Could we actually build an interstellar probe ?

[Simon Ross]

Hiya Folks

Reading and commenting on a recent post regarding the Orion nuclear drive system, it kinda got me thinking about unmanned interstellar probes.

Over the last decade or so, we have been identifying the existence of literally hundreds of planets around stars relatively close to our own, ranging from super Jupiter's to bodies not much bigger then our own Earth.

So the question is, could we actually reach any of them ?

Certainly we have the capability of building unmanned probes capable of remaining operational for decades, practical examples are out there now operating way beyond their original life expectancy.

While Orion will never be used for surface operations, it still remains the only possible interstellar drive we could actually build with our current technology. The problems are all in the engineering, not the theory.

I certainly don't see it happening any time soon, the political will is not there, and the cost would be simply horrendous, however....

I think we could actually do it now

Opinions ?

[NASAFanboy]

As of now, we could send a cubesat or two to Alpha Centauri for the cost of 50 Billion per probe, but not much beyond that.

Or we could do that 2-trillion dollar project Dadelus ship.

[Nibb31]

Orion is a non-starter, for so many reasons, but there is no reason we couldn't send a probe with chemical or NTR propulsion, although it would take a very long time to get there and it would probably be dead before it ever reaches its destination.

The biggest problem would be to design a power source that can last for centuries without refueling or maintenance and still have enough power to transmit data back to Earth.

[Ralathon]

We probably could if we really wanted to. The problem is the time scales involved and the actual science delivered.

Say we decided to send a probe now. in 2023 we finally finished the orbital construction of the Orion drive, the power source that can last centuries, the scientific instruments, the communication systems and the interstellar shield to protect the whole thing. We push the button and off it goes to Gliese 581. Let's be generous and say it manages to hit 5% of c. That means it takes 440 years to reach Gliese and another 22 to tell us about it.

So the people who'll get the data back are 12 generations removed from us. Furthermore, the probe doesn't have fuel to slow down, so it'll whizz through the Gliese system in a few days. Unless we're really lucky it'll only have a close encounter with a single planet. So it'd send back flyby information about a single planet and some blurry pictures of the rest. That's a rather pitiful payback for something that bankrupts our current economy.

Not to mention that the science instruments will be horribly outdated to the people of 2485. Even today the tech on the new horizons probe is hopelessly outdated. Perhaps they invented much more powerful fusion rockets around 2200 and the probe sent in 2223 actually caught up with our probe, trivializing ours. Or they invented workable Albuciere drives in the 2350's and their Gliese 581g colony goes stargazing to see our primitive scrapheap whizz through the system.

[K^2]

Not to mention that the science instruments will be horribly outdated to the people of 2485.

They'll probably have a crew standing by on Gliese 581 to document the arrival of the first interstellar probe.

I seriously shouldn't jump the gun on this. We should focus on our own system while we're stuck with propulsion techniques we have.

[Kilmeister]

Or they invented workable Alcubiere drives in the 2350's and their Gliese 581g colony goes stargazing to see our primitive scrapheap whizz through the system.

Wouldn't that be something. "Hey Guys, look at this pile of crap".

I mean we could send a probe to another star right now with current technology.... Just don't expect any return on that investment for a VERY long time, and the amount of info gained wouldn't be much more than a confirmation of what we already discovered from here.

So, yes, we could do something, but until there is a major breakthrough in technology, it simply isn't worth the effort.

[SargeRho]

I think a combination of ion engines and solar sails could get a small probe + impact shield to a significant portion of the speed of light (5% maybe?). That would still take like 100 years to get to alpha centauri, but Voyager I has been active for what, 37 years? It's not that unfeasible.

But maybe that Quantum Thruster thing works out, and we have an engine that carries no propellant, and gives 1N per kilowatt power, and we'll have an powersource that can provide enough fuel for long enough to accelerate to 2/3 the speed of light (I think accelerating at half a g for 1 year does the trick), and can then do the same at the target. That would be neat as well.

