What are the limits of interstellar travel?

[skippy1729]

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

 

[Astronuc]

Energy supply (stored energy, specific energy) and specific power are the limits - as is time.

 

[rhyshanan]

Energy supply (stored energy, specific energy) and specific power are the limits - as is time.

agreed. Stars are like oasis in the space desert. A spaceship would need to farm and store energy from stars, hopping from one to the next. The barrier of time may be overcome by traveling through bent space which is theoretially possible, though a long way off.

 

[Astronuc]

agreed. Stars are like oasis in the space desert. A spaceship would need to farm and store energy from stars, hopping from one to the next. The barrier of time may be overcome by traveling through bent space which is theoretially possible, though a long way off.

The fuel supply would have to come from gas clouds or planets. The stellar plasma emanating from stars is too tenuous.

Bending space requires tremendous mass which resists acceleration.

 

[Kevin_Axion]

You can confine energy into a small region of space by using new principles of Quantum Mechanics that may be discovered in Quantum Gravity/Superstring Theory, very speculative but Nature is like that.

 

[rhyshanan]

The fuel supply would have to come from gas clouds or planets. The stellar plasma emanating from stars is too tenuous.

Bending space requires tremendous mass which resists acceleration.

True. Couldn't a carriership deliver 'solar satellites' to collect a stars energy and direct that energy in a straight line ... creating a kind of solar sail? I picked the concept up from one of stephen hawkings documentaries. I guess they would run out of satellites if they get left behind.

 

[jreelawg]

Daedalus would be constructed in Earth orbit and have an initial mass of 54,000 metric tonnes, including 50,000 tonnes of fuel and 500 tonnes of scientific payload. Daedalus was to be a two-stage spacecraft . The first stage would operate for two years, taking the spacecraft to 7.1% of light speed (0.071 c), and then after it was jettisoned the second stage would fire for 1.8 years, bringing the spacecraft up to about 12% of light speed (0.12 c) before being shut down for a 46-year cruise period. Due to the extreme temperature range of operation required (from near absolute zero to 1,600 K) the engine bells and support structure would be made of molybdenum TZM alloy, which retains strength even at cryogenic temperatures. A major stimulus for the project was Friedwardt Winterberg's inertial confinement fusion drive concept[2][1] for which he received the Hermann Oberth gold medal award. [3]
This velocity is well beyond the capabilities of chemical rockets, or even the type of nuclear pulse propulsion studied during Project Orion. Instead, Daedalus would be propelled by a fusion rocket using pellets of deuterium/helium-3 mix that would be ignited in the reaction chamber by inertial confinement using electron beams. The electron beam system would be powered by a set of induction coils tapping energy from the plasma exhaust stream. 250 pellets would be detonated per second, and the resulting plasma would be directed by a magnetic nozzle. The computed burn-up fraction for the fusion fuels was 0.175 and 0.133 for the First & Second stages, producing exhaust velocities of 10,600 km/s and 9,210 km/s, respectively. Due to the scarcity of helium-3 it was to be mined from the atmosphere of Jupiter via large hot-air balloon supported robotic factories over a 20 year period.
The second stage would have two 5-meter optical telescopes and two 20-meter radio telescopes. About 25 years after launch these telescopes would begin examining the area around Barnard's Star to learn more about any accompanying planets. This information would be sent back to Earth, using the 40-meter diameter second stage engine bell as a communications dish, and targets of interest would be selected. Since the spacecraft would not decelerate upon reaching Barnard's Star, Daedalus would carry 18 autonomous sub-probes that would be launched between 7.2 and 1.8 years before the main craft entered the target system. These sub-probes would be propelled by nuclear-powered ion drives and carry cameras, spectrometers, and other sensory equipment. They would fly past their targets, still traveling at 12% of the speed of light, and transmit their findings back to the Daedalus second stage mothership for relay back to Earth.
The ship's payload bay containing its sub-probes, telescopes, and other equipment would be protected from the interstellar medium during transit by a beryllium disk up to 7 mm thick and weighing up to 50 tonnes. This erosion shield would be made from beryllium due to its lightness and high latent heat of vaporisation. Larger obstacles that might be encountered while passing through the target system would be dispersed by an artificially generated cloud of particles, ejected by support vehicles called dust bugs, some 200 km ahead of the vehicle. The spacecraft would carry a number of robot "wardens" capable of autonomously repairing damage or malfunctions.

http://en.wikipedia.org/wiki/Project_Daedalus

 

[inflector]

Bending space requires tremendous mass which resists acceleration.

