Interstellar travel

[orionrider]

Why do you say we could not receive a transmission from a probe at that distance? If it's transmitter were powerful enough, our largest radio telescopes could certainly pick up a signal.

The damned Inverse Square Law again: the signal gets fainter with the square of the distance.

It takes a 8m diameter dish antenna to receive the signals from Voyager 1 (farthest man-made object). The probe has a 20 watt transmitter. Currently the spacecraft is almost 13 light-hours away. When it will be 10 times farther away, (assuming it still works) we will need a 10x10 = 100 times larger antenna...

Imagine a new space probe with an impossibly powerful transmitter, say 200 kilowatts or 10,000 times what Voyager has. It could go 100x farther than Voyager and using the 8m antenna we could still hear it. That means about 50 light-days.
The Centauri system is still 30 times farther away.

Then there is the problem of these stars completely washing the faint signal from the probe. It would have to record everything to transmit it later, when it's clear of the star's electromagnetic glare.

using laser

Laser transmission is basically light. It answers the same inverse square law. If there was an Earth-like planet around this system, we could not directly see it, even using the Hubble. Yet it would be radiating more than 50,000,000,000,000,000 watts of reflected light.

The inverse square law is math, it won't improve with the years, whatever progress makes possible: better antennas, better transmitters, compression algorithms, lasers, you name it.
Space is HUGE. Get used to it.

 

[MeteorWayne]

Why do you say we could not receive a transmission from a probe at that distance? If it's transmitter were powerful enough, our largest radio telescopes could certainly pick up a signal.

The damned Inverse Square Law again: the signal gets fainter with the square of the distance.

It takes a 8m diameter dish antenna to receive the signals from Voyager 1 (farthest man-made object). The probe has a 20 watt transmitter. Currently the spacecraft is almost 13 light-hours away.

A tad optimistic methinks...the DSN consists of 34 meter and up dishes, and for Voyagers:

"Arraying of antennas within the three DSN locations is also used. For example, a 70-metre (230 ft) dish antenna can be arrayed with a 34-meter dish. For especially-vital missions, like Voyager 2, the Canberra 70-metre (230 ft) dish can be arrayed with the Parkes Radio Telescope in Australia; and the Goldstone 70-meter dish can be arrayed with the Very Large Array of antennas in New Mexico. Also, two or more 34-metre (112 ft) dishes at one DSN location are commonly arrayed together"

 

[orionrider]

Thanks MW. I thought the antenna was 8-meter, obviously I was wrong.
There is one thing that could save the day though: quantum communication via entanglement.
But it is still not clear if it will ever be possible to read the state of separated entangled particles without a physical link. If I understand correctly, at the moment you still need a 'traditional' communication method between the emitter and the receiver to make it work.
http://www.physorg.com/news193551675.html

 

[ZenGalacticore]

Yes sir! That's always been the factual conundrum. (There's that word again, conundrum.)

Now, going 10% the speed of light, there will be no significant time dilation. IOWs, the time it takes the travelers to get to their destination, won't be that far off the time perceived by those (all of us) left behind on the Earth. So, it will take our pioneers, traveling at 10% c., 360 years to reach 55-Cancri, 36 light-years distant. (Of course, the time duration onboard the ship will be less than what we observe. But not substantially.)

And then, back on Earth, some jackwad loveable son-of-a-beautiful woman (or ugly woman, but hey, it's all relative) figures out how to propel a craft at 86.4% the speed of light! And so, we can now build ships that can get to 55-Cancri in about 50 years, earthtime. By the "time" the original explorers who set out for 55 Cancri get there, the "later" designed starships will have already colonized the system, and would have been there already for over 300 years.

Ok, technology always keeps improving. What's your point, Zen? In the future of space travel there will always be the question of - "do we go now or wait for 10 or 50 or 100 more years until technology gets better?" And just as that question will always be there, the answer will always be "GO" for the people who spent thier money and thier time investing in the ship and learning how to fly her.

