Infrastructure for a robotic mission to Alpha Centari

willpittenger · Sep 28, 2007

At the speeds in which we could launch probes today, even New Horizons, we would need centuries to get it into position for the observations you suggested. The Centari system is just too far away. <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>

willpittenger · Sep 28, 2007

<blockquote>In reply to:<hr />1. There is not much gained by sending a probe to Alpha Centauri to "monitor" things that we can also do with sophisticated space bound telescopes here.<hr /></blockquote> I wasn't arguing otherwise. Just noting a bunch of requirements. <blockquote>In reply to:<hr />3. The first interstellar probe will very likely not slow down at its destination, because this would make matters a lot more complex and the spacecraft by magnitudes heavier than an interstellar probe that does a flyby.<hr /></blockquote> A solar sail couldslow down. <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>

MeteorWayne · Sep 28, 2007

I'm not sure even a huge sail gets you much acceleration once you get past Neptune. It get it's oomph from solar radiation, which is falling off very rapidly out there. <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

mithridates · Sep 28, 2007

That would depend on how much of a different angle we would need. A shift of 1 degree would require 1.75% of the distance to the star, so 4,800 AU, way too far. I don't know how far would be required though. Any distance where a transit can be seen that can't be seen from Earth would make the probe viable. ( <div class="Discussion_UserSignature"> ----- http://mithridates.blogspot.com </div>

j05h · Sep 28, 2007

Solar sails to other stars probably require lasers in orbit or similar. Something like gigawatt lasers at Jupiter's Trojan points or other deep future tech. Sails can slow down by ejecting outer part and using that to reflect laser light back at the the inner sail and payload. Josh <div class="Discussion_UserSignature"> <div align="center">We need a first generation of pioneers. </div> </div>

holmec · Sep 29, 2007

<blockquote>In reply to:<hr />It is not a question of an ion thruster running 20 years, it is a question of building one (or clusterin more) that large that it can propel your spacecraft with the required propellant for such a long time.<hr /></blockquote> Agreed. (BTW I never suggested scalability). <blockquote>In reply to:<hr />Technically spaceprobes with current ion-drive technology can not go much faster than for instance the Voyager probes (that ultimately gained a lot of speed by gravity assists)<hr /></blockquote> Not because the technology doesn't allow it. Also I only know of three spacecraft with ion propulsion, Deep Space 1 (a test bed for technologies, not a real probe), Smart-1 (ESA lunar orbiter with a Russian ion thruster), and DAWN (just launched). You could make a large ion thruster with large amounts of gas and a large reactor to make electricity. The thing is to make it go faster, you need to increase the gas output through the grid, that requires more Amps. And I think you can increase the Voltage for to try to get the gas to flow faster. I'm talking design phase not scalability. All this means a really big reservoir of gas. The good news is that such a trip is finite, so with the advancement of technology it should be doable. <div class="Discussion_UserSignature"> "SCE to AUX" - John Aaron, curiosity pays off </div>

MeteorWayne · Sep 29, 2007

As I understand it, some Geostationary satellites use ion propulsion for stationkeeping as well, since it's very efficient, and perfect for the job. <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

holmec · Sep 29, 2007

<blockquote>In reply to:<hr />A solar sail could slow down.<hr /></blockquote> That's correct, at least in theory. A real mission would probably make use of a combination of solar sail, ion thrusters, and gravitational navigation. And maybe even staged ion thrusters and solar sails. That way you have less mass in slowing down. (ie one solar sail for launch out of our solar system, and one tailored for Alpha to slow down). <div class="Discussion_UserSignature"> "SCE to AUX" - John Aaron, curiosity pays off </div>

holmec · Sep 29, 2007

<blockquote>In reply to:<hr />As I understand it, some Geostationary satellites use ion propulsion for stationkeeping as well, since it's very efficient, and perfect for the job. <hr /></blockquote> I only heard talk about it. Know of any currently in Geostationary orbit using ion thrusters? <div class="Discussion_UserSignature"> "SCE to AUX" - John Aaron, curiosity pays off </div>

