Interstellar travel - why travel so fast ?

[Sarevok]

Currently one of the main obstacles towards interstellar travel is the time required. Even the best chemical rocket or ion engine used in practice would take tens of thousands of years to reach nearby star systems. Therefore the key interest has been towards faster and faster propulsion systems. Lightsails, nuclear fusion powered engines, antimatter based power generation etc various methods have been proposed. Each method for going faster is more expensive and complex than the previous. Every extra kilometer / second of velocity required baloons the mission costs into the sky.

The question is why. Why does a interstellar spacecraft have to nearly break physics to get to Alpha Centauri and back within a single human lifetime ? Clearly the weakest point is the human passengers. It is infinitely easier to engineer them to live longer or make a new type of crew like robots than to construct a relativistic drive system.

Is it sensible to say it is pointless to send humans to interstellar missions ? Is it possible that any interstellar spacecraft would be launched by a future society that has very different idea of what it means to be human ? After all time is a relative thing. To a fruitfly a humans decade long lifespan is an eternity. To something else a humans lifespan might be but a blink of the eye.

[Kuroneko]

Therefore the key interest has been towards faster and faster propulsion systems. Lightsails, nuclear fusion powered engines, antimatter based power generation etc various methods have been proposed. Each method for going faster is more expensive and complex than the previous. ... Clearly the weakest point is the human passengers.

On the contrary, mass efficiency (specific impulse) has always been one of the primary issues, and for just about none of them is there an automatic assumption that there will be human passengers. People would prefer to see the fruits of their labors--regardless of whether the mission is manned or unmanned, doing it faster is preferable than not. But even neglecting timeliness as a priority, at some point it becomes more cost-effective to sink resources into making a faster, more power-intensive craft than making sure it stays operational over tens of millennia with very high probability.

[Napoleon the Clown]

The more time spent on a trip to the destination, the more chances some freak occurrence will take out your probe. The sheer number of things that could happen on a 4 light year trip are rather high. Even at speeds close to c that's a lot of time for something to happen that takes out the probe. At the speeds we can currently obtain, there's even more things that could happen that would make the probe useless or kill passengers aboard a vessel intended to carry a colony of people to another planet.


This isn't even taking into account the energy needs for running something the time it takes to get to a different solar system. Once you get beyond a certain point, you're going to need to generate power on-board. Solar panels won't cut it anymore. Nuclear powered spacecraft would have their fuel last longer, of course, but you still need rather staggering speeds to get anywhere in time for it to still provide power to transmit anything useful back. And batteries aren't going to hold a charge that long, either. A lightyear is an unimaginably huge distance.

[dragon]

Plus humans by nature are not very, hum crap don't know how to put it. We want results fast and not have to have our great, great, great grand kids receive the benefits. Plus as they pointed out the longer it takes the more that can go wrong as stuff does break.

[Mayabird]

Plus humans by nature are not very, hum crap don't know how to put it. We want results fast and not have to have our great, great, great grand kids receive the benefits. Plus as they pointed out the longer it takes the more that can go wrong as stuff does break.

Patient, dude. That's the word you're looking for.


Also on the OP, something to think about: we have a lot of trouble building technological stuff that lasts. Maybe it's because it's all so new. We've only been playing with electricity for a couple centuries, and only got serious about it in the last century. Or just that complex stuff has more things that can break down or go wrong. We're supposed to build a massive complex starship that can keep traveling for millennia, possibly longer than we've had actual civilization, when we can't build a cell phone that lasts more than five years?

Granted, I do think that real interstellar travel will probably be impractical for a very long time even in a best case scenario, but we've got plenty of solar system to colonize.

[dragon]

Plus humans by nature are not very, hum crap don't know how to put it. We want results fast and not have to have our great, great, great grand kids receive the benefits. Plus as they pointed out the longer it takes the more that can go wrong as stuff does break.

Patient, dude. That's the word you're looking for.


Also on the OP, something to think about: we have a lot of trouble building technological stuff that lasts. Maybe it's because it's all so new. We've only been playing with electricity for a couple centuries, and only got serious about it in the last century. Or just that complex stuff has more things that can break down or go wrong. We're supposed to build a massive complex starship that can keep traveling for millennia, possibly longer than we've had actual civilization, when we can't build a cell phone that lasts more than five years?

Granted, I do think that real interstellar travel will probably be impractical for a very long time even in a best case scenario, but we've got plenty of solar system to colonize.