The point is, that if we ever achieve 1% c., then sensible people will think twice before mounting any light-years distant voyages because it would be very likely that more advanced ships would be racing ahead of the 1% c ships in a relatively short period of time.

1% c, or even 0.5% c, would be great for exlporing the Solar System, though. 186,000 miles in about a minute and 1/2 would be a wonderful inter-planetary speed. To Mars in a few hours or a half a day. Yeehah!

 

[StarRider1701]

The point is, that if we ever achieve 1% c., then sensible people will think twice before mounting any light-years distant voyages because it would be very likely that more advanced ships would be racing ahead of the 1% c ships in a relatively short period of time.

1% c, or even 0.5% c, would be great for exlporing the Solar System, though. 186,000 miles in about a minute and 1/2 would be a wonderful inter-planetary speed. To Mars in a few hours or a half a day. Yeehah!

And why are you so locked onto a mere 1% of C? As others, orionrider I beleive, have pointed out we even now have plans for engines that should be capable of considerably higher speeds than that. 20 yrs ago I remember reading something about a nuclear spaceship engine that was being tested in a lab somewhere that should have been capable of 12% of C. As I recall I was reading an article in Newsweek while sitting in the waiting room at my docs.

OK, 1% of C seems way fast to us still limited to chemical rockests, yet out in interstellar space it isn't very fast at all. But I'd be willing to bet that the first time we actually build an engine in space it will be capable of far more than that!

 

[ZenGalacticore]

The point is, that if we ever achieve 1% c., then sensible people will think twice before mounting any light-years distant voyages because it would be very likely that more advanced ships would be racing ahead of the 1% c ships in a relatively short period of time.

1% c, or even 0.5% c, would be great for exlporing the Solar System, though. 186,000 miles in about a minute and 1/2 would be a wonderful inter-planetary speed. To Mars in a few hours or a half a day. Yeehah!

And why are you so locked onto a mere 1% of C? As others, orionrider I beleive, have pointed out we even now have plans for engines that should be capable of considerably higher speeds than that. 20 yrs ago I remember reading something about a nuclear spaceship engine that was being tested in a lab somewhere that should have been capable of 12% of C. As I recall I was reading an article in Newsweek while sitting in the waiting room at my docs.

OK, 1% of C seems way fast to us still limited to chemical rockests, yet out in interstellar space it isn't very fast at all. But I'd be willing to bet that the first time we actually build an engine in space it will be capable of far more than that!

The ideas for the Bussard ramjet and hydrogen scoop and all of that have been around for almost 50 years. They all use one form or another of either nuclear fission or fusion. (There's a great chapter in 'Cosmos', Travels in Space and Time, where he shows schematic blueprints and artist renditions of the Daedalus, Orion, and Bussard starships, and discusses the feasibility of each.)

The Bussard Ramjet is the only one that could reach relativistic speeds. But the hydrogen scoop would have to be the size of Texas in order to scoop up enough H to power the thing. (And there are fewer H atoms per cubic centimeter of space than was once thought.) And there are other technical problems, such as the induced cosmic rays that would occur as the ship approached any appreciable fraction of c. If the occupants are not properly shielded, they would be "fried", as Sagan put it.

I'm not "locked" into anything. I use 1% c., because that would be a milestone and landmark achievement, and as things are going now, I think achieving 1% is at least a semi-realistic goal and conservative scenario. At any rate, going 12% c is also kind of slow relative to the distances involved, and if we could attain that speed, then faster ships might come on line just a few years or decades later, still overtaking and racing past the earlier, slower vessels.

It's kind of similar to what we're doing now. Why spend billions on new chemical rockets when we've got people researching magnetic-pulse and other, more advanced propulsion systems. Maybe it's time for some brainstorming and drawing-board work before we commit to buiding something that will be obsolete in a decade or two.

PS- I don't think we've actually tested an engine that can attain 12% c, or even 1%. They've done concept studies, nothing more that I am aware of.

 

[crazyeddie]

I'm not "locked" into anything. I use 1% c., because that would be a milestone and landmark achievement, and as things are going now, I think achieving 1% is at least a semi-realistic goal and conservative scenario. At any rate, going 12% c is also kind of slow relative to the distances involved, and if we could attain that speed, then faster ships might come on line just a few years or decades later, still overtaking and racing past the earlier, slower vessels.

