Getting to Alpha Centauri

[Octavo]

I was reading this article on New Scientist http://space.newscientist.com/article/dn13393-nearest-stars-wobbles-could-reveal-earths-twin.html?feedId=online-news_rss20

I was wondering if someone here could come up with some realistic numbers for an attempt to travel to Alpha Centauri.

A nuclear reactor would be essential, so let's say you had a commercial reactor onboard capable of producing 2000Mw. How fast could you get there, given you can only accellerate for half the journey? How much uranium would you get through?

Let's say your ship has a mass of about 1000 tons (given the ISS is expected to weigh in at about 470 tons upon completion, I think a ship twice the size of the ISS would be reasonable for a journey to Alpha Centauri).

What about the hazards of interstellar space - Would micrometeorites be a problem?

 

[Cuddles]

Alpha Centauri is around 4 light years away. Going from memory of calculations done on this before, it would take a little more than 2 years to get there using with a constant 1g acceleration for the first half and then the same to slow down for the second half. According to the Australian Uranium Association, a typical 1000MW reactor requires about 25 tonnes of enriched uranium per year.

Using your estimate of a 2000MW reactor, that would therefore need about 100 tonnes of fuel for the reactor. However, I doubt that such a spacecraft would need anywhere near that amount of power. On the other hand, there would also be problems of non-fuel consumables. For example, the water, or whatever else, used as coolant become radioactive and needs replacing. Ideally, everything on board would be recycled, but waste water would not be suitable for anything involving humans, and would likely need to be kept away from any living things, including being unsuitable for growing food. On the plus side, nuclear waste wouldn't be a problem since any spent fuel could simply be jettisoned into space.

As for hazards, I'm not too sure. Micrometeorites would certainly be an issue. However, interstellar space will have much less of them than the space we already fly in. I think there have been cases of equipment being damaged, but other than needing a procedure for patching any holes which did occur, I doubt meteroites would be too much of a problem. What would be a much bigger problem is gas. When travelling at a significant fraction of the speed of light, even single molecules have enough energy to do some damage. This wouldn't be an immediate problem, but the front of the ship would wear away, and would therefore need to be significantly thicker, and therefore much heavier, than would be the case for a slower ship.

The only other real hazard would be radiation. Again, this would be more of a problem near the stars than through most of the journey. Space weather is an area that really still isn't understood that well. It's not even clear if manned missions to other planets are feasible at the moment. The only radiation monitor (that I know of) that tried to check the radiation dose astronauts would recieve on a trip to Mars burned out after a CME, so all we know is that things could be a lot worse than they were previously thought to be.

 

[Acleron]

Alpha Centauri is around 4 light years away. Going from memory of calculations done on this before, it would take a little more than 2 years to get there using with a constant 1g acceleration for the first half and then the same to slow down for the second half. According to the Australian Uranium Association, a typical 1000MW reactor requires about 25 tonnes of enriched uranium per year.

If the two year estimate is from the perspective of the passengers, how long would the return trip take from the perspective of earth?

 

[RecoveringYuppy]

I assume that 2000Mw reactor is to power the engines? Don't think there's any feasbile way for so little power to get you to Rigil in a reasonable time frame.

 

[Just thinking]

If the two year estimate is from the perspective of the passengers, how long would the return trip take from the perspective of earth?

Funny thing about this issue ... from the perspective of the astronauts, the journey would take about 2 years each way since it takes about 1 year at 1G of acceleration to get to near the speed of light (measured between ship to Earth). For the folks on Earth, the one-way journey would elapse to around 6 years. Two years for the ship to get to the speed of light (from 0 to c averages 0.5c) and travel 1 light year -- 2 more years at c (2 more light years) -- and then a final 2 years to get back to 0 velocity at your destination (the final and 4th light year). Now, what was that funny thing I mentioned earlier? Oh yes, those 2 years at c (as seen from Earth). To the travelers, they go by in a flash (maybe 2 days to 2 weeks, depending on how close to the speed of light they get). So, whether they choose to go to the nearest star, or the Andromeda galaxy, the journey to them will last around 2 years each way --- the big difference will be how long a time elapses on Earth in the meantime. The nearest star, 6 years; Andromeda 2 million years.

 

[shadron]

Don't forget, as well, that you have to have reaction mass to throw besides the fuel to heat it up, and you have to accelerate that as well, at least half of it up to relativistic speeds. Radiation is indeed a problem, as those small molecules occurring along the way will begin to cause significant radiation in themselves as they wear away the shielding, besides the cosmic and stellar sources..