Crap yes that's the word. Hate when something on the tip of the tongue and you can't remember it.

[GrandMasterTerwynn]

Currently one of the main obstacles towards interstellar travel is the time required. Even the best chemical rocket or ion engine used in practice would take tens of thousands of years to reach nearby star systems. Therefore the key interest has been towards faster and faster propulsion systems. Lightsails, nuclear fusion powered engines, antimatter based power generation etc various methods have been proposed. Each method for going faster is more expensive and complex than the previous. Every extra kilometer / second of velocity required baloons the mission costs into the sky.

The question is why. Why does a interstellar spacecraft have to nearly break physics to get to Alpha Centauri and back within a single human lifetime ? Clearly the weakest point is the human passengers. It is infinitely easier to engineer them to live longer or make a new type of crew like robots than to construct a relativistic drive system.

Because the ship itself has to last through the trip, and not break in such a way that it cannot be fixed using whatever resources it's carrying whilst in the middle of interstellar space. The best way to guarantee this is to minimize the amount of time the ship must spend out in the middle of nowhere. Ergo, the ship must go as fast as possible; we'd prefer that it gets to where its going well within the rated time-between-overhauls of its critical systems. (Alternately, you could make the ship so redundant and haul around so many resources that you could rebuild the goddamn thing three times over without needing to put into port. This dictates an extremely slow ship. Suitable for short colonization hops. Not so suitable for probes.)

Is it sensible to say it is pointless to send humans to interstellar missions ? Is it possible that any interstellar spacecraft would be launched by a future society that has very different idea of what it means to be human ? After all time is a relative thing. To a fruitfly a humans decade long lifespan is an eternity. To something else a humans lifespan might be but a blink of the eye.

Quite sensible. Sending humans on interstellar visits is pointless, since you could accomplish the same thing with a sufficiently large interferometer and a smaller starship crewed by a suitably advanced AI or post-biological human uploads. Once you've gotten that out of the way, you'd still want to eventually send people out, but it'd be on one-way colonization trips . . . but we'd still want the ship to go as fast as possible, solely to minimize the time it spends out in the middle of nowhere.

[bz249]

Currently one of the main obstacles towards interstellar travel is the time required. Even the best chemical rocket or ion engine used in practice would take tens of thousands of years to reach nearby star systems. Therefore the key interest has been towards faster and faster propulsion systems. Lightsails, nuclear fusion powered engines, antimatter based power generation etc various methods have been proposed. Each method for going faster is more expensive and complex than the previous. Every extra kilometer / second of velocity required baloons the mission costs into the sky.

The question is why. Why does a interstellar spacecraft have to nearly break physics to get to Alpha Centauri and back within a single human lifetime ? Clearly the weakest point is the human passengers. It is infinitely easier to engineer them to live longer or make a new type of crew like robots than to construct a relativistic drive system.

Is it sensible to say it is pointless to send humans to interstellar missions ? Is it possible that any interstellar spacecraft would be launched by a future society that has very different idea of what it means to be human ? After all time is a relative thing. To a fruitfly a humans decade long lifespan is an eternity. To something else a humans lifespan might be but a blink of the eye.

For exactly the same reason noone starts a computer simulation which takes 10 years, since with the computer next year you can make in a mere 5 years (thus 6 years in total). If a starship is not reasonably fast it is always better waiting a bit and improving it, since you will gain back that time with the shorter journey time.

[Zixinus]

A have a question regarding interstellar travel:
The faster you go, the better, right? The more you approach light-speed the less time you will have to wait. And at one point, you'll become relativistic when you approach the speed of light enough.

However, won't you be effectively be travelling forward in time then? A trip that by classical mechanics should take about a decade at near-lightspeed can effectively actually take hundreds, thousands of years?

[Samuel]

A have a question regarding interstellar travel:
The faster you go, the better, right? The more you approach light-speed the less time you will have to wait. And at one point, you'll become relativistic when you approach the speed of light enough.

However, won't you be effectively be travelling forward in time then? A trip that by classical mechanics should take about a decade at near-lightspeed can effectively actually take hundreds, thousands of years?

No, I don't see how you get that. Relativistic effects just mean the crew feels time go by slower- it still takes the same amount of time for outside observers.