MeteorWayne · Sep 29, 2007

I don't offhand, but I read it somewhere........ <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

themanwithoutapast · Sep 29, 2007

"Not because the technology doesn't allow it. Also I only know of three spacecraft with ion propulsion, Deep Space 1 (a test bed for technologies, not a real probe), Smart-1 (ESA lunar orbiter with a Russian ion thruster), and DAWN (just launched). You could make a large ion thruster with large amounts of gas and a large reactor to make electricity. The thing is to make it go faster, you need to increase the gas output through the grid, that requires more Amps. And I think you can increase the Voltage for to try to get the gas to flow faster. I'm talking design phase not scalability. All this means a really big reservoir of gas. The good news is that such a trip is finite, so with the advancement of technology it should be doable. " Scalability has its limits. As I said, it is technically not feasible to build large enough ion-drives or cluster that much + include enough nuclear reactors and enough propellant to reach the speeds you indicated to reach Alpha Centauri in 26 years. Engines need to be at the back-end of a spacecraft, the longer they run, the more propellant you need, which again increases the number of engines you need because of weigh increases which again increases power requirements etc. etc. - at some point you reach a limit where it is technically just not feasible any more to cluster more engines together at the back of your spacecraft and have gas as well as energy diverted everywhere (for the speeds you indicate we would be talking about clustering millions or even billions of ion thrusters together).

themanwithoutapast · Sep 29, 2007

"You could make a large ion thruster with large amounts of gas and a large reactor to make electricity. The thing is to make it go faster, you need to increase the gas output through the grid, that requires more Amps. And I think you can increase the Voltage for to try to get the gas to flow faster. I'm talking design phase not scalability. All this means a really big reservoir of gas. " To explain my post above further, here is a calculation why your (currently available) ion-drive has a limit to "top-speed". What scaling up a spaceprobe does is increasing the percentage of propellant compared to the total mass of the spacecraft. That is for instance Smart-1 had a mass of 387 kg of which 80kg was propellant for a propellant/mass ratio of about 22%. Now, it does not matter how large your spaceship will be, at some point you will reach a limit with this percentage. Currently I would say it is for sure below 90%, but for the sake of argument, let's say you can build a spacecraft with 98% propellant and only 2% of the mass for structure (tanks, pipes, electronics, heat-resistance etc.), engine and payload. Such a spacecraft with ion-engine(s) with an isp of 3000 would allow you to acclerate the spacecraft by about 100km/s or about 0.03% lightspeed. The trip to Alpha Centauri would thus take the spacecraft 12000 years. As you indicate, you would need to have the "gas flow faster", that is use another technique than the available ion-thruster, for instance such as VASIMR (which is currently not an available engine due to technological problems). But even with VASIMR, which has a max. thrust of 30000 sec and the above assumed (rather unrealistic) 98% propellant/mass ratio a spaceprobe would require 1200 years to Alpha Centauri.

gunsandrockets · Sep 29, 2007

Any rocket just isn't going to work very well for an interstellar probe. The problem is a practical rocket can't exceed the speed of it's exhaust by much more than a few multiples, or a speed about 3 times higher than the exhaust velocity. The NSTAR engine of Deep Space 1 had an exaust velocity of about 29 km/s. An experimental 3-grid ion engine that ESA is working with has a potential maximum exhaust velocity close to 200 km/s. Even with the better engine, that still puts the maximum speed of an ion engined interstellar probe at 600 km/s. The speed of light is 500 times faster than that, so the probe would take over 2 thousand years to reach Alpha Centauri. That travel-time demonstrates the magnitude of the difficulty of an interstellar probe. It's almost insanely difficult. [math error corrected!]

