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PlutonianEmpire
2012-Sep-07, 05:01 AM
Inspired by multiple factors, really, including the recent thread on the same star system. And I couldn't figure out a better subforum to post this, so this is going here.

Anyways, lets say that over time, as our instruments and detection methods and techniques get better and better (within reason, of course), we eventually figure out that, short of actually going there to find out for sure, Alpha Centauri (Proxima included) simply does *not* have any planets whatsoever? (Dunno how much of a stretch this actually is...)

Then what? What happens next? Do we just sit here, assuming that we're not pretty much trapped here anyway as discussions here and elsewhere (off-site) seem to imply? Or do we try to figure out what we're going to do next?

As far as I can tell, the next 10 or 11 stars out after Alpha Centauri A, B, and C are M dwarves, which will almost certainly result in tidally-locked dystopias, an A star, and a brown dwarf. The nearest star that has a marginally better chance of supporting us is a K star about 10 light years away, which also happens to have its own exoplanet system; albeit gaseous.

So, yeah.

What do we do if Alpha Centauri turns out to be a disappointment in the exoplanet realm?

Noclevername
2012-Sep-07, 05:10 AM
As far as I can tell, the next 10 or 11 stars out after Alpha Centauri A, B, and C are M dwarves, which will almost certainly result in tidally-locked dystopias, an A star, and a brown dwarf. The nearest star that has a marginally better chance of supporting us is a K star about 10 light years away, which also happens to have its own exoplanet system; albeit gaseous.

Exoplanets are unlikely to support us anyway, even the rare earthlike ones will probably need extensive terraforming, and a starship is just a long-wear space habitat with a big honkin' engine. If we can survive generations of the long journey to another star in a closed vessel, we can much more easily survive in vessels with sunlight and asteroids available. Planets will not be necessary at that point.

Jens
2012-Sep-07, 07:20 AM
Plus, I'm not sure how much difference it would make. A centauri is like 4 light years away, right? That's pretty much just as close to infinity than 10 light years is. I think probably that if we can overcome the difficulties of reaching a star at 4 light years, we can probably get to one at 10 light years. Either way you need a ship that can last hundreds or thousands of years without any external power source.

Noclevername
2012-Sep-07, 08:16 AM
Plus, I'm not sure how much difference it would make. A centauri is like 4 light years away, right? That's pretty much just as close to infinity than 10 light years is. I think probably that if we can overcome the difficulties of reaching a star at 4 light years, we can probably get to one at 10 light years. Either way you need a ship that can last hundreds or thousands of years without any external power source.

Well, it's not that simple. There's no "close to infinity"-- distances always matter. Going 2.5 times the interstellar distance is much more than 2.5 times as hard. It requires either a much greater amount of delta V and/or an even more massive increase in travel time (My math sucks, can someone help me out with the relevant calculations?), during which the ship has to be kept functional and life support has to be powered.

It's also many more generations in which the crew can get into trouble, lose knowledge or purpose, fragment socially, etc.

(Nitpick: A Centauri (http://en.wikipedia.org/wiki/A_Centauri) is not the abbreviation for Alpha Centauri.)

EDIT: There are several proposed starship concepts that use partial external power sources; beam riders, impact riders, fuel tracks, electrostatic ramjets, RAIR, etc.

Selfsim
2012-Sep-07, 08:22 AM
Why beat around the bush ?

If the destination target is physically unreachable by humans, and meaningful remote interstellar probe communications over light year distances has never been shown to be practically feasible, (even in the slightest); why are we bothered at all when a remote 'Earth-like exoplanet' is detected ?

Having posed such an unpopular question as that, I suppose it would be reasonable to be 'bothered' if we received an intelligent signal from such an 'Earth-like exo-planet', but that is about the only reason I can think of which would be worthy of a somewhat raised eyebrow ... (??)

Noclevername
2012-Sep-07, 08:29 AM
If the destination target is physically unreachable by humans, and meaningful remote interstellar probe communications over light year distances has never been shown to be practically feasible, (even in the slightest)

Neither claim is correct. What is your basis for these statements?

To clarify: Human starflight will not happen in the forseeable future, but the future will last a lot longer than we can foresee. There's nothing physically impossible about it. We know the physical parameters needed, they're massive, but not infinite.

As for communication, the only thing proven impossible at LY distances is detection of our normal, everyday Earthside communications-- commercial radio and TV transmissions.. A laser signal of sufficient strength could be detectable across interstellar distances.

