# Is a trip to explore the Alpha Centuri system actually feasible?

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Can the journey be completed within the lifetime of a human adult?

Assuming we have got around little problems like having an adequate form of propulsion, shielding from radiation, and avoiding deadly collisions with milligram sized dust particles.
Let's say our engines can provide constant acceleration then deceleration at 1g.

If the return journey can be done within the travellers lifetime, then will there be anyone still alive on Earth who they knew before departing?, or not so because those on Earth will have aged relatively faster than the travellers.

russ_watters
Mentor
The word "feasible" doesn't really apply here. What you are really asking is whether it is mathematically possible given the 1g acceleration constraint. Some quick, non-relativistic math tells me that in 6 months you'd be going half the speed of light and could make the round trip in about 20 years at that speed. I'd bet, but am not sure if I know how to calculate, that with constant acceleration you could make it in 10-11 years from Earth's perspective.

bob012345 and rootone
phyzguy
If you're talking about sending a 1 gram probe to Alpha Centauri in a human lifetime, it looks like that might be feasible. If you're talking about sending a group of people, with all of their support requirements, I don't think this is energetically feasible in a human lifetime. We're talking either generation ships, or some type of suspended animation.

rootone and anorlunda
Janus
Staff Emeritus
Gold Member
The word "feasible" doesn't really apply here. What you are really asking is whether it is mathematically possible given the 1g acceleration constraint. Some quick, non-relativistic math tells me that in 6 months you'd be going half the speed of light and could make the round trip in about 20 years at that speed. I'd bet, but am not sure if I know how to calculate, that with constant acceleration you could make it in 10-11 years from Earth's perspective.
Accelerating at 1g (proper acceleration) Until you reach 0.5 relative to the Earth will take 0.533 yrs ship time and a distance of 0.15 light years ( as measured from the Earth). This means that you would coast for ~ 6.928 yrs ship time and then spend 0.533 yrs decelerating for a total trip time of just under 8 years, one way.
By Earth time, it would take 0.56 years during acceleration and deceleration legs and 8 years coasting for a total of 9.12 years, one way.
However, even if we used a perfectly efficient fusion rocket, it would take ~10,108 kg of fuel per kg of payload to make just the outbound trip. If you need to bring the fuel for the return trip with you, this jumps to 102,171,664 kg for every kg you want to deliver to Alpha C and bring back.

bob012345, CalcNerd, Ryan_m_b and 3 others
Thanks for the interesting replies.
Anything much further than AC, is probably not.
and you certainly need to be sure to refuel at AC, otherwise getting back just ain't happening.

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Can the journey be completed within the lifetime of a human adult?

Assuming we have got around little problems like having an adequate form of propulsion, shielding from radiation, and avoiding deadly collisions with milligram sized dust particles.
Let's say our engines can provide constant acceleration then deceleration at 1g.

At 1g, you attain relativistic velocity in about one year. After this, still accelerating at 1g, increasingly dramatic length contraction makes it possible to reach just about any place in our local Virgo Supercluster (and farther) "within the lifetime of a human adult" on board the ship.

You "only" need that magic constant-1g drive.

rootone and bob012345
From an amateur author's perspective, I must admit, 'Not Without Cheating'.

FTL is often the one waiver that 'Hard SciFi' allows, just so the story can go beyond our Oort Cloud in practicable time. New Physics, loopholes in derivations, anti-matter and/or negative matter may provide. Buying or reverse engineering a functional Alien Star Drive just 'passes the buck'...

IMHO, you should take a look at the BIS' wondrous Project Daedalus. Though now a bit dated, and sadly failed by the lack of progress on pulsed fusion power, the reasonably extrapolated tech would take sizeable fly-by probes to our near-by stars in a couple of decades.

