Talk:Interstellar travel/Archive for 2009

Travel by humans vs. travel by robots

Interstellar travel by humans is a fantasy. Humans will be replaced by robots long before interstellar travel becomes possible.

I don't think that's a consensus opinion, you haven't stated any reasons why you think that's the case, and interstellar travel

by robots or extra-terrestrials is worth discussing in any case. Robert Merkel

The question of the future evolution of the human species (pure biological, robot, or some kind of hybrid) is very interesting, but essentially an independent issue. Of course "The Nature of Man" and the future of human civilization affects most of the articles in this encyclopedia, but we cannot let be more than an interesting footnote to this article, or we'll go way off track. IMHO. Wwheaton (talk) 21:33, 2 March 2009 (UTC)

Sub-light travel section

This whole section appears to be wrong according to this website

http://www.nasa.gov/centers/glenn/research/warp/scales.html

As it states, no method for sub light travel, even antimatter would accelerate you that fast without vast amounts of fuel, not sure about solar sails though. The snare 07:30, 2 June 2007 (UTC)

Interstellar Travel

If we suppose that we eventually have the ability to harness enormous resources, but do not uncover new laws of physics, then it will always take individual humans years to travel between the stars. The problem is that we can't accelerate faster than our bodies can survive. So, if we assume that the passengers want to experience the journey at an acceleration of 1 g, then how much travel time do they experience on an interstellar journey?

The difficulty that we have to work through is that the traveler isn't in an inertial frame of reference. That is, v keeps changing. The traveler starts at rest and undergoes a constant rate of acceleration g (in the traveler's frame of reference). What is the traveler's velocity (relative to the original frame of reference) at any time?

Let's define some coordinates. The position of the traveler in the original frame of reference is (x, t). (Here I'm using "position" to refer to both space and time.) The velocity of the traveler as measured in the original frame of reference is v. (The traveler sees the same velocity, but has a different distance scale...) The cumulative elapsed time that the traveler has experienced is τ. We want to define the relationship between these coordinates. To do so, we define two additional sets of coordinates: The coordinates in the traveler's inertial frame of reference are (x1, t1). The traveler doesn't really have an inertial frame of reference, since he/she is accelerating constantly, but this is the inertial frame that the traveler would be in if the acceleration were turned off briefly. The traveler is at position x1 = 0. When we envision turning off the acceleration briefly, we will take that moment to correspond to t1 = 0. At that moment, we will also want to consider another set of coordinates (x0, t0) in the Earth's inertial frame of reference. These coordinates are defined by the Lorentz transformation:

(1) x1 = γ (x0 – v t0) t1 = γ (t0 – v x0 / c2)

x0 = γ (x1 + v t1) t0 = γ (t1 + v x1 / c2)

where γ = √[1 – v2 / c2] –1 There will be an offset between x and x0 and between t and t0, but while we imagine the acceleration to be turned off, the offest is constant. That is, dx = dx0 and dt = dt0. Similarly, there is an offset between τ and t1, but dτ = dt1.

To make the physics perfectly clear, let's consider the acceleration to be a series of discrete boosts in velocity. That is, the traveler instantaneously shifts from the frame (x1, t1) into another frame of reference (x2, t2). The change in velocity dv1 happens at intervals dτ, such that dv1 = g dτ = g dt1. So, the relative velocity of the coordinate system (x1, t1) with respect to the coordinate system (x0, t0) is v, and the relative velocity of the coordinate system (x2, t2) with respect to the coordinate system (x1, t1) is dv1, but what is the relative velocity of the coordinate system (x2, t2) with respect to the coordinate system (x0, t0)?

We find

(2) x2 = γ1 (x1 – v t1)

=  γ1 γ (1 + v dv1 / c2)   [  x0  –    (v + dv1) c2 

---

c2 + v dv1 t0 ]

t2 = γ1 (t1 – v x1 / c2)

=  γ1 γ (1 + v dv1 / c2)   [  t0  –    v + dv1 

---

c2 + v dv1 x0 ]

where γ1 = √[1 – (dv1 / c)2] –1 Now we can recognize that this must also be a Lorentz transformation. Therefore, we must have

(3) v2 = (v + dv1) c2

---

c2 + v dv1

γ2 = γ1 γ (1 + v dv1 / c2)

where γ2 = √[1 – (v2 / c)2] –1 A little painful algebra verifies that the third equation follows from the first two. If dv1 is actually an infinitesimal change in velocity, then we only care about the first order term.

(4) v2 ≈ v + (1 – v2 / c2) dv1 dv = (1 – v2 / c2) dv1

Since dv1 = g dτ, we can integrate to find the velocity at any time τ.

(5) ∫ dv

---

1 – v2 / c2

  =   ∫ g dτ 

 

c tanh–1 (v / c)

  =  g τ 

 

v

  =  c tanh (g τ / c) 

This gives the velocity in terms of the traveler's elapsed time. From here it is a simple process to integrate this equation and find the position as a function of time. It is not, however, a trivial process, because the velocity is not equal to dx / dτ. The velocity is equal to dx / dt, so we need to determine the relationship between dt and dτ before we can integrate. We want to consider this relationship from the point of view of the traveler at x1 = 0, since that is where Equation 5 applies. Looking at Equation 1, we see that dt0 = γ dt1. Since dt = dt0 and dτ = dt1, we find

(6) dt = γ dτ

Now we can use Equation 5 to write γ = cosh(g τ / c), and it is trivial to integrate Equation 5.

(7) ∫ dx = ∫ v γ dτ

x = ∫ c sinh (g τ / c) dτ

=  (c2 / g) [cosh (g τ / c) – 1] 

This is very close to the formula that we want. We want to know the value of τ when the traveler has made it halfway to the destination, because then the deceleration starts. If the total distance is X, then the total travel time T is given by

(8) X / 2 = (c2 / g) [cosh (0.5 g T / c) – 1]

T = (2 c / g) cosh–1 (1 + 0.5 g X / c2)

If X = 4.3 light-years, then T = 3.6 years. Dozens of stars could be reached in five to six years. In fact, a traveler could even go the Andromeda galaxy in under 29 years if a constant acceleration could be maintained.

steve

That's all interesting, but your bit about LINAC still has no sources to allow verifiability. Please provide a reliable source before adding that information again. PubliusFL 18:39, 18 August 2007 (UTC)

Well, that math is definitely over my head, I think wikipedia is supposed to be written in layman's terms, you might want to submit that to a scientific journal instead, I have no idea how right or wrong it is, but........are you talking about the perceived trip, correct? We can't get to "dozens" of stars in a few years with sub-light speeds The snare 02:52, 19 August 2007 (UTC)

How do we get to Andromeda in 29 years? It's over 2 million light years away. I understand what you're saying about time dilation, maybe you would perceive the trip as 29 years, while in reality millions and millions of years would have passed. But, like I've been saying, we can't get anywhere near lightspeed in order for it to kick in enough with the huge fuel requirements the link above references. and to accelerate constantly we'd need fuel ALL the way there, look how much it takes just to get to Alpha Centauri. The snare (talk) 03:18, 16 February 2009 (UTC)

Very unlikely danger

There is a great difference between what is possible and what is probable. User:AfDproXX is concerned about collisions in interstellar travel, due to later, faster ships overtaking older, slower ones. Of course ITC (Interstellar Traffic Control) could be invoked to avoid such dangers, and it would be nice to hope the traffic could be heavy enough to make it advisable someday. But consider the size of space and the odds before worrying about it. Two fairly large ships might collide if they passed withing a km if each other, center-to-center. But let us make them 100X larger, so 100 km of clearance would be needed. And let us suppose further that the target is a planet orbiting its star as the same distance as we orbit the Sun, which is close to 150,000,000 km ( = 1 AU). Then the faster ship might reasonably be expected to pass within 1 AU of the first, assuming random lauch trajectories just designed to get the ships to the target star and planet. The area of the target ship is π×100² square km, ~ 31,500 sq km. The area of the 1 AU circle the following ship might pass through would be on the order of π×150000000² sq km. The ratio of these two numbers is 1/1500000², which is 1 in 2.25 million million, which works out to 0.00000000000044 or there abouts. This number is small enough that we do not need to worry about it. To put it another way, if we did the experiment 2.25 trillion times we might expect expect one collision. Actually since stars move and the ships would be launched and arrive at different times, the risk would be even less, or zero. There are other much more serious dangers, like wandering interstellar dust and debris, or dwarf planets that must surely exist out there. Wwheaton (talk) 06:54, 6 February 2009 (UTC)

Bussard ramjets

Something on Bussard ramjets, as a possible way of getting to high enough speeds that time dilation comes in?

unfortunately, it turns out that the density of interstellar hydrogen is an order of magnitude less than it was thought to be when Bussard first proposed the design. But I guess it should still be mentioned, as an idea which didn't pan out. BD

another problem with the current text: as the reaction providing the energy is the proton-proton reaction converting less than 0.1% of mass, the problem is essentially the same as for a fusion rocket engine, max speed at most 0.1c. —Preceding unsigned comment added by 88.195.199.91 (talk) 12:38, 12 April 2009 (UTC)

I am no friend of the Bussard ramjet concept (which I think absurd in practice), but the fundamental energy limitation that prevents rockets from reaching relativistic speeds does not apply to them, because they take their energy from the interstellar medium, not from fuel carried along. Thus the rocket equation does not apply. Their problems have to do with the mass and drag involved in the vast (? magnetic?) input scoop, and the problems of the losses involved in compressing and burning ¹H effectively to produce thrust, which entail mind-boggling engineering difficulties. Wwheaton (talk) 15:54, 12 April 2009 (UTC)

Motivation for interstellar travel

I think this article definitely needs a chapter about the motivation for interstellar travel. I'd be happy to add it myself, but I've been unable to find good sources which clearly say why we should launch interstellar missions. For unmanned missions, the intuition is that a probe would be able to gather information from a star system that normal methods of observation could not. This is implied in passing in the following article: "If the astronomers succeed in detecting a planet there, it will be a scientific bombshell -- and it will raise the question of how we might someday send a probe to get a closer look."[1]. But does anyone know of a source which says what exactly the benefits of using a probe to normal observation would be? Offliner (talk) 14:55, 12 June 2009 (UTC)

Gravitationally isolated space-time

Section Gravitationally isolated space-time is unsupported as it stands and vulnerable to deletion at any time. Note that the wikilink'ed SlipString Drive is also completely unreferenced and therefore highly vulnerable to deletion if not soon supported with reliable sources. Wwheaton (talk) 17:43, 28 May 2009 (UTC)

No references having been supplied, I have deleted this section. Wwheaton (talk) 00:32, 15 July 2009 (UTC)

Paragraph headings

Some random headers for paragraphs I will write later if no-one does it first.

possibility of new physics allowing faster than light travel

possibility of reaching other stars by travelling slower than light, faster version

and slower version

I would recommend a graph or two to summarize the issues, rather than whole sections. Then you might want to wrap a section around a really good graph. Wwheaton (talk) 08:32, 27 July 2009 (UTC)

Possibility of interstellar travel (2009 thread)

The impossibility of human interstellar is established by the energy required. As an example, a calculation based on the kinetic energy equation shows that the energy required to accelerate the International Space Station to a velocity which would give a 20-year trip to the nearest star would be equivalent of almost 40,000 thermonuclear bombs, regardless of the propulsion system. I think this makes the whole article irrelevant except for a discussion of voodoo physics which might apply.Paulkint (talk) 15:57, 25 July 2009 (UTC)

It's a sliding scale. If you're willing to have a trip that takes thousands of years, you can do it with a nuclear-electric drive with off-the-shelf parts (delta-v of 600 km/sec would give you one light-year per thousand years during flight, and ability to stop at the destination). If you want the trip to take decades, on the other hand, you need a specific energy of about 5e+14 J/kg (using a cruising speed of 10% C, an exhaust velocity of 10% C, and a craft that's about 90% fuel which is also used as reaction mass). This is about the right ballpark for fusion. For technologies that don't require future breakthroughs, nuclear reactors get about 2.4e+13 J/kg, and fission bombs get about 8.8e+13 J/kg (from Energy density (edit | talk | history | protect | delete | links | watch | logs | views)). In both cases, you can build systems that put most of their reaction energy into moving their reaction products. Assuming a craft that's 90% fuel, these power sources give you 2.3% C and 4.4% C, respectively. A bomb-driven Orion-style craft, using the second value, would take about 23 years per light-year, which is fast enough to be useful for generation ships and probes. A staged craft that was 99% fuel could make a trip to the nearest stars within a human lifetime.

