[Need some help] Simulation on mission to Alpha Centauri

[andrewas]

Oars are much more efficient, or pedal boats would be used for things other than entertaining tourists.

[cantab]

I'm not sure about more efficient. Rowing is powerful, largely because you can put your whole body into it, and it's mechanically simpler which was a factor in historic times when gears and bearings weren't as good as we can build them today. A cycling action though can be very efficient, and if you coupled it to a propeller rather than the traditional paddle wheels it would be even more so.

[Hattivat]

Sweet kraken, guys, this isn't relevant. That entire silly analogy of mine was about it not making sense, not about whether or not it was theoretically possible. I'm sure if you had 1000 Tour-de-France-class cyclists attempt to pedal their way across the Atlantic in waterproofed paddle boats, then at least one of them would manage to get to the other shore alive. So yes, it is certainly theoretically possible. But the question is: does it make sense to attempt this if there is any practical reason to your crossing (e.g. you want to do something on the other shore, which kinda presupposes that you want a reasonable chance of not dying or being forced to turn back)? And the answer to that one is a resounding no, that much is obvious in my opinion. The reason why this is so obvious is that if you can build an ocean-rated paddle boat, then you sure as hell also can build a sailing boat, and a sailing boat, no matter how primitive, is by definition an infinitely better choice (even though its safety and speed still leave a lot to be desired).

If we really were to insist on attempting to launch an interstellar probe this decade, there are a couple of better suited (though still not at all sufficient) options for propulsion than an all-chemical rocket, ion engines being the most obvious one.

[rpayne88]

Automatic star sensors and guidance systems aren't that difficult to make. OK so an interstellar trip adds a few wrinkles, but it's still a simple, well-defined problem with uncomplicated image processing needs (stars on a black background, how much simpler can you get?)

Anyway, presumably we do think that ground control is going to keep an eye on this for tens of thousands of years, since it needs to receive the close-ups when the probe finally reaches Alpha Cen. Otherwise what's the point of the mission?

But can we build something that will survive an airless, near absolute zero, highly irradiated environment for 40K years?

[cantab]

If we can't, we postpone the mission until we can. Otherwise we basically end up spending a lot of money chucking a brick at Alpha Cen.

Although more realistically the probe's primary mission would be to investigate the edge of the solar system. In that case it doesn't matter if it's a brick by the time it gets anywhere near another star, or if nobody's listening. But then neither does it matter if it's way off-course, indeed it doesn't need to be aimed at any specific star at all.

[peadar1987]

Considering that at the dawn of anno 28014 humans will in all likelihood be either

a) extinct / technologically regressed, or

possessing telescopes capable of collecting data on Alpha Centauri with much better fidelity than an ancient 250 kg probe on a highly imprecise course

I'm afraid that my conclusion is that there is no point, other than purely symbolic value.

Launching an interstellar probe with chemical rockets makes about as much sense as crossing an ocean with a paddle boat.

Well, actually the resolution we can get from telescopes is limited by the wavelength of light, it's not just a matter of making better optics. That's why we have electron microscopes, because electrons have a shorter "wavelength" than any form of light, and we can get really high resolution by bouncing them off stuff.

Others on these forums know more about cutting-edge physics than I do, but I don't think we'll ever be able to just look at Alpha Centauri with a telescope. We're going to have to actually go there.

[kurja]

Here's a question: suppose we did send a probe to alpha centauri and it had arrived today, or we just magically apparated a rover there or whatever, what kind of equipment would we need to have on it and here, to send instructions and receive data?

[-Velocity-]

Others on these forums know more about cutting-edge physics than I do, but I don't think we'll ever be able to just look at Alpha Centauri with a telescope. We're going to have to actually go there.

I'm not so certain about that. Massive space-based construction and manufacturing would be required to travel to Alpha Centauri. This same technology could be used to construct some incredibly massive telescopes. They've already been talks about building a 100 meter ground-based telescope- it's not funded yet, but probably someday we'll build one that big. But in space? There's no gravity or weight to contend with. If you can build the telescope IN space, then it can be MASSIVE.