Chronos
Gold Member
The technological challenges are currently too great to consider even a probe to Alpha Centauri. The fuel requirements alone are absolutely mind boggling. Even a hypothetical antimatter drive operating at say 50% efficiency [usable propulsion energy vs total liberated] would demand an enormous mass of antimatter. We have no clue how to produce or store more than nanograms of antimatter, much less an engine capable of converting the energy it liberates into a useable form of propulsion. With our current, most advanced ion drives, a one way trip would take around 80,000 years and cost trillions of dollars. It's not going to be easy to convince the appropriations committee such an effort offers anything resembling a reasonable ROI.

rootone
sophiecentaur
Gold Member
I have never been too much in agreement with the necessity of sending actual humans to do exploration. Sending a probe with a hefty transmitter would be far more realistic and the time to get the project under way and for information back would be far shorter. I say 'hefty transmitter' because the achievable data rate would need to be high (more than a few kB) and that would require Power. If a probe could be put into orbit around a star, its electronics could be 'Solar' powered. A beacon, in orbit would have a chance of getting noticed by the locals (if their civilisation were at a level something like ours).

rootone
I have never been too much in agreement with the necessity of sending actual humans to do exploration.

There is no telling what would count as "humans" in ~500 years. Nearly fully cyborgized bodies? Or even no organics left, brain structure scanned and then its operation simulated by computers? Or even AIs which succeeded humans as "actual brains" of civilization (the case where they turn out to be much more clever than we)?

On another front, building space-based ~kilometer scale individual telescopes and/or ~million km baseline imaging interferometers should be significantly faster than crossing interstellar distances - I'd imagine 100-200 years from today.

rootone
sophiecentaur
Gold Member
I could go along with most of that but the “500 year” figure is way too short, I think.
Technology has limits which are imposed by politics and human psychology. You are proposing a much more extreme revolution than anything we’ve experienced so far. Tech advance has to be funded by you and me. Remember the euphoria about the Moon landings?! That faded pdq once the public could see no immediate benefits ( the space race had been ‘won’). Tech spin-offs have often not been identified as coming from aerospace work. Timescales are too long for the public or political memory so projects have to be of modest scale.
Perhaps finding a trace of real alien life could change that.

rootone
I could go along with most of that but the “500 year” figure is way too short, I think.
Technology has limits which are imposed by politics and human psychology. You are proposing a much more extreme revolution than anything we’ve experienced so far.

Predicting the pace of changes is difficult, sure, so treat it as +-50% or even more.

I based my "500 years" on what happened in last 500 years. In 1500, we basically had no medicine to speak of, no understanding of chemistry, no calculus (even concept of first derivative was not well-understood), crude understanding of basics of physics. And we had awfully inefficient economic models, wasting human potential.

rootone
'hefty transmitter' because the achievable data rate would need to be high
Could we launch a set of relay probes at the same time, spaced out over the distance, to relay the signal? Since the power required is the square of the distance, doing it in two hops halves your total power needs. Ten hops cuts it by 90%.
They would need to be nuclear isotope powered.
The final hop back to Earth can be longer because we already have big dishes here.

Asymptotic, rootone and sophiecentaur
sophiecentaur
Gold Member
Predicting the pace of changes is difficult, sure, so treat it as +-50% or even more.

I based my "500 years" on what happened in last 500 years. In 1500, we basically had no medicine to speak of, no understanding of chemistry, no calculus (even concept of first derivative was not well-understood), crude understanding of basics of physics. And we had awfully inefficient economic models, wasting human potential.
I understand what you are saying. We are into the unknown here, of course. However, the technological and scientific advances may not be as relevant as the cultural ones. I would say that there has been very little real change between the development of language, the development of writing and the recent digital revolution. I would say that those are what have changed humans more than anything else. We are talking in terms of thousands of years between those steps.
I have worries that the human race could have more problems with the new Communications Explosion, although they don't seem to be mentioned a lot. Can we develop the appropriate mental and emotional skills to cope and to remain a reasonably stable species? What you are suggesting would be a further, enormous destabilising change, even if the technology were developed. It would never surprise me if humans entered another 'dark age'.

rootone
We have no clue how to produce or store more than nanograms of antimatter, much less an engine capable of converting the energy it liberates into a useable form of propulsion.
How big of a hurdle is this? We also didn't know how to store more than a few nanograms of plutonium once and had no real need to until we got wind that the Nazis were trying. Could the USA theoretically create the necessary fuel if it devoted its resources the way we did during the Manhattan project? Obviously, there is no reason to do that now, but if we did, I was under the impression that we could.

