What are the limits of interstellar travel?

[qraal]

Slightly on-topic: It's been said that the proposed Bussard Interstellar Ramjet could not work near here due to the super-nova-blown 'local bubble'. Outside the bubble, of course, is another ball-game...

IIRC, this potential answer to 'Where Are They' fell down on the potential observability of multiple doppler-shifted fusion sources...

As much as I hate to admit it a Bussard Ramjet is barely TRL 1-2. There aren't even realistic reactor concepts which can burn interstellar hydrogen via the CNO cycle, let alone pure proton-proton reactions. We need something more effective than fusion to realize ramjets IMO, but just what I don't know. RAIR systems might prove workable, allowing a boost to ~3-4 times the final velocity of a pure rocket, thus pushing 0.2-0.3 c at a stretch. Laser-powered ramjets probably sit at TRL 2 as well. We just have no experience with large scale plasma diversion and no working examples of self-sustaining burning fusion plasmas either. If the NIF achieves a Q of 10-30 then it's closing on break-even and thus controlled fusion designs might jump to TRL 3. Fusion pulse rockets sit there currently.

I think the closest interstellar concept with a TRL of 4 would be a VASIMR system powered by an advanced reactor, which might hit a burn out speed of ~300 km/s - thus 4000 years to Alpha Centauri. Alternatively a beryllium balloon sail might hit ~450 km/s and reach Alpha Cen in ~3000 years. Those are the current front-runners in the TRL stakes. But it could change in a hurry.

 

[D H]

That stuff sits at TRL 2 only in the mind of a theoretician who has zero connection with reality, and that includes VASIMR for interstellar exploration.

You are not talking about the VF-200 to be demonstrated on the ISS in a year or two. That device is projected to provide 5 Newtons of thrust with an ISP of 5000 seconds, or 49 km/s. Achieving a burnout velocity of 300 km/s would require a mass ratio of 456 for a single stage vehicle, or a vehicle that is 99.78% fuel. Staging would help a bit -- but that would require a tad more oomph than a paltry 5 Newtons.

You need to scale things by orders of magnitude to make VASIMR even close to a likely candidate for interstellar missions (or interplanetary missions for that matter). For example, a nuclear-powered VASIMR propulsion system. That is a back-to-the-drawing board concept: TRL 1.

The above completely ignores that fact that at 300 km/s we might as well just stay home and work on the problem for a while. Certainly in the 4000 years needed to go to Alpha Centauri as a fly-by mission humanity will have come up with something better than a dead vehicle going at 300 km/s.

 

[planethunter]

The best solution to this problem would indeed be self-replicating robots capable of reproducing and repairing themselves. We have already advanced in the field of AI and I feel that this will definitely be the next step in our quest for interstellar travel.

As far as humans joining this journey, I believe that unless we can discover how to bend spacetime to create a wormhole joining two distance corners of our universe, that we will be sending probes rather than humans in the meantime!

 

[qraal]

That stuff sits at TRL 2 only in the mind of a theoretician who has zero connection with reality, and that includes VASIMR for interstellar exploration.

Which TRL definitions are you working off? As near as I can tell VASIMR is TRL 5 by the DoD's definitions and an interstellar application (for a precursor mission) is only a linear extrapolation from there, but drops down to TRL 4 due to the low power levels of current space-reactors. The 200 kW version doesn't use hydrogen propellant thus its lower Vex, but that's for ease of demonstration and optimization for cis-lunar applications, not a lack of readiness.

As a precursor out to 1,000-10,000 AU a VASIMR would be near ideal. It's hardly a disconnected theorist's masturbatory fantasy.

 

[qraal]

The best solution to this problem would indeed be self-replicating robots capable of reproducing and repairing themselves. We have already advanced in the field of AI and I feel that this will definitely be the next step in our quest for interstellar travel.

As far as humans joining this journey, I believe that unless we can discover how to bend spacetime to create a wormhole joining two distance corners of our universe, that we will be sending probes rather than humans in the meantime!

Self-replicators capable enough to explore space and survive would be human-equivalent or higher. Better to go ourselves than create a new species with utterly unknown consequences. Definitely TRL 2, since we know self-replication can work, but we can't yet demonstrate, for example, a Rep-Rap that can assemble itself from components it has stereoprinted from a tank of feed-stock.

 

[jreelawg]

Self-replicators capable enough to explore space and survive would be human-equivalent or higher. Better to go ourselves than create a new species with utterly unknown consequences.

I wonder what kind of safety features we could program into self replicating probes to prevent unwanted consequences?

For example, we don't want them mining Earth in the masses. Somehow we need to make sure that if something goes wrong, they know not to mess with earth. But then there is the possibility they find an Earth like planet in another solar system with intelligent life on it, we don't want them necessarily taking them over do we?

Should we program them to only mine and build factories on bodies void of life? Maybe program them to non-invasively observe planets with life on them?

Should these probes be designed so that they cannot self program? No artificial intelligence, just a very large set of blueprints.

 

[planethunter]

Self-replicators capable enough to explore space and survive would be human-equivalent or higher. Better to go ourselves than create a new species with utterly unknown consequences. Definitely TRL 2, since we know self-replication can work, but we can't yet demonstrate, for example, a Rep-Rap that can assemble itself from components it has stereoprinted from a tank of feed-stock.

Ok, but at THIS time the best solution would be robots, since we obviously cannot live longer than say 100 years at the max and we have not been able to harness dark matter or other exotic energy to create a possible wormhole connecting interstellar space.

 

[qraal]

Ok, but at THIS time the best solution would be robots, since we obviously cannot live longer than say 100 years at the max and we have not been able to harness dark matter or other exotic energy to create a possible wormhole connecting interstellar space.