Well, we certainly don't know of any way to do it right now. But we can't say that it's not possible to bend space without tremendous mass until we know:

A) What mass actually is.

B) How mass bends space.

Understanding this will probably require a theory of quantum gravity at minimum, and perhaps other new science we can't imagine yet. We are far away from that judging by the progress of the last 80 years or so. But who knows what might be possible once we understand gravity and it's relationship with mass at a more fundamental level.

Our understanding of gravity is about where our understanding of electricity was when Maxwell wrote his equations. We can describe the phenomena in the domain of our experience very well, but we don't know how gravity works just like we didn't really understand electromagnetism and electricity until we had a decent model that included electrons and protons and eventually quantum mechanics.

We still don't have the electrons and protons of gravity and mass yet, let alone the specific detail of the gravitational equivalent of quantum mechanics.

Further, our world is ruled by the electromagnetic interaction. So until we have a theory that relates electromagnetism to gravity (if one even exists), we don't know what conversions between the two might be possible.

 

[Nabeshin]

Well, we certainly don't know of any way to do it right now. But we can't say that it's not possible to bend space without tremendous mass until we know:

A) What mass actually is.

B) How mass bends space.

Understanding this will probably require a theory of quantum gravity at minimum, and perhaps other new science we can't imagine yet. We are far away from that judging by the progress of the last 80 years or so. But who knows what might be possible once we understand gravity and it's relationship with mass at a more fundamental level.

Our understanding of gravity is about where our understanding of electricity was when Maxwell wrote his equations. We can describe the phenomena in the domain of our experience very well, but we don't know how gravity works just like we didn't really understand electromagnetism and electricity until we had a decent model that included electrons and protons and eventually quantum mechanics.

We still don't have the electrons and protons of gravity and mass yet, let alone the specific detail of the gravitational equivalent of quantum mechanics.

Further, our world is ruled by the electromagnetic interaction. So until we have a theory that relates electromagnetism to gravity (if one even exists), we don't know what conversions between the two might be possible.

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.

 

[Chronos]

We need way more physics and way more engineering before we can even conceive a way to send humans [alive] to even the nearest star system. Designing a craft that would survive a multi thousand year journey through deep space would be a phenomenal engineering accomplishment in itself. Preserving humans that long . . . priceless.

 

[inflector]

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.

At this point, we have no direct experiments for which GR does not predict the correct outcome. That's what I meant by: "We can describe the phenomena in the domain of our experience very well." I did not mean to imply that GR didn't predict new phenomena or that it was created for the purpose of describing only then-existing phenomena.

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.

I wasn't talking about doing electromagnetism in curved space. Clearly, we understand that. I was talking about the potential for conversion between the two forces. We don't know, at this point, whether this can even be done in principle. If we ever find a theory that unifies gravity and electromagnetism, we may find a way to convert.

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.

Don't confuse descriptions of phenomenology with descriptions of underlying mechanisms. Though, of course we know the formulas which describe the curvature of spacetime due to mass, that's what the Einstein Field Equations of GR do. But we don't know the mechanisms behind the curvature of spacetime. We don't even know what spacetime is beyond a collection of phenomenological field interaction descriptions and formulas for the various fields. If we did, then perhaps we would know how the forces all unify with gravity, or if they do at all.

Now, you may chalk this up to philosophy and say that it doesn't matter. I should point out that the same could have been said about electromagnetism at the time of Maxwell. We never would have developed electronics as an industry if we hadn't dug further to find the actual mechanisms of electricity at the subatomic level. We could have said that electricity is just the flow of electromagnetic potential and left it at that. But we dug further and found the carriers of electricity in the form of charged subatomic particles and found how these particles interacted with the materials of physical objects like conductors and insulators, and eventually semi-conductors.

Of course, every time we dig into a phenomena we find a deeper question that still remains unanswered. For electricity, these currently include: what is charge? why is charge quantized in the way that it is? and, how does charge interact with spacetime, i.e. what is the mechanism behind the propagation of EM photons? QED describes the phenomena but it doesn't provide a "real" explanation because the particles involved (virtual photons) are not real photons but are a mathematical construct which gives the right answer. We may never get a deeper explanation for these questions, but that doesn't mean that some scientists aren't looking and that the questions don't have answers.