What if your mirror was simply a very thin sheet of reflective material that was micro-actuated into a parabolic shape? That would never work on Earth, but in space, you might be able to get a thin, foil-like mirror that could be a hundred km across or more. This may sound ridiculously large, but consider a possibility where we are using machine intelligences to mine asteroids and run gigantic, space-based factories. Also, a thin-foil telescope doesn't use much in the way of raw materials anyway. Anyway, the diffraction limit on a telescope that large would be around 6 picoradians. You could easily surface features a few hundred km across on any planets orbiting Alpha Centauri. Make a telescope 1000 km in diameter, and you can resolve objects the size of cities.

Very large space-based optics would also have other uses- laser optics for light sailing. Besides light sails requiring a very lightweight reflective sail, if your light sail is laser-pushed, then you need massive laser optics to keep the laser focused on the sail out to great distances. So the same tech that allows telescopes hundreds of km across or more might be able to double as laser optics used for laser-pushed light sail interstellar probes.

I guess my point is, in the future, if we ever do get the ability to send probes to distant stars, our telescope technology will also be likely to have greatly improved as well. So exploration of space would likely be a combination of robotic missions and telescopic observation, just like today. You gotta have the telescopes to find interesting targets for close exploration anyway.

Edited September 26, 2014 by |Velocity|

[magnemoe]

I'm not so certain about that. Massive space-based construction and manufacturing would be required to travel to Alpha Centauri. This same technology could be used to construct some incredibly massive telescopes. They've already been talks about building a 100 meter ground-based telescope- it's not funded yet, but probably someday we'll build one that big. But in space? There's no gravity or weight to contend with. If you can build the telescope IN space, then it can be MASSIVE.

What if your mirror was simply a very thin sheet of reflective material that was micro-actuated into a parabolic shape? That would never work on Earth, but in space, you might be able to get a thin, foil-like mirror that could be a hundred km across or more. This may sound ridiculously large, but consider a possibility where we are using machine intelligences to mine asteroids and run gigantic, space-based factories. Also, a thin-foil telescope doesn't use much in the way of raw materials anyway. Anyway, the diffraction limit on a telescope that large would be around 6 picoradians. You could easily surface features a few hundred km across on any planets orbiting Alpha Centauri. Make a telescope 1000 km in diameter, and you can resolve objects the size of cities.

Very large space-based optics would also have other uses- laser optics for light sailing. Besides light sails requiring a very lightweight reflective sail, if your light sail is laser-pushed, then you need massive laser optics to keep the laser focused on the sail out to great distances. So the same tech that allows telescopes hundreds of km across or more might be able to double as laser optics used for laser-pushed light sail interstellar probes.

I guess my point is, in the future, if we ever do get the ability to send probes to distant stars, our telescope technology will also be likely to have greatly improved as well. So exploration of space would likely be a combination of robotic missions and telescopic observation, just like today. You gotta have the telescopes to find interesting targets for close exploration anyway.

yes, add that you can couple telescopes together to increase resolution.

As for signaling back, laser aimed at one of the big telescopes would work.

And it would be an nice idea to know if its something cool where you send the expensive probe. As I understand Alpha Centauri don't have gas giant like Jupiter at the same distance or closer or we would detected them so Avatar kind of fails here, no Na'vi

It might hold earth like planets but we don't know, the detection methods are pretty crude.

[cantab]

Yeah, in terms of resolution interferometry is where the real potential is. It'd take a lot of precision, but a set of telescopes in high Earth orbit would have serious resolution. Ramp it up to scattered around the solar system and we'll be imaging in incredible detail.

[Hattivat]

Well, actually the resolution we can get from telescopes is limited by the wavelength of light, it's not just a matter of making better optics. That's why we have electron microscopes, because electrons have a shorter "wavelength" than any form of light, and we can get really high resolution by bouncing them off stuff.

Notice how I wrote "data", not "images". That's because I assumed these telescopes would not operate in the visible spectrum. To my admittedly limited knowledge images in the visible spectrum are nice for publicity, but they are usually not what you want if your aim is to actually learn something about the observed object.