Space isn't empty. There are hydrogen atoms at a very slight density between the stars, as well as other molecules. Seems like, if I understand it, Star Trek's Enterprise had a collector that absorbed hydrogen and fed it to the fusion reactors. There is simply no way to carry enough fuel on board to make it happen unless, hyper-space/wormholes are real and you can dimensionally slip between vast distances.

I find the whole thing depressing in the extreme. That coupled with an article in the current issue of Scientific American which states that, while life may exist on other planets in our galaxy, the odds of having it reaching (or exceeding) our level of intellectual evolution is slim to none. While there are literally billions of planets, since it seems that every star has it raft of planets as a "normal" component of its creation, the chain of random and fortuitous events that led us to this day on Earth simply doesn't seem feasible elsewhere. It starts with our distance from our star, the effect of the moon to keep us from tilting too far off the elliptic, the vegetation that produced O2, the liquid mantle with the iron core producing the magnetic field that shields the planet from ionizing radiation, etc., etc.

It's depressing because this is probably the only planet we'll ever have and humans seem to be too shortsighted to realize that we could really screw it up. Interstellar travel just might not develop in time, if it ever develops at all, to save us from ourselves.

Buzz Bloom
I find the whole thing depressing in the extreme. That coupled with an article in the current issue of Scientific American which states that, while life may exist on other planets in our galaxy, the odds of having it reaching (or exceeding) our level of intellectual evolution is slim to none.

This is a good thing in my book.
It's much better to meet aliens in their home planet's orbit, than in ours.

stefan r and Auto-Didact
Janus
Staff Emeritus
Gold Member
Space isn't empty. There are hydrogen atoms at a very slight density between the stars, as well as other molecules. Seems like, if I understand it, Star Trek's Enterprise had a collector that absorbed hydrogen and fed it to the fusion reactors.
The Bussard collectors. Based on the concept of the Bussard ramjet suggested by Robert Bussard in 1960. The idea being that as the craft traveled through space, it collected hydrogen, which it fused and used to create exhaust for propulsion.
One problem is that the interstellar medium turned out to be thinner than we thought it was when he came up with idea.
The analysis of one design concluded that the drag caused by collecting the fuel would exceed the thrust of the rocket.
Even if we could increase the efficiency of the thrust, I don't see such a craft ever being capable of velocities of more than a few percent of c.(There is just so much you can get out of a fusion rocket, and the faster you go, the greater the drag from fuel collection, and they eventually just cancel each other out.)
There is a variation where you don't use the hydrogen as fuel, but just reaction mass. This solves the problem of having to match the captured hydrogen to ship velocity and reduces the drag, but requires carrying a separate energy source to produce in order to produce thrust from it.

How big of a hurdle is this? We also didn't know how to store more than a few nanograms of plutonium once

Uh, wrong? There were never any problems with storing Pu. The problems lied in production.

stefan r
The Bussard collectors. Based on the concept of the Bussard ramjet suggested by Robert Bussard in 1960. The idea being that as the craft traveled through space, it collected hydrogen, which it fused and used to create exhaust for propulsion.
One problem is that the interstellar medium turned out to be thinner than we thought it was when he came up with idea.

I imagine the bigger problem was how exactly would you fuse Protium??

Janus
Staff Emeritus
Gold Member
I imagine the bigger problem was how exactly would you fuse Protium??
The two suggested methods would be the proton-proton chain, or the CNO cycle (where Carbon, Nitrogen, and Oxygen are used as catalysts for the reaction.)
Of course, any discussion of such a craft has to assume that we have learned how to make practical use of these reactions.

Yeah, sure, but both seem to be technically intractable. p-p requires enormous densities and pressures, and CNO in addition requires you to decelerate incoming hydrogen (ouch), react it, extract CNO catalyst nuclei, and re-accelerate exhaust.

bob012345
Gold Member
I have never been too much in agreement with the necessity of sending actual humans to do exploration. Sending a probe with a hefty transmitter would be far more realistic and the time to get the project under way and for information back would be far shorter. I say 'hefty transmitter' because the achievable data rate would need to be high (more than a few kB) and that would require Power. If a probe could be put into orbit around a star, its electronics could be 'Solar' powered. A beacon, in orbit would have a chance of getting noticed by the locals (if their civilisation were at a level something like ours).
It's not necessary to send humans but it's also not necessary to explore in the first place. But if society is going to spend a fortune exploring I say send humans because exploring is not simply about gathering information, it's about going places and being there. Probes are fine for the first phases of exploration but ultimately they should be followed by humans.