Jens
2012-Sep-07, 08:46 AM
Well, it's not that simple. There's no "close to infinity"-- distances always matter. Going 2.5 times the interstellar distance is much more than 2.5 times as hard.

That might be true, but I was thinking in terms of something like this: suppose you have to cross a one-mile freezing lake and a two-mile freezing lake. The effort won't be that much different, because, since you can't swim, you have to build yourself a boat, and once you have the boat, it will take you two hours instead of one hour. So I was thinking that if you travel 4 or 10 ly, still you are going to have to build a mother of a spaceship with a self-contained habitat that can last maybe thousands of years. So even if you have the same general speed, designing a habitat that can last for 2,000 years doesn't seem that much more difficult than one that can last for 1,000 years.

Noclevername
2012-Sep-07, 08:57 AM
That might be true, but I was thinking in terms of something like this: suppose you have to cross a one-mile freezing lake and a two-mile freezing lake. The effort won't be that much different, because, since you can't swim, you have to build yourself a boat, and once you have the boat, it will take you two hours instead of one hour. So I was thinking that if you travel 4 or 10 ly, still you are going to have to build a mother of a spaceship with a self-contained habitat that can last maybe thousands of years. So even if you have the same general speed, designing a habitat that can last for 2,000 years doesn't seem that much more difficult than one that can last for 1,000 years.

If you can only row for one hour, it makes a big difference.

By the time it becomes remotely plausible to cross those distances, we will probably have had self-sustaining habitat communities at the edge of the Solar System for some time. So making the transition from that to a starship will be just a matter of gathering sufficient quantities of energy in a storable form (a several-mile ball of deuterium ice, for example), and stocking up on raw materials that can't be replenished in midflight. And building a big honkin' engine.

swampyankee
2012-Sep-07, 09:40 AM
Why beat around the bush ?

If the destination target is physically unreachable by humans, and meaningful remote interstellar probe communications over light year distances has never been shown to be practically feasible, (even in the slightest); why are we bothered at all when a remote 'Earth-like exoplanet' is detected ?

Having posed such an unpopular question as that, I suppose it would be reasonable to be 'bothered' if we received an intelligent signal from such an 'Earth-like exo-planet', but that is about the only reason I can think of which would be worthy of a somewhat raised eyebrow ... (??)

The Voyager probes use 23 watt radios. I don't know what the gain on their antennas is, and, like telescope resolution, it's a function of diameter and wavelength, so a larger antenna using the same frequency won't be worse. When Voyager was closer -- at Jupiter -- it could transmit at 115 kilobyte/second. Jupiter is about 5 AU (8e-5 light years). A sufficiently powerful transmitter -- about 57 billion watts -- would give the same bit rate from Alpha Centauri. The most powerful transmitter I could find is about 50 megawatts, so it would take some work to build one of 57 gigawatts, but there's no reason -- except money -- it can't be done.

Jeff Root
2012-Sep-07, 11:19 AM
I don't know enough about either the theory or the
specifics to say anything definitive, but I suspect that
you were comparing the power used by the Voyager
radio against the *effective* power in a transmitted
beam. 50 megawatts has to be way, way, way more
than the actual power supplied to the transmitter.

-- Jeff, in Minneapolis

antoniseb
2012-Sep-07, 11:37 AM
Going back to the OP's question... Noclevername has already given something close to my answer, but I'd like to elaborate.

If Alpha Centuari has no planets, it probably does have asteroids and comets. Any probe we send, whether manned or not, will want resources when it arrives, such as light and easily accessed minerals... which will be available in the asteroids and comets. If it IS a manned mission, it seems likely it would be either a giant ship on a thousand-year voyage, in which case the residents on arrival wouldn't be thinking about changing lifestyle by moving to something so wild and unkempt as a planet (they might have storms and earthquakes and supervolcanos!)... or perhaps some collection of frozen embryos, waiting to be grown after arrival... again, no particular need for a planet-based culture after that.

If it isn't a manned mission, we'll probably send nano-factories that will make the most of what-ever is there for the purposes of manufacturing what we want-need for observations, communications, probes to yet other stars, and maybe habitats for future arrivals.

NEOWatcher
2012-Sep-07, 11:45 AM
If you can only row for one hour, it makes a big difference.
But if the technology is to "push off" and coast, there's no difference.