It's kind of similar to what we're doing now. Why spend billions on new chemical rockets when we've got people researching magnetic-pulse and other, more advanced propulsion systems. Maybe it's time for some brainstorming and drawing-board work before we commit to buiding something that will be obsolete in a decade or two.

PS- I don't think we've actually tested an engine that can attain 12% c, or even 1%. They've done concept studies, nothing more that I am aware of.

Once we develop deutronium annihilation, we will be able to get to Alpha Centauri in about 5 and a half years..........

 

[Mee_n_Mac]

Laser transmission is basically light. It answers the same inverse square law. If there was an Earth-like planet around this system, we could not directly see it, even using the Hubble. Yet it would be radiating more than 50,000,000,000,000,000 watts of reflected light.

The inverse square law is math, it won't improve with the years, whatever progress makes possible: better antennas, better transmitters, compression algorithms, lasers, you name it.
Space is HUGE. Get used to it.

The difference between reflected light and laser light is that the latter is collimated. Were a beam to be perfectly collimated (theoretically impossible) then you would recover all the power transmitted, except that lost to scattering and absorbtion through the ISM (no 1/r^2 loss), assuming your collector was beam sized and the beam was perfectly aimed. The trick then is to point the beam where the receiver will be when said beam arrives. Not so easy at 4+ LY !!

So given "real life" lasers what might their loss be at 4 LY ? Well how about a 10M diameter beam (hey this is some years in the future) and in the near IR band. The theoretical divergence is something <0.13 uR, let's use 0.2 uR as "real life". The resulting beam would then be 15 million km in diameter when it gets to Earth space (at least pointing accuracy is helped). If we used a collector the same size as Arecibo (but now it would have to be in orbit due to the IR transmission) we'd get a loss of about 155 dB (neglecting ISM effects). Could we ever have enough power, concentrated across 10M no less, to make up for that loss ? I don't know, I'm not sure what the MDS for a present day IR telescope (or collector, in my terms above) is. Did we beat the 1/r^2 losses ? No but with about 175 dBi of "antenna gain" we gave it our best shot ... I think.


In effect it has a incredibly high "antenna gain" term.

 

[eburacum45]

The inverse square law only applies to signals which are broadcast in all directions equally. Lasers diverge according to the laws of diffraction, so that a short wavelength beam diverges less than a long wavelength beam. Use a blue laser for best effect, or something even shorter.

 

[space_tycoon]

Well, there's always the Khan Noonian Singh approach: take a conventional interplanetary vessel and equip it with a reeeaaalllyyy reliable cryogenic system. Not to mention a good shielding system.

I think that suspended animation research needs to be dusted off and treated seriously. Even for solar system travel.

 

[StarRider1701]

That just caused a wierd idea to pop into my head!

A baby's brain is basically an empty slate, a total blank. Store the consiousness of a person on a data storage device during the voyage then, perhaps slowly over the period of months or even a few years, download it into a newborn baby's brain. Have the ship carry frozen fertelized eggs, then implant them into artifical wombs and grow the baby when the ship arrives at destination. When the baby is born, begin downloading the complete knowledge and memories of a person. That baby will become the person as it grows up. Instant trained astronaut, colonist, scientist or farmer, or engineer or whatever. Sounds strange, but it could work, maybe.

 

[space_tycoon]

Raises too many ethical, moral,and spiritual questions.

 

[Mee_n_Mac]

The inverse square law only applies to signals which are broadcast in all directions equally. Lasers diverge according to the laws of diffraction, so that a short wavelength beam diverges less than a long wavelength beam. Use a blue laser for best effect, or something even shorter.