 

[RecoveringYuppy]

Someone should double check my numbers but even if I've slipped in a half dozen extra zeros, a 2GW reactor isn't anywhere near the right ballpark.

A 1,000 ton ship is approximate a million kilograms. One percent of the speed of light is 3 million meters/second. So the energy contained in a ship moving at 1 percent of the speed of light is in the neighborhood of 5 quintillion joules. A 2GW reactor would take a century and a half to deliver that energy and that's before accounting for conservation of momentum which is going to require some energy also be delivered to some reaction mass somewhere.

Cuddles suggestion of accelarating at 1g for 2 years requires a force of 10 meganewtons to act over about 20 quadrillion meters (2 light years). That's around 200 sextillion joules. Several million years for that 2GW reactor to deliver that kind of energy.

 

[dudalb]

If the two year estimate is from the perspective of the passengers, how long would the return trip take from the perspective of earth?

There have been a lot of science fiction novels written about how FTL might distort time.
"The Forever War" by Joe Halderman is a good example.
There is nothing I would like to see happen more then a practical way to Go To The Stars,but I do not see it happening anytime soon,if ever.

 

[Corsair 115]

It's not even clear if manned missions to other planets are feasible at the moment. The only radiation monitor (that I know of) that tried to check the radiation dose astronauts would recieve on a trip to Mars burned out after a CME, so all we know is that things could be a lot worse than they were previously thought to be.

Seems to me the easiest way to handle it is to build into the spacecraft a "radiation shelter" which the crew can retreat to in case of increased radiation due to a solar flare or CME. They stay protected in the shelter until the radiation danger passes.

The downside would presumably be the great amount of weight such a shelter would add to the spacecraft.

 

[godless dave]

Forget about astronauts. I suspect if we ever send missions to the Alpha Centauri system, they will be unmanned. So then we don't have to worry about radiation or life support - just producing a hell of a lot of thrust.

 

[mhaze]

In the last decade we've had several space probes that collected intersteller dust in an aerogel collector, and returned them to earth. Most of these if I recall are about one micron in size. Taking the frontal area of a spacecraft and looking at the cubic volume it would sweep out on a one way or two way trip to Alpha Centauri would give an order-of-magnitude guess at how much stuff will hit that spacecraft.

Too much dust to accomodate with shields? Has this been estimated?

 

[godless dave]

We've collected dust inside the solar system, but we really don't know how much there is in interstellar space. Voyager 1 and 2 are almost there, but what we can find out with their instruments is pretty limited.

 

[dudalb]

Forget about astronauts. I suspect if we ever send missions to the Alpha Centauri system, they will be unmanned. So then we don't have to worry about radiation or life support - just producing a hell of a lot of thrust.

I don't know if a unmmaned mission for just pure scientific reasons would ever get much support.
The old saying is still true,No Buck Rogers..No Bucks.

 

[Loss Leader]

We've collected dust inside the solar system, but we really don't know how much there is in interstellar space. Voyager 1 and 2 are almost there, but what we can find out with their instruments is pretty limited.

Unless the answer to the question "What is the density dust in interstellar space?" is "Beep, beep, beep," the Voyager probes aren't really going to be able to tell us much.

 

[BenBurch]

In practical terms, the solid bits of matter in interstellar space limit the speed of any starship to around 5% of the speed of light. So 80 years is about the minimum time you can contemplate for a trip to the nearest star. However, if you are going to use NERVA type nuclear rocket, likely you cannot carry enough reaction mass to achieve this velocity; More likely 1% of the speed of light will be the speed, at which time 400 years is more like the amount of time required.

So, start thinking about building a civilization in a bottle, and sending it off to deep space; More time will elapse than the total time since the Jamestown Colony was founded.

 

[Octavo]

Alpha Centauri is around 4 light years away. Going from memory of calculations done on this before, it would take a little more than 2 years to get there using with a constant 1g acceleration for the first half and then the same to slow down for the second half.
<snip>
For example, the water, or whatever else, used as coolant become radioactive and needs replacing.

That's pretty amazingly fast. The question then becomes, how much energy do you need to accelerate and decelerate at 1g for 4years?