No matter how you look at life support and human consumables, everything is going to have to be recycled, remanufactured, grown, etc. for either ship. There's probably loss in all these systems, but it's still a fraction of the increase.

Depending on the power source (independent of thrusting or whatever) might not scale up linearly.

kzb
2012-Sep-07, 03:00 PM
In the unlikely event there are no planets whatsoever in the alpha-cent system, that will mean the planetary system formation theorists will have some more firm data to chew on. A negative result can be just as informative as a positive result in science.

PlutonianEmpire
2012-Sep-07, 11:49 PM
I apologize for taking so long to answer. I really need to stop making new threads at times when I'm supposed to be going to bed. :doh: :lol:


Going back to the OP's question... Noclevername has already given something close to my answer, but I'd like to elaborate.

If Alpha Centuari has no planets, it probably does have asteroids and comets. Any probe we send, whether manned or not, will want resources when it arrives, such as light and easily accessed minerals... which will be available in the asteroids and comets. If it IS a manned mission, it seems likely it would be either a giant ship on a thousand-year voyage, in which case the residents on arrival wouldn't be thinking about changing lifestyle by moving to something so wild and unkempt as a planet (they might have storms and earthquakes and supervolcanos!)... or perhaps some collection of frozen embryos, waiting to be grown after arrival... again, no particular need for a planet-based culture after that.

If it isn't a manned mission, we'll probably send nano-factories that will make the most of what-ever is there for the purposes of manufacturing what we want-need for observations, communications, probes to yet other stars, and maybe habitats for future arrivals.
Hmm, I got an interesting idea when I finished reading this, that even if Alpha Centauri had no planets, it might make for a useful "pit stop" for a manned mission to a more distant system, such as, collecting resources from whatever asteroids or comets that Alpha Cen may have lying around, and using their resources to aid in the continuation of the voyage.

Might that have any feasibility at all, depending on the travelers' (manned or not) final destination?


In the unlikely event there are no planets whatsoever in the alpha-cent system, that will mean the planetary system formation theorists will have some more firm data to chew on. A negative result can be just as informative as a positive result in science.
That's true. :)

potoole
2012-Sep-08, 12:47 AM
As I understand (probably wrongly so) Alphas 'A' and 'B' orbit a center of gravity, which would be located between the two sun-like stars. (I think)

Proxima is a distant small star that probably orbits that same center of gravity. In effect (maybe?) it orbits the two larger stars. Perhaps, there are planets orbiting the two larger stars in the same manner as Proxima. Planets that could be somewhere in between Proxima and Alphas 'A' and 'B', and such planets would be receiving energy from both large stars.

Perhaps there is a goldilocks zone at a certain distance from the center of gravity.

Just my thoughts
PO'T

Noclevername
2012-Sep-08, 02:02 AM
Hmm, I got an interesting idea when I finished reading this, that even if Alpha Centauri had no planets, it might make for a useful "pit stop" for a manned mission to a more distant system, such as, collecting resources from whatever asteroids or comets that Alpha Cen may have lying around, and using their resources to aid in the continuation of the voyage.

Might that have any feasibility at all, depending on the travelers' (manned or not) final destination?


Probably not, the majority of starship designs will require a signifigant existing infrastructure to launch-- so it's basically one start, and if you're lucky, one stop. (Unlucky means no stop.) If you make a "pit stop", it will be for the rest of your life-- and if the crew and their descendents can survive at Alpha Centauri long enough to build up a launching industry, they'll probably be so settled-in that they won't bother to build it or to continue with the rest of the trip.

Also, most of the nearby stars in the direction beyond AC are not suitable for Earthlike planets. If that's what you're looking for, then another part of the sky would make a better destination.

Noclevername
2012-Sep-08, 02:05 AM
As I understand (probably wrongly so) Alphas 'A' and 'B' orbit a center of gravity, which would be located between the two sun-like stars. (I think)

Proxima is a distant small star that probably orbits that same center of gravity. In effect (maybe?) it orbits the two larger stars. Perhaps, there are planets orbiting the two larger stars in the same manner as Proxima. Planets that could be somewhere in between Proxima and Alphas 'A' and 'B', and such planets would be receiving energy from both large stars.

Perhaps there is a goldilocks zone at a certain distance from the center of gravity.

Just my thoughts
PO'T

A and B are 85 AU apart, so there are two GL zones-- one for each star. Proxima, not so much.