Inverse square law applies to anything that radiates and diverges. While it's certainly true that an isotropic radiator exhibits ISL, any radar (for instance), even one that has a reasonably narrow beamwidth, with exhibit a power density in the beam that follows ISL. Now to be fair I simplified my laser example above. I don't think the power density across an actual laser's beam will be equal after 4LY of travel (any ISM effects neglected). As the beam diverges it will follow ISL for any small area that stays equidistant (angularly) from the beam center. The intensity of the far field beam, at differing angular seperations from the center, with look like an Airy disc (Fraunhofer diffraction). But just as the radiation pattern for any radio antenna has non-equal power distribution across it's mainlobe, sidelobes and backlobes, they all follow ISL ... as will a laser with it's Fraunhofer diffraction.


I think ....


Think of it this way ... put a sphere with radius R around whatever radiator you have. Different areas on the sphere may have different power densities depending on mainlobe, sidelobes, backlobes. Now with a sphere of radius 2R ... those same size areas, at the same angular locations, will have 25% of the power densities they had with the original sphere.

 

[eburacum45]

Laser beams are almost parallel, but not quite. At the emitter, the beam will be the same width as the aperture; at distance d the width will be the width of the aperture (a) plus a tiny fraction x; at 2d the width will be a+2x and so on. x is dependent on wavelength. The beam only starts to (almost) follow the inverse square law when nx is significantly larger than a. If you have a very large aperture or a very small wavelength, that distance will be quite far.

However I suppose that over interstellar distances the aperture would need to be unfeasibly large if you want to avoid an inverse square law drop-off; you aren't likely to be able to take an emitter the size of a solar system with you on every mission...

 

[ZenGalacticore]

Our nearest star is Alpha Centauri

It is not. Proxima Centauri is the closest at 4.2LY.

If we sent a probe to Proxima or Alpha Centauri, we could not receive its transmissions, it's much too far.
So what would be the point?

I am compelled to respond to this, OrionRider.

You are so right about the distance from our system to Proxima, ie, 4.2 LY.

So, beyond 4.2 LY, what is the distance to Alpha Centauri A? 4.3 light-years? No. It's just a tiny fraction more than 4.2 LY.

If you're going to give me a hard time about being "technical", I suggest you look in the mirror!

I mean really, there's hardly any difference between the distance of Earth to Proxima, or Alpha A and B. The differential is irrelevant.

 

[orionrider]

Sorry Zen, I didn't mean no offense, it's just that many people think Alpha is in fact Proxima, ie: the same star.
You're right that there is no real difference between the distance of the components of this multiple star system. Anyway, since they probably orbit each other, at the end of its 40,000 years journey the probe could reach any of the stars first.

 

[space_tycoon]

This discussion raises an interesting thought.

As it stands right now, travel within our solar system is a real possibility, albeit a daunting one. Interstellar travel, on the other hand, is kind of outside of our reach, from an economic and technical standpoint.

But let's say, for the sake of argument, a civilization such as ours had developed in a system like Alpha Centauri. They would find themselves with a neighbouring star system, Proxima, which on a scale of difficulty would lie somewhere between interplanetary and interstellar exploration. Assuming Proxima is a system with at least some planets.

Such a civilization, having mastered in-system travel, would then be in a position to challenge themselves to step out a little further into space, refining and improving their propulsive and life support technologies before going out into the depths of interstellar space. Sort of an intermediate step.

One might see our Kuiper Belt and Oort Cloud as filling the same niche. Once we´ve become masters of the Solar System, we have at least one more frontier to cross before taking the plunge into the galaxy.

 

[StarRider1701]

Raises too many ethical, moral,and spiritual questions.

The good thing about all those semantic and/or phillosophic issues is that when practical necessity calls for it, humans can and will throw all that other crap out the window. There are literally thousands of ethical, moral and spiritual issues regarding WAR, but we humans seem to be very good at doing that dispite all those questions.

Yes, the Oort Cloud and Kuiper Belts might be good stepping stones, especially if we find a few more dwarf planets out there. Not to mention that it might be a great place to stop and refuel, filling up the old water tanks and such on the way to the next solar system in our Generation Ships!

 

[Mee_n_Mac]

Raises too many ethical, moral,and spiritual questions.