sophiecentaur
Gold Member
it's about going places and being there.
Actually, it's about somebody else going places and being there - and it's me who's paying for it. Does that sound like a good deal?

bob012345
Gold Member
Actually, it's about somebody else going places and being there - and it's me who's paying for it. Does that sound like a good deal?
We can be represented vicariously by people or by machines but the moon landing/walk just wouldn't have been the same watching a machine.

sophiecentaur
Gold Member
We can be represented vicariously by people or by machines but the moon landing/walk just wouldn't have been the same watching a machine.
I guess that is also why people like watching sport on the TV. Personally , I tend to find that a bit boring - with a few notable exceptions. I wonder how much people would be prepared to pay for this "vicarious" satisfaction because it could well involve a huge portion of the GDP (or equivalent) of the planet in future. The way that the popularity of NASA pretty soon faded after the Apollo programme (and that was actual people out there) suggests to me that funding would be 'difficult', unless there were some extra factor to waken long term interest.
But I tend to be a grumpy old gimmer where manned space exploration is concerned. What we have learned from unmanned missions has been orders of magnitude more significant.

Chronos
Gold Member
Retreating to stark reality is sometime refreshing. Returning to the prospect of anti matter powered engines, a 2016 anti matter projection pegs the production cost of anti matter at $100 trillion [US] per gram. The anti matter needed to power a probe to Alpha Centauri is estimated at 100 - 1000 TONS [re: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160007059.pdf] To draw some historial comparisons:the cost of aluminum in the 1850's was about$550 per pound [~$1.20/gm] and fell to around$.20 a pound by 1930. At the same rate of decline the price of anti matter will plummet to a bargain price of \$36 billion per gram by the year 2100.

In summary, it's only feasible given as yet unknown technology and engineering.

Please continue though, unimaginable technology and engineering has happenned before.

Check out this project: Breakthrough Starshot. Whether it can be pulled off, who knows. But its goal is to do just what you've asked about.

https://breakthroughinitiatives.org/initiative/3
"Breakthrough Starshot aims to demonstrate proof of concept for ultra-fast light-driven nanocrafts, and lay the foundations for a first launch to Alpha Centauri within the next generation. Along the way, the project could generate important supplementary benefits to astronomy, including solar system exploration and detection of Earth-crossing asteroids."

sophiecentaur
Gold Member
There is a variation where you don't use the hydrogen as fuel, but just reaction mass.
Do you assume a mean velocity of molecules of about zero (in the frame of the local part of the Galaxy) and a 'thermal' spread of speeds? That would be low, I imagine (?).
I don't find it surprising that the sums are not very encouraging for this idea. After all, we assume / observe that drag is very low in deep space (even within the Solar System, on the grounds that Cosmic Dust particles of microgram mass still arrive at Earth at very high velocity.

russ_watters
Mentor
In summary, it's only feasible given as yet unknown technology and engineering.
In other words, not anywhere close to feasible. You're trying to make a spot of moisture sound like an almost full glass of water.
Please continue though, unimaginable technology and engineering has happenned before.
No, not like this. That's a common dreamer's refrain, but it misses the critical other side of science and technology's coin: it doesn't just make previously not possible things possible, it give us a better understanding of what is actually not possible.

...and that's in addition to the fact that most popular examples of this are wrong.

Staff Emeritus
2021 Award
In other words, not anywhere close to feasible.

Was there really any doubt? We can't get people to Mars, for heaven's sake. Alpha Centauri is 500,000 times farther away.

russ_watters
Mentor
Was there really any doubt? We can't get people to Mars, for heaven's sake. Alpha Centauri is 500,000 times farther away.
Evidently, from the last few posts it seems there is.

Also, while I think the OP is stretching the definition of "feasible" literally by light-years, I think you're over-compressing it. "Can't" is too strong a word to describe the feasibility of a trip to Mars, given that plans are underway to make such a trip*. We haven't yet proven it is feasible (doable), but I doubt many scienctists/engineers doubt that it is.

*Assuming the seriousness of such plans...

Chronos