Solfe
2012-Sep-08, 02:34 AM
A and B are 85 AU apart, so there are two GL zones-- one for each star. Proxima, not so much.

Is the distance to Alpha Centauri measured from the center point of the system to the center point of our Solar System?

Hornblower
2012-Sep-08, 02:37 AM
Proxima and the AB pair are in orbit around the overall barycenter of all three, which according to my estimate is roughly 1/20 of the way from AB to Proxima. That distance is several times the separation of A and B. I will show the math if anyone has any question about my reasoning.

potoole
2012-Sep-08, 02:53 AM
Proxima and the AB pair are in orbit around the overall barycenter of all three, which according to my estimate is roughly 1/20 of the way from AB to Proxima. That distance is several times the separation of A and B. I will show the math if anyone has any question about my reasoning.

Would there be such a thing as a goldilocks zone in this mess?

How far is proxima from A and/or B?

Noclevername
2012-Sep-08, 03:10 AM
Would there be such a thing as a goldilocks zone in this mess?


The A Goldilocks zone would be 1.25 AU from A, B's Goldilocks zone would be 7/10 of 1 AU from B. Proxima, being a small red dwarf flare star, is unlikely to have a livable world, but if it did, it would be somewhere between 0.023–0.054 AU away.


How far is proxima from A and/or B?

0.123 LY, or about 738 trillion miles.

potoole
2012-Sep-08, 03:30 AM
The A Goldilocks zone would be 1.25 AU from A, B's Goldilocks zone would be 7/10 of 1 AU from B. Proxima, being a small red dwarf flare star, is unlikely to have a livable world, but if it did, it would be somewhere between 0.023–0.054 AU away.

0.123 LY, or about 738 trillion miles.

Noclevername
"Proxima and the AB pair are in orbit around the overall barycenter of all three, which according to my estimate is roughly 1/20 of the way from AB to Proxima. That distance is several times the separation of A and B. I will show the math if anyone has any question about my reasoning. "
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Could there be a planet, or planets, orbiting that barycenter, which is 1/20 of the distance from AB to Proxima? Rather than orbiting any of the individual stars.
PO'T

Noclevername
2012-Sep-08, 03:56 AM
Noclevername
"Proxima and the AB pair are in orbit around the overall barycenter of all three, which according to my estimate is roughly 1/20 of the way from AB to Proxima. That distance is several times the separation of A and B. I will show the math if anyone has any question about my reasoning. "
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Could there be a planet, or planets, orbiting that barycenter, which is 1/20 of the distance from AB to Proxima? Rather than orbiting any of the individual stars.
PO'T

I think not, too gravitationally unstable at that distance, but if there were it would be nowhere near any goldilocks zone. About 37 trillion miles from the nearest star.

potoole
2012-Sep-08, 04:46 AM
I think not, too gravitationally unstable at that distance, but if there were it would be nowhere near any goldilocks zone. About 37 trillion miles from the nearest star.

Thank you
PO'T :)

Hornblower
2012-Sep-08, 01:44 PM
Noclevername
"Proxima and the AB pair are in orbit around the overall barycenter of all three, which according to my estimate is roughly 1/20 of the way from AB to Proxima. That distance is several times the separation of A and B. I will show the math if anyone has any question about my reasoning. "
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Could there be a planet, or planets, orbiting that barycenter, which is 1/20 of the distance from AB to Proxima? Rather than orbiting any of the individual stars.
PO'T

Please be careful with quotes. You somehow attributed one of my posts to Noclevername.

potoole
2012-Sep-08, 05:09 PM
Please be careful with quotes. You somehow attributed one of my posts to Noclevername.

My apologies, and thank you for the information.

PO'T

Romanus
2012-Sep-08, 06:20 PM
Nothing significant will happen, IMO.

1.) A light-year is a light-year; going 20 in a reasonable amount of time with feasible technology won't be much harder than going one. We'd be no less "trapped" with planets at Alpha Centauri than without; the time, effort, technology, and money necessary are godlike and far beyond the horizon.
2.) Alpha Centauri has never been the sine qua non of exoplanet studies.


As far as I can tell, the next 10 or 11 stars out after Alpha Centauri A, B, and C are M dwarves, which will almost certainly result in tidally-locked dystopias, an A star, and a brown dwarf. The nearest star that has a marginally better chance of supporting us is a K star about 10 light years away, which also happens to have its own exoplanet system; albeit gaseous.