Making a human-equivalent robot is no closer than life-extension technologically speaking.

 

[planethunter]

Making a human-equivalent robot is no closer than life-extension technologically speaking.

True, but what I was trying to say is that a robot can be made to withstand harsher elements than a human being can, so even if we were to be able to extend human life, it still is a fragile life in comparison to the elements that a robot can withstand.

Maybe as nanotechnology advances we will be able to send robots to a distant galaxy that will come together at a certain time to form a larger robot from smaller robots. Since their mass would be at the nano level as they travel to that galaxy, this makes their velocity faster and thus would reach their objective quicker.

 

[D H]

Which TRL definitions are you working off?

Not this one:

Self-replicators capable enough to explore space and survive would be human-equivalent or higher. Better to go ourselves than create a new species with utterly unknown consequences. Definitely TRL 2, since we know self-replication can work ...

On a scale of 1 to 9, Self replicators have a negative TRL.

Regarding VASIMR: Make almost any change and readiness suffers. Make a large change and it suffers a lot. Make a huge change (a couple orders of magnitude or more) and you are back to the drawing board. You are not using the same technology. Engineering does not scale up by orders of magnitude. You are postulating a huge, huge change in thrust, size, power, reliability, and longevity. TRL 2, maybe.

 

[Sagittarius A]

Can the mass of a body be dramatically altered?

 

[Nik_2213]

"Can the mass of a body be dramatically altered?"

Not with the Physics we know, IIRC.

Possible 'gotcha' is the reported, but as yet unrepeatable, anomalous behaviour of some superconductors in magnetic fields, though even that is marginal...

FWIW, I'd like to be able to wire up an array of ambient superconducting tunnel diodes, adjust the drive phasing and evade gravity...

 

[Quantimez]

For simplicity consider an unmanned probe to a star in our neighbourhood say 10 to 20 LY.

Most discussions of this concentrate on speed and power. But what is the fastest realistic velocity? 40,000 MPH (~ 1/10 of 1% of light speed)? Even at this speed interstellar dust banging away at the ships hull for hundreds of thousands of years is going to be a big problem. Not to mention the occasional pebble or golf ball. The kinetic energy of the impacts increasing with the velocity squared.

If we accept hundreds of thousands of years and build a suitable hull, what about the innards? There will have to be some moving parts. Take a valve for fuel to thrusters for example. It will not be used often but can we build one that will actually function after sitting for half a million years? The conductor traces and semiconductor junctions in the ships electronics are so small that random diffusion may cause shorts and open circuits in such a time frame.

So we want to go as slow as possible to minimize hull damage while not making the trip so long as to allow the inner workings of the probe to "die" of old age. These facts alone place a maximum range for an unmanned probe, unless we want to consider a robotic ship that is capable of remanufacturing itself from extra raw material and recycled stuff. But then we are probably talking about a ship the size of a small city.

Is anyone aware of studies that consider these kind of problems.

You might question the value of an unmanned probe that takes hundreds of thousands of years to reach its target. That might be an interesting topic but I am not concerned with it here.

Skippy

Well, it depends. Engineers made the lunar lander's hull (Apollo 11) extremely thin to a fraction of an inch. So despite the dangers you would have to hope an area of space is cleared out by another massive object's gravtitation sufficiently enough to lower the risk. I have heard of micrometeorites making cracks and or impressions. I'm sure artificial space junk present an issue as well. Hopefully insterstellar gas areas might be detected beforehand for an altering of a given movement. Also let's think about unconventional flight. Bending space and time (Which is gravity in Einstein's general Relativity) compressing space in front and expanding it behind so that the craft is not moving itself but the space and time around it is moving. Perhaps this can conquer the light barrier seeing that gravity would be in use which is a function of black holes to overpower light.

 

[Quantimez]

The best solution to this problem would indeed be self-replicating robots capable of reproducing and repairing themselves. We have already advanced in the field of AI and I feel that this will definitely be the next step in our quest for interstellar travel.

As far as humans joining this journey, I believe that unless we can discover how to bend spacetime to create a wormhole joining two distance corners of our universe, that we will be sending probes rather than humans in the meantime!

Yes in General Relativity the result in the bending of spacetime is gravity. Which is in enormity in black holes which in turn overpowers light :).

 

[Quantimez]

I am abundantly more pessimistic than you.

It is not as though we merely came up with the theory of GR to summarize our experimental predictions -- quite the opposite, GR was invented before we had any real experimental evidence to suggest we needed it! (Of course, there was much theoretical motivation, but that's another story alltogether.) So I surely wouldn't say we're only at the stage of predicting within the realm of our experience -- GR has predicted much more beyond that.

Also, it's not as though we can't combine gravity and electromagnetism. On the contrary, it's really not that difficult to do electromagnetism in a curved spacetime. The non-existence of a theory of quantum gravity just doesn't factor into this.

And too, do note that GR perfectly well answers both of your questions you think it doesn't. The Einstein Equations tell you precisely how mass curves space-time, and as to the question of what mass is, well it's just one manifestation of energy in the SET. If you want to dig deeper into what mass "really" is, that's like asking what charge "really" is. Philosophy.

Whoa, I think I know what he is pointing toward. We know through general relativity the functions, behaviors and tendencies of mass and it's effect on spacetime as a result of measureble or observable effects. The the entire working of it is in question. Possibly we might be able to use electromagnetism to inflict a "bending of space and time" (gravity). If we know how to manipulate the nuclear forces of atoms which holds molecules together and composes mass which mass in turn creates that bend in space time proportional that mass. So with this known we could safely assume we can "warp" spacetime to a given degree with a given level of electromagnetic energy. I'm optimistic :D.