For sailcraft, the energy cost is higher, but not unfeasibly so. A light-sail gets a power-to-thrust ratio of about C, requiring about 9e+15 J/kg to send a probe on a flyby trajectory at 10% C. A particle beam sail could in principle get much better efficiencies (comparable to the specific energy needed for carrying your own fuel, but without having to carry your own fuel), assuming you can keep the beam focused over the boost distance.

Now that I've shown the math for the energies required, I'll address your statements about these energies being unfeasible. Uranium is about as common as lead. Thorium is even more common. If you're running a fission reactor on the ship, you have no shortage of fuel. Assuming a generation-ship craft dry-weight of 1000 tonnes, you'd need 10,000 tonnes for a half-speed craft and 100,000 tonnes for a full-speed craft (cruising speed expressed as a fraction of the exhaust velocity). This is not cheap, but certainly feasible (you'd use a breeder reactor, so you can process the same depleted uranium that's being used for bullets and armour). For a bomb-driven craft, you need to process about 150 times as much unenriched uranium as you want U235 fuel (abundance is 0.7%). This is more expensive, but still feasible (it's the price of going faster).

For a light-sail, a 1-tonne probe would require about 9e+18 J; say 1e+20J at a 10% wall-plug efficiency. This is about 3 TW-years. The electrical energy production capacity of the world (from World energy resources and consumption (edit | talk | history | protect | delete | links | watch | logs | views)) is a bit more than 1 TW (and 5-10 times this counting fossil fuel that's burned directly for power). Given that the cruise time of a 10% C probe is decades, a boost time of several years is tolerable. Total cost, at 10 cents per kWh, would be about $3 trillion. This is high, but not impossible. This is a high estimate, as electricity is considerably cheaper than this, and with a multi-$trillion budget, you'd spend some of it making more efficient lasers (off-the-shelf industrial CO2 lasers are 10%-20% efficient; diode lasers are much more efficient than this, but are expensive to scale up). A particle beam sail would have up to 10x lower electricity cost, but would be more expensive to develop (the beam is harder to collimate than a laser).

Long story short, interstellar probes and ships are buildable; just expensive. The real constraint to worry about isn't total energy required, but power to weight ratio of the systems that have to handle that energy (having enough delta-v to get to 10% C doesn't help if it takes you a thousand years to accelerate to that speed).

This doesn't mean that anyone's going to build an interstellar probe any time soon. My point is that we have the _ability_ to without any major breakthroughs or drastic reductions in price being required.

I hope this information addresses some of your concerns. --Christopher Thomas (talk) 20:10, 25 July 2009 (UTC)

We are roughly in the situation now w/r interstellar travel that we were w/r to local (ie, Solar System) space travel 150 years ago. Some could see then that travel to the Moon was at the edge of the possible. (A debunker had fun calculating the rocket size needed as being roughly the size of Mt. Everest, which was probably correct for his assumption of black powder as the propellant.) Today we can see that large (? aircraft-carrier size) ships traveling at ~10% of c are similarly at the edge of the possible, with no technical magic required. The trouble is, technology changes so fast, and so unpredictably, that a mission design today would surely be obsolete long before the voyage could be completed. So we are not yet at the point where such a project can be rationally proposed, and really don't know when we will be. Thus we think about the possibilities, and watch the engineering realities as they change.

Re power to weight & thrust to mass ratios, I agree completely. A possibly useful rule of thumb is that 1g acceleration gets you up to roughly c in 1 year. So 0.01g gets you to 0.1c in 10 years, very nearly. I would envisage a 10-year acceleration phase at that rate, 50 years coasting, and 10 years to decelerate, for a 6 lt-yr trip. The numbers you get for such thinking highlight the fact that the power-to-mass ratio of the propulsion system is really the critical issue for us be thinking about now.

Today my standard guess is "about one thousand years from now, maybe less" for when the first mission will arrive at its destination. A daunting interval, but not vast even on the scale of human history. In 1860 I might have thought roughly the same about the Moon. As it turned out, that was quite a bit less than a millennium. So I think we are wise to stress the difficult realities for those who do not understand them and expect to reach the stars in fifty years; but not foolish to keep chewing on the problem as a realistic aspiration for the longer term future of the human species. Wwheaton (talk) 08:26, 27 July 2009 (UTC)

I find it's most useful to pick an exhaust velocity (or specific energy), and a power-to-weight ratio, and then find the resulting acceleration and delta-v, rather than starting with a desired acceleration and delta-v and working back to the other two. Every so often you find a thread (here or elsewhere) that says a mass fraction of 1e+6:1 or what-have-you is needed, when you can take the same system, reduce the cruising speed by a factor of (in the 1e+6 case) 3, and get a reasonable mass fraction.

Useful numbers for comparison for power to weight ratio are about 1 kW/kg for internal combustion engines, and about 1 to 10 MW/kg for jet turbines. Higher values exist, but as a ballpark, this works. Sail power to weight ratios depend on how thin you assume you can make it, but at 0.1 kg per square metre and 3000 K operating temperature, you end up with about 100 MW/kg for a light-sail craft that had most of its weight in the sail. The light-sail value gives you an acceleration of about 0.3 m/s2, and the 10 MW/kg ratio gives you an acceleration on the order of 10m/s2 for a 0.1% C exhaust velocity and modest fuel fraction, assuming the non-fuel part of the ship is mostly engine. In both cases, practically achievable accelerations will be lower. --Christopher Thomas (talk) 21:22, 27 July 2009 (UTC)

You are right that the limitations of interstellar travel based on rockets is basically defined by the Tsiolkovsky rocket equation which is: ship velocity = reaction velocity x the log of (1 + reaction mass/ship mass). A ratio of ten gives a value log (11) = 2.4 which means the maximum ship velocity is 2.4 x reaction velocity. I think you wrote that the reaction velocity attainable with nuclear propulsion is 300,000 m/s (compared to that of present space rockets, around 5,000 m/s!) which I calculate would give a travel velocity of 720,000 m/s, resulting in a 4000 year round trip to Alpha Centauri. Increasing the mass ratio to 100 would just cut the trip to around 2000.years.since the log of 100 is only 4.6 But increasing the launch weight of the Space Station by a factor of 100 would give a launch weight of 25,000 tons. Considering that the weight of a battleship is 35,000 tons or so, that would pose a considerable launch problem. Actually, unless the reaction velocity can be increased by a factor of hundred or so, it seems me that interstellar travel by even nuclear powered rockets is impossible.

That seems to leave only solar sails, powered either by the sun or a earth based laser. Here I think energy requirement becomes germane, for to achieve 10% light velocity, assuming the mass of the Space Station , the energy required is some 24-billion TNT-tons or that of 1.2 million 20-kiloton fission bombs I cannot visualize this accomplished by photons. Note that the trip would still take some 90-years.

Further, especially in the case of the laser, how does the ship shed this energy at its designation, land for exploration, and then reinstate it for the return trip? Especially of concern is shedding that energy upon return before it enters our atmosphere. Failing to do so would obviously have very unpleasant consequences.70.144.105.158 (talk) 12:54, 27 July 2009 (UTC)

I think it parochial to consider 35,000 tons a truly insurmountable problem. As has been said, we have at our disposal now the means (300 km/s, if you like) to colonize nearly the entire Solar System (excluding of course the rather substantial exceptions of the surfaces of Jupiter and the other giants, and of Venus). The asteroids in particular make billions of tons plausibly accessible, on a time scale likely to be short compared to that separating us from the great terrestrial explorations of the 16-18th centuries. I agree that light sails, either solar or laser assisted, seem inadequate to solve the core difficulties, though they may be useful as parts of a larger scheme. I see no way that this program can be accomplished in anything like a traditional human generation of 25—30 years; we have to reconcile ourselves to the long-term view (barring unlooked-for help from fundamental new physics). One thing that I think has received inadequate attention is the possibility of useful and interesting destinations nearer than the closest stars, say in free-floating Jupiter/Europa-like systems or Plutino-like objects. Deep IR surveys (especially proper-motion surveys), should reveal (or limit) such possibilities fairly soon, and large (? Texas-sized!) objects escaped from the edges of other solar systems really must be fairly common floating in interstellar space. Not too cozy a prospect maybe, but like the first life emerging from the ancient ocean to colonize the land, we must be prepared to make use of what we find. Cheers, Wwheaton (talk) 22:39, 27 July 2009 (UTC)

300 km/sec was the exhaust velocity I quoted for nuclear-electric rockets built with off-the-shelf reactors and electric thrusters, with easily-attainable power to weight ratios. The actual maximum exhaust velocities are far higher (2.3% C and 4.4% C for breeder reactors accelerating their waste, and U235 bombs, respectively, using specific energy values quoted above). For cruising speeds, I assumed a rocket that was about 90% fuel, with total delta-v of about twice its exhaust velocity. In principle, D+D fusion can get exhaust velocities a few times this, but for purposes of this thread I'm sticking with technologies that we have in-hand. --Christopher Thomas (talk) 21:04, 27 July 2009 (UTC)

Regarding slowing a sail down, Forward proposed a scheme where the main sail keeps moving, but the craft deploys a smaller secondary sail for decelleration. The main sail reflects laser light back at the secondary sail. In practice, this imposes serious design challenges (laser drive distance has to be tens of light-years rather than one light-year or less, and the main sail has to be a well-behaved optical surface rather than a scattering reflector). That's why I proposed sailcraft only as probes above. Any manned trip via sailcraft would likely be one-way, with the expectation that the colonists would build the infrastructure needed to send ships back at some point in the future.

If you have a Forward-type decelleration scheme, and don't want humans to stay in the target system long, you could use the main sail to give a 2x cruising velocity boost rather than 1x. This would send the ship back towards Earth, rather than stopping it in the target system, but you'd have very little time to do research there (so I'm not sure why you'd send a manned ship vs. just an unmanned flyby probe in that case). On return to Earth, you'd use the original launch laser to slow down the returning sailcraft. --Christopher Thomas (talk) 21:29, 27 July 2009 (UTC)

Regarding launch weight, 35000 tonnes would cost about $350 billion at current launch prices ($10,000/kg for unmanned cargo launches to LEO). This is much less than the cost of electricity needed for a sailcraft.

Regarding sailcraft power requirements, I remind you that world electrical production is about 1 TW. This gives you the equivalent of a 1 kT bomb every 4 seconds, or about 8 million kT per year (per TW of power generation built for the project).