Nevertheless, though we may never get to the end of the questions, we have in the past made progress on specific sets of questions, so I expect the same to hold for gravity in the future. When that happens, though it may only lead to more questions, we might perhaps understand how or even if it might be possible to convert electromagnetism to gravity.

 

[twofish-quant]

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)?

With laser sails and/or fusion rockets, you can get up with a few tenths of the speed of light. It's a massive engineering problem, but it's an engineering problem, no new physics required.

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

Lots. Wikipedia is a good place to start for links to people that have thought of these things.

 

[Max™]

First problem: easy access to space, something better than rockets, preferably not an unobtanium project like elevators. I've rambled about lofstrom loops in other threads, we would build them with current tech for a couple billion dollars.

Second problem: gathering materials/developing construction methods in space, asteroids + time.

Third problem: getting up to speed, the way I figure, there is a FREAKING HUGE ball of nuclear fire, and lots of room. Laser sails to get a ship up to speed, and mass drivers to begin launching fuel packages for the ship to capture. The inertia gained from slowing down the fuel packages would speed the ship up, and the effort of getting the fuel up to speed could be performed elsewhere with a mass driver stationed in the solar system.

Burn the fuel however works best to get up to speed as needed, catch the fuel needed for deceleration/return trip for the final bit of acceleration, and you're on your way.

 

[jreelawg]

The only limit to interstellar travel is time. Most people could care less about a mission which will take more than the length of a human life. A proposal in the seventies suggested we could expect to achieve about 12% the speed of light, and make it to another star in 48 years. Of coarse this proposal was to send a probe, not a human.

I see no good reason to send people anyways, unless we are bailing on our solar system. We probably won't need to do that for quite a while, but when we do, no need to worry about getting there fast. We could survive in our massive space ships for however many generations it takes to get there.

 

[D H]

But what is the fastest realistic velocity? 40,000 MPH (~ 1/10 of 1% of light speed)?

Correction: That figure is off by just a bit. 40,000 MPH is about 1/75 of 1% of light speed. 1/10 of 1% c is 670,000 MPH.

Daedalus would be constructed in Earth orbit and have an initial mass of 54,000 metric tonnes, including 50,000 tonnes of fuel ...

http://en.wikipedia.org/wiki/Project_Daedalus

Daedalus is an impossible pipe dream. 50,000 tonnes of fuel, fissionable fuel, ready for detonation, launched into space?

Common sense precludes doing this. Rockets are far from 100% reliable. How many launches are needed to get 50,000 tonnes of fissionable material into space? (That 50,000 tonnes is a small fraction of the total payload mass. The material will need to be shielded.) Some of those launches will fail given the current track record.

International law precludes doing this. Environmentalists protest at launch sites every time a paltry 11 kilograms of fissionable material is launched into space. Countries don't protest such launches because 11 kilograms does not constitute a weapon. 50,000 tonnes does constitute a weapon, many, many times over.

With laser sails and/or fusion rockets, you can get up with a few tenths of the speed of light. It's a massive engineering problem, but it's an engineering problem, no new physics required.

I disagree. Both laser sails and fusion rockets are currently the domain of physics rather than engineering. We haven't the foggiest about doing either of those.

Laser sails: A continuously operating 1.21 gigawatt laser will produce a paltry 8 Newtons of thrust, and that ignores beam dispersion over astronomical distances and assumes the sail is orthogonal to the laser beam. A continuously operating terawatt laser is more in line with what is needed for a laser sail. (A 5 petawatt laser would achieve Saturn V thrust levels.) Given that a continuously operating megawatt laser is currently still in the realm of science fiction, I would not can laser sails an engineering problem. It is a physics problem.

The same goes for fusion rockets. Physicists (not engineers) have been working for over half a century on the problem of controlled fusion. They aren't there yet. A fusion rocket that can accelerate a payload to 0.1c is still in the realm of physics.
So far the discussion has focused on velocity. I'll add two more bumps in the road: Power and communications. The power problem can be overcome, perhaps. The Voyager spacecraft after all are still operating 33 years after launch. Extending that by a factor of 6 is an engineering problem. The Voyager spacecraft , however, are currently operating on rather meagre power. Most of the spacecraft instruments were powered down long ago. This interstellar probe will need to be fully operational, including transmitting a signal back to Earth, when it reaches its target star. That ups the ante quite a bit. A factor of 6 won't suffice. We are facing orders of magnitude improvement here.