[The Duchess of Zeon]

Bleh, if you want to travel to another solar system, upload your brain as the ship's computer. It makes so much more sense--you can spend transit cheerfully running simulations and data analysis on whatever you please, then arrive, scan the place to your heart's content and seed it with whatever stored life you have aboard, and carry on to the next system. Speaking of which I've been mulling over a hard SF novel focused around the journeys of a little commune of CIs (computational intelligences) living aboard a sublight ship on a journey of galactic exploration. At the rate of current computer development versus implementation of the materials and engine and power source designs required for instellar travel, we will not need to worry about taking our limited fleshbags along for the ride by the time we do get to interstellar exploration.

[Starglider]

Note that current computing hardware suffers from both electromigration and (unless your ship is a huge hollowed out asteroid) progressive radiation damage. MTBF on current server-class CPUs is somewhere around 15 years - then there's the memory, storage, networking etc, all more things that can fail. That's under near-ideal conditions; existing satellites and probes use rad-hardened, relatively obsolete CPUS, and still frequently suffer from computer hardware faults - which are usually mitigated by redundancy, but that strategy only works over a mission lifetime of a couple of decades.

There are a variety of speculative designs for much more reliable computing hardware, but AFAIK few if any have been built. For pretty much the same reason that the rest of our technology isn't designed to last more than a few decades (if that); there just isn't the demand for it.

[bz249]

Note that current computing hardware suffers from both electromigration and (unless your ship is a huge hollowed out asteroid) progressive radiation damage. MTBF on current server-class CPUs is somewhere around 15 years - then there's the memory, storage, networking etc, all more things that can fail. That's under near-ideal conditions; existing satellites and probes use rad-hardened, relatively obsolete CPUS, and still frequently suffer from computer hardware faults - which are usually mitigated by redundancy, but that strategy only works over a mission lifetime of a couple of decades.

There are a variety of speculative designs for much more reliable computing hardware, but AFAIK few if any have been built. For pretty much the same reason that the rest of our technology isn't designed to last more than a few decades (if that); there just isn't the demand for it.

Yes but the main issue is still resource management. Imagine a situation where the goal is to reach a becon at 1 lightyear (way closer than the closest star). Imagine that it is possible build a probe which can reach 300km/s (way faster than current technology). Thus travel time will be 1000 years. Let´s have two competing groups, group A launches their probe so it will reach the target 1000 years later. While group B tries to make a better design and they will launch their probe 500 years later. If they were able to build a probe which can reach 600km/s till that time, they will win the race. It is highly probable that they will capable of that so group B is the likely winner. Thus the good strategy is to not launch anything.

Any probe we can launch today will most probably be caught soon by a newer one launched 100 years later, thus launching a probe today would be a waste of resources.

[montypython]

In a sense, things to seem to require a certain critical mass of interrelated technologies to make interstellar travel truly feasible, much like the difference between self-powered ocean-going vessels vs primitive river craft.

[GrandMasterTerwynn]

Note that current computing hardware suffers from both electromigration and (unless your ship is a huge hollowed out asteroid) progressive radiation damage. MTBF on current server-class CPUs is somewhere around 15 years - then there's the memory, storage, networking etc, all more things that can fail. That's under near-ideal conditions; existing satellites and probes use rad-hardened, relatively obsolete CPUS, and still frequently suffer from computer hardware faults - which are usually mitigated by redundancy, but that strategy only works over a mission lifetime of a couple of decades.

There are a variety of speculative designs for much more reliable computing hardware, but AFAIK few if any have been built. For pretty much the same reason that the rest of our technology isn't designed to last more than a few decades (if that); there just isn't the demand for it.

Yes but the main issue is still resource management. Imagine a situation where the goal is to reach a becon at 1 lightyear (way closer than the closest star). Imagine that it is possible build a probe which can reach 300km/s (way faster than current technology). Thus travel time will be 1000 years. Let´s have two competing groups, group A launches their probe so it will reach the target 1000 years later. While group B tries to make a better design and they will launch their probe 500 years later. If they were able to build a probe which can reach 600km/s till that time, they will win the race. It is highly probable that they will capable of that so group B is the likely winner. Thus the good strategy is to not launch anything.

Any probe we can launch today will most probably be caught soon by a newer one launched 100 years later, thus launching a probe today would be a waste of resources.

This is a logical fallacy. There will come a point where any probe we can launch today won't go slower than a probe we launch 100 years from now. Barring some sort of highly unlikely new physics that will routinely allow us to flout Einstein, I fully expect that day will likely come around 1000 years from now, and that's only because self-assembling solar-powered antimatter production farms can only be built so fast; and a solar collector array of a given size, at a certain distance from the Sun, can only generate so much power. (By then, ship travel time will be constrained by how much fuel the ship can carry, because the technology allowing us to extract a given delta-vee per kilogram of fuel will have matured long before that.)