This kind of infrastructure is expensive, but well within the bounds of what could be built in a relatively short timeframe if we wanted to. --Christopher Thomas (talk) 23:31, 27 July 2009 (UTC)

I initiated this discussion because I thought that the “Difficulties of Travel” section in the main article should include a discussion of the required energy for interstellar travel for that seems to me is a basic measure of the difficulty. I propose something like this to follow the first paragraph of “Difficulties of Travel”

Another measure of the difficulty is the energy which must supplied to obtain a reasonable travel time. This is primarily a function of the velocity since the required energy goes up as the square of the velocity based on the fundamental kinetic energy equation e = ½mv^2 . The required velocity for a lifetime round trip to the nearest star is thousands of times greater than the present velocities which have been achieved for space travel. This means millions times as much energy must be supplied as now supplied to such vehicles as the Space Shuttle by chemical rockets . This energy is well beyond their capability and new methods of propulsion not yet developed would be required.

I apologize for introducing such subjective judgments as “impossible” and getting argumentative which have no place in an encyclopedia. Let me know your thoughts. Paul KintnerPaulkint (talk) 17:33, 28 July 2009 (UTC)

Looks good; by all means update the article text. The main concern (with quite a lot of this article, not just your suggested change) will be finding appropriate references (per WP:V, even if statements are correct, we need to show that someone else published them before including them in the article). Add it either way, but if you can find a reference to cite that talks about it, even better. --Christopher Thomas (talk) 19:48, 28 July 2009 (UTC)

I agree. I think it is essential for a credible article to face the problems squarely. I assume we will be making heavy use of extra-terrestrial materials and nuclear energy, but I will be quite surprised if it is not at least a few centuries before anyone begins to go beyond straw-man mission designs. At least we are making revolutionary progress on the astronomical observations side of the problem, so we should have a pretty good idea of where we are going, and what we will find there, before we depart. Cheers, Wwheaton (talk) 02:54, 29 July 2009 (UTC)

You need to give a source for this, otherwise it's WP:OR. Offliner (talk) 14:36, 29 July 2009 (UTC)

I would not put such speculative opinions into the article w/o sources, of course. John Lewis's book Mining the Sky makes a compelling case for the advantages of extraterrestrial materials. Re. the time scale, I would simply challenge anyone to present a credibly sourced scenario, with non-magical technology, that will get us to other stars in less than a century or two. I'm not saying it is impossible, or that it won't happen, of course; only that nobody has a believable scheme, based on what we know how to do today, even painting with a pretty broad brush. My claim about the observational astronomy is non-controversial I hope, but peripheral to the core of the article in any case. Wwheaton (talk) 06:25, 31 July 2009 (UTC)

I was referring to Paulkint's addition. He hasn't given a source so it looks like WP:OR. Offliner (talk) 17:38, 31 July 2009 (UTC)

That's a popular-press book, not a scientific work. It's great for showing that an idea exists, and can be referenced as such, but it's not an engineering study. Regarding building an interstellar craft from non-terrestrial materials, I *really* don't see the point. Launch of materials isn't the dominant cost of the project (energy is for sailcraft, refining the fuel probably is for fission craft, and building the reactor probably is for fusion craft if we assume they're practical). But I'm handwaving too, of course.

Regarding trips with short travel times that have been seriously proposed, Project Longshot was a NASA study that proposed building an ICF-driven craft backed by a conventional fission plant, with Project Daedalus being a more ambitious variant of the design proposed by the British Interplanetary Society. The Starwisp sailcraft design has received a modest amount of attention in scientific literature, and (unlike Longshot) requires no new technologies at all. For craft that carry humans, the Project Orion craft could certainly be built with off-the-shelf technology, and would be capable of interstellar travel on century timescales if most of its mass was fuel. So, I question your "nobody has a believable scheme" statement. Believable schemes have been published for years. --Christopher Thomas (talk) 17:36, 31 July 2009 (UTC)

Forgive me, but I cannot consider those serious solutions to the problem of interstellar travel. Several of them suggest interesting pieces of a solution that may be useful, like pieces of a jigsaw puzzle. In particular, I think fly-by probes do not really qualify.

Lewis is a respected planetary chemist, widely published, and surely an adequate source for extra-terrestrial material use in this context.

A complete solution must solve all of several terribly difficult problems:

• The velocity problem obviously, to achieve interstellar distances in something like a human lifetime.
• The acceleration time problem, noting that a = 0.01 g needs ten years of thrusting merely to reach v = 0.1 c.
• The power-to-mass problem, a huge issue for reasonable accelerations, especially for high-exhaust velocity rocket concepts.
• The heat transfer problem, to keep the ship from burning up due to heat leakage from the propulsion system resulting from the acceleration and power requirements. Extremely high efficiency seems necessary, possibly using a non-thermal power concept.
• Any kind of reaction ("rocket") concept must accept the rocket equation, implying enormous initial mass for a given payload.
• Light-sail concepts have to accelerate very fast (while near the Sun) if they plan use sunlight: eg, a constant 3g acceleration would require 100 AU to get up to 0.1c, where the intensity of sunlight would be 10^-4 of that at Earth's orbit. Beamed concepts face serious diffraction problems requiring huge space-based optics to keep the beam focused on a reasonable size sail. Either of these still has to address the detail of how to stop, which I consider necessary for anything called travel.

So I stand by my "nobody has a believable scheme" statement; I guess we may just have to agree to disagree. I consider a very large (? aircraft carrier size, at least?) fusion rocket (possibly an ion or plasma drive), with acceleration maybe 0.01g to ~10% of c, trip time around a century, the best possibility we can imagine at this time (ugly as it is), and that is not yet near anything that could go to a serious mission feasibility study, in the usual sense of the term.

Cheers, Wwheaton (talk) 20:44, 1 August 2009 (UTC)

Re interstellar probes (which I think of as pieces in the search for a solution to the Great Interstellar Travel Problem Jigsaw Puzzle, not really as solutions in themselves), this is interesting, although it is not clear how seriously one must take the conclusions. It might have a place in the article text, somehow. Wwheaton (talk) 17:22, 3 August 2009 (UTC)

The article was interesting, but I'm puzzled by a couple of the assumptions it makes. First, it assumes that a very small number of probes would be built. Given tens of millions of years (far longer than the transit time across the galaxy), I'd think it more reasonable to assume that they'd have a factory producing N probes per year than that they'd stop at some relatively modest number. Second, it assumes a very low colonization rate (this hinges on an explicit assumption that they make about chosen population dynamics that they take from another paper). I'd think it more reasonable to asssume that if they've decided to colonize the galaxy at all, they'd adjust their birth rate on periphery worlds so as to supply colonists at a faster rate (if ferrying them instead of creating in-situ), and to cause colony worlds to reach their desired final populations more quickly. That said, the article correctly notes that any assumptions about alien behavior and goals is arbitrary. I just think that they've fine-tuned their choice of assumptions, and that an exploration of all likely areas of assumption-space produces a different conclusion (probes all over the place in a much shorter time). Still, a good read. --Christopher Thomas (talk) 05:59, 5 August 2009 (UTC)

I think we're mostly disagreeing on what constitutes acceptable interstellar travel (discounting probes as solution candidates is the biggest, but not only, part of this).

I'm going to nitpick your statement about the rocket equation, though: when people cite this, they're usually picking a velocity and solving for mass fraction. Per above, set the mass fraction, and that gives you the velocity you get (for a given specific impulse) or specific impulse/exhaust velocity needed (for a given cruise velocity). The scenarios I described usually assumed a craft that was 90% fuel for flyby probes and 99% fuel for the fastest non-flyby craft. Downgrading to 90% doubles (but only doubles) the travel time. What the rocket equation actually says is that a practical ship's delta-v is limited to 2 to 4 times its exhaust velocity (20 to 40 times its Isp).

Regarding use of extraterrestrial material, I have several objections to your reference (as best I can evaluate it without the book in-hand). First, its attempt to place a market value on mined material assumes that you can find a market for several $trillion worth of base metals (this is why people usually cite it to me; I recognize that your reasons may be different). By and large, we have enough base metals here on Earth to saturate the market already (Earth's crust is metal ores and silica, and we don't have to perform space-mining to get it). Secondly, regarding use of extraterrestrial material in space, this certainly makes sense on the surface (due to reduction - not elimination - of transport costs), but it's only worthwhile if it results in a substantial net cost reduction for the project being considered. For launching sailcraft, for instance, the cost of the power to drive the beams dwarfs the cost of lifting terrestrial material for the craft, so I'm not sure why we'd spend the effort to develop extraterrestrial mining technology in that scenario (if using a beam array based on Earth; a space-based array could easily change that conclusion). Thirdly, I've been given the impression that this book has nowhere near the level of detail in design studies needed to accurately price out the development and operation costs of space mining technologies (from transport to mining to smelting to maintenance and so forth). To get a really good estimate of development cost (with "really good" being at best a relative term, given the uncertainties involved), you'd need a design study with level of expertise and detail comparable to NASA's old moonbase design study. My understanding is that this book does not present such a study. Long story short, I don't feel that it's a foregone conclusion that your application of that book is within the scope of the book's assumptions, and I remain skeptical of its claims about both the value of the mined material and the cost of the development of the technologies for mining it.

That said, I suspect we're "violently agreeing" on most points about space mining and disagreeing on assumptions for the scenario we're applying mining to, so I'm willing to drop it if you are. Actually, that probably applies to most of this thread. In case I'm leaving the wrong impression, I'd also like to say that I respect the fact that you've been willing to take the time to discuss this and the fact that you've provided well-reasoned arguments (even if I disagree with your conclusions).--Christopher Thomas (talk) 05:59, 5 August 2009 (UTC)

Hi, I agree that our disagreements are largely semantic and probably would be better settled over a few beers (a la Gates/Obama) than debated here. I do appreciate your comments, which have been knowledgeable and defensible. My motivation for writing so extensively here has been mostly to try to explain, to some of the less experienced space editors who might happen by, just how terribly difficult this problem is, and yet preserve some credible grounds for believing that it really is likely to happen in a time short compared to geological and astronomical time scales, even without some "magical" technology (which is probably likely, but beyond all predicting). So I am thinking in terms of "worst case", long-term scenarios, trying to interest those who might believe the whole subject is ridiculous on the one hand, and educate those who do not appreciate the massive difficulties, on the other—all to the end of helping to better focus the article. Best, Wwheaton (talk) 21:27, 5 August 2009 (UTC)

A benchmark for consideration is considering the diffulty of intestellar travel

Assuming a mass of the International Space Station, the energy required for a round trip travel time of 50 years to Alpha Centauri is 74.7 trillion tons of TNT, equivalent to 3.7 million 20-kiloton fission bombs and 7,470 10-megaton thermonuclear bombs. These values will vary proportionally to the mass and decrease as the square of the increase in travel time.

The calculation is based on the classical kinetic energy equation e = ½ mv^2 joules where m = 227.267 kilograms, and v = 17.48% of light, 52,440,000 meters/second. The conversion factor is one TNT-ton is equal to 4.184 E+9 joules. paulkint70.144.110.34 (talk) 18:24, 2 August 2009 (UTC)

Unsourced additions

The recent additions by User:Paulkint are unsourced and thus WP:OR. I will remove them soon if no sources are provided. I know that there are some other unsourced pieces of text in the article as well, but let's not get into the habit of inserting even unsourced stuff. I think we should remove unsourced additions immediately, and not wait until someone discovers that something about them is wrong. Offliner (talk) 20:19, 1 August 2009 (UTC)

These are both obvious consequences of high school physics and the uncontroversial distances to nearby stars. I recommend leaving them. Wwheaton (talk) 20:58, 1 August 2009 (UTC)

High school physics or not, we are not allowed to publish original research in WP. We should not write examples and calculations ourselves without sources.

