The History Channels idea for traveling at half the speed c.

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kewell_

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Ok, they proposed the idea of using sails to travel in the cosmos (obviously not the first time come up with this idea.) But is this plausible? <br /><br />They said light leaving the sun mimics gust of wind in outer space.. (solar wind). The idea was to perhaps have a battery of lasers on the moon. And have a solar sail. Focus the many laser cannons of light on the solar sail and in theory that would propel the sail to maybe half the speed of light. Could this even physically work? Debunk?
 
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vogon13

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Sagan liked the idea.<br /><br />I am not sure he ever advanced an upper limit of .5C however.<br /><br />Niven and Pournelle had a largish proposed light sail at .07C and even it seemed to be pushing the realm of barely possible.<br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>
 
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vogon13

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2nd generation Orion Nuclear Impulse Spacecraft seem feasible (technically if not economically) at perhaps up to ~.1C and with payloads in the 25 million <i><b>ton</b></i> range.<br /><br />Performance a lightsail would be hard pressed to match.<br /><br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>
 
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alokmohan

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We shall be too heavy to enjoy the travel.Whats the point?
 
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siarad

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Never worked out how to steer it without a keel but apparently can be done.
 
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vogon13

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The point in sending a 25 million ton payload at .1C is to carry a viable human colony to another star system so they can settle a new planet.<br /><br />A light sail cannot do this for you unless it is enormous (100,000 km across ??) and is augmented by enough earth and earth orbit bound lasers so as to literally fill the sky.<br /><br /><br />Thanx for providing the impetus for me to finally contemplate something less feasible than an Orion Nuclear Impulse Interstellar Vehicle.<br /><br /><br /><img src="/images/icons/smile.gif" /><br /><br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>
 
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venator_3000

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I'm not sure about "pure" solar propulsion as in a sail that utilizes the solar wind from the sun.<br /><br />However, Bob Forward proposed a laser propelled "sail" that could possibly exceed 0.1 c. Here is a journal reference that I used in a paper I wrote:<br /><br />Forward, R.L. : "Roundtrip Interstellar Travel Using Laser-Pushed Lightsails". Journal of Spacecraft & Rockets: vol 21 (Mar/Apr 1984), pp 187-194<br /><br />I don't have payload or crew capacity. If I recall there were some issues around assuring the laser was left on during the acceleration phase from home and deceleration at the other star system. But that was more social/financial than technical! Another issue was reflectivity/efficiency of the sail materials.<br /><br />Nothing about it being a 1/2 c vessel, however. <br /><br />IIRC GK O'Neill also speculated about building a Bernal Sphere around some type of matter-antimatter propulsion. Sort of a "fast" colony ship. Its top speed was about 0.1-0.2 c.<br /><br />BTW, aside from interstellar travel the laser-sail system could revolutionize travel in the solar system. If it was technically feasible to build such a large laser as well as perfect materials for the sails then you could send small payloads to say, Mars, in a little over a week. Bigger systems could get there in longer periods of time.<br /><br />v3k <div class="Discussion_UserSignature"> </div>
 
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qso1

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IMO, were way to early in our technological development to know if any of the ideas ever proposed would work except maybe Orion. The problem with Orion is expense but the idea you mentioned has one major flaw if its anything other than a flyby mission. And that is, laser cannons to slow it down at the destination star.<br /><br />For a flyby, it might work getting the sail out of our solar system even when accelleration is factored in. The laser battery will have to be as coherant when the probe is nearly out of our system when it hits .5 light speed after months of being accellerated to that speed. Now the questions become, will it get the sail to .5 light and will it be more or less expensive than the Orion concept? <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>
 
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kewell_

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I still dont understand the concept of how a laser can push or accelerate a solar sail in space. How would the laser make an object accelerate the way it has been proposed.
 
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brigandier

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I'm confused about that too.. <br /><br />Is this light sail like a conventional sail in that it uses flowing matter from the Sun to accelerate it? Or does it use a shiny surface to reflect light to propel it?<br /><br />I've heard of the latter. I don't see how light can be used to push a sail
 
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derekmcd

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Newton's third law and the fact that photons, though massless, do have measurable momentum.<br /><br />Solar sails do not use solar wind. <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>
 