Finally, comm. If the vehicle cannot send a signal back to Earth and have it received on Earth, what's the point? The free space path loss over 20 light years alone is rather high. Space isn't quite empty. A 20 light year long path will have quite a bit of dust to disperse the signal. Sending a signal across 20 light years away that doesn't attenuate to nothing is a huge challenge, and that is assuming that the background noise is very small.

The background noise will not be small. Suppose we send a probe to Gliese 581g (if it exists). Unlike the Voyager spacecraft which have a relatively noise free background, our probe will be broadcasting right next to a star. The angular separation between Gliese 581 and Gliese 581g as seen from the Earth is 24 milliarcseconds, max.

 

[Astronuc]

Laser sails: A continuously operating 1.21 gigawatt laser will produce a paltry 8 Newtons of thrust, and that ignores beam dispersion over astronomical distances and assumes the sail is orthogonal to the laser beam. Given that a continuously operating megawatt laser is a technical challenge A continuously operating terawatt laser is more in line with what is needed for a laser sail. (A 5 petawatt laser would achieve Saturn V thrust levels.) Given that a continuously operating megawatt laser is currently still in the realm of science fiction, I would not can laser sails an engineering problem. It is a physics problem.

Some additional context to DH's commentary - In 2008, total worldwide energy consumption was 474 exajoules (474×10¹⁸ J). With 3.1536x10⁷ s/yr, this gives an average annual consumption rate of ~15 TW - for the entire world! Compare that to a PW laser for one Saturn V.


For those who still advocate interstellar travel by laser sail:

And where is this PW laser located? On Earth's surface? What about the rotation of the earth? How many MW lasers will be required?

In orbit? What will it cost to put a PW laser in orbit? Think about $10k/kg, or $10M/t.

 

[jreelawg]

"Due to the scarcity of helium-3 it was to be mined from the atmosphere of Jupiter via large hot-air balloon supported robotic factories over a 20 year period."

http://en.wikipedia.org/wiki/Project_Daedalus

 

[jreelawg]

Project Icarus is a theoretical design study aimed at designing a credible nuclear fusion-based interstellar spacecraft that will stand as a blueprint for a possible unmanned mission. The project is currently being run under the guidance of the Tau Zero Foundation (TZF) and the British Interplanetary Society (BIS), and was motivated by Project Daedalus, a similar study that was conducted between 1973 and 1978 by the BIS.[1]
The project is planned to take five years and began on September 30, 2009[2]. An international team of twenty scientists and engineers has been assembled.
The primary propulsion system for the Icarus design will be mainly-fusion based which would have the capability to accelerate the spacecraft to an estimated 10% to 20% of the speed of light.
Many of the original assumptions of the Daedalus systems will be completely re-examined and Project Icarus will likely be a redesign with only some common elements with Daedalus.

http://en.wikipedia.org/wiki/Project_Icarus_(Interstellar_Probe_Design_Study )

 

[D H]

"Due to the scarcity of helium-3 it was to be mined from the atmosphere of Jupiter via large hot-air balloon supported robotic factories over a 20 year period."

http://en.wikipedia.org/wiki/Project_Daedalus

And this makes Daedalus better? Seriously? At least Project Orion was somewhat attached to reality. This is TRL 0. (Technology Readiness Level 0, and do note that the technology readiness scale goes from 1 to 9).

 

[qraal]

Point of fact but "Daedalus" was powered by electron-beam inertial confinement fusion of deuterium/helium3 NOT fission. Orion was proposed as fission bomb propelled, but only interplanetary speeds. The interstellar version used fusion bombs.

As for non-fission triggered nuclear fusion, it's more advanced than you might think. The real engineering difficulty is powering the initiating lasers efficiently. Back in the 1970s when laser confinement was first explored lasers had dreadful efficiency, but slowly they're inching towards practical levels. New fusion techniques, such as direct drive fast-ignition, have been developed and will be implemented in the next few years. More speculative options like ultradense deuterium are being intensively researched too.

But even old fission has a few surprises yet, but that's for another post...

 

[qraal]

And this makes Daedalus better? Seriously? At least Project Orion was somewhat attached to reality. This is TRL 0. (Technology Readiness Level 0, and do note that the technology readiness scale goes from 1 to 9).

TRL 1 at worst. The theoretical aspects are well established thus TRL 1. Mining helium is tricky, but near-term if we have the will. Pointless if other fusion options prove better. And Jupiter is NOT the best choice.