3.) If we've learned anything in the 20 years since the first exoplanets were found, it's that we shouldn't second-guess what we haven't discovered yet.

wd40
2012-Sep-08, 11:51 PM
Wouldn't it be fun if it turned out that those poor deluded souls who believe that the stars are only light-days away were right after all

eg

http://astronomyinformation.org/index1.htm

http://fixedearth.com/Redshift%20Fraud.htm

http://archive.org/details/DeLaboreSolis

http://ncse.com/cej/2/2/moon-spencer-small-universe

and that Voyager suddenly reported back to us that its coming up on Alpha Centauri 10,000 years too soon, something like in "The Truman Show"!

cjameshuff
2012-Sep-10, 02:04 AM
Well, it's not that simple. There's no "close to infinity"-- distances always matter. Going 2.5 times the interstellar distance is much more than 2.5 times as hard. It requires either a much greater amount of delta V and/or an even more massive increase in travel time (My math sucks, can someone help me out with the relevant calculations?), during which the ship has to be kept functional and life support has to be powered.

Except it isn't more than 2.5 times as hard, or even 2.5 times as hard. It doesn't require any more delta-v, the same delta-v gives a trip time that's only about 2.5 times as long...a bit less because the acceleration and deceleration periods are a smaller portion of the trip. You're almost certainly going to have a highly self-sustaining habitat on the spacecraft, extending the travel time by 2.5 times only requires perhaps a minor increase in some supplies that are imperfectly recycled. Difficulty is likely to be almost independent of distance, except for the most extreme cases.

cjameshuff
2012-Sep-10, 02:45 AM
The Voyager probes use 23 watt radios. I don't know what the gain on their antennas is, and, like telescope resolution, it's a function of diameter and wavelength, so a larger antenna using the same frequency won't be worse. When Voyager was closer -- at Jupiter -- it could transmit at 115 kilobyte/second. Jupiter is about 5 AU (8e-5 light years). A sufficiently powerful transmitter -- about 57 billion watts -- would give the same bit rate from Alpha Centauri. The most powerful transmitter I could find is about 50 megawatts, so it would take some work to build one of 57 gigawatts, but there's no reason -- except money -- it can't be done.

Difficult, but not impractical, and taking some severely conservative approximations. The Voyager antenna is 3.66 m in diameter. Parabolic antenna gain scales with the square of linear dimensions. Arecibo is 305 m in diameter. Taking your 57 GW figure for a Voyager-sized dish and adjusting for an Arecibo-sized dish, I get 8.2 MW.

And that's doing it the hard way, with a direct radio link. Relay stations would allow you to reduce power further or operate at higher bandwidth, and laser communication would allow considerable further improvements. The only thing keeping us from constructing an interstellar communications link with present-day technology is the difficulty of putting hardware at the far end.

Selfsim
2012-Sep-11, 02:32 AM
I'm not quite sure what's going on in this thread now, but the weakness of a space probe's radio signal results from restrictions placed on the size, weight and power supply of the spacecraft, by the payload and weight-lifting limitations of the launch vehicle. As a result, the design of the radio link ends up being engineering tradeoffs between spacecraft transmitter power and antenna diameter, and the sensitivity that can be built into the ground receiving system.

To date, the EIRP (Effective Isotropically Radiated Power) of the spacecraft ends up being around the same, regardless of the generation of spacecraft. (Voyager 1 for instance has an EIRP of ~84.7dB and Mars Reconnassance Orbiter is ~86.5dB). Voyager 1's 23 watt transmitter equates to about 42.5dBm which when added to the high gain antenna gain of 48.2 dBi, (less carrier suppression of ~-6dB), comes out to be 84.7dBm, which is close to 1 Megawatt effective power in the beam.

The information throughput reduces with the square of the distance as the noise on the path increases, reducing the number of unerrored bits getting through. (Information encoding can improve this latter aspect however).

To calculate the signal-to-noise ratio (SNR), assume the path loss of an 8.4 GHZ signal over 1 light year is ~370 dB, which is subtracted from the EIRP.
For Voyager;
the total SNR =84.7-370+(receive antenna gain)-176 dBm (assume: Tsys~189K and moon noise 3.3dB).