For example: "But that speed is 7,785 times the average speed of a typical present space vehicle, the International Space Station which orbits at 7,707 meters per second This means that if the spaceship had the same mass as the Space Station. it would possess an energy 7,785 squared or approximately sixty million times as much energy as that the Station possesses" - how do we know this is correct? Who says the ISS "orbits at 7,707 meters per second"? Why is the ISS "a typical present space vehicle"? Offliner (talk) 21:09, 1 August 2009 (UTC)

I cannot specifically source the calculations since I have not found any place where the probable energy requirements for interstellar travel have been ennumerated. Generally, it seems to have been ignored.I find it inexplicable that the energy is not mentioned once in the orginal article for to me the fundamental barrier is energy. The problem is it cannot be appreciated unless some sort of benchmark is given. I say this is the energy required for a space vehicle with this mass traveling at velocity giving this travel time to nearest star. There is not getting around it only by decreasing the mass or increasing travel time which are easy to calculate.

I have referenced my main equation and all the constants used in the in Wikepedia articles. For example, the mass value is given at International Space Station. Would you rather I just say a space vehicle with this much mass? Cannot my calculations be verified by simply computing them?Paulkint (talk) 20:54, 4 August 2009 (UTC)

What I think you may be overlooking is that the energy problem is equivalent to other problems (the nearest of which would be the problem of specific energy, not absolute energy). What I recommend you do is look up the published studies about Project Longshot and Project Daedalus, and use the wording that they use when making their "only fusion has enough specific energy/specific impulse/what-have-you to make the trip" claims (which themselves depend on other assumptions, but that's beside the point). One of these was by NASA, and the other was by BIS, so both should be publicly available. Just remember that we're not allowed to change the wording of sources, or draw too many inferences from them (like computing specific energy from specific impulse, even though it's a trivial calculation). --Christopher Thomas (talk) 05:22, 5 August 2009 (UTC)

Paulkint, you suppose a returning ship would be aimed at Earth, rather than come to a slow speed, appropriate for local travel, somewhere near Earth. Earth is a tiny target within the Solar System, so the probability of a random collision would be of the order of P ~ ((10⁵ km)/(10⁹ km))² = 10^-10. So this is not a reasonable concern, and easily reduced further by simple precautions. The problems of interstellar travel, based on current technology, dwarf those of local space settlement, so I think the notion of a round-trip junket to any known nearby star is unrealistic, almost to the point of being quaint. At this moment we have no certainty whether life as we know it on Earth is a common phenomenon, or unique. If it is rare, we will go one-way, intending to expand Earth biology and human life to other star systems, not for tourism or academic science. Once established elsewhere, people may come back, but they are very unlikely to be the first travelers. Energy, metals and silicon being abundant, the construction of thousand-km sized solar collectors at convenient locations (eg, Sun/Earth L1, L2, L4 or L5) cannot be dismissed, and human mastery of plasma confinement and heating in a century or more is reasonably likely to make fusion energy available as well. But there is surely no reasonable way to estimate the cost of energy a century or more from now, at a time when humans should have made a good start at settling the Solar System. It is a bit like someone in 1720 CE trying to assess the prospects for our global ~1 TW energy budget, based on the Newcomen steam engines of that day. ("It would require all the timber in England for the walking beams!") These problems you raise are less serious than the core problems of propulsion, sustainability, trip duration, and of course, politics. Unforeseeable technological advances in the next century may well short-circuit the whole process, but we cannot say more about the deep unknown than acknowledge the possibility. Wwheaton (talk) 19:22, 8 August 2009 (UTC)

Energy Requirements

To computer the energy requirements for interstellar travel, I have constructed a spreadsheet based on the Einstein energy equation e = mc^2 x (1/sort(1-(v/c)^2-1 joules. (I use the spreadsheet convention “^” to indicate an exponent. “v” is the ship velocity, “ c” is the velocity of light and “m” is the mass of the space vehicle.) For lack of any other guide, I assume the mass of the International Space Station 303,663 kilograms (see Wikepedia article) which at least is designed for long term human habitation. However, it does not carry a propulsion system which the interstellar space vehicle certainly requires, so it may be too small a figure

The maximum velocity referred to in the main article is 10% of light or v/c = 0.1. Placing this and the “m“ value into the equation gives a result of 1.37 x 10^20 joules. This is translated into tons of TNT by the so called “TNT equivalent” (see Wikepedia article ) which is 4.184 x 10^9 joules. Dividing this into the result gives an energy of 32.6 trillion tons of TNT. This is pretty hard to comprehend-- perhaps this gives a better picture: it is equal to 1.64 million 20-kiloton fission bombs and 3,291 ten-megaton fusion bombs.

Even if achievable, this requires 94 years for a round trip to the nearest star Alpha Centauri. For a “lifetime” trip of 47 years based on 20% of light velocity. the energy goes up to the equivalent of 6.7 million fission bombs and 13,469 fusion bombs. Going the other way, 500 years would still require the energy of 50.000 fission boms and 100 fusion bombs. It takes a trip time of over 5000 years to get it down to one.

Traveling near light velocity has been employed in science fiction to get the effect of time dilation. Thus if the velocity was 90% of light there would be dilution factor of 2.3 based on the equation 1/sqrt(1-(v/c)^2) (See Wikepedia article) This would cut the otherwise travel time of 9.7 years down to 4.2 years, at least for the travelers. ( For the rest of us, they will still be gone 9.7 years.) This sounds great. Unfortunately, the energy goes up to that of 845,000 fusion bombs.

For the ultimate, travel at something like 0.9999 of light velocity. The dilation factor will be 70 and the round trip to Alpha will be just a few weeks for the travelers. All it takes is to figure out how to provide an energy equal to 45-million fusion bombs!72.150.191.160 (talk) 16:26, 6 August 2009 (UTC)

Energy requirements are discussed extensively in the "Possibility of interstellar travel" thread near the top of this talk page. The specific energy of chemical fuels (and substances like TNT) is too low to achieve this, but it's within reach of fission and fusion, which have vastly higher specific energies (2-9e+13 J/kg for fission, and 3-6e+14 J/kg for fusion, compared to 4e+6 J/kg for TNT). You also seem to be misreading the distance to Alpha Centauri. It's about 5 light-years, for a travel time of about 50 years with the cruising speed of 10% C you assume (delta-v, not cruising speed, is 20% C in that scenario). --Christopher Thomas (talk) 17:05, 6 August 2009 (UTC)

I am asuming a manned round trip. I have simply enumerated the kinetic energy requirements. I have no comment on feasability. paulkint72.150.191.160 (talk) 17:14, 6 August 2009 (UTC)

You stated, "Obviously, those who talk about a 10% velocity have never bothered to compute the energy.". You then edited your previous post to make it look like you hadn't said this. The "history" tab makes it very easy to see this type of thing, so please don't do it. If you want to retract a statement, use the strikeout markup tags instead.

Regarding "simply enumerating kinetic energy requirements", that's been done, repeatedly, already. If you feel that statements in the article are inaccurate, say so. If you aren't commenting on the article's content, then you should probably be posting at Wikipedia:Reference desk/Science rather than here (this page is for discussing changes to the Interstellar travel article).--Christopher Thomas (talk) 19:45, 6 August 2009 (UTC)

Thanks for the lecture. I was not aware discussions were restricted specifically to the contents of the article If so, this should be stated at the beginning of the discussion section.

Assuming you are correct, I will discuss this statement “However, special relativity offers the possibility of shortening the travel time: if a starship with sufficiently advanced engines could reach velocities approaching the speed of light, relativistic time dilation would make the voyage much shorter for the traveler” However, I show above that the resultant energy required of an International Space Vehicle traveling at 0.999 of light velocity would be equivalent to that of 845,000 fusion bombs. I suggest that. based on this. special relativity does not offer any such possibilityPaulkint (talk) 13:48, 7 August 2009 (UTC)

Sorry, the assumed velocity is 0.9999 of light, which indentally would give a dilation factor of 70, and the energy is 45-million fusion bombs.70.144.93.85 (talk) 14:19, 7 August 2009 (UTC)

I agree that it's almost certainly impractical to use time dilation to shorten subjective travel times for any means of interstellar travel we presently have the technology to achieve. I've tweaked wording in that section to indicate that near-C travel and wormhole travel are speculative technologies.

Regarding talk pages, the guidelines regarding them are described at WP:TALK. There's a template (Template:Talkheader) that's sometimes added to spell this out when an article attracts too much off-topic talk-page traffic, but it's usually only used when this is causing problems (e.g. at Talk:Black hole, where for a while almost all of the traffic was people asking questions about black holes, not talking about improving the article). I don't think this talk page is quite at that stage yet (though it could stand to have old threads archived). --Christopher Thomas (talk) 17:32, 7 August 2009 (UTC)

Thank you. I have another bone of contention with the article. I am not on as firm ground on this as energy.Would you pleae confirm it or show where it incorrect?

The article states that Orion (a proposed nuclear powered rocket) would be able to reach a velocity of 10% of the speed of light and is “one of very few known interstellar spacecraft proposals that could be constructed entirely with today's technology“. However it appears that this cannot be true based on the Tsiolkovsky rocket equation This equation defines what ship velocity is achievable based on the exhaust velocity and the amount of reaction mass relative to the ship mass. The equation is . v = e x log (1 + m) where v = ship velocity, e = exhaust velocity and m = ratio of reaction mass to ship mass. In the Project Orion article it states that an exhaust velocity can reach 30,000 m/s. Assume m = 3, i..e., the reaction mass is twice the ship mass. Since the log of 3 is 1.09, the maximum ship velocity is essentially equal to the reaction mass velocity, which is 30,000 meters per second whereas 10% of light velocity is 30,000,000 meters per second. It appears that even a nuclear powered rocket with present technology has a long way to go to reach 10% of light70.144.103.203 (talk) 18:01, 7 August 2009 (UTC)

Numbers quoted in the Project Orion (nuclear propulsion) article vary widely. The "Performance" section lists 2e4-3e4 m/s at the top, but values from 1e5 to 1e7 m/sec later in the article. 1e+7 would give you a cruising velocity of 10% C with a cargo fraction of e^-6 (about 500 times as much fuel as structure/cargo). In practice, you'd use a cruising velocity of 5% C and a much more reasonable fuel fraction if you had that exhaust velocity (per the previous thread, I find it much more useful to assume a fuel fraction and solve for cruise velocity than to assume a cruise velocity and solve for fuel fraction, as the former is always guaranteed to give a feasible craft configuration). I suspect that the high values for exhaust velocity are someone's calculations from first-principles, as they aren't properly sourced. Per the previous thread, it's easy to show that fusion weapons have enough specific energy to produce an exhaust velocity this high, but that doesn't mean someone found a feasible craft configuration that could do it.