 

[D H]

Of course its TRL 1 at worst. The scale starts at level 1, after all. Saying this is TRL 0 is akin to somewhat jokingly saying that some organization is at CMMI 0 (aka the "Dilbert Gone Wild" level of maturity).

Suppose a couple of physicists go out for some Saturday lunch with their families, including young kids, in tow. The server brings crayons and some sheets of paper to keep the kids amused. Now suppose the physicists, just for fun, try to flesh out some borderline notion in a science fiction book they just read. Lacking writing materials, they steal the kids' paper and crayons. Those scribbled down notes in crayon are at TRL 1.

 

[qraal]

Of course its TRL 1 at worst. The scale starts at level 1, after all. Saying this is TRL 0 is akin to somewhat jokingly saying that some organization is at CMMI 0 (aka the "Dilbert Gone Wild" level of maturity).

Suppose a couple of physicists go out for some Saturday lunch with their families, including young kids, in tow. The server brings crayons and some sheets of paper to keep the kids amused. Now suppose the physicists, just for fun, try to flesh out some borderline notion in a science fiction book they just read. Lacking writing materials, they steal the kids' paper and crayons. Those scribbled down notes in crayon are at TRL 1.

Actually the basic technologies are spread between TRL 2-5 by the current definitions of those levels. "Daedalus" was meant to be an extrapolation of 1970s technology and it did stick close to what was known. The only "futuristic" components were the main engine and the autonomous computer system - the rest were known technologies applied to a different task. Even the gas-core ramjets weren't without technological precedents since both the US and the Russians had gas-core systems in the 1960s close to operational readiness.

If deuterium had been the propellant of choice then the TRL would be higher overall, but the neutronicity would've been much higher. Better DD fusion designs, developed since then, confining the neutrons in the implosion and adding their energy to the fusion product plasma, mean the neutron-heating load could be much lower, eliminating the need for mining 3He totally.

This is not to say "Daedalus" would've been 'easy' or a 10-year effort. But the designers didn't say that either. They knew, better than anyone, it was pushing the boundaries of the possible - but it wasn't "crayons on paper" either.

 

[D H]

Depends on who is doing the TRL rating. Here's a howler where some scientists rated the technology readiness of some advanced propulsion techniques without having a clue of what those technology readiness levels mean: http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/20238/1/98-1132.pdf

Some examples:

• Solving the problem of laser beam spread at astronomical distances: TRL 5-6.
• Creating a 1 petawatt continously-operating laser: TRL 3-4.
• Increasing antimatter production by a factor of 10¹²: TRL 2-3.
• Forming and storing high-density solid anti-H₂: TRL 2-3.
• Attaining controlled nuclear fusion with a gain of 1000: TRL 3-4.
• Mining ³He from lunar soil or from Jupiter's atmosphere: TRL 3-4.
• Directing fusion products to generate thrust: TRL 2-3.
• Attaining ¹H-¹H fusion: TRL 2.

They haven't the foggiest clue as to what TRL 1 means. Just because an idea exists and does not violate the laws of physics does not vault the idea beyond TRL 1 status. You can't just say "extract ³He from lunar soil or from Jupiter's atmosphere" and have the TRL number magically jump to 3 or 4 just because you were somewhat specific about the source. We don't even know if there is helium 3 on the surface of the Moon, let alone knowing how to mine it. That is TRL 1-2. Too many known unknowns. We haven't the foggiest idea of how to build something that can mine helium 3 from the atmosphere of Jupiter. Too many unknown unknowns. Mining ³He from Jupiter's atmosphere is TRL 1, period.

Requiring anything beyond an order of magnitude improvement in existing capabilities (computer technology being a notable exception) typically sends the concept back to ground zero. These guys aren't just talking about an order of magnitude improvement. They are talking many, many orders of magnitude. Improving anything by twelve orders of magnitude is not a TRL 2-3 concept, and that includes things such as computation speed where an order of magnitude improvement happens just by waiting for 3 years or so. Increasing antimatter production by a factor of 10¹² is exactly the kind of thing I would call TRL 0.

 

[qraal]

There is 3He on the Moon. It was found in all the Apollo samples, down to the limits of the core they drilled. As for mining Jupiter "haven't the foggiest" is nonsense. Cryogenic extraction from a feedstock that's 13.6% helium is quite straight forward and has been analyzed in depth by several different teams.