Arecibo is not a parabolic reflector (nor was it designed for deep-space comms), but if it was, its gain would be ~86.8dBi at 8.4GHz, (assuming efficiency of ~66%), giving a receive signal-to-noise-ratio of -22.7dB (1Hz) for the Voyager signal, which is well below the noise floor. To bring the signal above the noise and make the message quality (~as per Voyager's out solar system messages), whilst keeping the same antenna, the transmitter power on the probe would have to be (42.5+~40)dBm = 82.5 dBm, or ~178 KW (or 423 times the total usable power generated by the 3 RTGs onboard Voyager).

A 178 KW supply would be about 600 times the mass of the Cassini/New Horizons/Galileo probe supplies, or about 34.3 tonnes .. and that's just to provide the extra power for the radio link (assuming an Arecibo sized receiving antenna, and a 3.6m probe transmit antenna at 1 lightyear)!

If 238Pu is used as the power supply fuel, some of its functions would continue for a best case maximum lifespan of about 50 years (the comms signal would disappear into the noise again at around ~35-40 year lifespan mark).

So, to make just the comms viable, at 8.4 GHz and with a probe antenna of 3.74m diameter, a transmit power on the probe of 174KW and an Arecibo sized receiving antenna, the 1 light year trip would have to be done in about 35-40 years, and an extra 34 tonnes still has to be incorporated into the probe. These are your design constraints for this mere 1 year option. Alpha Centauri is over four times that distance !
Good luck, folks !

Footnote: I think my calculations are correct but I'm happy to correct them if they're not. Changing the assumptions, (ie: like transmit frequency, transmitter antenna size, etc), would result in an entirely different link budgetting exercise, however … and until the recalculation is done, the (non)viability, is simply not clear.

Noclevername
2012-Sep-11, 02:37 AM
I'm not quite sure what's going on in this thread now, but the weakness of a space probe's radio signal results from restrictions placed on the size, weight and power supply of the spacecraft, by the payload and weight-lifting limitations of the launch vehicle.

Are you assuming the probe is being launched directly from a planetary surface? It could be a modular unit, assembled in orbit from several launches.

Selfsim
2012-Sep-11, 04:08 AM
Are you assuming the probe is being launched directly from a planetary surface? It could be a modular unit, assembled in orbit from several launches.All probes launched thus far have been launched from the planetary surface.

My assumptions rarely contain 'coulds' .. why should they ?

cjameshuff
2012-Sep-11, 04:38 AM
Are you assuming the probe is being launched directly from a planetary surface? It could be a modular unit, assembled in orbit from several launches.

An interstellar probe would very nearly have to be assembled in space...the most feasible concepts I've seen require extensive infrastructure in space in addition to the probe. Assuming it must be launched in one piece is just ridiculous.

There is no reason to assume the antenna would be a copy of the one on Voyager. Even with a single launch, it could easily be much, much bigger. The Trumpet reconnaissance satellites are reported to have had 106-150 m antennas. Arecibo's exact geometry and purpose are quite irrelevant, I mentioned it as a point of comparison because it is quite reasonable to consider a 305 m antenna for an interstellar probe. And of course, we could deploy even larger reflectors in the solar system to pick up the signal. And this is all ignoring the fact that it'd likely be considerably easier to use laser communications...

And the assumption of RTGs...they decay continuously and deliver peak power at the start of the mission, exactly what you don't want for an interstellar probe. A fission reactor is a much more likely choice, able to stay inert or operate at low power for the trip and come up to full power once the destination is reached. They are also capable of providing rather more power for their mass than an RTG...the SAFE-400 gets 100 kW of electrical power out of 1.2 tonnes.

So, if you assume unsuitable technology is used, well of course the numbers don't look great. However, even with those terrible assumptions, you get a system that's basically feasible. 178 kW isn't an unreasonable transmit power for an interstellar probe, nor is a power supply in the range of tens of tonnes, and they're especially not unreasonable for a manned interstellar spacecraft.

Van Rijn
2012-Sep-11, 05:47 AM
All probes launched thus far have been launched from the planetary surface.

My assumptions rarely contain 'coulds' .. why should they ?

Well, some have been launched from the Space Shuttle. For instance:

http://en.wikipedia.org/wiki/Magellan_%28spacecraft%29

But more generally, do you have a particular interstellar probe concept in mind? Like cjameshuff, I would expect that space assembly would be pretty much a given. Maybe it would help if we understood what kind of interstellar mission you were assuming?

Jeff Root
2012-Sep-11, 06:53 AM
Are you assuming the probe is being launched directly
from a planetary surface? It could be a modular unit,
assembled in orbit from several launches.
That's ridiculous. There is no "could be" about it.
Any interstellar probe will definitely be assembled
in orbit from multiple launches. Existing launchers
would suffice.