The good thing about all those semantic and/or phillosophic issues is that when practical necessity calls for it, humans can and will throw all that other crap out the window. There are literally thousands of ethical, moral and spiritual issues regarding WAR, but we humans seem to be very good at doing that dispite all those questions.

Yes, the Oort Cloud and Kuiper Belts might be good stepping stones, especially if we find a few more dwarf planets out there. Not to mention that it might be a great place to stop and refuel, filling up the old water tanks and such on the way to the next solar system in our Generation Ships!

Let me amend your thought above and say a KBO might make a good place to launch a refueling ship from. I think it likely any interstellar trip would originate from the inner solar system. If that's true then why would we want to waste the deltaV built up to get to the Kuiper Belt to stop and refuel ? Even more so for any Oort cloud object. But perhaps a small outpost on any such object could launch an automated ship with fuel for the main mission ship. It boosts and matches the main ship. And as that thought crosses my mind perhaps it might make sense to send out a flotilla of such ships all along the flight path of the main mission ship. Each ship has whatever supplies might be needed so the main ship doesn't have to have everything for the whole trip onboard at launch.

Then again maybe we just hollow out a "good" KBO and make it into a generational ship.

 

[ZenGalacticore]

Well, it's interesting to think that the Voyager spacecraft will, within a decade or two, officially enter interstellar space.

So, technically, we ARE an interstellar spacefaring species! Yay! (Um, well, our probes anyway.)

Those babies are still transmitting data. Gotta love 'em!

 

[MeteorWayne]

m

Our nearest star is Alpha Centauri

It is not. Proxima Centauri is the closest at 4.2LY.

If we sent a probe to Proxima or Alpha Centauri, we could not receive its transmissions, it's much too far.
So what would be the point?

I am compelled to respond to this, OrionRider.

You are so right about the distance from our system to Proxima, ie, 4.2 LY.

So, beyond 4.2 LY, what is the distance to Alpha Centauri A? 4.3 light-years? No. It's just a tiny fraction more than 4.2 LY.

If you're going to give me a hard time about being "technical", I suggest you look in the mirror!

I mean really, there's hardly any difference between the distance of Earth to Proxima, or Alpha A and B. The differential is irrelevant.

Yeah the inaccurate closest star distance was a little tweak. It doesn't change the point of the post that at 4.2 or 4.3 LY, we could not receive the transmissions.

 

[ZenGalacticore]

Yeah the inaccurate closest star distance was a little tweak. It doesn't change the point of the post that at 4.2 or 4.3 LY, we could not receive the transmissions.

I thought that's what I was saying... :?

Anywho, it's not big deal or anything, and no offense was taken, OrionRider.

But let me ask you a question, Wayne. If we couldn't even receive a signal from as close as Alpha C, then what are we listening for with all of our radio telescopes devoted to SETI?

 

[Mee_n_Mac]

But let me ask you a question, Wayne. If we couldn't even receive a signal from as close as Alpha C, then what are we listening for with all of our radio telescopes devoted to SETI?

A long shot ?

I think the idea is to detect a civilization that's sending a very strong radio signal in our direction. This is different than trying to catch random signals from their equivalent of TV broadcasts or even a signal from a spaceship. In these latter cases the expected power sent our way is relatively small compared to what a civilization could do IF it wanted to shout out and say "H%1*fgY@t".

Now I wonder how far away the Arecibo Message could be "heard" ?

 

[MeteorWayne]

Yeah the inaccurate closest star distance was a little tweak. It doesn't change the point of the post that at 4.2 or 4.3 LY, we could not receive the transmissions.

I thought that's what I was saying... :?

Anywho, it's not big deal or anything, and no offense was taken, OrionRider.

But let me ask you a question, Wayne. If we couldn't even receive a signal from as close as Alpha C, then what are we listening for with all of our radio telescopes devoted to SETI?

There's some confusion here. Omnidirectional signals would be far too weak. Focused signals deleiberately broadcast could be strong enough. The original assertion was that our leakage would be strong enough to detect. Ain't happenin'. But directed beams could be detected; that is what we are searching for.

MW