holmec · Sep 30, 2007

<blockquote>In reply to:<hr />The problem is a practical rocket can't exceed the speed of it's exhaust by much more than a few multiples, or a speed about 3 times higher than the exhaust velocity. <hr /></blockquote> Is that based on relatively? Because Newtonian physics allows for it. <blockquote>In reply to:<hr />The NSTAR engine of Deep Space 1 had an exaust velocity of about 29 km/s.<hr /></blockquote> according to Deep Space 1 site: "The xenon ions travel at about 35 kilometers/second (77,000 miles/hour). " <div class="Discussion_UserSignature"> "SCE to AUX" - John Aaron, curiosity pays off </div>

billslugg · Sep 30, 2007

At the risk of revealing that I have no idea what I am talking about, I believe the exhaust speed multiple limitation might only apply within an atmosphere. In space, it should make no difference whatever, what speed something is cast out the back. If it has mass and is cast with speed, the result will be an increase in velocity of the ship. <div class="Discussion_UserSignature"> </div>

themanwithoutapast · Sep 30, 2007

<blockquote>In reply to:<hr /> Is that based on relatively?<hr /></blockquote> No, that is based on what I have explained above. You get to a point where you just can't increase the propellant/total mass ratio of a spacecraft. Ion-thrusters have exhausts speeds of about 30 km/s which gives you at the very theoretically outmost a top speed of your craft of 100 km/s.

holmec · Sep 30, 2007

<blockquote>In reply to:<hr />No, that is based on what I have explained above.<hr /></blockquote> I'm asking for the math, I don't want a reiteration of what you said. In space as long as you can 'burn' you can accelerate. Anyway what you posted is a separate issue that what gunsandrockets posted. And while your post is probably right it doesn't preclude me from exploring other things wrong with the idea. <div class="Discussion_UserSignature"> "SCE to AUX" - John Aaron, curiosity pays off </div>

holmec · Sep 30, 2007

<blockquote>In reply to:<hr />At the risk of revealing that I have no idea what I am talking about, I believe the exhaust speed multiple limitation might only apply within an atmosphere.<hr /></blockquote> That's how I understand it. <div class="Discussion_UserSignature"> "SCE to AUX" - John Aaron, curiosity pays off </div>

billslugg · Sep 30, 2007

So regardless of where you are; if you are throwing mass out the back, eventually you run out of mass? So the ship velocity to exhaust velocity ratio is based on practical considerations of how much payload is left over after all the mass has been thrown out the back? Thus, you might have a million to one ship speed to exhaust speed if your payload was comprised of a speck of matter? Is this correct? <div class="Discussion_UserSignature"> </div>

holmec · Sep 30, 2007

Sounds about right: Except its product not ratio. That is multiplication rather than division. Is momentum mass x velocity <div class="Discussion_UserSignature"> "SCE to AUX" - John Aaron, curiosity pays off </div>

holmec · Sep 30, 2007

Another technology that could be used is the nuclear tech from Project Orion. There's 4.3 light years between here and Alpha. That's a lot of environment. Why not use nuclear blasts between the two solar systems to get to Alpha? I would expect the contamination to be extremely low for either solar system, that is if the residue of the explosions end up in either solar system over time. <div class="Discussion_UserSignature"> "SCE to AUX" - John Aaron, curiosity pays off </div>

themanwithoutapast · Sep 30, 2007

<blockquote>In reply to:<hr />So regardless of where you are; if you are throwing mass out the back, eventually you run out of mass? So the ship velocity to exhaust velocity ratio is based on practical considerations of how much payload is left over after all the mass has been thrown out the back? Thus, you might have a million to one ship speed to exhaust speed if your payload was comprised of a speck of matter? Is this correct?<hr /></blockquote> Not entirely, what you are describing is only technically possible if you consider a staged spaceship approach. I was talking above of a one-stage spacecraft that is accelerating. Such a spacecraft needs a certain dry mass for its structure, engine and payload which results in a maximum speed of about 100 km/s no matter who large you make your spacecraft. Of course you can consider a multi-stage approach with ion-thrusters, but if you do the calculations you very soon come up with unfeasible large masses although your top speed is not that high. Consider a 10 kg payload (the actual probe). Further consider that it is possible to build a spacecraft exisiting of 80% propellant, 10% structure (tank, engines etc.) and the 10% payload. This one-stage probe with an ion-thruster could propel the 10kg payload assuming an isp of 3000 sec to 50km/s. If you build a second stage which has a mass of 1000 kg and which payload is the 100kg first stage you can similarly achieve another 50km/s accleration. The problem of course is, this staged approach exponentially increases your spacecraft's mass. At 250km/s for your end-spacecraft (or 5 stages), the mass of your inital spacecraft would need to be 1000 tons or more than 2 times the ISS-mass. In order to reach 1000 km/s or about 0.3% of lightspeed your spacecraft needs to be nearly as heavy as the Moon. And to reach 3000 km/s or 1% of lightspeed with your spacecraft you will require an

