager out of the solar system back in , I'm not concerned about getting there in a timely manner, this would be a one way trip

@LuísHenrique Science doesn't progress along all avenues at the same time, solving problems in physics is different to creating some human-like constructs, both are equally implausible in my mind at the moment

@ChrisJ - True, but there are degrees of difficulty. Would a civilisation develop brain surgery without developing metallurgy first? (and yes, I think the difference between "reconstruction" and controlled fission is of the same order - or even bigger). We know what nuclear fission is, that it exists, and the problem is how to control it in practice. We don't know whether reconstruction is even possible (or, rather, due to Heisenberg's principle, we pretty well "know" it is impossible; we would have to have a (huge) paradigm shift to even start posing the correct questions in that field).

@LuísHenrique apologies, reconstruction is the term I've used which is misleading, it's more like building a human shaped sack of meat and giving it a human-like personality for the first generation. whilst the second generation will be cultured and be more human

I see... actually there is no crew at departure, the crew is going to be created at the end of the trip? Then the problems are of another nature, closer to sociology and linguistics. I suggest you read something about "feral children", so that your trip planners take the adequate (or, at least, credible) measures about the creation of human beings ex-nihilo.

@LuísHenrique that's where I'm going with this :) what kind of messed up monsters would we be with perfect parents

Ah, so you understand the problems involved in the experiment. Then that's what important for your narrative; the practicality of your scientific-sounding technicalities about both "reconstruction" and accelerating enough to make an interstellar trip should be a lesser concern - they should be the subject of suspension of disbelief, rather than of an extensive scientific research on their viability (and maybe it is just me, but if you can get me to believe in "reconstruction" for the sake of the story, then you can get me to believe in any kind of propulsion, nuclear or not, for the same end).

@LuísHenrique mostly I wanted to know the timeframe, I wanted it to be long enough that a generational ship was out of the question but long enough for problems to creep into the system

@DonaldHobson, I would love to see a source on that