Infrastructure for a robotic mission to Alpha Centari

gunsandrockets · Sep 30, 2007

<Is that based on relatively? Because Newtonian physics allows for it. > No. It's all basic Newtonian physics and the principles of rocket propulsion. Any single stage rocket has a practical upper limit on the mass fraction dedicated to propellant due to the limits of real world engineering. And even worse, the payoff in increased burnout speed does not go up in proportion with increased propellant fraction. More specifically... Mass ratio is the mass of a rocket with full propellant tanks divided by the mass of that rocket with empty propellant tanks. For a rockets burnout speed to equal it's exhaust velocity requires a mass ratio of about 2.7. For a rockets burnout speed to equal double it's exhaust velocity requires a mass ratio of about 7.5. For a rockets burnout speed to approach triple it's exhaust velocity requires a mass ratio greater than 15! The reason why rockets are so inefficient this way (in contrast to gun propulsion), is that rockets must carry the propellant which feeds their engine. A rocket accelerates it's entire mass, so increases in propellant doesn't pay off in a linear way because added propellant also increases the total mass of the rocket. A rocket has to push the stored propellant plus the payload and structure of the rocket. So added propellant has to push the added propellant! <according to Deep Space 1 site: /> Here is a superior source telling you all you could possibly want to know about the NSTAR engine used on Deep Space 1... http://www.boeing.com/defense-space/space/bss/factsheets/xips/nstar/ionengine.html ... so according to the source, to be more precise the exhaust velocity of the Deep Space 1 engine is 30.38 km/s (and not 29 km/s as I said earlier).

gunsandrockets · 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?... Is this correct? > Yes! You have a basic grasp of the underlying concept. <Thus, you might have a million to one ship speed to exhaust speed if your payload was comprised of a speck of matter? /> A million to one? It would be a very very tiny speck of matter! And don't forget there isn't just payload to take into account, there is also the structure of the rocket itself used to contain the payload and contain the propellant. So increases in propellant means an increase in the rocket structure. Think of it this way, the maximum possible mass ratio of a rocket can be no greater than the mass ratio of the propellant tanks all by themselves. (hence the appeal of ideas like the Popsicle-Rocket )

qso1 · Sep 30, 2007

mithridates: Since Deep Impact is going to be used to search for transiting planets in front of stars (I think), and since the more angles you have to look at a star the higher the chance is of finding a transiting planet, Me: It depends on how much difference you can make in the viewing angles. Even the nearest star system is too far for any spacecraft from solar orbit to be able to see much difference due to angular change. That is, if you have a spacecraft in solar orbit say orbiting at the distance of the asteroid belt. No amount of change will be significant enough to make it any easier to detect transiting worlds except in the rarest of cases. The distances are just too great for these changes to have much of an effect. You will see what I mean if you take a sheet of 8.5"11" paper and take a pen and place two dots at each end of the paper. The distance between the dots represent the distance between Sol and Alpha Centauri. This means that each dot is somewhere between 25 and 50 billion miles in diameter. It would take two spacecraft in solar orbits several tens of billions of miles in diameter to make enough angular distance to have any chance of achieving significant results IMO. <div class="Discussion_UserSignature"> My borrowed quote for the time being: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. </div>

mithridates · Oct 1, 2007

Yes, it's quite a colossal distance and perhaps it's impossible at present. What I'm wondering more than anything though is what minimum difference of angle one would need to have a chance of seeing a transit where we might not see one from Earth. It's simple enough to calculate the distance using basic trig but first one would need to know the required angle. Actually, that brings up a different subject: transits of Venus only occur about once every century (except for the two that occur once every eight years) but would it be possible to have a probe in an elongated enough polar orbit around the Earth that it would be able to catch a lot more of them than we do? That would be interesting if we were able to put up a satellite that could do that, probably combined with something else since catching the odd solar transit isn't reason enough to put up a satellite. <div class="Discussion_UserSignature"> ----- http://mithridates.blogspot.com </div>

holmec · Oct 1, 2007

Thanks for info. <blockquote>In reply to:<hr />I am not sure what you are talking about. If you accelerate your craft it never "becomes constant speed." <hr /></blockquote> That refers to relatvism. Because your limited to an amount of energy you have for your thruster, you can only attain to a fraction of light speed. If you graph energy vs speed , at high speeds its not linear but curves. And for this long of a distance there is a distinct possibility of reaching high speeds. Thus that will also preclude how the ability of such a craft. <div class="Discussion_UserSignature"> "SCE to AUX" - John Aaron, curiosity pays off </div>

holmec · Oct 1, 2007

Very good info, thanks. <div class="Discussion_UserSignature"> "SCE to AUX" - John Aaron, curiosity pays off </div>

