Hellbound
Merchant of Doom
There are over a hundred or so within, say, 20 light years radius (IIRC, been a while since I examined the data). The closest (Proxima Centauri) is about 4 light years.
Metaphysics vs. Atheism
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There are over a hundred or so within, say, 20 light years radius (IIRC, been a while since I examined the data). The closest (Proxima Centauri) is about 4 light years.
Yes, it will. More accurately, the speed of light combined with the energy budget needed.
Could you (or someone else) elaborate on that or us please? Why are we unlikely to be able to afford the energy costs of traveling to nearby star systems in the next few hundred years?
Psiload
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You'd need to travel very fast in order to get to even the nearest star system (4.3 light years away). Even travelling at the speed of light (a physical impossibility) it'd still take you 4.3 years to reach the nearest star system.
And this will give you some idea of the energy required to do even that...
http://en.wikipedia.org/wiki/Faster-than-light
Are you getting the picture?
Hellbound
Merchant of Doom
Psiload:
To be fair, I think we should take a more realistic scenario.
Let's assume it's a manned mission. Because of this, we'll make a few "design specifications" to our craft.
Maximum acceleration: 1G. We don't want to crush our astronauts or make them too uncomfortable during the trip.
Minimum Acceleration: .1G I can't recall, but I believe about this level is the minimum needed to prevent bone/joint deterioration. We want them to be physically able to explore, or simply walk when they return.
Size: This is difficult. Most hypothetical designs I've seen for actual interstellar craft weigh in about 10 tons (IIRC, this was for a very small crew). So we'll use that as our weight.
Distance: 4.3 light years.
Travel Characteristics: We can do three calculations. First, we'll see about accelerating halfway, then deccelerating the rest, at a minimal acceleration (say speed = .9c at the halfway point). Then, we'll see about full acceleration to halfway and full deccel to end (1G), again with a max cruising speed of .9c. Finally, we'll do a minimal expenditure calculation using our minimum acceleration (.1G) and see where that takes us.
I'll work on these figures, but if someone with more know-how wants to run with them to figure energy requirements, feel free. I have to look up some data and equations, so I might be a while.
To be fair, I think we should take a more realistic scenario.
Let's assume it's a manned mission. Because of this, we'll make a few "design specifications" to our craft.
Maximum acceleration: 1G. We don't want to crush our astronauts or make them too uncomfortable during the trip.
Minimum Acceleration: .1G I can't recall, but I believe about this level is the minimum needed to prevent bone/joint deterioration. We want them to be physically able to explore, or simply walk when they return.
Size: This is difficult. Most hypothetical designs I've seen for actual interstellar craft weigh in about 10 tons (IIRC, this was for a very small crew). So we'll use that as our weight.
Distance: 4.3 light years.
Travel Characteristics: We can do three calculations. First, we'll see about accelerating halfway, then deccelerating the rest, at a minimal acceleration (say speed = .9c at the halfway point). Then, we'll see about full acceleration to halfway and full deccel to end (1G), again with a max cruising speed of .9c. Finally, we'll do a minimal expenditure calculation using our minimum acceleration (.1G) and see where that takes us.
I'll work on these figures, but if someone with more know-how wants to run with them to figure energy requirements, feel free. I have to look up some data and equations, so I might be a while.
Consider the energy costs of accelerating a 1kg object to 1% of light speed (which would be necessary, at a minimum, to get to a nearby star-system in the "next few hunded years." ) (I get about 4.5 times 10^12 joules.)
Assuming we use a perfectly-efficient pinpoint fusion reactor (which could get about 3x10^8 J out of a single kg of fuel), we'd still need to use 100kg of fuel to get that much energy. And we'd need to carry that much fuel with us in order to decelerate at the end, so we'd need 100 times as much fuel as payload, and we'd need fuel to move the fuel, and so forth.
And these calculations don't take into account reaction mass!
Not yet, because I don't think anyone is positing that the exploring vessels would travel at or close to the speed of light.
Let's say the vessel to the nearest star system travels at 1/5 the speed of light. So that the trip each direction takes 21.5 years. What are the best estimations of the energy costs?
Of course, if we substantially increase human lifespan in the next several hundred years, longer voyages could be feasible. So how about the energy costs of going 1/40th the speed of light? The vessel might depart 100 years from now, and reach the nearest star system in 172 years (if my math is correct) -within our "several hundred year" timeline. Let's say it carries a minimal human crew: 1 or 2 passengers.
Finally, we could send nanorobotic craft, which I think would greatly reduce the fuel needs. But I don't think that's in the spirit of the original speculation that we'll be exploring nearby star systems in a few hundred years. I think he/she meant human exploration. Because technically we're exploring star systems now with telescopy.
Thanks. Awesome.
(full disclosure, I think this thought experiment has been done multiple times before, by Carl Sagan and within science fiction. But I'm still curious as to the best informed results of it).I don't think you need to do this much work. Just calculate the maximum speed the ship will hit in mid-flight, then use 1/2 m v^2 to figure out how much kinetic energy it would be packing at that point.
Using your numbers and a local envelope, I get a cruising speed of 270,000,000 m/sec and a mass of 10000kg, giving me 3.645 x 10^20 Joules. I don't know what you're planning on using for rocket fuel, but if it's something like hydrogen, that's "only" about 10^18kg of hydrogen. To put that in perspective, that's something like a hundredth of a percent of the mass of the moon.....
And then remember that the spacecraft not only has to gain that velocity, but it has to lose it as well at the end of the trip.
Hellbound
Merchant of Doom
Oh, cool!
I found answers for my 1G acceleration speed. See here.