Have you actually looked at the source material? Your apparent ignorance of basic facts is surprising given the certainty with which you state your opinions.

 

[D H]

We do not know if there is sufficient ³He on the Moon for mining. Apollo astronauts happened to find some. That is one reason why NASA has made experiments to further study the abundance of ³He on the Moon a moderate (but not high) priority item. It is not high priority because we don't know how to mine it. ISRU is hard enough for something that is plentiful such as silicon and oxygen. It is a bit tougher for things a bit further down the abundance scale such as metals. For things way down on the abundance scale, TRL 1.

Regarding mining from Jupiter: Just because helium does constitute 13% of Jupiter's atmosphere does not suffice for vaulting the TRL level of mining ³He from Jupiter's atmosphere to TRL 3-4. There is the slight problem of dealing with Jupiter's gravity, atmosphere, and weather.

For an idea of what TRL 3-4 truly is: Aspects of performing a pinpoint landing amidst hazardous terrain remain on the Moon or Mars remain at the TRL 3-4 level. Phoenix lander was a step on the way, but a 25+ km miss does not constitute a precision landing, let alone a pinpoint landing. NASA's goal is to bring autonomous landing and hazard avoidance technologies to TRL 6 in a couple more years.

 

[Nik_2213]

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...

 

[qraal]

We do not know if there is sufficient ³He on the Moon for mining. Apollo astronauts happened to find some. That is one reason why NASA has made experiments to further study the abundance of ³He on the Moon a moderate (but not high) priority item. It is not high priority because we don't know how to mine it. ISRU is hard enough for something that is plentiful such as silicon and oxygen. It is a bit tougher for things a bit further down the abundance scale such as metals. For things way down on the abundance scale, TRL 1.

I think you're being excessively dismissive of the work by people who have analysed the samples and extrapolated. You don't really have a good reason for your scepticism. But fair point.

Regarding mining from Jupiter: Just because helium does constitute 13% of Jupiter's atmosphere does not suffice for vaulting the TRL level of mining ³He from Jupiter's atmosphere to TRL 3-4. There is the slight problem of dealing with Jupiter's gravity, atmosphere, and weather.

I agree Jupiter is excessively challenging. Better options exist further out from the Sun. TRL2 is probably a better assessment though. Would be better to not bother at all with 3He, but it does have a certain allure.

For an idea of what TRL 3-4 truly is: Aspects of performing a pinpoint landing amidst hazardous terrain remain on the Moon or Mars remain at the TRL 3-4 level. Phoenix lander was a step on the way, but a 25+ km miss does not constitute a precision landing, let alone a pinpoint landing. NASA's goal is to bring autonomous landing and hazard avoidance technologies to TRL 6 in a couple more years.

Let's hope they don't get their SI and Imperial measures mixed up while trying to do so.

 

[jreelawg]

And this makes Daedalus better? Seriously? At least Project Orion was somewhat attached to reality. This is TRL 0. (Technology Readiness Level 0, and do note that the technology readiness scale goes from 1 to 9).

Just addressing your previous rant about launching fuel into space, and the law, and all that.

I don't know if it makes it more feasible, but getting the facts strait makes the discussion more accurate and based in reality.

Your arguments about readiness tell us what we, and the people who wrote Daedalus, already knew. I don't think anyone had the impression, that we can launch the mission right now.

I guess maybe we should define "limits". I was thinking in terms of what is possible.

 

[D H]

Your arguments about readiness tell us what we, and the people who wrote Daedalus, already knew. I don't think anyone had the impression, that we can launch the mission right now.

I don't think they do know. Look at the document to which I linked in [post=2939070]post #24[/post]. CP Snow's Two Cultures addressed the growing gap between the humanities and the sciences. Within the sciences a similar culture gap has grown between theoreticians and experimentalists, designers, and developers. Theoreticians see saying "mining ³He from Jupiter's atmosphere via balloons", "increase antimatter production by a factor of 10¹²" or "create a 1 petawatt continously-operating laser" as an end point of their research. Next! To the people who have to implement these ideas, those aren't solutions. They are hand waves.

I guess maybe we should define "limits". I was thinking in terms of what is possible.

Your idea of what is possible is very different from some other person's idea. Possible to me means quite a bit more than not violating laws of physics.

Aside: The focus to date has been on getting there. Let's assume for the sake of argument that that is a solved problem. Without a power source that lasts for centuries and a communications link that spans light years, what is the point of getting there?

 

[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.