[montypython]

Note that current computing hardware suffers from both electromigration and (unless your ship is a huge hollowed out asteroid) progressive radiation damage. MTBF on current server-class CPUs is somewhere around 15 years - then there's the memory, storage, networking etc, all more things that can fail. That's under near-ideal conditions; existing satellites and probes use rad-hardened, relatively obsolete CPUS, and still frequently suffer from computer hardware faults - which are usually mitigated by redundancy, but that strategy only works over a mission lifetime of a couple of decades.

There are a variety of speculative designs for much more reliable computing hardware, but AFAIK few if any have been built. For pretty much the same reason that the rest of our technology isn't designed to last more than a few decades (if that); there just isn't the demand for it.

Yes but the main issue is still resource management. Imagine a situation where the goal is to reach a becon at 1 lightyear (way closer than the closest star). Imagine that it is possible build a probe which can reach 300km/s (way faster than current technology). Thus travel time will be 1000 years. Let´s have two competing groups, group A launches their probe so it will reach the target 1000 years later. While group B tries to make a better design and they will launch their probe 500 years later. If they were able to build a probe which can reach 600km/s till that time, they will win the race. It is highly probable that they will capable of that so group B is the likely winner. Thus the good strategy is to not launch anything.

Any probe we can launch today will most probably be caught soon by a newer one launched 100 years later, thus launching a probe today would be a waste of resources.

This is a logical fallacy. There will come a point where any probe we can launch today won't go slower than a probe we launch 100 years from now. Barring some sort of highly unlikely new physics that will routinely allow us to flout Einstein, I fully expect that day will likely come around 1000 years from now, and that's only because self-assembling solar-powered antimatter production farms can only be built so fast; and a solar collector array of a given size, at a certain distance from the Sun, can only generate so much power. (By then, ship travel time will be constrained by how much fuel the ship can carry, because the technology allowing us to extract a given delta-vee per kilogram of fuel will have matured long before that.)

Just as post-Newtonian physics segued into Einsteinian physics, it would be no surprise to see a post-Einsteinian development occur as well, especially as there are still far too many aspects of physics that is unknown to be firmly nailed down as being effectively immutable.

[bz249]

This is a logical fallacy. There will come a point where any probe we can launch today won't go slower than a probe we launch 100 years from now. Barring some sort of highly unlikely new physics that will routinely allow us to flout Einstein, I fully expect that day will likely come around 1000 years from now, and that's only because self-assembling solar-powered antimatter production farms can only be built so fast; and a solar collector array of a given size, at a certain distance from the Sun, can only generate so much power. (By then, ship travel time will be constrained by how much fuel the ship can carry, because the technology allowing us to extract a given delta-vee per kilogram of fuel will have matured long before that.)

It is not that difficult to estimate what to do... check what current hardware is capable. Extrapolate what will be the capabilities of the hardwares when you are half finished the job with the current one. If the hardware that time will be more than twice as efficient than the current one it is better wait till that time and use the resources today on something different. Because any effort made today would be irrelevant in the big picture. This is how life works.

Since we have advanced chemical and very basic electrostatic drives it is not that bold to expect that 10.000 years later they will be able to produce something at least twice as fast as today.

[Patrick Degan]

It is not that difficult to estimate what to do... check what current hardware is capable. Extrapolate what will be the capabilities of the hardwares when you are half finished the job with the current one. If the hardware that time will be more than twice as efficient than the current one it is better wait till that time and use the resources today on something different. Because any effort made today would be irrelevant in the big picture. This is how life works.

No, that is not how life works. Life works by using the best tools you've got on hand for the job at hand, not by waiting for the perfect tool to appear one day in the far future.

Since we have advanced chemical and very basic electrostatic drives it is not that bold to expect that 10.000 years later they will be able to produce something at least twice as fast as today.

So... that means that we should not do anything at all in space until nuclear electric rockets or whatever FuturTech™ handwavium drives "forseen for the future" are perfected? How does that even fall out as remotely logical?

[Ariphaos]

Just as post-Newtonian physics segued into Einsteinian physics, it would be no surprise to see a post-Einsteinian development occur as well, especially as there are still far too many aspects of physics that is unknown to be firmly nailed down as being effectively immutable.