-- Jeff, in Minneapolis

Jeff Root
2012-Sep-11, 06:56 AM
Ha! I posted that before I read cjameshuff's reply!

-- Jeff, in Minneapolis

Selfsim
2012-Sep-11, 07:47 AM
An interstellar probe would very nearly have to be assembled in space...the most feasible concepts I've seen require extensive infrastructure in space in addition to the probe. Assuming it must be launched in one piece is just ridiculous.Agreed.
I think the ISS (for example) is about 450,000 kg.

There is no reason to assume the antenna would be a copy of the one on Voyager.To be successful, I think that's pretty clear.

Even with a single launch, it could easily be much, much bigger. The Trumpet reconnaissance satellites are reported to have had 106-150 m antennas. The Trumpet satellites were reportedly intended to be for monitoring Earth sourced communications. I understand they were launched between 1994 and 1997 by the US. Such an application would have different requirements from a deep-space comms application. Any idea of what those differences might be?


Arecibo's exact geometry and purpose are quite irrelevant, I mentioned it as a point of comparison because it is quite reasonable to consider a 305 m antenna for an interstellar probe. And of course, we could deploy even larger reflectors in the solar system to pick up the signal.Once again I am simply querying the assumptions behind the 'could'. I have no arguments from a theoretical perspective … the practical aspects are what I'm querying. Practical feasibility requires that the technologies used, be within the tolerances called for by the requirements of the specific application. The materials and systems used on the ground would be different from those used in space. Serviceability is also one aspect easily overlooked, when making the assumption of applicability for space-based deployments. Admittedly serviceability may be offset with say (for eg) redundancy. This would come at a cost to the probe design (and a build budget ... cost factors into practical feasibility, also).


And this is all ignoring the fact that it'd likely be considerably easier to use laser communications…Can you demonstrate for us that this technology is practical for deep-space purposes? What reliability figures can we count on for the voyage duration? How long do these technologies last in an unmaintained ground-based environment? Are the assumptions valid for deep-space applications? What requirements does this technology impose on the power/fuel systems?


And the assumption of RTGs...they decay continuously and deliver peak power at the start of the mission, exactly what you don't want for an interstellar probe.I agree .. they do represent the best solution so far demonstrated for lengthy interplanetary flights as far power per unit mass and volume. Nonetheless, no matter what size our probe ends up being, these criteria will always be important for spaceflight.

A fission reactor is a much more likely choice, able to stay inert or operate at low power for the trip and come up to full power once the destination is reached. They are also capable of providing rather more power for their mass than an RTG...the SAFE-400 gets 100 kW of electrical power out of 1.2 tonnes.What is the expected lifetime of this technology? Are any expendables involved? How long are the turbines expected to last? … Looks interesting ...


So, if you assume unsuitable technology is used, well of course the numbers don't look great. However, even with those terrible assumptions, you get a system that's basically feasible.What 'numbers' apply for other suggested technologies in this post? (For supporting the practical feasibility of what you've mentioned?)

178 kW isn't an unreasonable transmit power for an interstellar probe, nor is a power supply in the range of tens of tonnes, and they're especially not unreasonable for a manned interstellar spacecraft.Ok .. so what impacts do they have on the overall design of the spacecraft, the RF feed and its proposed goals?

Footnote:
Folks, were speaking from different perspectives here.

It seems all here are implying that I have made certain assumptions of feasibility. I haven't. Others making suggestions are the ones assuming feasibility without quantifying exactly why it is feasible. As far as I'm concerned, feasibility needs to be demonstrated for the application in question.

All I've done is 'show the numbers' behind what has so far been demonstrated as practical in spaceflight, and what is already known.

Jens
2012-Sep-11, 09:13 AM
It seems all here are implying that I have made certain assumptions of feasibility.

All?

cjameshuff
2012-Sep-11, 03:39 PM
That's ridiculous. There is no "could be" about it.
Any interstellar probe will definitely be assembled
in orbit from multiple launches. Existing launchers
would suffice.