Another thing is that my impression is that most of you guys still don't seem to appreciate the incredible difficulty of achieving interstellar travel within manageable timeframes. There is a well known study on the feasibility of building an interstellar probe, called Project Daedalus. Dozens of scientists and engineers cooperated on it with the aim of coming up with the most feasible and achievable design for an interstellar probe that we could build using technologies that we can reasonably expect to have at out disposal in the future. What they came up with was this:

That's a 54 000 tonnes behemoth propelled by a inertial confinement fusion engine with a nozzle the size of an olympic stadium, requiring a large-scale mining operation on Jupiter to provide fuel. That's what the easiest feasible way to get a probe to another star system within a century using imaginable technology is like. And that's just for quick fly-by, getting it into orbit there would require a probe four times bigger and more complex. Even more interestingly, when this result was published, it was considered shockingly optimistic by the scientific community, because the operating assumption among the physicists up to then had been that achieving any sort of practical interstellar travel either required technology that we wouldn't even be able to imagine yet, or was for all intents and purposes simply impossible.

So yeah, in comparison to this thing above, I think that putting a large space telescope at the Sun's gravitational lensing spot (550 AU) is a relatively sane and achievable proposition.

[cantab]

I thought Orion drive (nuclear pulse propulsion) was up to the job of a quick-ish interstellar trip. It would still require a big ship, probably needing something like lunar or asteroid mining to make costs tolerable, but use a proven method of nuclear fusion rather than something still in the distant future.

As for telescope wavelengths, well astronomers like to use the full range of the electromagnetic spectrum because there's useful information everywhere. Visible light is quite good for general use because it's the range that many stars peak in - it's because the Sun peaks in those wavelengths that our eyes evolved to see them as visible light in the first place. For the specific task of studying exoplanets infrared may be more useful, since stars don't outshine planets quite as badly at such wavelengths.

Regardless, for all but the extremes of the EM spectrum a telescope can produce an image - a set of intensity measurements each corresponding to a certain patch of sky. Whatever the original wavelength we're free to display it in greyscale or false colour for our appreciation. For example look at the maps of the Cosmic Microwave Background.

[magnemoe]

Notice how I wrote "data", not "images". That's because I assumed these telescopes would not operate in the visible spectrum. To my admittedly limited knowledge images in the visible spectrum are nice for publicity, but they are usually not what you want if your aim is to actually learn something about the observed object.

Another thing is that my impression is that most of you guys still don't seem to appreciate the incredible difficulty of achieving interstellar travel within manageable timeframes. There is a well known study on the feasibility of building an interstellar probe, called Project Daedalus. Dozens of scientists and engineers cooperated on it with the aim of coming up with the most feasible and achievable design for an interstellar probe that we could build using technologies that we can reasonably expect to have at out disposal in the future. What they came up with was this:

http://d1jqu7g1y74ds1.cloudfront.net/wp-content/uploads/2008/08/daedalus_starship_630px.jpg

That's a 54 000 tonnes behemoth propelled by a inertial confinement fusion engine with a nozzle the size of an olympic stadium, requiring a large-scale mining operation on Jupiter to provide fuel. That's what the easiest feasible way to get a probe to another star system within a century using imaginable technology is like. And that's just for quick fly-by, getting it into orbit there would require a probe four times bigger and more complex. Even more interestingly, when this result was published, it was considered shockingly optimistic by the scientific community, because the operating assumption among the physicists up to then had been that achieving any sort of practical interstellar travel either required technology that we wouldn't even be able to imagine yet, or was for all intents and purposes simply impossible.

So yeah, in comparison to this thing above, I think that putting a large space telescope at the Sun's gravitational lensing spot (550 AU) is a relatively sane and achievable proposition.

Note that an smaller scale fusion ship might work if we get fusion. If not we have more serious problems.

An laser pumped solar sail would be far smaller again. Still the telescopes are nice for targeting they would be cost effective even if we had good faster than light.

