A few questions about space travel

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baulten

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Greetings,<br /><br />I'm a new member to the forum, though I've been browsing space.com for a while. Skipping the formalities, I'm writing a paper on interstellar space travel, and I've got a few questions.<br /><br />I read in another thread on the forums (Unfortunately, once I got my account activated, I could not find the thread) that if you accelerate fast enough the cosmic microwave background radiation will become blueshifted and essentially fry you and your ship. Is this true? If so, what speed would this occur at?<br /><br />My second question is: Would it be possible to use a solar sail-like device to slow down upon reaching another solar system? In the paper, I'm trying to outline a rough plan for an interstellar probe, and I was wondering if such a device would work for breaking at least somewhat, and at what speed a probe would need to be traveling for it to be a viable method of breaking if it is indeed possible.<br /><br />Last question: Would using Proxima Centauri as a gravity-assist point work for a mission toward Alpha Centauri A and B? I'm a bit of an amateur at the physics of space travel, so I apologize in advance.<br /><br />I hope someone can help, and thanks in advance.
 
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billslugg

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Welcome aboard! I don't know whole lot about your questions but I'll give it a shot. At some very high speed the background radiation will be blue shifted into the x-ray and gamma ray. That would not happen except at extremely high speeds - like 99.999999c. Prior to that, things like cosmic rays, interstellar atoms and starlight would kill you. So you don't have to worry about the 3 Kelvin background.<br /><br />A solar sail would help but it produces a very small force so I don't think it would slow you too much. Of course if you used a solar sail to escape the sun and when you got half way there, you turned it around to slow you down then it would exactly balance out.<br /><br />You cannot use a body to accelerate you in the direction you want to go, unless the body is moving in that direction. I don't know what direction Proxima Centauri is headed, so I don't know if it would help.<br /><br />I read somewhere that if you sent the space shuttle orbiter to Alpha Centauri in a period of time short enough for one person's life it would require the mass of the universe in fuel.<br /><br />Interstellar travel is possible. Fast interstellar travel is difficult. <div class="Discussion_UserSignature"> <p> </p><p> </p> </div>
 
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baulten

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Thanks for the response. That cleared up the whole MBR thing, and I'll assume you meant .99999~c instead of 99.999, since FTL is impossible and all.<br /><br />As for the Proxima Centauri thing, I guess I was thinking of it in a different way. What I was meaning is if, since it lies between us and the two other stars in that system, is using Proxima as a gravity assist would be more fuel and/or time efficient than a direct route to the central two stars. The reason for this is that in my paper and the rest of the project I was thinking that dropping an orbiter around Proxima for scientific research purposes might be a good idea, as more knowledge about red dwarfs is something that might prove useful. If the gravity assist method around Proxima would not work or would cost more fuel or time (or both) then I may drop that idea.<br /><br />I also read the part about the amount of fuel to reach A. Centauri being more than the universe, but I believe it was debunked and consisted of some wrong math, or something along those lines.<br /><br />A bit more information: My plan was to lay out rough designs for a probe using a Project Orion-style nuclear pulse propulsion system to reach the system in between 50 and 100 years. I'm still working on the details and physics of it, but from the various other projects (Orion, Daedalus, Longshot, etc.) it seems that such a mission would be possible. <br /><br />Edit: Forgot to talk about the starlight, interstellar medium, etc. that would "kill you" as you put it. I realize that at significant fractions of the speed of light such things become very deadly, but right now this is only a robotic probe I am laying out, and I believe I have read that at around .1c those things would not be as deadly and would be able to be shielded against. I'll have to do some research.
 