But as of right now, it's not really feasible.

Edited December 17, 2013 by SargeRho

[Neil1993]

Personally, I think it would be best if we humans held off on interstellar probes for now. The previous users got it right. By the time a probe with our current technology reaches another star, we may already have the technology to travel there in the blink of an eye (Maximum Warp, Engage!). I think it would be better if we only travelled within the abilities of our current propulsion systems. That's not to say that we should confine ourselves. We should still be tirelessly researching and innovating so as to get the best possible propulsion system for interstellar travel.

[NASAFanboy]

Agreed. At the earliest, we should wait atleast fifty years before pursuing a serious program of interstellar exploration (Serious, as in building prototypes, testing the systems, and actually planning to do it along with constructing mission parts). As for me, I believe interstellar travel to our nearest stars is a 22nd century dream, not the 21st.

For now, we should focus on our own solar system, and slowly build infrastructure to expand to Mars and the Moon. This will greatly help our efforts when we are truly ready to send out the first interstellar probe.

And why Glise 581g?

Alpha Centauri, Sirius, and Eplision Enrandi interest me more, and they are all within 12 LY of earth

[-Velocity-]

We could probably build an interstellar probe if we focused on fusion propulsion or building laser/maser-pushed light sails, but our money right now is better spent exploring our own solar system, and building bigger and better space telescopes. Doing this, we will naturally develop the technology to go to the stars (there are folks investigating fusion pulse propulsion for Mars missions, for example). Additionally, the space telescopes will answer the kinds of questions we need to ask right now, such as-

How common are Earth-like habitable worlds in the galaxy?

Where are some nearby ones?

Do they harbor an Earth-like biosphere?

Are they inhabited?

Space telescopes will answer these questions, and for a FRACTION of the price of an interstellar probe. Eventually though, if we continue to develop space technology, then the costs of interstellar probes may drop enough that they will begin to compete with space telescopes. Eventually, you get to a point with space telescopes that it becomes impossible to build them much larger, and to get better information, you have to send an actual probe to your exploration target. But that won't come for a long time, if ever.

[DaveofDefeat]

I'd rather go to Europa.

[-Velocity-]

I'd rather go to Europa.

Yup, same. We need to explore Mars, Europa, Enceladus, Titan, and the whole host of solar system bodies in much more detail before we spend a lot of money on interstellar probes. If it becomes possible to create an interstellar probe for "only" like today's equivalent of like $100B, and we find a VERY exciting target to explore (like a nearby habitable world with signs of life) then we should launch the mission. Otherwise, our money is better spent on solar system exploration until we either run out of exploration objectives in the solar system and/or the cost of interstellar missions comes down enough to allow us to mix them economically into our exploration program.

But... in answer to the poster's original question- yes, again, we could probably build an interstellar probe, not with today's technology but with near-future technology developed after a couple decades of a focused tech surge on things like fusion-pulse propulsion. But we probably won't for quite some time, because it just wouldn't be a wise investment. Exploration wise, it would be "putting the cart before the horse".

Edited December 17, 2013 by |Velocity|

[brdavis]

We could probably build an interstellar probe if we focused on fusion propulsion or building laser/maser-pushed light sails…

Agreed. Daedalus was a looong time ago, and other than minor economic problems*, they had touched on most of the problems and possible solutions of fusion/orion style missions. For laser sails, I've always thought Starwisp would be an interesting way to go (really really cheap in some sense). Beamriders (using smart pellets, not energy-based beams) are another possibility. Forget (currently) mythical warp drives of one form or another - there's ways to do this under current physics and understandings. But…

Space telescopes will answer these questions, and for a FRACTION of the price of an interstellar probe…

Sadly I have to agree there too. It's not that we can't do these things… it's just that right now, there are better and cheaper ways to accomplish most of the goals of such interstellar missions. Which doesn't make me stop wanting them… but does make them hard to justify.