holmec · Oct 1, 2007

This post is great. I love how everybody brought in different points of view and technology. To me there seems to be a picture forming, to send a permanent probe to Alpha, you need a stack (probe and propulsion systems). I propose that it should use many propulsion technologies as well as gravitational sling shot technique. 1. Chemical rocket launch and chemical booster to escape earth orbit. 2. gravitational sling shots around our planets to set up for a solar flyby. 3. on solar flyby deploy solar sail to launch out of our solar system. 4. use nuclear pulse propulsion system (derived from Orion project) to span the 4 light year distance. Hopefully cutting the time down to hundreds of years. 5. deploy final solar sail on reaching Alpha. 6. use ion thrusters to navigate the system. 7. The probe needs to be autonomous enough to navigate and select points of interest. <div class="Discussion_UserSignature"> "SCE to AUX" - John Aaron, curiosity pays off </div>

qso1 · Oct 1, 2007

Catching a transit of Venus would be a lot easier to do IMO than transits of planets orbiting other stars simply because the baseline distance is much smaller. I would not say that its impossible to increase the detection of transiting extrasolar planets in the way you mentioned. <div class="Discussion_UserSignature"> My borrowed quote for the time being: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. </div>

rfoshaug · Oct 2, 2007

I guess the best approach for an interstellar probe would be: - Nuclear electric propulsion, i.e. an ion drive with a thrust several orders of magnitude greater than anything built so far. I'm talking about a high-powered fission or fusion power plant. - It would have to be a multistage vehicle when parked in Earth orbit. The first couple of stages could be chemical hydrogen/oxygen stages to get it up to solar system escape speed (so that it doesn't use a decade just to break Earth orbit). The rest of the vehicle would either have to be several ion engine stages or a single stage that could drop empty tankage and structures as they became expended. Even with efficient ion engines, it would be very difficult to get a good enough mass fraction (empty vs. full of fuel) to get the desired delta-v. - It would need a very very big and powerful data transmitter if we are to be able to pick up its signal from more than 4 lightyears away. - The ideal thing would be to stop at the destination star system and enter orbit in order to really check out the star system and its planets, however that would require a vehicle several times larger. <div class="Discussion_UserSignature"> ----------------------------------My minds have many opinions </div>

holmec · Oct 2, 2007

I'm not familiar with "magsails". Do you mean solar sails that are made up of plasma trapped in a magnetic field? <div class="Discussion_UserSignature"> "SCE to AUX" - John Aaron, curiosity pays off </div>

willpittenger · Oct 2, 2007

Try this link: Magnetic Sail. <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 · Oct 2, 2007

<blockquote>In reply to:<hr />It would need a very very big and powerful data transmitter if we are to be able to pick up its signal from more than 4 lightyears away.<hr /></blockquote> A fusion reactor would help there. However, are we capable of building space going transmitters that can handle that much power? I figure the dish would be at least 50 meters in diameter. <blockquote>In reply to:<hr />The ideal thing would be to stop at the destination star system and enter orbit in order to really check out the star system and its planets, however that would require a vehicle several times larger.<hr /></blockquote> A solar sail (or perhaps two, one of Sol and one for the destination) would help reduce the mass. <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>

holmec · Oct 2, 2007

Thanks, I love that comparison it has with a solar sail, but they leave out something, the powersource of the magnetic field. That will throw off your mass. <div class="Discussion_UserSignature"> "SCE to AUX" - John Aaron, curiosity pays off </div>

holmec · Oct 2, 2007

The transmitter probably won't have to be as big as you might expect. Just have a big receiver. Some complexity would be in the way you transmit. Since a transmittion will take just over 4 years to get there. You might have to gather a lot of data and transmit at once and repeat the transmission for a month or a time, so you have a window of time to receive it. A real challenge would be from earth the the probe transmission. If you keep the probe permanently there you might want to reprogram it for several fact finding missions and for that you need a big receiver. Now that I've said that it seems that a separate interstellar transmitter/receiver craft is in order. So it can do the big transmissions and receptions and relay to the exploration craft. <div class="Discussion_UserSignature"> "SCE to AUX" - John Aaron, curiosity pays off </div>

willpittenger · Oct 2, 2007

<blockquote>In reply to:<hr />The transmitter probably won't have to be as big as you might expect. Just have a big receiver. Some complexity would be in the way you transmit. Since a transmission will take just over 4 years to get there. You might have to gather a lot of data and transmit at once and repeat the transmission for a month or a time, so you have a window of time to receive it.<hr /></blockquote> Part of the problem would be that an autonomous probe would have to tell us what is coming and how much to expect before it sends anything else. Also, we won't know when such a probe would start transmitting. <blockquote>In reply to:<hr />Now that I've said that it seems that a separate interstellar transmitter/receiver craft is in order. So it can do the big transmissions and receptions and relay to the exploration craft.<hr /></blockquote> Make the relay craft be the mother craft. That craft would need to slow down, but not enough to enter orbit. The daughter would detach and enter orbit on its own. <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>