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vogon13

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Quite a bit of work on Orion Nuclear Impulse has been done here.<br /><br />You might want to consider .1C as an absolute upper limit for speed, and be prepared for an enormous amount of engineering to achieve. Also, the acceleration and deceleration phases will take decades. Flight time is actually going to be long despite the .1C speed since a large portion of the journey will be spent in accel and decel mode.<br /><br />Additionally, Orion works better as it gets bigger.<br /><br />This .1C vehicle will be sized to transport a viable human colony to the Centauri system. Payload will be millions of tons, and note most of your propulsion nukes are going to be in the 20 to 30 megaton range. Weapon cores may be distributed across the pusher plate to reduce demands on the shock absorber system, and in flight assembly of the nukes will be required, as well as periodically shedding annular rings from the pusher plate as the bomb load is burned off. This is to extract the maximum performance from the propulsion system, the closer to optimum size you keep the pusher plate, the more effecient the vehicle becomes.<br /><br />Radiation channel 'filler' will be poo from the crew, btw. A low Z material is needed for this function to spread the impulse out over time. loading the bombs also give the crew something to do during the flight.<br /><br /><br />Don't bother with Proxima Centauri. Detour not worth the bother or added risk. If you want to put an instrument package on some of the shed pusher plate material and send it in that direction, go ahead. Proxima encounter would be interesting, but of no consequence in establishing a colony. <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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billslugg

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I think the planet or star you want to use for a gravity assist has to be moving in the general direction of where you are going. You could always use it to change your direction. I think you could visit it, swing by and end up pointed at your final destination. No additional fuel needed. You could not stop and orbit for a while. That would cost extra fuel. <div class="Discussion_UserSignature"> <p> </p><p> </p> </div>
 
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baulten

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@vogon13<br /><br />I do already realize many of things you are saying. I believe that in the Project Longshot pdf I read it stated 100 years as the time it would take said project to enter orbit around the system. That's why I said 50 to 100 years, since I was unsure of the exact flight time for acceleration, cruise, and deceleration.<br /><br />However, as to the part about Orion working better as it got bigger, I hadn't heard that. Is this to say that a manned colony aboard the ship would in fact make it a more viable mission than a probe? That seems to be what your post is saying.<br /><br />And you literally mean that, in a manned crew, the radiation filter would be... bodily wastes? I'd always assumed that either the left over slag or other waste materials from construction would be melted down and formed into the shielding. Hadn't heard that idea before.<br /><br />Thanks. <br /><br />Edit: @ Bill,<br /><br />The main idea for the flyby was the possibility of deploying a secondary probe for a long term mission around the red dwarf. I had figured it might be a good idea to do since the star is technically "on the way" to A. Centauri A/B. I understand that said object needs to have the same relative direction as where you're using it as a gravity assist to, which is what I'm asking now (earlier I was indeed inquiring as to if it were possible). Obviously going into orbit and then leaving orbit would take more fuel.<br /><br />Thanks<br /><br />Edit 2: Another thing I'd like to ask is if anyone has a link to a good document that would help me better understand the mathematics and physics involved in determining how I would fire this probe at A. Centauri and how I would slow it down. I know the basic ideas, but mathematically, I'm a bit lost.<br /><br />Edit 3 (No I'm not just editing for the fun of editing, I keep noticing things I missed! <img src="/images/icons/tongue.gif" />):<br />I do believe I read an article that said a pusher plate sprayed with oil would not be wo
 
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vogon13

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Scaling Orion Impulse down to an interstellar probe isn't feasible.<br /><br />I suppose a full size unit could be launched with only a probe for a payload, but that is getting pretty weird.<br /><br />Efficiency of nukes goes up with size, and an interstellar vehicle needs to squeeze every bit of efficiency out of every parameter.<br /><br />Hence the shedding of pusher plate material enrout.<br /><br />Additionally, bomb load needs to be graduated in size. As the mission progresses, mass of bomb load burned off necessitates a smaller pusher plate and realtively smaller nukes. <br /><br />Mass of vehicle at launch is ~90% bombs. Performance of vehicle changes enormously as bomb load is utilized. A 25 megaton nuke at the end of the mission is grossly to large. Something around 3 meagtons would be used for final velocity trim to enter orbit around the target planet.<br /><br />Alpha Centauri doesn't need to be final destination. Once the thing is accelerated, cruise duration of voyage can be as long as human generational crew can be trusted to complete the mission.<br /><br />We don't want to use 'slag' for radiation channel filler. Metal oxides to high Z material. poo contains H, C, O, and N, perfect material.<br /><br />Also, slag is too heavy to carry for that purpose. Having 'food cycle' open to a degree will be more efficient.<br /><br /><br />.1C Orion vehicle needs literally every once of efficiency and performance possible. Thats why bomb materials go on the pusher plate. Drastically cuts demands on suspension system, and distributes mass across pusher plate allowing a thin plate to be used.<br /><br />We will need advanced materials, very efficient nuke technology, and cutting edge engineering to make it work.<br /><br /><br />Also, construction time is 200 to 400 years for the vehicle, and I do not see how we can avoid using Orion Impulse to launch construction materials from earth's surface, as the vehicle has to be assembled in orbit.<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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billslugg