Checking the articles in question, it looks like the quote about Orion being capable of 8-10% the speed of light was from Carl Sagan's Cosmos book (a collection of non-fiction essays). My best guess is that he was doing a similar first-principles calculation. I'll apply wording tweaks to clarify this. --Christopher Thomas (talk) 19:08, 7 August 2009 (UTC)

I would like to raise a point the main article does not cover: at some point the kinetic energy has to be dumped. As I have shown, this energy can be mammoth. For example, a hundred year trip would require a velocity 8.7% of light and the kinetic energy would be equivalent to 2,486 fusion bombs assuming a mass of the International Space Station. This incredible energy has to be dissipated upon return to earth. I am not concerned with how it done, only that if there is a failure and the spaceship plunges into the earth’s atmosphere carrying this much energy, there would be an explosion beyond comprehension. Consider the Tunguska explosion which occurred in 1908 in Siberia and which knocked down some 10-million trees. That energy is estimated to be that of just one fusion bomb

No matter small the probability, it cannot be reduced to absolute zero. Consequently, I cannot envisage politically any round trip ever being allowed.Paulkint (talk) 12:33, 8 August 2009 (UTC)

Under normal conditions it ends up dumped as the kinetic energy of the exhaust, for both reaction drives and sailcraft. You're correct in noting that an interstellar craft that doesn't turn around but instead rams its target would have a very high explosive yield. If you can find references satisfying WP:RS that discuss the problem, a paragraph can be added to the article that talks about it. --Christopher Thomas (talk) 16:42, 8 August 2009 (UTC)

Beamed propulsion feasibility

The main article states “Beamed propulsion seems to be the best interstellar travel technique presently available“ A referenced article is Forward, R.L. (1984). "Roundtrip Interstellar Travel Using Laser-Pushed Lightsails". J Spacecraft 21 (2): 187–195. However, retrieving the summary of the article, indicates requirements are far from being presently available. It begins with “The laser power system uses a 1000-km- (620 miles) diameter lightweight Fresnel zone lens that is capable of focusing laser light over interstellar distances“ It proposes for a “rendezvous mission” to Alpha Centaur a “:payload“: lightsail 30-km (18-miles) in diameter and one 100-km (62-miles) “decelerator”. (These are so outlandish that they may be typos. Perhaps the author met meters although a 1,000 meter ( 3,048 foot) Fresnel lens would be a considerable challenge.

The author apparently proposes to decelerate the space vehicle at its designation and accelerate it for the return with the laser beam. , reversing its thrust with a “reflector” sail. However, the precision by which this must done does not seem to have been considered. Alpha Centaur is 24,600 trillion miles distant, based on a distance of 4.3 light years and a light velocity of 186,000 miles per second. A simple calculation shows if there is an pointing error of one-millionth of a degree, the beam would be off target by 429,000 miles. (Assume the distance is the radius of a circle. Multiply by 2 pi to get the circumference and divide by 360 x one million.)

The cost of the energy supplied to the laser equally does not seem to have been considered Assuming a velocity of 20% of light, 60,000,000 meters per second, and a mass that of the Space Station ,303,663 kilograms, the energy is 5.47 x 10^20 joules. based on the kinetic energy equation e = ½ mv^2, (A small amount of relativistic energy is not included) The National Institute of Standards defines the energy of a kw-hr as 3.6 x 10^6 joules. This gives an energy of 1.52 x 10^14 kw-hrs Assuming a cost of 8 cents per kw-hr this defines a cost of 12.1 trillion dollars.

Since these figures seem to indicate that a manned interstellar round trip based on beamed propulsion is impossible, both from an engieering and cost viewpoint, reviewing the assumptions and figures for accuracy would be appreciated.Paulkint (talk) 19:52, 9 August 2009 (UTC)

The figures are correct, and there is no mistake. Note that the article does not claim that beamed propulsion missions listed in the table are feasible, which is what you seem to be arguing against here, so I don't think there's a need to change anything. I inserted the table from Forward's paper because I thought it would be a nice way to give the reader a small overview of what would be required. If anyone can think of a better way (such as similar examples from more recent papers) to do this, feel free to edit. Offliner (talk) 21:13, 9 August 2009 (UTC)

I ran numbers for beamed propulsion power requirements in our previous thread near the top of this article, which you may wish to review. Power requirements are comparable to the total world production of electrical power, indicating that they are not beyond reason (just very expensive). Regarding the sizes for sails and laser focusing arrays needed, huge sails and optics are needed due to diffraction (see Airy disk for a discussion of the specific type of diffraction affecting this type of system). Using a smaller array or smaller sail causes emitted light to spread out more, and most of it to miss the sail (increasing the total system power requirements for a given sail drive power). My own back-of-the-envelope analysis says that the most expensive part of a beam-sail propulsion system would be the laser array, but that's sensitive to the assumptions made. A lower bound to cost can be estimated from power considerations, and is quoted in the previous thread. --Christopher Thomas (talk) 00:55, 10 August 2009 (UTC)

The statement in the main article “Beamed propulsion seems to be the best interstellar travel technique presently available“ clearly implies feasibility. However, as I show, it is feasible only if a laser beam can be projected across thousands of trillion miles of space to a target of a few meters and an electric bill of trillions of dollars can be paid. To me, especially the projection problem makes it completely unfeasible. I don't think it should even be suggested as a possibility.

Incidentally, in the Forward Table, he lists the mass of an “outbound manned vehicle” as 78,500 t. I assume “t” means tons. If so, that is the mass of a couple of battleships. Quite a feat lifting that with a laser beam to escape velocity70.144.105.231 (talk) 13:37, 10 August 2009 (UTC)

"The statement in the main article “Beamed propulsion seems to be the best interstellar travel technique presently available“ clearly implies feasibility" - nope. It says "this seems to be the best technique presently available." Read what the source in question says: Lightsails are being tested for interplanetary travel, solar-pumped lasers and high power solar-electric sources for electrically powered lasers have been demonstrated for space applications, and methods for combining many laser beams into one coherent beam and pointing that beam accurately have been demonstrated for space weapon purposes. Interstellar beamed laser propulsion systems will be larger than anything presently considered, but there is no new physics involved.

Also, could please cut down on your original research and speculation? This talk page is about discussing how to improve article, not for posting personal opinions or your own calculations. If you disagree with what the article says, please provide sources that support your view. Offliner (talk) 13:48, 10 August 2009 (UTC)

Please actually take the time to read the material before making bizzare statements like "projected thousands of trillions of miles to a target of a few metres". You've overestimated the distance, and the sails used are huge precisely in order that a laser array can focus on them with minimal diffractive losses. The distance to the nearest star is about 5e+16 metres (for 5 light-years). Acceleration baseline (and so drive beam range) for a flyby probe would be shorter, at about 1.5e+15 metres (at 0.3 m/s^2, for 1e+8 s). The smallest feasible sail/emitter combination for the flyby probe is about 40 km wide, and the smallest feasible sail/emitter combination for the Forward-style probe is about 200 km wide (assuming a drive laser wavelength on the order of 1 micron, which is is typical for near-infrared diode-pumped Nd:YAG lasers). Keeping large arrays phase-matched is a solved problem (it's the same type of technique used to keep guide stars in focus with segmented-mirror telescopes). You can reduce the sail size by increasing the emitter array size by the same factor, and vice versa (in practice, you'd use a larger emitter to reduce power densities in the lasers, and use a smaller sailcraft to reduce the total energy requirements of the project for unmanned missions). Long story short, the math has been done, and there's no question that we _could_ build a sailcraft launch array if we had a good enough reason to. --Christopher Thomas (talk) 17:52, 10 August 2009 (UTC)

I suggest Mr, Thomas read Wikepedia: Civility One can point out errors without sarcasm and such adjectives as “bizarre”. The distance value I used in my calculations was the correct one, 2.5E+ 13 miles. (I used the value of 4.37 light years for distance) . I simply misread it when I formatted it as a standard number

I showed that a millionth of a degree pointing error would cause the spaceship to miss the target by 436,000 miles.(4.36E+5) But a millionth of a degree accuracy is probably a very unrealistic assumption. Consider a hundredth of a degree error which may be achievable. But now the miss is 4.36 billion miles (4.36E+9) Question: What keeps the spaceship precisely centered on the beam?

The main article states “In theory a light sail driven by a laser or other beam from Earth can be used to decelerate a spacecraft approaching a distant star or planet, by detaching part of the sail and using it to focus the beam on the forward-facing surface of the rest of the sail” referencing an article by Forward, R.L. (1984)." However, I am highly suspicious . To me, something is something wrong with the idea of pointing a beam at some system and it then it comes towards you by bouncing the light around inside the system

To Offliner The statement in the main article is it:“Beamed propulsion seems to be the best interstellar travel technique presently available, since it uses known physics and known technology that is being developed for other purposes,[3] and would be considerably cheaper than nuclear pulse propulsion.” It appears to definitely be referring to ”interstellar travel

You chide me for placing my research and calculations into a discussion without giving sources. .However, Wikepedia: Talk Pages says “ A talk page is a space for editors to discuss improvements to articles and other pages.”, nothing about requiring sources. Indeed, these discussions abound with calculations and speculations which have not been sourced. Mr. Thomas’ reply is a good examplePaulkint (talk) 13:59, 11 August 2009 (UTC)

It is not uncivil to expect you to read a) the article, and b) responses to your own previous comments, when constructing further arguments. I've been assuming that you're acting in good faith, but points a) and b) are challenging that assumption.

Regarding pointing accuracy of the laser array, this is - again - the same solved problem that's used for focusing large segmented-mirror telescopes. Resolving accuracy for the Keck telescope is about 0.04 arcsecond (10 millionths of a degree, or 0.2 microradian). The largest telescope under construction has a designed resolving accuracy on the order of 0.001 arcsecond (0.3 millionths of a degree, or 5 nanoradians). So, if your "hundredth of a degree" number referred to laser pointing accuracy, you're apparently unfamiliar with what segmented array optics can presently do. They can generally reach accuracy comparable to the diffraction limit of the combined aperture of the array (by phase-adjusting each individual mirror to match the wavefront produced by a "guide star" laser spot, or by back-reflected laser light in the case of a sailcraft).

Regarding direction of the craft itself, it's trivial for the craft to actively track the laser beam by tacking the sail (angling it so that light spills off slightly more towards one side than the other). This is done by making small adjustments to the length of the cables holding the probe pod to the sail. Because the laser array is tracking reflected light from the craft, this lets the craft make fine adjustments to its trajectory as it approaches the target system. This is covered in most published proposals of sailcraft, so your objections are puzzling.

Regarding the Forward decelleration scheme, this is the same optical arrangement as you find in just about all reflective telescopes. Light from Earth hits a big mirror-sail. Big mirror-sail focuses it on the small mirror-sail. Both of these sails are along the same line (the laser's direction of projection). A small amount of light hits the back of the small sail, but the vast majority of the light striking the small sail is light focused from the big sail, so the net effect is a strong decelleration. The main difficulty with the Forward scheme is that the big sail is required to act as a controlled optical surface, which probably increases its complexity and weight.

All of this is explained in the references. --Christopher Thomas (talk) 16:56, 11 August 2009 (UTC)

Some thought shows that the Forward deceleration proposal is based on a fallacy. First, the two sails are a closed system. Second, it is self evident that a closed system cannot be decelerated through a reverse force contained entirely within the system, which the beam becomes when it is reflected. The effect is the same as replacing the reflecting sail by an equivalent light source, carried by vehicle, projecting a light beam onto the target sail Obviously, it would not decelerate the vehicle. In fact, Forward’s scheme is precisely equal to “pulling one self up by the bootstraps”

Based on this Forward’s proposal should be excised from the main article.Paulkint (talk) 16:54, 14 August 2009 (UTC)

Unless you're a published scientist, we're not going to remove anything from the article based on your personal opinion. Please read WP:OR and don't come back before you do. Offliner (talk) 16:59, 14 August 2009 (UTC)

The description did need work. I've tweaked it. --Christopher Thomas (talk) 18:07, 14 August 2009 (UTC)

You appear to have missed several important points, here. 1) The closed system is the two sails, the laser array back home, and the beam of photons driving the sail. That's how sailcraft accelerate in the first place (exchanging momentum with the beam of photons). 2) Only the small sail decellerates. The large sail accellerates. The net effect is to exchange momentum between the two sails, with the small sail losing momentum and slowing down, and the big sail gaining momentum and speeding up.