holmec · Sep 30, 2007

<blockquote>In reply to:<hr />Not entirely, what you are describing is only technically possible if you consider a staged spaceship approach. <hr /></blockquote> He was talking about thrust regardless of stages. <div class="Discussion_UserSignature"> "SCE to AUX" - John Aaron, curiosity pays off </div>

holmec · Sep 30, 2007

<blockquote>In reply to:<hr />In order to reach 1000 km/s or about 0.3% of lightspeed your spacecraft needs to be nearly as heavy as the Moon. <hr /></blockquote> But what acceleration do you get? When you talking about thrust is space your talking acceleration. And that is all that matters on long trips. For instance if you have a constant acceleration of 30Gs you could, theoretically, reach Alpha in 112.6 years. But practically you run into problems with relatively because you cannot travel faster than light. so it would take you much longer. Also bear in mind that you cannot sustain speed of light because it require infinite energy. So with a certain amount of thrust you end up with a point where your acceleration just becomes constant speed. Someone should have a graph out there to show what that would be. DOH! At high speeds [v- />c] momentum is redifined as: p= [lorentz factor]mv p= momentum c= light speed [lorentz factor] = 1/sqrt(1-(v/c)^2) v = velocity Shoot I should use this to make a graph in Excel. <div class="Discussion_UserSignature"> "SCE to AUX" - John Aaron, curiosity pays off </div>

themanwithoutapast · Sep 30, 2007

<blockquote>In reply to:<hr /> He was talking about thrust regardless of stages. <hr /></blockquote> I think you don't understand my post. When I say that an ion-thruster has an isp of 3000 sec I speak of thrust. 3000 sec means an exhaust velocity of about 30 km/s. <blockquote>In reply to:<hr />But what acceleration do you get? When you talking about thrust is space your talking acceleration. And that is all that matters on long trips. <hr /></blockquote> Exactly, as I mention above one stage in my hypothetical gives you an acceleration from a hypothetical 0 km/s to 50km/s. The next stage accelerates the spacecraft from 50km/s constant speed to 100 km/s constant speed, and so on... <blockquote>In reply to:<hr /> For instance if you have a constant acceleration of 30Gs you could, theoretically, reach Alpha in 112.6 years. But practically you run into problems with relatively because you cannot travel faster than light. so it would take you much longer. Also bear in mind that you cannot sustain speed of light because it require infinite energy. <hr /></blockquote> 1. With an acceleration of 300m/s (=30G) you would reach relativistic speeds (that is speeds where relativity matters, which is above ~90% the speed of light) after a bit below 1 million seconds or about 12 days. 2. What we are talking about here is nowhere near relativistic speeds nor are we talking about accelerations of 30G, rather accelerations of 0.0003 Gs... 3. The whole discussion is about the technically possible speeds of a spacecraft using ion-thrusters. This discussion is in the couple of hundreds km/s, so again relativity does not concern us at all. <blockquote>In reply to:<hr /> So with a certain amount of thrust you end up with a point where your acceleration just becomes constant speed. Someone sh</blockquote>

Infrastructure for a robotic mission to Alpha Centari

willpittenger

Guest

willpittenger

Guest

MeteorWayne

Guest

mithridates

Guest

j05h

Guest

holmec

Guest

MeteorWayne

Guest

holmec

Guest

holmec

Guest

MeteorWayne

Guest

themanwithoutapast

Guest

themanwithoutapast

Guest

gunsandrockets

Guest

holmec

Guest

billslugg

Guest

themanwithoutapast

Guest

holmec

Guest

holmec

Guest

billslugg

Guest

holmec

Guest

holmec

Guest

themanwithoutapast

Guest

holmec

Guest

holmec

Guest

themanwithoutapast

Guest

Similar threads

TRENDING THREADS

Latest posts

Share this page