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vogon13<br />Isn't the low z thing for neutrons? Don't you also need high z for gamma? <div class="Discussion_UserSignature"> <p> </p><p> </p> </div>
 
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baulten

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Ah, that makes total sense. I can't believe I was completely overlooking the mass of bomb payload.<br /><br />If nuclear pulse propulsion is not a viable way to send an interstellar probe, then what propulsion system would be more viable? Planning out a manned mission seems out of my league, as even this robotic probe is proving difficult to come up with a good plan. <br /><br />Are robotic probes, despite their initial favorable appearance (No close life support system needed, much smaller, etc.) just not viable due to the difficulty of a propulsion system that can accelerate it?
 
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billslugg

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The ultimate speed that can be obtained is by the ratio of the launched mass to the payload mass (among other things). If you could start with a launch mass of several million tons and end up with a payload mass of a few grams then you would have it made. <div class="Discussion_UserSignature"> <p> </p><p> </p> </div>
 
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vogon13

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Just an idea, but ejecting 100 or so deuterium pellets per second out the back of a small pusher plate, and zapping them into fusion reaction with an x-ray laser would be the 'sleek' form of Orion instead of the 'blunderbuss' form.<br /><br />100 little pulses per second would still generate intersting thrust levels, and would be scaled to the size range you want.<br /><br /><br />Advantage to big Orion is that we know how to make big nukes.<br /><br />Disadvantage to little Orion is we still don't have technology to efficiently fusion ignite little dueterium pellets.<br /><br /><br />Little Orion could even be made modular, and if you put enough modules together, you could make something as large as big Orion, or anything in between. And fusion is 100 times more energetic than fission, and fission is a sizeable percentage of the big Orion's nukes. This really give vehicle dynamics a good shove up the graph.<br /><br />You can either go faster, take more stuff, or trade of both for what you want to do.<br /><br />(assuming x-ray laser turns out be manageable in size, cost, performance and engineering)<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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baulten

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So the main reason that Orion's pulse propulsion system is somewhat ineffective for robotic probe missions is because it can't be scaled down due to the dynamics of nuclear weapons?<br /><br />That seems to leave anti-matter catalyzed nuclear pulse propulsion (Possible, but extremely costly) and like you said, micro-nuclear fusion propulsion. Am I correct in this assumption?<br /><br />Outside of pulse propulsion systems, what about Fission Fragment Rockets? I believe I saw quoted something like 3-5% of the speed of light, which I would think would also be easier to slow down that upwards of 10% of the speed of light, making travel time about the same? (Slightly more with a FFR, obviously, but less time spent decelerating upon reaching the star, no?)
 