Well, it's conceivable that a small probe could be assembled on the ground and launched in one piece...if you don't include the launch and assembly of the beam propulsion station required to get it out of the system. But if you have that, you may as well assemble your probe in orbit too, just for the reduction in complexity, mass, and risk from self-deployment.

cjameshuff
2012-Sep-11, 04:01 PM
It seems all here are implying that I have made certain assumptions of feasibility. I haven't. Others making suggestions are the ones assuming feasibility without quantifying exactly why it is feasible. As far as I'm concerned, feasibility needs to be demonstrated for the application in question.

No, you've made an assumption of unfeasibility, and you took further assumptions to support the conclusion you started out with...assuming a tiny Voyager-sized transmitter, in spite of how incredibly absurd that is for an interstellar probe, assuming power supplies that are completely unsuited for the purpose, etc.

Feasibility of a technology does not have to be re-demonstrated for every new use it is applied to. Parabolic reflectors don't suddenly become omnidirectional or impossible to construct when intended for interstellar communications. Your claim that:


meaningful remote interstellar probe communications over light year distances has never been shown to be practically feasible, (even in the slightest)

...is just factually incorrect.

Selfsim
2012-Sep-13, 12:30 AM
Having taken some time in considering the content of the immediately prior post, and in attempting to reply, without diverging too much from the OP question, I have chosen to respond and explain my position on the other related thread here. (http://cosmoquest.org/forum/showthread.php/137967-Interstellar-Probe-Comms-Link-Design)

PlutonianEmpire
2012-Sep-13, 12:39 AM
Nothing significant will happen, IMO.

1.) A light-year is a light-year; going 20 in a reasonable amount of time with feasible technology won't be much harder than going one. We'd be no less "trapped" with planets at Alpha Centauri than without; the time, effort, technology, and money necessary are godlike and far beyond the horizon.
Why the quotation marks? From what I've been reading on here during my lurking, it's pretty much a given that we're simply not going anywhere.

(If this statement is treading ATM, I'd be willing to stop. :o )

2.) Alpha Centauri has never been the sine qua non of exoplanet studies.
My mistake, I had presumed it was on some sort of high priority due to its proximity.

3.) If we've learned anything in the 20 years since the first exoplanets were found, it's that we shouldn't second-guess what we haven't discovered yet.
The "second-guesses" I've made through-out my life have nasty habit of turning out to be correct after all. :/

Noclevername
2012-Sep-13, 12:54 AM
From what I've been reading on here during my lurking, it's pretty much a given that we're simply not going anywhere.


Not soon, no. But forever is a very long time. It would surprise me greatly if the planet reached the end of its habitability and no human or posthuman had ever gone to another star. The amount of energy needed is monumental, but calculable and achieveable. It's just more than our present single-planet economy can spare.

PlutonianEmpire
2012-Sep-13, 01:06 AM
Not soon, no. But forever is a very long time. It would surprise me greatly if the planet reached the end of its habitability and no human or posthuman had ever gone to another star. The amount of energy needed is monumental, but calculable and achieveable. It's just more than our present single-planet economy can spare.
That's true, you never know. :)

I will admit I spend most of my time on another forum where the two subjects not allowed here are openly discussed there, and quite often (the beating-a-dead-horse kind of often :o ), so it's easy for me to get demotivated by the stuff I see happening in the world we live in now. :/

Noclevername
2012-Sep-13, 01:12 AM
That's true, you never know. :)

I will admit I spend most of my time on another forum where the two subjects not allowed here are openly discussed there, and quite often (the beating-a-dead-horse kind of often :o ), so it's easy for me to get demotivated by the stuff I see happening in the world we live in now. :/

Keep in mind that what we see happening is never more than a tiny fraction of the whole picture; The world is just too big for us to take it all in, and the "if it bleeds it leads" mentality of the news media (not to mention the "I'm a troll!" attitude of much of the internet) means we're seeing negatively filtered views most of the time.

cjameshuff
2012-Sep-13, 04:06 AM
Why the quotation marks? From what I've been reading on here during my lurking, it's pretty much a given that we're simply not going anywhere.

It would be extraordinarily difficult and costly to send humans to another star, but it's not outright impossible. However, barring some unforeseen discovery that completely changes the problem, the things that would be necessary also make the difficulty rather insensitive to distance. A manned mission would need to be highly self sufficient, not relying on expendable supplies that are depleted during the trip...not a trivial achievement.

However, once you have that, it doesn't matter much if the trip takes 3 times longer...it means a slight increase in initial stockpiles to account for losses in recycling, and a longer wait. It also makes the lack of nearby earthlike worlds rather moot...if you can cross the gap between stars, you don't need them.