Here's a diagram for you:

Does this clarify the situation? --Christopher Thomas (talk) 17:24, 14 August 2009 (UTC)

How can the large sail be speeding up and the small sail be slowing down when they are connected by a physical structure?Paulkint (talk) 20:07, 14 August 2009 (UTC)

They aren't connected during the decelleration phase. Per both the original and revised descriptions in the article, when the sailcraft wants to slow down, it splits into two sails, with the larger continuing on and the smaller slowing down. The diagram right above your response clearly shows two unconnected craft, so I'm puzzled as to where your confusion comes from. --Christopher Thomas (talk) 20:46, 14 August 2009 (UTC)

Sorry, I did't realize that they were to be disconnected. I was assuming round trips which this seems to preclude. So is beamed propulsion limited to one way trips? Paulkint (talk) 22:45, 14 August 2009 (UTC)

That depends on the assumptions made. If the big sail is light enough, the ship can pack a second one (folded up), deploy it when they want to go home, and use it to reflect the starting system's laser on to the small sail to boost them back towards the original star system. Alternatively, if they don't mind having only a short time to explore the destination system, they could use the original big sail for this (depending on system parameters, it could be usable for up to several years before getting too far from the small sail). --Christopher Thomas (talk) 22:59, 14 August 2009 (UTC)

It appears to me that the “separated sail” idea to return the spaceship to earth will not work for several reasons.

1. The laser beam cannot be kept focused on the reflecting sail, light years away, for it would typically require a pointing accuracy by the laser array of less than a millionth of an arc sec, even for a 1000-km sail. Even if this precision could be achieved, the laser beam is being pointing blindly for there is no way tracking the reflecting sail from the laser array. .

2. The focal point of the reflecting sail would change as the sails separate which would require adjusting the curvature of the reflecting sail. How this is done with a 1000-km sail is not apparent.

3. As the sails separate by light years, the required pointing accuracy of the reflecting sail becomes on the order of millionths of an arc sec which would require an extraordinary stability. . However, it is floating in space with no apparent mechanism of keeping it stable and focused on the small sail and certainly continuous corrections would be required, with no apparent way of making them.

It might be pointed out that even if the target can be observed, that tracking and controlling over interstellar distances is impossible. This is because there could be light year delays between receiving the error information and getting the correction to what is being controlled. You cannot control something based on information years old and when it takes years to make the correction Obviously, by the time the correction gets there it is hopelessly out of date.Paulkint (talk) 11:07, 17 August 2009 (UTC)

With regards to 1), per my repeated explanations above, we can get any pointing accuracy we please (up to the diffraction limit of our laser array's combined aperture), as long as we have a point source of light to use as a reference. The reflected light from the sail is exactly this kind of point source. With regards to lag time in making these adjustments, as long as the sailcraft can tack to ride the beam, lag isn't an issue (the base station can only change its pointing direction slowly, but the craft can make small adjustments to stay in the right place very quickly).

With regards to 2) and 3), the large sail has to have the same type of phase-aligned optics as used by the laser projector. The constraints aren't quite as tight (distance between the sails is at most about 1/7 of a light-year; this is the "flyby probe" acceleration baseline given previously in this thread). Given that type of optics, pointing accuracy is limited only by the size of the sail. I agree that building a large sail that's a controlled optical surface is the biggest difficulty with Forward's scheme, but saying that it's impossible is premature (I can think of a couple of approaches for doing it). All that can really be said right now is that a sail of this type would almost certainly be heavier and more expensive than a non-focusing sail (and I acknowledged this repeatedly already in this thread).

The interesting thing to look for right now would be papers published by Forward or by others that talk about how they intended to build a focusing sail. --Christopher Thomas (talk) 21:21, 17 August 2009 (UTC)

You have not addressed the matter of the required pointing accuracy of the reflecting sail on the return journey which has a much smaller targetPaulkint (talk) 11:35, 18 August 2009 (UTC)

That is addressed up where I gave the baselines and sizes for flyby-only and Forward-style sails. If you want to calculate values yourself, keep the baseline length for a flyby sail (as it's set by an assumed maximum power to weight ratio of the sail material), and use the relation "d1 times d2 equals 1.5e+9 square metres" (actually equals baseline times the wavelength, with a near-unity fudge factor from the Airy disc derivation that I've ignored, but close enough for our purposes). If the large sail was 200 km wide (i.e., we assume that the laser array back home and the big sail were the same size, per previous), the small sail would have to be at least 8 km wide, requiring a pointing accuracy of about 5e-12 radian at most. Pointing accuracy of the emitter back home hitting the big sail at that distance is 4e-12 radian. So the pointing accuracy is actually quite close to being the same.

In a real system, you'd use a larger emitter array and smaller cruising sail. This means the laser array at home needs improved accuracy, but the large sail's accuracy targetting the decelleration sail can be lower. Minimum size of the large sail is about 40 km, as it can't be any smaller than the small sail for power-to-weight reasons (and even that is tricky, as the small sail (which is now as big as the large sail) can't be blocking the beam from Earth, and so must be launched at an angle in that scenario). --Christopher Thomas (talk) 06:23, 19 August 2009 (UTC)

1-radian is equal to 3420 arc sec. 5E-12 of a radian is 1.7E-08 or 17-billionths of an arc-sec, You seem to be saying that a 162 mile reflecting sail floating in space could be pointed millions of times more accurately than the resolving power of the largest telescopePaulkint (talk) 11:31, 19 August 2009 (UTC)

Yes, it can, just as easily as a ground-based array can, as long as 1) you have a pointlike reference (light reflected from the small sail, in this case), and 2) you can adjust the position of all regions of the sail's surface to an accuracy of about a tenth of a micron (this accuracy stays the same no matter how small or how big the sail or mirror array is; that's why we can make arrays with arbitrarily good pointing accuracy).

It's requirement 2) that makes a Forward-type sail difficult to build, but this is a question of "could it be done within a reasonable weight limit", not "could it be done at all" (trivial version would be a truss with a conventional mirror array on it, but that would be a thousand times heavier than the sail assumed in previous calculations). A MEMS-based approach would meet the weight requirements but be prohibitively expensive to build with present fabrication techniques, as another possible example. The key point is that the sail doesn't have to be _rigid_, and can even be flapping around, as long as its flapping motions are slow compared to the speed at which you can phase-adjust any given patch of the sail. The sail doesn't have to conform to _one_ specific geometry; acceptable optical surfaces are a set of nested shells one wavelength apart from each other. Any given piece of the mirror surface has to be on _one_ of these shells, but this doesn't have to be the _same_ shell as other pieces (it's acting more or less like a Fresnel reflector). So, moving half a wavelength in either direction will always take you to a position that works. Again, this is the same type of technology that segmented mirror telescopes use. --Christopher Thomas (talk) 17:48, 19 August 2009 (UTC)

You are confusing the optical requirements of a telescope and the positioning requirements. The first is done by a lens or mirrors, the second by mechanical control systems. The telescope only has to be pointed in the general direction, view a large field, and resolve objects optically within the field. So the pointing requirement is far less than the optical requirement.

In contrast, the laser beam has to be pointed precisely at the object, which has nothing to do with the focusing ability of the laser array. The significant thing is that in beam propulsion the beam has be projected across trillions of miles and strike a speck, based on some mechanical system controlling the direction of the beam. .

However, in the case of Mr. Forward’s deceleration scheme, although it requires a extremely precise angular orientation of the reflecting sail after it is released, there is no control system whatever to accomplish thisPaulkint (talk) 15:15, 20 August 2009 (UTC)

What you appear to be missing is that forming an image with a telescope, and projecting a laser beam on a target, are exactly the same optical problem (just run the light rays through the system backwards to see why this is true). To make a mirror system do this, or to do anything else you like (limited only by the diffraction limits of the combined array), you have to be able to adjust the position of the optical surface to sub-wavelength accuracy (about a tenth of a wavelength will give you good results, though you can get away with lower). See active optics and adaptive optics for the systems we use for this. The sail scenario and the telescope scenario are setting the mirror for different focal lengths, but the precision needed doesn't change. --Christopher Thomas (talk) 17:38, 20 August 2009 (UTC)

Why beam propulsion is not feasible 1. The accuracy required to point a beam across trillions of miles of space and hit a target is hundreds of times more than what has been achieved by the finest telescopes.

2, Even this is based on targets over a hundred of miles in diameter as assumed by the advocates. Obviously this is impossible The accuracy required is actually at least a hundred times greater if a reasonable sail size is assumed.

3. The advocates assume that the laser beam will be perfectly collimated through space. This is not true, even a laser beam diffuses due to diffraction. A beam was projected to the moon and diffused to five miles wide. The diffusion would be thousands times greater targeting an object in deep space.

4. The beam is projected from a rotating earth and so will lose contact for a substantial part of the day.. Even with the most powerful telescopes the vehicle could not be observed for months during the year since it would be appearing in daylight. Contacting the vehicle again would be virtually impossible.

6. Beamed propulsion would be prohibitively expensive The cost of the laser electricity for a 50-year round trip to the nearest star is estimated at approximately eight trillion dollars.

7. An extremely power laser beam would be projected through the paths of commercial airliners and satellites, If struck they would be destroyed. Although low, the probability is finite and never would be allowed.

The figures are based on spreadsheet calculations. They will be furnished on requestPaulkint (talk) 10:36, 21 August 2009 (UTC)

Please, this talk page is not a forum for discussing the feasibility of beamed propulsion. You can continue this discussion per email if you want. I fail to see how any of this has to do with improving the article. Paulkint, no one is interested in your personal opinion on this, so please stop discussing it here. Offliner (talk) 11:02, 21 August 2009 (UTC)

These are not personal opinions, but scientifically based facts. They show overwhelmingly that the statement in the main article “Beamed propulsion seems to be the best interstellar travel technique presently available since it uses known physics and known technology” is completely false and that Mr. Forward’s proposal is nothing but scientific fiction fantasy, It has no business in a encyclopedia of fact being presented as factual and the only improvement that can be made on this article is to remove it. I am sorry if you are irritated but you have been duped.