We know there will be 'post Einsteinian' physics already. There is no guarantee, however, that
1) It will permit any sort of FTL effect at all
2) If it does, that it will be in any way usable
3) That if it is in fact usable, that it it can be used as a drive of some sort, rather than requiring an established presence at the target

Each of those is a pretty big if.

[bz249]

No, that is not how life works. Life works by using the best tools you've got on hand for the job at hand, not by waiting for the perfect tool to appear one day in the far future.

Except if using the best current tools have absolutely zero chance of achieving any succes. Or are you try to build a fusion power plant in your backyard. The first part of any at least semiserious project is to check what is available currently.

So... that means that we should not do anything at all in space until nuclear electric rockets or whatever FuturTech™ handwavium drives "forseen for the future" are perfected? How does that even fall out as remotely logical?

Nope it means we should not do interstellar travel because whatever effort we made today will be meaningless. We'd better explore the asteroid belt, the moons of the Jupiter, send dozens of probe to Mars, or put a new telescope into orbit whatever. Because that is what is possible with our current tech and this where there is a chance to discover something useful. Sending a probe to Alpha Centauri to be outraced when it have made 2% of the overall journey is a waste of precious resources which should be devoted to explore our own Solar system.

[Simon_Jester]

I think bz has a good point; there's a tremendous disparity of scale between interplanetary and interstellar travel, one that makes the difference between paddling a canoe to an offshore island and sailing a galleon to another continent look small. At some point, you have to limit your plans to match what you have the toolkit to do.

Probes to the nearest stars will realistically have to wait until we can build a drive to accelerate to ~0.1c, and ideally a way to slow them down when we get there. Once we approach that point, it starts making sense to launch the probes, because with a 45 year trip to Alpha Centauri the odds that someone will come up with a breakthrough in propulsion that halves the trip time in the next 20 years are relatively low. To more distant targets, the minimum advisable speed is higher, because the time window in which waiting for a better engine will get you the data sooner is longer.

It all depends on the projected curve of top speeds you can manage for your interstellar craft design as a function of time, of course; if you're confident that no serious improvement in propulsion will take place for the next 200 years it's OK to launch a slowboat on a 300 year voyage.

[Gilthan]

The interstellar Orion design published in 1968 or a variation of a similar idea remains to this day and the foreseeable future the least expensive method of delivering many tons to another star, with a century or so in transit (as low as maybe a half century if you have the vast bulk of starting mass be propellant, or multiple centuries if you want only a minority to be propellant). Details can change, but the basic concept remains that nuclear explosions result in an exhaust velocity several percent of lightspeed.

Higher velocities can be obtained by lightweight laser or maser propelled sailcraft, as also by antimatter instead of nuclear propulsion. However, the expense of electricity or laser energy, let alone antimatter, is astronomically higher than unconfined nuclear energy per megaton. Accordingly, with anything resembling present economics, interstellar sailcraft are restricted to far lesser weight than nuclear starships, mere kilograms if not mere grams.

So, in other words, by the time we build such a nuclear interstellar spacecraft, we'll know that it is not going to be overtaken enroute by a later starship unless either:

1) Vastly more expense is put into that later starship, probably mainly a possible scenario if a Singularity-like change in economics occurs from development of self-replicating technology within the 1 century following launch that it takes the original starship to reach its destination.

2) That later starship is far smaller and lighter but accordingly having far lesser capabilities, short of, again, self-replicating technology.

3) Known laws of physics turn out to be wrong in some very convenient way, so we get to discard our nuclear engines for exotic-physics drives (improbable).

A smaller variant of similar principles was NASA's Project Longshot.

Project Longshot is a design for an interstellar spacecraft, an unmanned probe intended to fly to Alpha Centauri powered by nuclear pulse propulsion. Developed by the US Naval Academy and NASA, Longshot was designed to be built at the Space Station Alpha, the precursor to the existing International Space Station. Unlike the somewhat similar Project Daedalus, Longshot was designed solely using existing technology, although some development would have been required.

Unlike Daedalus' closed-cycle fusion engine, Longshot would use a long-lived nuclear fission reactor for power. Initially generating 300 kilowatts, the reactor would power a number of lasers in the engine that would be used to ignite inertial confinement fusion similar to that in Daedalus.

Longshot would have a mass of 396 metric tons at the start of the mission, including 264 tonnes of Helium-3/Deuterium pellet fuel/propellant. The active mission payload which includes the fission reactor but not the discarded main propulsion section would have a mass of around 30 tonnes.