I'm not sure that keeping the reactor cool would be a problem on an interstellar mission. Whatever liquid is used for coolant can easily be recycled as all that is required is to cool it back down - simply pipe the coolant close enough to the skin of the spacecraft and the cold of space will do the rest.

Cuddles suggestion of accelarating at 1g for 2 years requires a force of 10 meganewtons to act over about 20 quadrillion meters (2 light years). That's around 200 sextillion joules. Several million years for that 2GW reactor to deliver that kind of energy.

Aaah... so a bit more than 2000Mw then... damn.

Too much dust to accomodate with shields? Has this been estimated?

Good question - how thick would the shielding have to be? Is there any way we could deflect dust/gas/micrometeorites from the ships path with some sort of beam, thus negating the need for most shielding?

In practical terms, the solid bits of matter in interstellar space limit the speed of any starship to around 5% of the speed of light. So 80 years is about the minimum time you can contemplate for a trip to the nearest star. However, if you are going to use NERVA type nuclear rocket, likely you cannot carry enough reaction mass to achieve this velocity; More likely 1% of the speed of light will be the speed, at which time 400 years is more like the amount of time required.

So, start thinking about building a civilization in a bottle, and sending it off to deep space; More time will elapse than the total time since the Jamestown Colony was founded.

That's pretty much the answer I was expecting - a generational ship would be the only viable solution given our current technology and even then it's a major stretch. Things like giving birth in space, artificial gravity and various psychological effects would all become major obstacles.

 

[Cuddles]

There have been a lot of science fiction novels written about how FTL might distort time.
"The Forever War" by Joe Halderman is a good example.
There is nothing I would like to see happen more then a practical way to Go To The Stars,but I do not see it happening anytime soon,if ever.

That was probably a good book. Unfortunately the edition I have is by far the worst edited book that has ever been published, and somehow managed to replace the entire middle third of the book with the first third. The start was OK, but not so great I wanted to read it twice.

Seems to me the easiest way to handle it is to build into the spacecraft a "radiation shelter" which the crew can retreat to in case of increased radiation due to a solar flare or CME. They stay protected in the shelter until the radiation danger passes.

The downside would presumably be the great amount of weight such a shelter would add to the spacecraft.

And that's really an impressively big down side. The big problem is that we really don't even know how much radiation we'd need to protect against. We know about how much radiation astronauts recieve under normal conditions, but we really don't know how bad things can get.

Forget about astronauts. I suspect if we ever send missions to the Alpha Centauri system, they will be unmanned. So then we don't have to worry about radiation or life support - just producing a hell of a lot of thrust.

I think this is pretty unlikely. The timescales are just too big for unmanned craft to be practical. The first problem is that mechanical things just don't last that long. Hundreds of years in unknown conditions with no maintanence or support? Not going to happen. The second problem is the actual research. What are the chances that we'll even remember we sent a probe hundreds of years ago, let alone be able to pick up and decode a signal? Although I think there might be a decent book there - humans send probe to nearby star, hundreds of years later the signal arrives, but since the records have long since been lost it's held up as the first proof of alien life. Finally, as Dudalb says, where's the money going to come from for a project that has absolutely no chance of achieving anything in the forseeable future, let alone while anyone alive now is still around? NASA and other space agencies have enough problems with funding as it is.

I'm not sure that keeping the reactor cool would be a problem on an interstellar mission. Whatever liquid is used for coolant can easily be recycled as all that is required is to cool it back down - simply pipe the coolant close enough to the skin of the spacecraft and the cold of space will do the rest.

OK, firstly I was a bit of an idiot in my last post. The coolant used in the reactor core is a closed system which transfers heat through a heat exchanger to another coolant system which actually produces the electricity. While the coolant in the core becomes radioactive, it doesn't ever leave the reactor and so isn't a problem (unless it leaks of course, but then you've got rather bigger problems to worry about). The problem with waste water isn't radioactivity, but simply the heat being dumped into the countryside, which is the same problem pretty much all power plants have.

That said, you've now got me thinking about something which might be more of a problem. Things are usually cooled by conduction and convection, with radiation being basically irrlevevant until you get really hot. However, in space there is no conduction of convection. Any heat produced onboard will stay onboard, and will only radiate away very slowly. I haven't done any calculations, but I suspect this would actually be quite a big problem. I know some sci-fi has covered this sort of thing, with spacecraft needing big heat sinks sticking out the side, but I don't know if even that would be sufficient. A few gigawatts is an awful lot of heat.

