I think we have a good chance of sending interstellar probes to the nearest stars within a century or so. For that matter we do already have spacecraft leaving our solar system - just very slow and no way to report back to us.
There have been several projects investigating this - Project Longshot - Project Daedalus - and Project Icarus (interstellar) which is ongoing
And article about it here Futuristic Interstellar Space Probe Idea Revisited and here Icarus: Revisiting the Daedalus Starship
Various ideas such as a neutral beam smart particles - instead of using a laser from Earth focused on the distant receding starship - which seems impossible to achieve, such tight focus - you use it instead to accelerate many smaller mini solar sail type particles just a few mms across - and they can accelerate to a decent fraction of the speed of light - and with smart electronics - they steer themselves towards the receding star ship and homing in on a beacon - collide with it to drive it further away.
As for decelerating at your destination - some ingenious ideas described here. Slowing Down The Icarus Probe & Induced Deceleration
I particularly like the "fry by" - that by doing a close fly by of Proxima Centauri - that you can use the atmosphere of the star itself for aerobraking, enough to reduce your speed for an eventual capture later by Centaurus A or B.
Anyway - one way or another - if we continue for some centuries - I'd be surprised if we don't manage to send some lightweight interstellar probes to nearby stars at least (especially with miniaturization - and also greater reliability of electronics).
If we become a stable civilization lasting for millennia - then the stars will come to us. Because - from time to time, then our solar system will have close encounters with other stars. Not close enough so they pass within our solar system, probably, but close enough to be easy for an advanced civilization to get there.
Another way it can happen, suppose also that humans come to have lifetimes of thousands of years, even millions of years in the future - that's not impossible surely. Then it might be practical to cruise around our neighbourhood even at speeds far slower than the speed of light within a single lifetime. Alpha Centuari wouldn't seem so far away if a one century voyage is, say, just 0.0001% of your life span or less.
And another way - that if you can build self sustaining settlements using materials from the Oort cloud, and with say fusion power for energy - well the Oort clouds of neighbouring stars mingle - so - even without meaning to do it, you could end up orbiting another star as you spread through the Oort cloud.
Indeed - myself - I'm more concerned the other way. Because - there is no imminent danger of Earth being destroyed for a few hundred million years. Are we mature enough to start exploring the galaxy? What would our descendants do to it? If you imagine - just take the Earth's population as it is now - and spread it over say a million stars with seven thousand people per star - and then give them all the capability to make von neumann self replicating probes - which they can use to literally reshape the galaxy to your wishes - demolish planets and make them into Dyson spheres, maybe even transform stars eventually - and to make cyborgs, interstellar star ships, to create new biological entities by manipulating DNA, etc etc -would that be a decent galaxy to live in?
Seems more likely that it would be a galaxy full of horrors, worse than any ETs we imagine invading Earth in our movies. A danger to us and also for other more peaceful ETS or ones with perhaps no technology at all.
So - what can we do about that? Well - I think we can do something. Maybe the very fact that we are still here shows that it won't happen - because other ETs have probably already evolved in our galaxy and not managed to trash it. Or maybe the ones that would be likely to be reckless in the way they seed the galaxy are also not good at forward thinking when it comes to themselves also, and always destroy themselves first.
Maybe there is some insight we need to explore the galaxy responsibly ourselves. Hopefully by maturing rather than destroying ourselves.
Or it might be that we are the first, if so have to be especially careful here.
On the bit I said about "no need to panic", see Why We Can't "Backup Earth" On Mars, The Moon, Or Anywhere Else In Our Solar System - Opinion Piece
Have plenty of time to go interstellar - if that is what we do eventually. I don't agree at all that we need to do this urgently, now, or in the next few million years even.
There have been several projects investigating this - Project Longshot - Project Daedalus - and Project Icarus (interstellar) which is ongoing
And article about it here Futuristic Interstellar Space Probe Idea Revisited and here Icarus: Revisiting the Daedalus Starship
Various ideas such as a neutral beam smart particles - instead of using a laser from Earth focused on the distant receding starship - which seems impossible to achieve, such tight focus - you use it instead to accelerate many smaller mini solar sail type particles just a few mms across - and they can accelerate to a decent fraction of the speed of light - and with smart electronics - they steer themselves towards the receding star ship and homing in on a beacon - collide with it to drive it further away.
As for decelerating at your destination - some ingenious ideas described here. Slowing Down The Icarus Probe & Induced Deceleration
I particularly like the "fry by" - that by doing a close fly by of Proxima Centauri - that you can use the atmosphere of the star itself for aerobraking, enough to reduce your speed for an eventual capture later by Centaurus A or B.
Anyway - one way or another - if we continue for some centuries - I'd be surprised if we don't manage to send some lightweight interstellar probes to nearby stars at least (especially with miniaturization - and also greater reliability of electronics).
If we become a stable civilization lasting for millennia - then the stars will come to us. Because - from time to time, then our solar system will have close encounters with other stars. Not close enough so they pass within our solar system, probably, but close enough to be easy for an advanced civilization to get there.
Another way it can happen, suppose also that humans come to have lifetimes of thousands of years, even millions of years in the future - that's not impossible surely. Then it might be practical to cruise around our neighbourhood even at speeds far slower than the speed of light within a single lifetime. Alpha Centuari wouldn't seem so far away if a one century voyage is, say, just 0.0001% of your life span or less.
And another way - that if you can build self sustaining settlements using materials from the Oort cloud, and with say fusion power for energy - well the Oort clouds of neighbouring stars mingle - so - even without meaning to do it, you could end up orbiting another star as you spread through the Oort cloud.
Indeed - myself - I'm more concerned the other way. Because - there is no imminent danger of Earth being destroyed for a few hundred million years. Are we mature enough to start exploring the galaxy? What would our descendants do to it? If you imagine - just take the Earth's population as it is now - and spread it over say a million stars with seven thousand people per star - and then give them all the capability to make von neumann self replicating probes - which they can use to literally reshape the galaxy to your wishes - demolish planets and make them into Dyson spheres, maybe even transform stars eventually - and to make cyborgs, interstellar star ships, to create new biological entities by manipulating DNA, etc etc -would that be a decent galaxy to live in?
Seems more likely that it would be a galaxy full of horrors, worse than any ETs we imagine invading Earth in our movies. A danger to us and also for other more peaceful ETS or ones with perhaps no technology at all.
So - what can we do about that? Well - I think we can do something. Maybe the very fact that we are still here shows that it won't happen - because other ETs have probably already evolved in our galaxy and not managed to trash it. Or maybe the ones that would be likely to be reckless in the way they seed the galaxy are also not good at forward thinking when it comes to themselves also, and always destroy themselves first.
Maybe there is some insight we need to explore the galaxy responsibly ourselves. Hopefully by maturing rather than destroying ourselves.
Or it might be that we are the first, if so have to be especially careful here.
On the bit I said about "no need to panic", see Why We Can't "Backup Earth" On Mars, The Moon, Or Anywhere Else In Our Solar System - Opinion Piece
Have plenty of time to go interstellar - if that is what we do eventually. I don't agree at all that we need to do this urgently, now, or in the next few million years even.
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
4.8m answer views110.4k this month
Top Writer2017, 2016, and 2015
Published WriterHuffPost, Slate, and 4 moreHuffPost, Slate, Forbes, Newsweek, Business Insider, and Science 2.0