To pull relevant portions:
The time for the trip with 1G accel is 3.6 years on-board the ship.
The fuel needed is 38kg per kg of payload (assuming 100% efficiency, all matter to energy). That's a significant amount of energy. Using E=mc2 it works out like so:
E=(38kg)*(300,000,000m/s)*(300,000,000m/s)
E=3,420,000,000,000,000,000kgm2/s2 with is Joules
So, that's 3,420 TJ of energy, per kg of our ship.
So for our 10 tons (about 9000kg), we'll need (9000*3,420) 30,780,000 TJ.
Now, from the USGS[/url, the U.S produces 72.9 Quadrillion BTUs per year. That works out to 76,909,500 TJ (1055 Joules/BTU).
So, we're looking at about half of the energy production in the U.S. for an entire year, in order to get this thing to the nearest star. This does not include the return trip, either, so we'd have to double it.
I found answers for my 1G acceleration speed. See here.
To pull relevant portions:
The time for the trip with 1G accel is 3.6 years on-board the ship.
The fuel needed is 38kg per kg of payload (assuming 100% efficiency, all matter to energy). That's a significant amount of energy. Using E=mc2 it works out like so:
E=(38kg)*(300,000,000m/s)*(300,000,000m/s)
E=3,420,000,000,000,000,000kgm2/s2 with is Joules
So, that's 3,420 TJ of energy, per kg of our ship.
So for our 10 tons (about 9000kg), we'll need (9000*3,420) 30,780,000 TJ.
Now, from the USGS[/url, the U.S produces 72.9 Quadrillion BTUs per year. That works out to 76,909,500 TJ (1055 Joules/BTU).
So, we're looking at about half of the energy production in the U.S. for an entire year, in order to get this thing to the nearest star. This does not include the return trip, either, so we'd have to double it.
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Hellbound
Merchant of Doom
Yeah, I was going to get to fuel later. And I was trying to consdier both acceleration and decceleration. Found a few figures that give numbers for to and from Proxima, so I posted that.
Also, don't forget that relativistic effects come into play, which was the part I was going to be looking up.
Awesome. I don't have the time or energy to recreate your work and check your numbers (nor do I have the aptitude to do it quickly). But if those numbers and calculations hold up, it seems quite possible and achievable to do several hundred years from now. Even if our energy conversion ratio is significantly lower than 50% and more in the realm of the best energy conversion ratios we can achieve with today's technology.
In fact, your work seems to imply that we could send a human to the next closest star with today's technology, although it would require massive social investment. Are their any current technological barriers to prevent us from doing so?
Sidenote: I forgot about the difference between on board ship time and time on Earth, so I think my previous comments about human aging are pretty irrelevant.
Hellbound
Merchant of Doom
Actually, I miscalculated, because the 10 ton figure for craft weight was for crew compartments and stores...otherwise, you have the weight equivalent of two people for your payload.
I've recalculated with the 10 ton figure as the payload weight, which is more reasonable. You need crew quarters, 8 years or so of food (or means to produce it artificially, still not cheap), water, medicines, exercise areas, etc.
That sounds like a lot, but it may not be prohibitive. To continue the parallel example to the energy used by the U.S. each year, what percentage of the mass of the moon does the U.S. use in energy each year?
okay, looking forward to your fleshed out appraisal of these factors.
Fuel is the big killer, though.
The United States happily burns through zillions and zillions of Joules of energy each year, mostly by burning millions of tonnes of fuel from a huge geological tank that's been accumulating solar energy since time immemorial. The mere energy isn't a problem -- but packing it up for transportation is.
I don't believe that "total conversion" matter/antimatter power generation will be practical, now or ever, and there's literally nothing else in theory that will even come close to making a practical fuel source; even if we could bind protons directly to make helium nuclei, that achieves less than 1% of the energy density of total conversion -- "the best energy conversion ratios we can achieve with today's technology" is about 0.01% of TC.
The other killer problem is simply reaction mass. Merely having "energy" doesn't help unless you've got mass to move in the other direction (another reason I don't think that matter/antimatter drives will ever happen). If you're going to assume the development of a reactionless thruster, why not simply assume fairies riding warp speed unicorns? And if you're going to use reaction mass for deceleration -- how are you carrying it?
Hellbound
Merchant of Doom
I corrected them in the original post. I did not know I'd already been quoted before I began editing in the correction. It works out closer to 40-50% the energy cost.
And, as drkitten has noted, I did not take any account of reaction mass. THe figures I've made assume the absolute best technology (impossible technology, actually, since no process can be 100% efficient), so this is an absolute limit on the amount of energy needed, under the best possible conditions of technology.
As drkitten said, with anything in current or foreseeable technology, when you add in actual efficiency, fuel costs, and reaction mass, that figure is going to skyrocket. Because if you add, for example, 1 ton of reaction mass, you need 38 more tons-worth of energy to push it. I'd expect actual figures to require much, much more than this (at least a factor of one hundred) for anything in the foreseeable future.
Also, consider that even assuming antimatter is used, you have another problem. Conversion of energy into antimatter is, at best, 50% efficient. Any creation of matter from energy creates equal parts of matter and antimatter. So even before we consider reaction mass and other factors, we've doubled our energy requirement again.
Kaarjuus
Critical Thinker
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According to these stats, world oil production in 2005 was 84,361,000 barrels per day. The weight of oil depends on its source, but for about 8 barrels per tonne, this makes 3848970625000 kilograms per year. So, about 3.85×10^12 kilograms. The weight of the moon is about 7,35×10^22 kilograms. So the moon has about 2×10^10 times greater mass than the oil produced in 2005 in the whole world. So a lot more than the hundredth of one percent.