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vogon13

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IIRC, nuclear yields in the 1 to 10 kiloton range can loft a 4000 ton craft off the earth's surface.<br /><br />This an order of magnitude bigger than the Saturn 5, and virtually all of the 4000 tons makes it to orbit, instead of just the relatively small capsule.<br /><br />(a demonstration of the efficiency of nuclear reactions compared to chemical)<br /><br /><br />I once facetiously pointed out an interstellar craft could conceivably carry the Great Pyramid of Cheops if we were short of payload to send.<br /><br /><br />Probably makes more sense to work out a pulsed fusion device. I am only slightly familiar with the fission fragment concept, and I remain skeptical it would be practical. Although the environmentalist folks might hate that one more than Orion Impulse.<br /><br /><br />Regarding peak speed, you just have to have enough 'juice' to slow down at the end, whatever the peak is.<br /><br />Burning off the pusher plate in the destination star's photosphere would be an interesting, if unsurvivable, alternative to a braking series of nukes. Might work on a probe only mission, however . . . <br /><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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nexium

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A few experts have argued that the Orion pusher plate would be distroyed too soon even if it converted only 1% of the H bomb yeild to acceleration of the the intersteller ship. We have not tested the pusher plate idea, so it is mostly math at this point.<br />At present perhaps the ion engine and several simular engines are most promising in my opinion. They need to be scaled up more than a million times the present puny thrust. The ion engine has the advantage that it would provide a small amount of artificial gravity during accelleration and deceleration. Is 1/5 g better for the health of the crew than free fall? We have not tested this, but we may soon, when suitable tethers are available to spin two space craft around each other with perhaps a kilometer of tether.<br />If a = 2 meter per sec per second: t = 146 million seconds: S = 1/2at squared = 21,316 trillion meters = 21.316 trillion kilometers = half way to Centarii A or B. We then decelerate the remaining 1/2 of the trip, so we can make a soft landing in the Centarii system. It may be practical to swish past Centarii Proxima. We might even want to do a sling shot manuver, if we are operating at reduced power, which is likely after 290 million seconds = 80,555 hours = 3356 days = 9.2 years, ship time. More time elapsed back on Earth as we reached about 0.9 c at the half way point. Protecting from radiation and micro meteorites is far beyond present technology as the others suggested even at 0.1 c.<br />I agree the cosmic background radiation is not a problem, but the occassional sub atomic partical produces intence gamma radiation at 0.9 c or even 0.1 c. I don't think we can protect vital systems of even a probe from these sub atomic particles even at 0.1 c. My guess is we need many major technology advances to make a trip to the nearest stars practical. Neil
 
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vogon13

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Materials have survived immersion in nuclear fireballs virtually unscathed.<br /><br />Lew Allen's carbon spheres for instance. And computer simulations of weapons effects indicate pusher plate erosion can be small enought to be practical. <br /><br />Hydriding of a steel pusher remains a concern, although Dysons' suggestion of a DU plated pusher remains an interesting palliative.<br /><br />I have some concerns about fatigue cracking in the pusher, although a laminated and maintainable shield might be built. <br /><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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baulten

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From everything I read regarding the Orion propulsion system, it seems that it is indeed viable. Though off-hand I can't recall the materials needed for the pusher plates, I believe the general consensus was that erosion would not be a problem. <br /><br />Ion thrusters have the problem that they produce very, very low thrust. I don't know if it will ever be feasible to build a large enough thruster, since they take so much energy. <br /><br />This project is entirely theoretical and it's not plans to build one, just how it might be done. I realize that at the very minimum, it'll probably be 50 or (more likely) more years before we can send a mission outside of our solar system, depending on how anti-matter and fusion technology progress.
 
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nexium

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If Orion produces an average acceleration of 1/5g, my numbers apply. 80,555 nukes are needed at one per hour, and I think much oftener than one per hour is proposed. Likely Orion is thinking much longer than 9.2 years ship time; perhaps coasting for several centuries. Several centuries means hardly any of the equipment will be repairable at the destination. Even spare parts in the original box typically fail in about a century, some much sooner, without being used at all. Neil
 
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baulten

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I think your assumption that things would fall apart in the time it would take for a mission to a nearby star system is slightly false.<br /><br />For example, I was on a tour of a hydro-electric dam in my state a few days ago. The entire thing had been built over 50 years ago, had only minor replacements that never would have had to happen if it hadn't been overlooked in the first place, and is expected to run at least another 50 years without having to replace any major parts. Now I don't see why it's completely unreasonable to think that a spacecraft could be build that could last just as long.<br /><br />The pusher plate mechanism obviously will need testing and research, but the vast majority of data I have found says it really should work.
 