*here "minor economic problems" can be viewed as "make a unified world government and focus the bulk of the world GDP into a production program aimed at mining Jupiter's atmosphere wholesale using hundreds to thousands of floating atmospheric refineries, over a period of 50+ years… Just To Get The Fuel". So, yeah, you know… no serious practical problems

[SargeRho]

Actually, Telescopes can't answer if there's life on a planet. It can give you compelling indications, but it can't give you a deffinite answer, and a telescope could only answer if there's civilisation there, if they have night-time lighting. They could be nocturnal after all.

[Kryten]

There's no plausible mechanism other than life for a thick atmosphere with high levels of oxygen (or a bunch of other reactive gases), and this can be detected using spectral studies.

[Neil1993]

The best use for interstellar travel is for ensuring the survival of the human race by spreading ourselves across the galaxy. Since our current ability to do this is, shall we say, limited, we'll have to take small steps first. Kryten is quite right when he says that life can be discovered using spectroscopy. A good course of action would be to scout for interstellar targets using more and more advanced telescopes while we do as NASAFanboy suggested: "focus on our own solar system, and slowly build infrastructure to expand to Mars and the Moon." This is probably the way we're going right now, anyways.

Edited December 17, 2013 by Neil1993

[-Velocity-]

Yup, biomarker molecules can be detected in the atmosphere. Earth light has O2, O3, water vapor, and methane absorption lines that would be detectable for dozens of light-years by aliens utilizing the just the kind of technology we have today. These molecules, together, scream life- there is no known way for these molecules to coexist in large quantities outside of biological processes The JWST could possibly find an Earth-like planet with life, especially if they get that star shade made for it. It seems likely within 100 years we will have telescopes capable of detecting life on Earth-like planets out to 1000 light-years or more.

It gets better too- for example, it seems the spectral signature of chlorophyll is detectable in Earth light too- astronomers analyzed Earth shine on the Moon and found what appeared to be a weak signal from chlorophyll. Also, aliens looking at Earth would easily be able to tell our planet rotated in ~24 hours, by changes in the color and albedo, and graphing these. They could tell how much cloud cover Earth has, what percentage was land and what percentage was water. They could tell that some areas of land were brownish/orangish, while some were very green. With a really big telescope, of the kind we'll have in a few decades, they would be able to see the light pollution from our cities- for example, there is no reason a planet like Earth should have mercury emission lines, other than artificial lighting.

All this kind of information can be determined just by analyzing the light of a distant, point-source Earth. No need to resolve the disk.

[-Velocity-]

Oh by my calculations, you'd "only" need an interferometer baseline of around 22 km to resolve the disk of Earth at the distance of 50 light-years in visible light-

Diameter of the Earth ~13e7 meters

50 light years = 4.75e17 meters

Angular size of Earth's disk at 50 light-years:

13e7/4.65e17 = 2.74e-11 radians

Visible light wavelength: ~500e-9 meters

Require telescope diameter to resolve 2.74e-11 radians:

D = 1.22*500e-9/2.74e-11 = 2.24e4 meters = ~22 km.

An interferometer gets around having to actually have an impractically large telescope by combining the light gathered by two much smaller telescopes, precisely positioned a known and large distance apart from each other. With some clever signal processing and utilizing some of light's wave effects, you get the same maximum resolution as a telescope the diameter of the distance between the two smaller telescopes. And you can do with with alot more than two telescopes at a time. There are some drawbacks, you collect less information on lower spatial frequencies (you can "fill in" the gaps by using multiple, and shorter baselines). You also want a 2D, not one dimensional array, otherwise your interferometer only has high resolution in one dimension. Radio astronomy, which has been using interferometers for decades, has techniques to counter some remaining drawbacks as well (it is vastly easier to build radio interferometers than it is to build optical interferometers).

So you "just" need a fleet of space telescopes flying in formation across a few dozen kilometers of space, station-keeping with respect to each other with nanometer precision. Sounds tough, but this is achievable with today's technology, believe it or not. Just expensive. BUT A HELL OF A LOT LESS EXPENSIVE THAN AN INTERSTELLAR PROBE!!!! The cost of each space telescope would also be significantly lowered because you'd be effectively mass-producing them.