With that I have nothing more to say. Farewell.Paulkint (talk) 12:32, 21 August 2009 (UTC)

If it's a scientifically based fact, then it should be easy for you to provide a reliable source which explicitly says so. You have been asked many times to provide a source, but you have not. Your opinion may or may not be true, but as long as it's unsourced, it's not verifiable and therefore has no place in Wikipedia. Offliner (talk) 12:50, 21 August 2009 (UTC)

Discussion material does not need citations. In fact, most of the stuff in these discussion pages are opinions and are not cited. As for Forward, who seems to have been the chief proponent for beam propulsion, I note that in the table of his in the main article, he is proposing for his “manned output” stage launching a spaceship weighing 78,500 tons 50% more than the battleship Missouri (45,000 tons), carrying into space and deploying a sail 1000 km (620 miles) in diameter. Do I need a citation to state that the man is nuts?Paulkint (talk) 14:17, 22 August 2009 (UTC)

Do you need a citation for that? Yes, you do. Offliner (talk) 15:51, 22 August 2009 (UTC)

I told my bright 12-year old grandson that this encyclopedia says you can send a spaceship bigger that a battleship into space and get it up to 93,000 miles per second with a laser beam from earth. He said "That's the craziest thing I have ever heard" I told him to shut up. "You can't cite that"Paulkint (talk) 10:30, 23 August 2009 (UTC)

See Also cleaned up

As per the Manual of Style, I went through the See Also section and removed all links that a) already exist in the body of the article (like Bussard ramjet), or b) are space-related but not specifically with interstellar travel (like Eugen Sänger or Freeman Dyson). There are thousands of Wikipedia articles that deal with deep space travel in some fashion. If you can't come up with a good way of integrating a link into the article itself, please ask yourself why it would belong in the See Also section! YLee (talk) 00:17, 24 August 2009 (UTC)

Looks good; thanks. This article is overdue for a cleanup pass. --Christopher Thomas (talk) 01:25, 24 August 2009 (UTC)

Unsourced OR & original synthesis deleted

It seems to me that a number of edits by Paulkint (talk) have really gone way outside the bounds of Wikipedia's policies re. neutral point of view, original research, verifiability, and especially original synthesis. In the light of his edits and many comments here on the discussion page, I have come to believe that he is simply determined to "prove" that interstellar travel is impossible. He would be certainly correct to argue that it appears very difficult, and he would have little difficulty finding credible published support to back that claim. But when he claims it is impossible because it will require specific energies a million times that of typical current values, he must show that those energies are fundamentally impossible to achieve. Then he faces great difficulties, which his calculations simply do not address, because no one can say what will or will not be technically or economically easy, difficult, or impossible a century hence. A century is a long time for technology, but not long even on the scale of recorded history, let alone on the biological (evolutionary), geological, or astronomical time scales. Consider Jules Verne's voyage to the Moon: technically "impossible" along the lines he imagined, and yet we actually went there, in barely a human lifetime.

I have according deleted a couple of particularly egregious paragraphs of this sort, and ask that they not be re-instated without consensus here on the discussion page. Best, Wwheaton (talk) 06:45, 25 August 2009 (UTC)

In my editing I have never made the claim that interstellar travel is impossible in the future. I have only simply attempted to show the scale of the problem which can only be done by concrete examples based on simple calculations, and then let the reader come to his or her own conclusions. As for “he must show that those energies are fundamentally impossible to achieve“ that is an asinine statement. I can no more prove that the energies cannot be achieved in the future than you can prove they can be achieved In fact, it well known in logic that you cannot prove a negative, the burden of proof is on you to prove that they can be achieved.

The truth is I make no predictions about the future, I am not and have never been a seer.. However, whoever wrote the article is prophesying with “Given sufficient travel time and engineering work, both unmanned and generational interstellar travel seem possible” (Not cited) Prove that!

I have inserted a cited statement where engineers and scientists of the propulsion field do discuss the feasibility of the future of interstellar travel. Their opinions are not necessarily mine.

Why you object to the magnitude of the energy barriers being shown baffles me. Do you believe that readers of the “Interstellar Travel” article have no right to be made aware of the magnitude of the energy problem, of which there was not a hint before I entered the picture? Is it that interstellar travel enthusiasts cannot deal with inconvenient facts?Paulkint (talk) 19:24, 25 August 2009 (UTC)

I object because you have given no credible source for the cost of energy in space a century from now, and I believe you cannot do so, any more than Rutherford could guess the cost of nuclear energy in 1935. I seriously doubt that even the meaning of $1 can be defined over the relevant time scales. It is trivial to estimate the specific energy (per kg) of a payload moving at 0.1c, to six digits if you like. The extent that this number will pose a practical problem on the timescale of interest is not trivial, and central to the argument. But completing the chain of reasoning is original synthesis, unless sources explicitly doing so can be provided. A source can assert proposition A, and another can assert B, and A and B together may imply C, but unless an external reliable source makes that logical connection, we are not allowed to complete the chain and assert C in an article here. It is brutal, but that is the rule. And in this particular case, if A is the specific energy required, and B is the cost, then even B can probably not be specified, even to order of magnitude, from external sources. Wwheaton (talk) 14:28, 26 August 2009 (UTC)

The main article states that (beamed propulsion)would be considerably cheaper than nuclear pulse propulsion This statement seems to be taken out of thin air for no one knows the cost of nuclear pulse propulsion. However, at least one portion of the cost of beam propulsion can be easily computed, the cost of the electricity to power the laser beam. The minimum cost will be based on the kinetic energy of the spaceship since the beam must supply that energy. Assume a 300,000 kg. ship and a velocity of 20% of light for a 48-year round trip to Alpha Centauri. Placing these two values into the Newtonian kinetic energy equation give a energy value of 5.4E+20 joules. Based on 1 kw-hr = 3.5E+06 joules (NIST Standards), the energy is 1.5E+14 kw-hrs. At 5 cents/kw-hr the cost is 7.5E+12 or 7.5 trillion dollars.

Based on this should that statement be removed? Should the obvious titanic cost of beam propulsion be mentioned?Paulkint (talk) 11:08, 26 August 2009 (UTC)

The claim that beamed propulsion would be cheaper than nuclear pulse propulsion needs to be credibly sourced. I too am skeptical of that at this point. I imagine there may be consensus for the removal of that assertion; what do other editors think? The 7.5 $T figure is large, but would need to be related to the resources of the human species at a distant future time (and also to the perceived value of the project, compared to other possible expenditures) to be termed "impossible" (and sourced, of course). I agree it is a difficulty seen from the perspective of the present day; one of many. Wwheaton (talk) 14:49, 26 August 2009 (UTC)

I believe the intent of that section was to point out that a beamed propulsion system may be cheaper, per unit delivered cargo, due to not having to carry its own fuel. You can certainly pick scenarios where this is true (by picking a cruising velocity such that an arbitrarily large amount of fuel would be needed to move a given amount of cargo). For practical systems (i.e., where fuel fraction is limited to about 99%), the situation is less clear-cut. It'd depend on the relative costs of the fuel (bombs, D-T pellets, what-have-you) versus the cost of a sail and a laser array and the power to drive the laser array. If you can think of a good way to tweak the text to make the point clear, by all means do so. --Christopher Thomas (talk) 01:14, 3 September 2009 (UTC)

Before you take action consider this:I have pointed out many problems with interstellar travel, particularly the titanic energies required which seem to me to have been generally ignored. However, I freely admit that that these energies might somehow be supplied some way in the future. But I think I have finally come up with evidence that makes the feasability of beamed propulsion in particular essentially zero. This is the matter of laser beam and the fact that in order to strike an even large sail trillions of miles away it must be close to perfectly collimated. But as stated in Wiki:laser "The beam may be highly collimated, that is being parallel without diverging. However, a perfectly collimated beam cannot be created, due to diffraction”

However, I cannot compute the divergence because I don't have the necessary data and I am not an optical expert. However, I have found what might be called a benchmark, based on a project where a laser beam is projected to the moon to be reflected from a reflector left there by the Apollo astronauts. It is called the Ranging Retroreflector Experiment At the NASA Eclipse website http://eclipse.gsfc.nasa.gov/SEhelp/ApolloLaser.html you can get the data. It stats that the beam at the Moon's surface is roughly four miles (6.5 km) wide; i,e, .the beam has diverged four miles in traveling 384,000(3.84E+5)km, the distance to the moon.(The moon beam is focused at the state of the art by the 107-in. Harlan J. Smith Telescope of the McDonald Observatory )

It will continue to diverge proportionally for greater distances Accordingly at a distance of one light year it would have diverged by a factor of (9.46E+12 / 3.84E+5) = 2.4E+7 or 24-million. Multiplying that by the 6.5 km beam at the moon gives a beam width of slightly under 160-million km (multiply that by 4.37 for the distance to Alpha Centauri.)

Forward, the proponent of beamed propulsion, apparently knew he had a problem so he proposed the beam be focused by a 1000-km (620-mile) diameter Fresnel “zone lens”! (reference-9 Wiki: Interstellar Travel) I doubt that even this will solve his problem but I think you will agree that a beam projector that sticks 620 miles up in the air is something less than feasible. So it seems evident that if this is what it takes for beam propulsion, forget it. Whether the subject be deletedis up to you. But I don’t think something that requires the equivalent of a 620-mile diameter telescope be in a encyclopedia of fact labeled “Beamed propulsion seems to be the best interstellar travel technique based on known physics"Paulkint (talk) 15:03, 27 August 2009 (UTC)

I might point out that the Lunar Retroreflector Experiment can quantify the problem of pointing a beam at distant target. The problem is described at the NASA Website http://eclipse.gsfc.nasa.gov/SEhelp/ApolloLaser.html “When the beam is precisely aligned in the telescope, actually hitting a lunar retroreflector array is technically challenging. At the Moon's surface the beam is roughly four miles wide. Scientists liken the task of aiming the beam to using a rifle to hit a moving dime two miles away” Actually, this is simple compared to hitting a target one light year away. The distance to the moon 3.8E+4 km. A light year distance is 9.5E+12 km. 2.5E+8 times greater. Accordingly hitting a 200-km sail at one light year distance would be equivalent to hitting a target on the moon which is 200/2.5E+8 = 8E-6 km or 8-millimeters wide.Paulkint (talk) 11:27, 28 August 2009 (UTC)

In searching the web on the subject of beamed propulsion, I came across an announcement of an upcoming convention of the American Institute of Beamed Energy Propulsion where interstellar travel by beamed propulsion will be discussed. The announcement included this

The latter subject (interstellar travel) will always be associated with Robert Forward, whose studies of beaming technology and sails made us understand that reaching the stars was not necessarily impossible. Lasers were the key, as Forward learned through his work with Ted Maiman at Hughes Research Laboratory. Years later he recalled his ‘eureka’ moment: “I knew a lot about solar sails, and how, if you shine sunlight on them, the sunlight will push on the sail and make it go faster. Normal sunlight spreads out with distance, so after the solar sail has reached Jupiter, the sunlight is too weak to push well anymore. But if you can turn the sunlight into laser light, the laser beam will not spread. You can send on the laser light, and ride the laser beam all the way to the stars!”

Of course, Forward was wrong because laser beams, as with all light, spreads due to diffraction The divergence may be seem remarkably small, such as only 4 miles after traveling 238,000 miles to the moon as achieved by the Ranging Retroreflector Experiment. But that divergence becomes a monstrous 430-million miles after traveling the 26-trillion miles to Alpha Centauri Sadly, it is evident that humans will never ride a laser beam all the way to the stars. A inexorable fact of physics forbids it. —Preceding unsigned comment added by Paulkint (talk • contribs) 11:00, 30 August 2009 (UTC)

Hi — You are right that diffraction is an issue for beamed propulsion, as I think Christopher Thomas has discussed earlier. But it is not an impossible problem. The best possible beam spread, θ [in radians], for a diffraction-limited optical system, is proportional to λ/D, where λ is the wavelength of the light and D is the diameter of the optical system. So you have to use a large diameter laser (or an even larger microwave antenna), to be able to focus the beam onto a distant spot, of diameter d at a distance L, essentially:

D = λ L/d .