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richalex

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The most likely device for interstellar travel is an antimatter-induced fusion engine. A gram of anti-matter with several kilograms of fusionable material could produce enough thrust to make robotic probes to the nearest stars practical with a travel time of a decade or two. The probes also don't have to be very large and don't have to survive the effects of detonating nuclear bombs. Professional interest in antimatter augmented engines is increasing, and may be our best hope for exploring the outer solar system within our lifetimes.
 
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siarad

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<blockquote><font class="small">In reply to:</font><hr /><p>Last question: Would using Proxima Centauri as a gravity-assist point work for a mission toward Alpha Centauri A and B? I'm a bit of an amateur at the physics of space travel, so I apologize in advance.<p><hr /></p></p></blockquote><br />Gravity is not a form of energy, gravity assist is only the means of <i>transferring</i> inertia from the passed object. Therfore you have to passby & be dragged <i>sideways</i> changing direction but robbing some of the inertia. Note your speed hasn't changed as gravity isn't energy but your direction & therefore velocity has.<br />Sorry don't know the stars of which you speak but to offer gravity assist one must be passing the other not simply moving to or from, as in our planets passing each other.<br />Firing a rocked whilst in a heavy gravity field increases it's efficiency but isn't gravity assist as is generally understood. This could be done orbiting the Sun but obviously can't be used as gravity assist inside the solar system.<br />I'm not a scientist so this is a laymans view <img src="/images/icons/smile.gif" />
 
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baulten

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I understand the gravity assist method for the most part. I know it's not energy and that it only transfers your inertia, which is what I'm asking. I was a bit misunderstood, though, as I had assumed you used the gravity of an object to increase your rockets efficiency, slingshotting you toward the next target. Thanks for clearing that up.<br /><br />In reference to anti-matter catalyzed fusion reactions, that indeed would be the most effect method that we, theoretically, could do right now. Unfortunately, with anti-matter so rare and hard to produce, it doesn't seem as feasible as a propulsion method. Using a few nanograms for micro fusion/fission seems like a suitable method, but I'm not sure as to the feasibility of that (in terms of specific impulse, acceleration, etc.). Anti-matter is definitely looking like it will be the future of spacecraft propulsion, though. <br /><br />New question: How much of a problem would radiation be for a robot probe be, and how exactly would you shield against that?
 
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richalex

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<blockquote><font class="small">In reply to:</font><hr /><p>Note your speed hasn't changed as gravity isn't energy but your direction & therefore velocity has.<p><hr /></p></p></blockquote>Speed and velocity are the same thing. <br /><br />A gravity assist is capable in some instances of increasing the velocity of a passing spacecraft.
 
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derekmcd

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Not exactly. A gravity assist will not increase speed relative to the planet due to conservation laws. It does, however, change velocity which is direction dependent. <br /><br />Velocity is a certain speed in a particular direction. <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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billslugg

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Not exactly either. A gravity assist from a planet to a spacecraft will increase the spacecraft's speed, it will increase the spacecraft's velocity magnitude and will change the velocity vector. I believe it is momentum that is conserved. The spacecraft speeds up relative to the planet and the planet slows down. New Horizons approached Jupiter at 23 km/s relative to the Sun and left Jupiter at 27 km/s relative to the Sun. (21 and 25 relative to Jupiter). With a mass of 478 kg compared to Jupiter's 2E27 kg, Jupiter was slowed down about .1 micron per year. <div class="Discussion_UserSignature"> <p> </p><p> </p> </div>
 
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qso1

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Using any star as gravity assist is not likely to help much because the forward velocity is so high and the time it takes to decellerate would basically negate the possibility of gravity assist without coming in too close to the star I would think.<br /><br />Solar sail would maybe get around that problem but solar sailing could take much longer to decel. <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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