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venator_3000

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Hello,<br /><br />A solar sail is a gi-normous mirror-like sheet that reflects sunlight. Sunlight consists of photons. As the photons in the sunlight hit the sheet they will (given a certain efficiency of reflection) bounce off the sheet. When this happens they very gently (micro-grams of force) push the sheet by transferring momentum to the sheet. These very tiny and almost perfect elastic collisions add up. Because there are many, many, many photons in sunlight, and because they are continuously hitting the sheet, there is a continuous pressure (force per unit area, P=F/Area) exerted on the sheet that creates an acceleration that is constant, if very small. Imagine a space payload attached to this giant sheet and you have a sail.<br /><br />Although the force on the payload's sail is extremely below that of the thrust of a conventional chemical rocket the payload and its sail constantly accelerates over time and can ultimately achieve a very high velocity. Light sails would need enormous surfaces, several tens of kilometers or more, perhaps, to utilize solar photons.<br /><br />A laser sail uses the same principles as the solar sail (transfer of momentum from all those teeny photons across the enormous area of the payload's sail). However, the laser sail's designers envision a super laser pushing the sail instead of the light from the sun. The lasers would propel the payload much faster than a solar sail. This is because the photons from the sun are sort of like a steady mist on a breeze while the laser's are intense photon streams that are sort of like fire hydrants of light. You can imagine the difference in force between a pleasant summer mist and the fire hydrant. The same would be true for the photons from the sun versus the laser.<br /><br />The laser sail might use a slightly smaller surface area for its sail. Also, laser sails might use a system where a series of sails bounce the laser light to help improve efficiency.<br /><br />An Earth-based laser could push a sp <div class="Discussion_UserSignature"> </div>
 
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kewell_

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Thanks for the reply, that pretty much cleared everything up for me. I find it fascinating that light is capable of "pushing" objects like a solar or laser sail in space.<br /><br />Im going to check that book out, I am pretty sure they have it at my University.<br /><br /><blockquote><font class="small">In reply to:</font><hr /><p> They even had a demonstrator system that fired laser pulses at a model and elevated it several stories. Cool demo! <p><hr /></p></p></blockquote><br /><br />How does this work wouldnt gravity interfere with the experiement?
 
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venator_3000

Guest
The basic concept behind light propulsion is that the high powered laser can combust or ignite the air. This combustion, if captured and channeled correctly, is basically an explosion. The force of this explosion in turn lifts the payload. The payload has to be designed in such a fashion that it is light-weight and has a geometry to be aerodynamic as well as provide a combustion chamber for the ignited air. <br /><br />So rather than carry fuel and rockets like the space shuttle the laser propelled payload uses the force of ignited air. No pollution. Once in orbit the payload would need some type of fuel however, to maneuver and go places (unless it deployed a solar sail =:)<br /><br />Light propulsion is quite different than the sails we spoke of earlier. The primary similarity is the high powered laser as the propulsion source. Although I suppose it could also be accomplished with intense microwaves. In the test I saw the payload looked like a funny metal hat and was lifted quite quickly into the air. There was a putt-putt noise from the multiple and very rapid explosions. <br /><br />Enjoy the book!<br /><br />v3k <div class="Discussion_UserSignature"> </div>
 
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derekmcd

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You must be referring to the Lightcraft launch. I remember seeing this on the science channel a few years back. Looks like a fun project to work on. <br /><br />edit: fixed link <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>
 
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dragon04

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"Solar Sails" will work, because particles have mass. The sun (well, any Sun for that matter) emits streams of particles.<br /><br />Those particles have kinetic energy. Where's the guy asking about Relativistic Mass in the other thread?<br /><br />Now obviously, a single proton traveling out at, say 1 million MPH doesn't have the ability to push something so heavy as a 10,000kg space ship, but if you put a big enough, light enough sail out in front of it, it's now catching billions of protons moving at 1 million mph, and the cumulative force that they collectively exert can move mountains. Literally. <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> <em>"2012.. Year of the Dragon!! Get on the Dragon Wagon!".</em> </div>
 
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nexium

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Besides the mass of the sail cloth, a means of keeping it approximately flat and facing the optimum direction will at least double the mass, if we get practical sail cloth material at 1/10th gram per square meter. A million square kilometers is a trillion square meters times 0.2 grams = 200 billion grams = 200 million kilograms = 200,000 meteric tons. If it averages one billion kilometers per year per year accelleration, the distance traveled per year is: S = 1/2at squared = 1/2 billion kilometers, which puts it well into diminishing returns = out of range of both the sun and possibly the lasers, for the 2nd year of accelleration. v= at = one billion kilometers per year, for the rest of the trip which is fast for trips to the outer planets, but much too slow for intersteller. This enormous sail (1000 kilometers by 1000 kilometers) will accellerate faster if it carries a tiny payload, but it is unlikely anyone will fund a project that costs a billion dollars per ton of payload, even if it will travel one lightyear in less than ten million years. We can also do better, if we have lasers on some of the moons of the gas giant planets and on comets ect even farther from the Sun, but these come with very costly price tags with present technology. As we increase the accellerating period to several years, we have ask when will the sail be in taters? Neil
 
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derekmcd

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Dragon, <br /><br />Solar sails do not rely on particles from the solar winds. They are simply too slow and too few to have much, if any, effect on a payload. They rely on the actual photons. <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>
 
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brigandier

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<font color="yellow">Newton's third law and the fact that photons, though massless, do have measurable momentum.<br /></font><br /><br />So the sail changes the direction of the photon with reflectivity, right? If it is speculated that a portion of the photon's momentum is transferred, wouldn't that imply that the photon loses some of its speed? <br /><br />It has no mass, so the thing that must give is its speed, right? But isn't light always moving at c? (At least, in the absence of a medium (i.e.- in space))<br /><br />I know I'm probably overlooking something very simple.. and I don't have my textbooks to look through right now
 