 

[Octavo]

And that's really an impressively big down side. The big problem is that we really don't even know how much radiation we'd need to protect against. We know about how much radiation astronauts recieve under normal conditions, but we really don't know how bad things can get.

Who cares - it's a one way trip anyway

Seriously though, once in interstellar space I would think the only worry would be from stray gamma ray bursts and the risk of flying through one would be pretty minimal I'd think.

I think this is pretty unlikely. The timescales are just too big for unmanned craft to be practical. The first problem is that mechanical things just don't last that long. Hundreds of years in unknown conditions with no maintanence or support? Not going to happen.

Quoted for truth!

Any heat produced onboard will stay onboard, and will only radiate away very slowly. I haven't done any calculations, but I suspect this would actually be quite a big problem. I know some sci-fi has covered this sort of thing, with spacecraft needing big heat sinks sticking out the side, but I don't know if even that would be sufficient. A few gigawatts is an awful lot of heat.

I'm assuming heat wouldn't radiate away very well because there aren't very many particles in the vacuum of space to transfer the heat energy to, right? I hadn't thought of that before. I always just read that space was really really cold and thus assumed that reactor cooling would be one problem you *wouldn't* have in space. Guess not.

 

[PixyMisa]

We has a discussion similar to this a few month back. The most practical method I can think of is a multi-stage operation that would take a couple of centuries to set up.

Carrying all your fuel with you is impractical, so instead you start by building a huge solar-powered orbital laser array. (This is good, because once you have your huge solar-powered orbital laser array, funding for the rest of the project is assured.)

You use this laser array to drive the solar sails on a fleet of hundreds of small robot craft, first exploration and survey robots, and then following a few years behind them, mining and construction robots. These will be pretty slow, just a few percent of lightspeed, because they will need to carry the fuel with them to decelerate.

Once they arrive at their destination, the mining and construction bots (guided by information already received from the first wave of explorers) deploy to whatever handy asteroids, comets, or small moons there might be lying around, and start building another huge solar-powered orbital laser array.

And once that's ready, you can step aboard your big comfy spaceship and zip off to Alpha Centauri in just, oh, eighty years or so. You don't need to carry a big reactor with you, because all the power to accelerate and decelerate is provided.

Easy!

 

[Cuddles]

Who cares - it's a one way trip anyway

Seriously though, once in interstellar space I would think the only worry would be from stray gamma ray bursts and the risk of flying through one would be pretty minimal I'd think.

In interstellar space the risks would certainly be a lot less. The problem is that geting to interstellar space in the first place means spending a fair bit of time in stellar space, and again at the other end. CMEs aren't amazingly common, so you'd probably manage to get out without much problem, but the risk is there and we really don't know enough about how much risk there is.

The other problem is that even if we figure everything out at this end, there is no guarantee the stars at the other end will behave in the same way. To start with, it's a twin system (ignoring Proxima which is too small and too far out to have much effct), which will certainly influence the dynamics of the two stars. That also brings the problem that you can't just hide on the dark side of a planet, since with two stars there is no guarantee there is a dark side. We also don't know if there would be planetary magnetic fields to protect people once they arrive, or even if there are any planets at all. Finally, although we think the Sun is a pretty standard star, since we've never visted any others we really can't be sure. It's entirely possible that other stars are much more active than the Sun, and then any shielding which worked here would be useless at the other end of the trip. Of course, it's also possible that other stars are much less active, but it wouldn't be a great idea to count on it.

I'm assuming heat wouldn't radiate away very well because there aren't very many particles in the vacuum of space to transfer the heat energy to, right? I hadn't thought of that before. I always just read that space was really really cold and thus assumed that reactor cooling would be one problem you *wouldn't* have in space. Guess not.

Well, the radiation won't depend too much on the number of particles. The problem with there not being any particles is that there isn't any conduction, which is by far the most important way of losing heat (convection relies on conduction to transfer the heat to the fluid in the first place). Space is cold, in the sense that the average energy in the radiation in it is equivalent to about 3K. This means that the craft will absorb much less heat from radiation it would on Earth, so the radiative cooling will be more effective. This almost certainly wouldn't be enough to offset the absence of conductive cooling though. It's also worth bearing in mind that 3K is the average background temperature, the temperature near a star will be much higher, and so cooling will be that much more of a problem at the start and end of the trip.

Edit: For comparison, spacesuits can deal with about 600W of heating. That's just for the body heat from one person.