Edited December 17, 2013 by |Velocity|

[iDan122]

Titan! Never forget!

[MBobrik]

Sure. just add an exception to the partial nuclear test ban treaty and we can start building.

[K^2]

Oh by my calculations, you'd "only" need an interferometer baseline of around 22 km to resolve the disk of Earth at the distance of 50 light-years in visible light-

[...]

An interferometer gets around having to actually have an impractically large telescope by combining the light gathered by two much smaller telescopes, precisely positioned a known and large distance apart from each other. [...] So you "just" need a fleet of space telescopes flying in formation across a few dozen kilometers of space, station-keeping with respect to each other with nanometer precision.

There are a few problems with that which go a bit beyond engineering. First, I agree with the ~20km estimate, but that's just to resolve it as a point. Basically, you'd be able to confirm that it's not a point object, which we already know. If you want to image anything of its features, you'll need a significantly larger effective aperture. Just to say, "Hey, look, clouds" you'll need something like 10x larger size.

Of course, if we are doing this with a swarm, it's not really a huge problem. We can build an interferometer thousands of km across. In fact, anything smaller than Earth orbit is kind of pointless. But the larger you go, the more challenging it gets with precision. So lets talk about that.

Starting with there not being a terribly good way to build an optical interferometer telescope. It's easy enough in radio astronomy. We just build an array, record the actual time-dependent signal, and use computers to sort out the rest. We can't do that with a signal in optical frequencies. So we have to build an actual interferometer. And again, on ground, it's not impossible. You just have to tune everything, like you said, to nanometer precision. The fact that this is something that's been understood for many decades, and we still only have a couple of arrays that are actually capable of doing something useful, with a few more planned or under construction, should tell you how difficult a task it is. On the ground, where things don't shift around, and we can measure distances with incredible precision. Distance is still a limiting factor. Across a few hundred meters, we can use lasers to measure distances very, very precisely. As distances grow, you start having problems. Not only is it hard to measure distances, but they constantly change.

And then we go to the big problem. Space. First of all, holding station with required precision is impossible. Just forget about that. Things are going to move and drift, and you'll have to find a way to deal with that. No mechanical system is going to move with sufficient precision and with little enough vibration to allow this to work. You'll have to measure precise positions of the objects and find non-mechanical ways to adjust distances that beams have to travel. Fortunately, there are electro-optical systems that can work for you, but nearly all of them require polarized light, so you'll have losses there requiring larger mirrors on telescopes. But that's ok, we can deal with that. Measurement, though... Lets start with the fact that GPS can pinpoint your position on the ground to within a few meters. You'll have to do better. Even if we forget about doing this full scale, and try to build a swarm just 200km across somewhere, you'll end up doing 200x better than GPS on timing along. So we went from error in meters, to that in centimeters. Perhaps, millimeters. We are still 4-5 orders of magnitude short. You can do improvements with the right geometry, doing some adjustments, improve timing techniques used for positioning, and you might be able to shave a couple of orders of magnitude off that. But you are still way short.

Using interferometry to image exoplanets is a great idea, but we are not going to do this with modern technology. We can't. The only way we'll have chance is if we can do the same thing we do with radio interferometry and do computer processing. And to do that, we need optical computers. When that becomes a standard for computation, we can start talking about ways to build a large space array for imaging exoplanets.

[MBobrik]

" Lets start with the fact that GPS can pinpoint your position on the ground to within a few meters. You'll have to do better. Even if we forget about doing this full scale, and try to build a swarm just 200km across somewhere, you'll end up doing 200x better than GPS on timing along."