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bobw

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Less energetic photons have a lower frequency but the same speed. Maybe it comes in as visible light and reflects off as heat (infrared). <div class="Discussion_UserSignature"> </div>
 
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venator_3000

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That is pretty cool. The lighter the weight of the sail the more payload you can send. If the techniques to build, or as you describe it weave, a sail come to fruition, you might make standard launch packages. Can you imagine an entire propulsion system that is squeezed into a coffee can? The sail plus its payload might be launched by a conventional rocket (sort of the way Cosmos 1 was launched) and then the sail would spring out of the can and be deployed. <br /><br />You might even envision private organizations or universities using standard sail packages with their own specialty payload. Speed is everything. The Pioneers and Voyagers and New Horizon have taken years to travel several AU. This may sound a tad presumptive (pardon me) but if say, the SDC people at these boards built a little instrument and camera and attached it to the weave-sail that you describe, and then launched it today, that small package with modest sail could eventually reach a speed of about 240,000 kph. <br /><br />Assuming Voyager 1 is 15.8 billion kilometers from Earth...<br />...then the SDC Heliopause probe would overtake V#1 in 65,833 hours...<br />...if we assume 8765 hours per year then SDC-H passes V#1 in 7-8 years....<br /><br />That means if we launched today we might pass the Voyagers around the 2015-17 timeframe, despite the fact they were launched in 1977. And then SDC-H would keep on going. That type of flight-time could change our ability to sample the solar system. An SDC Probe Mission to the Heliopause could get out there in about a dozen years (maybe more). Likewise, missions to planets could also be sped up, as the sails can be designed to ultimately maneuver into an orbit. Little payloads sure, but if you have an opportunity to send many little payloads then some fairly good science can be done.<br /><br />That's very exciting, thanks for the info.<br /><br />v3k <div class="Discussion_UserSignature"> </div>
 
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venator_3000

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Thanks for that link. That is the video I saw. I think it was on the PBS Nova show.<br /><br />It is a very cool demo.<br /><br />v3k <div class="Discussion_UserSignature"> </div>
 
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derekmcd

Guest
Like Bob mentioned above, the EM wave loses energy when interacting with a medium. <br /><br />The speed of the photon will always be C no matter the medium. When a photon interacts with matter, it is absorbed which excites the electron (transferring energy). The electron then reemits a new photon still traveling at C. How often the photon is absorbed and reemitted is what gives the illusion of slowing light down.<br /><br />This absorption of energy by the electron is what creates the recoil via Compton Scattering. Depending on how reflective the surface of the sail is determines the direction of the scattering.<br /><br />I hope someone with a better understanding of QED can chime in. I might be a bit out of my league here.<br /><br /> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>
 
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venator_3000

Guest
I think it may have everything to do with a combination of the amount of photons from the Sun, momentum change, and the surface area of the solar sail.<br /><br /> I know that seems counter-intuitive but consider anything travelling at lightspeed. The photon has no "rest mass", but then photons are always moving, thus photons do have mass via the energy they carry.<br /><br />Here's my algebra and physics-lite solution:<br /> E=mc^2, so m = E/c^2 (carrot means "squared")<br /><br />momentum =p = mv = E/c.<br /><br />If our solarsailors have equipped their ship with a sail that is like a very reflective mirror-sheet then it has the ability to gain up to twice a given photon's momentum,<br />because the light isn't just stopped, it's reflected directly back in the direction from wence it came.<br /><br />This suggests that twice the change in velocity means twice the momentum developed.<br /><br />In terms of the math as it relates to earth orbit, where I assume our soalrsaoilors will begin...<br /><br />Sunlight "strength" is: Energy/(AREA x t) = 1kilowatt/m^2 = 1000 Joules/m^2-sec<br /><br />allowing that the photon is at lightspeed= c= 3x10^8 m/sec<br /><br />Developed Thrust on Sail/AREA = p/time/area = E/c/time/area = E/(AREA x t)/c where p=momentum<br /><br /> = 1000 J/m^2.sec / 3x10^8 m/sec = 3x10^-6 J/m^3-sec^2 = 3x10^-6 Newtons / m^2.<br /><br />So not a lot of force per photon but if you have a large enough area the effect adds up. The elastic collision between photon and sail is similar to a classroom physics experiment where two pucks collide on an air-hockey table. Plans for the solarsails entail kilometers-wide sheets that will allow as many photons as possible to hit them. As each photon-sail collision occurs, the photon transfers its momentum to the sail, pushing it away as the photon bounces off in the opposite direction.<br /><br />v3k<br /> <div class="Discussion_UserSignature"> </div>
 
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15203700700579

Guest
Well I'm probably not a genuis but why would the "History" Channel have futuristic shows on? And also wouldn't solar radiation ,sorry you just said it was a SOLAR sail! Well I would think if it wasn't done right it would cause drag.
 
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