A difference in the mission architecture between Longshot and the Daedalus study is that Longshot would go into orbit about the target star while Daedalus would do a one shot fly-by lasting a comparatively short time.

The journey to Alpha Centauri B orbit would take about 100 years, at an approx. velocity of 13,411 km/s - or 30,000,000 mph, and another 4.39 years would be necessary for the data to reach Earth.

http://en.wikipedia.org/wiki/Project_Longshot
http://ntrs.nasa.gov/archive/nasa/casi. ... 007533.pdf

A top question actually, though, is why are we going to the other star? We'll probably before then have massive space-based interferometry telescope arrays already telling us, for instance, that the Centauri system has no earthlike life-bearing planets, only a bunch of asteroids, comets, and maybe some lifeless planets, able to be colonized but mostly pretty boring.

Why bother even sending a probe or exploration mission if you can get your info from here by a lesser investment in advanced telescopes?

That observation suggests the main reason to go to another star would be if you could and wanted to deliver not merely probes but a colonization mission. In that case, since the starship must be big and well-equipped enough to found a new society upon arrival, its crew also should easily be able to repair any minor breakdowns enroute, and spending 100 years in transit should be no problem, provided they are posthuman enough that they live far longer than that.

Perhaps the crew just lets the years go by mostly immersed within their virtual reality version of World of Warcraft. Frankly, it is quite likely that eventually most of human civilization will move into the holodeck so to speak, even those not leaving this solar system.

[Gilthan]

Note an exotic physics method has particularly low odds because it must not just deal with relativity but also with inertia and conservation of energy.

After all, the basic barrier to getting big manned spacecraft to the stars faster than over decades to centuries in transit isn't even the Theory of Relativity. We'd be in almost the same situation for practical purposes under newtonian physics.

It is:
KE = 0.5 M V^2 (aside from relativity making kinetic energy requirements at high fractional-c even higher)
+
economics
+
maybe the interstellar medium if trying to go faster than a limited percentage of lightspeed

[Patrick Degan]

No, that is not how life works. Life works by using the best tools you've got on hand for the job at hand, not by waiting for the perfect tool to appear one day in the far future.

Except if using the best current tools have absolutely zero chance of achieving any succes. Or are you try to build a fusion power plant in your backyard. The first part of any at least semiserious project is to check what is available currently.

However, if Daedelus or Orion is the best thing you've got to send a ship to Alpha Centauri (for whatever reason such a project has become an imperative), that's what you use instead of waiting for whatever to appear in the far future.

So... that means that we should not do anything at all in space until nuclear electric rockets or whatever FuturTech™ handwavium drives "forseen for the future" are perfected? How does that even fall out as remotely logical?

Nope it means we should not do interstellar travel because whatever effort we made today will be meaningless. We'd better explore the asteroid belt, the moons of the Jupiter, send dozens of probe to Mars, or put a new telescope into orbit whatever. Because that is what is possible with our current tech and this where there is a chance to discover something useful. Sending a probe to Alpha Centauri to be outraced when it have made 2% of the overall journey is a waste of precious resources which should be devoted to explore our own Solar system.

The problem with that sort of "logic" is that there is actually no way to predict when something better will be developed or what form it will actually take. And if you don't gain engineering experience with lesser designs (which at the time seem the best thing you've got), you won't get to that more advanced design the form of which you can't even predict at this point. Doing nothing is not an option.

[Simon_Jester]

In some cases, you can broadly predict the rate of future technical advances (Moore's Law), or at least predict that advances will be made (as someone building steam engines in, say, 1830 might).

Right now it would make sense for us to wait, because even if we decided to pour the entire planetary GDP into building an interstellar probe, there's too much we don't know about building long-lived spacecraft. Working on other interplanetary projects first would almost certainly pay off enough to justify waiting until we actually know we can build objects with century-scale lifespans and operate them in space.

Questions of advances in drive technology depend on too many variables; nothing is certain, so it would be absurd to apply bz's argument in every imaginable situation and argue that it ALWAYS makes sense to wait. However, it would be equally absurd to pretend that it NEVER makes sense to wait, which is what it sounds like you're doing. One thing that bz's argument is right about, for instance, is that it makes no sense to use a drive design we know is highly unreliable or inadequate until we've at least tried to improve on it.

Do the job that's in front of you with the tools you have on hand, but don't take on jobs you know you can't do, or can't do cost-effectively, at the cost of wasting effort you could use to improve on your tools.