It is also not necessary that the transmitting optics be a single physical object of size D , as "synthetic aperture" techniques can be used, with an array of antennas or transmitters suitably distributed over a diameter of order D. Then the phases of the individual beams have to be controlled to achieve constructive interference on a small spot of size d. Radio astronomers already use this approach in inverse form, to image down into the milliarcsecond range with relatively small antennas separated by intercontinental distances. This approach can be extended over interplanetary distances for D, using a space-based transmitting array. I still consider beamed propulsion unattractive, primarily due to the problem of stopping, but it's not impossible in principle, and the achievable velocities really surprised me when I first heard of the idea back in the 1970s. Wwheaton (talk) 23:00, 30 August 2009 (UTC)

I made this point to him many days ago, and linked to relevant Wikipedia articles describing the techniques, but he's apparently decided to reject it out of hand. Long story short, you can build a segmented mirror as big as you like, because for this problem (keeping a boosting laser and returning light from the craft phase-matched), the system is "embarassingly parallel" (scaling it does not increase complexity or the precision constraints for control of individual units). I'd handwave that the optimal size of individual units is about 2-3 m diameter (i.e., as big as you can cheaply make individual mirror segments). A ground-based array could work, but atmospheric distortion is a serious problem. Ideally you'd put the array in space, or on the far side of the moon (to prevent it from being used to carve one's initials into cities), but that involves launch costs and the cost of putting the power plant (or solar array) in space with it. Placement isn't a concern for a straight feasibility argument, though. --Christopher Thomas (talk) 01:09, 3 September 2009 (UTC)

I have come across a fact at Minute of Arc which is that the most presently developed telescopes have achieved an angular resolution of 0.05 arc sec or 2.42E-07 radians If a sail of 10-km (6.2 miles) is assumed , 41.2 million km. is the maximum distance at which the image can be resolved. That is less that the distance to Mars, so it appears that even the present day super telescopes cannot track such spaceship with a 10-km sail beyond the solar system. What does it take? A10-km image at Alpha Centaur is 2.43E-13 radians viewed from earth, so the resolving capability of a telescope to track a space ship with a 10-km sail to Alpha Centaur has to be one-million times better than the present day best telescopes. .The beam generator has a formable problems as well. It must generate a beam whose width is 10-km at Alpha Centauri. That beam would be 2.3-inches wide at the moon The best that has been accomplished so far is four miles at the moon with the 107-inch Harlan J. Smith Telescope at the McDonald Observatory. But what poses impossible requirements is controlling the pointing of both the telescope and beam generator, well beyond the capability of any conceivable control system . A simple calculation based on the 2.43E-13 radian figure previously given ,shows that the accuracy of pointing is equivalent to hitting a dime with a bullet 46 million miles away. And this dime is moving since the pointing is being done from a rotating earth and the spaceship has to be tracked. On top of all this, if the spaceship ship is in mid-flight what is seen is where it was years previously So it would be firing blindly at the dime guessing where it was and the bullet could take years to get there. All this means that interstellar travel based on beam propulsion is overwhelmingly impossible at the present time and the probability of it being achievable in the future is vanishingly small.Paulkint (talk) 11:00, 7 September 2009 (UTC)

That is significantly incomplete and incorrect. For example, intercontinental baseline radioastronomy routinely achieves resolutions of ~0.001 arc-sec (or even significantly less, I believe) with wavelengths thousands of times longer than visible light. Space-based interferometry promises to achieve sub-micro arc-sec resolutions in a decade or less.

You are absolutely correct, Paulkint, that interstellar travel is an extraordinarily tough nut, probably beyond us for centuries. Yet—given the astounding scientific and technical progress we have made in the past few hundred years—to declare it unequivocally "impossible", and wage a campaign against it across the board as you seem to be doing, really verges on improper point-of-view pushing. You may feel it is so remote a possibility as to be uninteresting; if so, leave it alone. A few centuries is a short time in the history of life and the Earth. We are a part of that great process, and some think it is worthwhile to think about it in the large. Keep us honest, if you like, but do not tie up these pages with a vendetta. You are a smart person, and you know quite a lot, but you are not an expert on these matters (well neither am I; in truth nobody really is, yet). If you would like to discuss particular objections on our personal talk pages, I invite you to do so. Only please don't clutter up this venue implacably; it is inappropriate. All the best, Wwheaton (talk) 16:27, 7 September 2009 (UTC)

Wikepedia: Talk Pages states “A talk page is a space for editors to discuss improvements to articles” I first suggested an improvement in recognizing the central problem of supplying the energy for interstellar travel which had been completely ignored. It aroused a furious response and the facts I gave to show the magnitude of the problem were removed. Next I have shown at length that “Beamed propulsion seems to be the best interstellar travel technique presently available, since it uses known physics and known technology that is being developed for other purposes” is essentially a false statement. The truth is beamed propulsion is not presently available It may be centuries from now but that it not what the statement says. The article would be improved by removing the entire “Beam Propulsion” section for the statements are not supported by “known physics and known technology” which I have shown at length. It amounts to a science fiction imagination being presented as scientifically feasible which has no place in an encyclopedia of fact. I guess I could remove it under Wikepedia rules but I do not remove the contributions of others without their permission.72.150.181.61 (talk) 12:42, 8 September 2009 (UTC)

You have shown no such thing. You raised objections, and then ignored the responses to the objections and simply repeated your objections again. I gave up on trying to address your concerns many days ago, as you don't seem to be interested in collaborative improvement of the article. --Christopher Thomas (talk) 13:59, 8 September 2009 (UTC)

I kept repeating them because they have not been answered. There has been no credible answer to the fact that the resolving power necessary to view a reasonable size sail at Alpha Centauri is a million times greater than what the best telescope can offer today. There has been no answer at all to the fact that the control system must orient the telescope and beam generator to an accuracy equal to hitting a dime 40-million miles away.. There has been no answer to the fact that that for a significant portion of the year the spaceship will only appear during the daylight hours and cannot be viewed and the beam will be pointed blindly. There has been no answer to the fact that even at night the spaceship will be flying directly into the light of Alpha Centauri which will obliterate any reflection from the sail, just as planets cannot be viewed directly by the most powerful telescopes There has been no answer to the fact that, even if viewed, such information as the beam being off target could be several years old and useless for control purposes. There has been no answer to the fact that if the beam gets off center the spaceship will begin to spin like a waterwheel until the sails fly off. There is no answer to the fact that the cost of the electricity for powering the laser could be trillions of dollars Most of all there has been no answer to the fact that it is difficult to imagine a worse scheme for interstellar travel.. Tired of repeating, I depart from the discussion pages. A response will not be viewed —Preceding unsigned comment added by Paulkint (talk • contribs) 11:09, 9 September 2009 (UTC)

Yes, and a rocket capable of a round-trip to the Moon is "impossible", the size of Mt. Everest (...if fueled with the 19th century standard, black powder). Edward Everett Hale's "Brick Moon" space station (Atlantic Monthly, 1883 I think) is "impossible" because it cannot be launched with his spinning super-water wheel. Jules Verne's cannon was impossible as a way to go to the Moon. Rutherford declared the large-scale release of nuclear energy "moonshine" in 1935. Technology cannot be advanced if one is simply hostile and determined to kill off creative possibilities. Beamed propulsion has to be done from space, but there is no fundamental limit to its resolution, nor to beam control. Energy (especially Free energy, in the thermodynamic sense) is abundant in space, as are all the material elements required. Your objections show that you are not seriously considering the possibilities, even with today's technology, for a project that can hardly be expected for a century or more. It is impossible if you approach it with a closed mind, determined to imagine creative failure modes without serious consideration, or even real knowledge. This is much more than is appropriate for this kind of discussion on an article talk page. I remain willing to respond to serious question on user talk pages, but I am soon going to start deleting inappropriate material here without further discussion. Wwheaton (talk) 15:21, 9 September 2009 (UTC)

Energy cost

I have again reverted an edit per our original synthesis rules; not because it is incorrect, but besides being formally inadmissible, it contains unstated and unverifiable assumptions, in particular about the likely cost of energy in the context of a civilization that has expanded to settle much of the Solar System. In the same spirit I changed one "impossible" to "impossible for the foreseeable future", which I hope is acceptable. Wwheaton (talk) 19:43, 26 September 2009 (UTC)

Hi Wwheaton thanks for the lesson, you're a much more seasoned editor. Nonetheless I want to question whether the article wouldn't be better with some more explanation. Here's my case. You deleted:

"Accelerating one ton to one tenth of the speed of light requires at least 125 billion kWh, not accounting for losses. At current prices this energy costs about US-$ 10 billion per ton of a spaceship. Deceleration will require the same again."

I don't think I understand the way you define synthesis nor what assumptions you refer to. There's no synthesis in stating the amount of kinetic energy, that's equivalence derived from calculation, so many sources available that it superfluous to name any. Nor is there synthesis in stating the current price of energy, and it is neither stated nor suggested that energy prices could not change significantly. One can object to the relevance of putting the challenge of interstellar travel in terms that laymen can understand, but I don't think that was the intent of the synthesis rule. And I think it is relevant that the challenge is easily understood. I'm not saying it is impossible. There are smart people out there saying that the only chance for survival of humanity is to spread into space, on the order of hundreds of years. Realizing that "spreading into space" is difficult will make us work harder, both to make it happen and to extend the time we have to achieve it. I'm not sure how downplaying the difficulties can be useful. Thus as you say, my edit was a good faith edit, it was intended to make the article more understandable. I think something along these lines should be added to that paragraph. Rather than just doing it, let me try to discuss it first:

Accelerating one ton to one tenth of the speed of light requires at least 125 billion kWh, not accounting for losses. At current prices this energy costs on the order of US-$ 10 billion per ton of a spaceship. (Note that energy might be much cheaper in the future.)

Your other edit concerns something someone else wrote. But I don't think your edit is correct. There is belief that interstellar travel is impossible as cited later in the paragraph (and there also is belief that it is possible or will become so). Stating that there is belief that it is "impossible for the forseeable future" wrongly suggests that everyone believes it is possible in principle. I think the original version was true. --EphemeralKnowledge (talk) 21:12, 28 September 2009 (UTC)

Automated archiving

What do the lurkers here think about setting up automated archiving for this talk page? I feel that it would be a good idea, due to accumulated inactive threads, but a straw poll is probably in order before I actually add the archivebot activation code. Good idea? Bad idea? Neutral? --Christopher Thomas (talk) 05:58, 30 September 2009 (UTC)

Not exactly a lurker, but I favor archiving, as long as the time limit is fairly long, so we don't entirely lose our memory of older discussions & go in circles. Could we try 3 or 6 months? Bill Wwheaton (talk) 06:11, 30 September 2009 (UTC)

Good idea. But looks like 6 month interval would be enough. Offliner (talk) 06:23, 30 September 2009 (UTC)

Done, with a 180-day threshold. We'll find out in about a day whether or not I configured it properly. --Christopher Thomas (talk) 06:39, 30 September 2009 (UTC)

Update: Looks like it worked, with a couple of glitches (now fixed). I'm going to take some time over the next couple of days to date all of the unsigned/undated comments and split out the remaining headingless threads. After that, we'll probably see about half of the remaining threads archived, and be left with only recent threads. --Christopher Thomas (talk) 05:02, 1 October 2009 (UTC)

First-pass cleanup has been done. Accessing history prior to May 2005 doesn't seem to work, so anything from before that period has a bogus timestamp added in an HTML comment (which is supposed to be enough for MiszaBot). --Christopher Thomas (talk) 07:49, 3 October 2009 (UTC)