.

you don't need to know the absolute position, all information you need is the phase shift you have to add to the incoming signal. And that can be potentially inferred by letting other beams of known phase and multiple frequencies travel back and forth between parts of the interferometer. The engineering issues are going to be tremendous, though. just sending and receiving coherent laser light of several frequencies at once over hundreds of millions of kilometers would be tricky. not even mentioning the device that gathers this information and translates it into phase shift of the gathered signal...

[K^2]

.

you don't need to know the absolute position, all information you need is the phase shift you have to add to the incoming signal. And that can be potentially inferred by letting other beams of known phase and multiple frequencies travel back and forth between parts of the interferometer. The engineering issues are going to be tremendous, though. just sending and receiving coherent laser light of several frequencies at once over hundreds of millions of kilometers would be tricky. not even mentioning the device that gathers this information and translates it into phase shift of the gathered signal...

And that's different from the timing issue how, exactly?

Multimode helium-neon lasers have a typical coherence length of 20 cm, while the coherence length of singlemode ones can exceed 100 m. Semiconductor lasers reach some 100 m. Singlemode fiber lasers with linewidths of a few kHz can have coherence lengths exceeding 100 km. Similar coherence lengths can be reached with optical frequency combs due to the narrow linewidth of each tooth.

We are talking about a small fraction of coherence length here to achieve sufficient precision.

We just can't achieve a stable enough signal to get position precisely enough.

[-Velocity-]

K^2, I fail to see how your pessimism is justified. First of all, the 20 km diameter was clearly stated as simply being able to resolve an Earth-sized planet as a "disk" from 50 light-years. "Resolving as a disk" is the common term used when we talk about what resolution is required before a point source first begins to resolve. It is obvious I was talking about simply a fuzzy picture of something just barely larger than a point source. Clearly, if you want a true terrestrial planet imager, you'll need something like on the order of at least 100 km across, and then, you'd only be able to get halfway decent images of the very closest planets. I had almost added that if we could make an interferometer some day with a diameter of 1000 km, we could put the equivalent of 50 pixels across an Earth-like planet 50 light years away, but I cut that part out; I thought it was too much detail and too distant of a proposition. Perhaps I should have left it in to make my meaning more clear?

Perhaps I misspoke when I said we can do it with today's technology, but we should be able to do it soon, especially if more funding and study was put into it. I don't see where there are any major show-stopper issues. Perhaps I misspoke because what I really mean, when I think of "today's technology", is that it is something that we either already have, or could immediately begin research and development on, and have ready in a reasonable time span like 10 or 20 years. Clearly, if we directed NASA to build a space interferometer composed of a separately-flying, telescope constellation, we wouldn't be getting in for at least 10 years, probably more like 20- just look how long it took for just the "simple" JWST. But at least, we'd be working on it, and it would be coming.

I would assuming that station keeping would be accomplished with something like small electric ion thrusters. You can probably have some small adjustable optical elements to make up for really small, fast perturbations. You can determine the precise distance between the spacecraft with laser interferometry, and it is accurate to the required precision for optical interferometry. How can this not be extended to kilometer+ distances?

I would think that the lack of atmosphere, ground vibrations, the ability to smoothly change the distances between the telescopes means space is a far better environment to build an optical interferometer than the ground. I don't see what you find so worrisome about drift. In space, perterbation forces are extremely weak, as opposed to on Earth, where we have wind, Earth quakes, even the minor seismic distrubances caused by people walking around or cars passing by; temperature swings, etc. Space is empty and constant. We have the thruster and MEMS technology to precisely position the telescopes, or there is no reason to think we couldn't develop it. We have laser interferometers capable of measuring extremely precise distances, precise enough for the task. And if by some chance a visible light laser interferometer isn't good enough, why not just use a UV laser interferometer?

Where do I go wrong here?

Edited December 17, 2013 by |Velocity|

[-Velocity-]

Anyway, I was curious if gravity waves actually make a noise floor that makes optical interferometers larger than a certain size impossible, or could that be compensated too...

Still, if it does make a noise floor, it would probably not kick in till what... optical interferometers like the size of the solar system?

Edited December 17, 2013 by |Velocity|