holmec · Oct 2, 2007

<blockquote>In reply to:<hr />Part of the problem would be that an autonomous probe would have to tell us what is coming and how much to expect before it sends anything else. Also, we won't know when such a probe would start transmitting. <hr /></blockquote> Right. Well that's all in the header of the message. Also you would have to maintain a constant listening channel, or schedule when to send and receive. <div class="Discussion_UserSignature"> "SCE to AUX" - John Aaron, curiosity pays off </div>

holmec · Oct 2, 2007

<blockquote>In reply to:<hr />Make the relay craft be the mother craft. That craft would need to slow down, but not enough to enter orbit. The daughter would detach and enter orbit on its own.<hr /></blockquote> That doesn't sound too permanent. Have both orbit Alpha and then the explorer craft to bop around on a ion thruster and solar panels. <div class="Discussion_UserSignature"> "SCE to AUX" - John Aaron, curiosity pays off </div>

willpittenger · Oct 2, 2007

I don't think the mother should attempt to enter orbit around Alpha. Too much delta-V for a big heavy craft. Orbit around the entire Centari system? Maybe. <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>

holmec · Oct 2, 2007

<blockquote>In reply to:<hr />Orbit around the entire Centari system?<hr /></blockquote> is that possible? I mean are the stars so close together that you can orbit the whole bunch? <div class="Discussion_UserSignature"> "SCE to AUX" - John Aaron, curiosity pays off </div>

MeteorWayne · Oct 2, 2007

That would not be correct AFAIK. AC A & B are very close, so you could orbit around them. Proxima Cetauri ("C") is not even necessarily a member of the system, orbiting some 13,000 AU away from A & B, if it even orbits at all. Perhaps the intention was to obit only A&B, otherwise the orbit would be />100,000 years. That would be a long time to wait, and wouldn't get very close to the barycenter, around which any prospective planets would likely orbit. <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>

3488 · Oct 2, 2007

Hi MeteorWayne. The average distance between Alpha Centauri A & B is 23 AU, or approx 3.45 billion KM. The closest they come together is 11.2 AU (similar from the Sun to Saturn) & the furthest is 35.6 AU (similar from the Sun to Neptune). They orbit their barycentre once every 80 years. Alpha Centauri C or Proxima Centauri is approx 13,000 AU from the central pair & there is considerable doubt as you mention, that Proxima is actually gravitationally bound to the A & B pair. If so the orbital period is over 500,000 years!!!!!! Also there also appears to be an age anomaly. Alpha Centuari A & B are approx 6.5 GYO, somewhat older than the Sun's 4.6 GYO where as Proxima is thought to be no more than 800 million or 0.8 GYO, a youngster. Alpha Centauri stars, compared to Sol. Habitable Zones around the A & B pair @ Periastron. Sun as seen from Alpha Centauri. Alpha Centauri B as seen from Alpha Centauri A. Alpha Centauri A as seen from Alpha Centauri B. Alpha Centauri A & B pair as seen from Proxima Centauri. Sirius as seen from Alpha Centauri. Procyon as seen from Alpha Centauri. Andrew Brown. <div class="Discussion_UserSignature"> "I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before". Linda Morabito on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.http://www.launchphotography.com/ http://anthmartian.googlepages.com/thisislandearthhttp://web.me.com/meridianijournal </div>

christine16 · Oct 2, 2007

thanks 3488

willpittenger · Oct 2, 2007

<blockquote>In reply to:<hr />Perhaps the intention was to obit only A&B, otherwise the orbit would be >100,000 years.<hr /></blockquote> With such a big heavy mother craft moving at such high speeds, you might want such a long period. Note: It would be an elongated orbit. So the mother craft would spend little time deep in the system. <blockquote>In reply to:<hr />That would be a long time to wait, and wouldn't get very close to the barycenter, around which any prospective planets would likely orbit.<hr /></blockquote> As mentioned, the orbit of the mother craft would be quite elongated and would penetrate deep into the system. However, it would probably spend much of its time relaying data from the science craft that would be in a better data gathering orbit. <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>

3488 · Oct 3, 2007

Hi Christine16. You might like these from the Sirius system this time. Sirius B as seen from Sirius A. Sirius A as seen from Sirius B. Sun & Altair as seen from Sirius. Alpha Centauri as seen from Sirius. Procyon as seen from Sirius. Vega as seen from Sirius. Andrew Brown. <div class="Discussion_UserSignature"> "I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before". Linda Morabito on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.http://www.launchphotography.com/ http://anthmartian.googlepages.com/thisislandearthhttp://web.me.com/meridianijournal </div>

Infrastructure for a robotic mission to Alpha Centari

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