[Hattivat]

No, as far as I know, with an Orion drive we are talking about a trip taking hundreds of years, unless you assume that your ship has a set of magic weightless radiators. The most optimistic case calculated by Dyson, in which he basically assumed an ablative coating made out of unobtainium, would still take 133 years to reach Alpha Centauri.

As for the telescopes - what I basically meant was that you don't need to actually be able to "see" an object (in the sense that you see Mars on pictures transmitted by probes) in order to learn a great deal about it. What I posit is that a space telescope from anno 28 000 will be able to detect the atmospheric composition, size, shape and even true color of planets in the Alpha Centauri system in much better detail than a tiny probe from anno 2014 flying millions or even billions of kilometers away from these planets, even if it may never be able to produce an actual picture of them for use in screensavers.

About the smaller scale fusion ships - yes the US Project Longshot proposed just that. Even then, it still requires us to develop not just a working fusion drive, but also one that can operate non-stop for years. That is quite literally a "long shot", so I'd much rather put my money behind that 550 AU space telescope idea to use the Sun itself as a gravitational lens. That's still quite ambitious, considering that 550 AU is about 5 times further away than the voyager probes currently are, but is nothing in comparison to the interstellar distances.

[Tommygun]

I was just about to also suggest Project Longshot as a reference.

It is probably one of the more realistic proposals for an interstellar mission to the Centauri system.

PROJECT LONGSHOT:

http://en.wikisource.org/wiki/Project_Longshot

[metaphor]

Would a space telescope at 550 AU be useful at all? You could really only point it toward a single direction unless you wanted to burn massive amounts of fuel. And if it got there ballistically within a reasonable timeframe it wouldn't be within its operating range for more than a year or so.

Anyway, it's certainly possible to send a probe toward Alpha Centauri at 50 km/s using only chemical engines (and 50 kg is tiny as far as space probes go). Without gravity assists: you could launch on an Altas V with a Star 48, which can take about 1 ton to solar escape velocity. Then you would spend several years/decades getting to a high apoapsis, and burning about 2 km/s to drop to a low solar periapsis. By that point your mass is about 0.5 tons. Then you could drop by the Sun (at the right inclination/ArP for Alpha Centauri) at about 4 solar radii (the original mission plan for Solar Probe+), and burn at periapsis for maximum Oberth effect. Assuming hypergolic propulsion and another stage, you could probably get about 6 km/s, which would result in a solar hyperbolic excess velocity of about 60 km/s.

But it's much easier and shorter with gravity assists. The Atlas V can get a 5 ton payload to Earth escape velocity. You could go past Venus and use either VEE or VVE gravity assists to get to Jupiter, which would then be able to drop you to the Sun without any other burns (basically this trajectory). So with your 5 ton payload at 4 solar radii you could burn about 12 km/s (with hypergolic propulsion) and get a solar hyperbolic excess velocity of about 100 km/s.

Of course, you could use a bigger launch vehicle, like the SLS, and an improved currently available mode of propulsion, like solar thermal or high thrust ion engines (since there's huge amounts of solar energy that close to the Sun). Using the gravity assist method, your ~60 ton payload could get to ~180 km/s solar escape velocity using solar thermal, or ~280 km/s using ion engines. If you have enough shielding on your spacecraft to get even closer to the Sun, like within 1 solar radius of the surface, you could get 300-400 km/s. That would get you to 550 AU in 6 years, and to Alpha Centauri in 3000 years.

[kurja]

Notice how I wrote "data", not "images". That's because I assumed these telescopes would not operate in the visible spectrum. To my admittedly limited knowledge images in the visible spectrum are nice for publicity, but they are usually not what you want if your aim is to actually learn something about the observed object.

What are you suggesting these telescopes would record? Something other than electromagnetic radiation??

[Hattivat]

Would a space telescope at 550 AU be useful at all? You could really only point it toward a single direction unless you wanted to burn massive amounts of fuel. And if it got there ballistically within a reasonable timeframe it wouldn't be within its operating range for more than a year or so.

From what I read, the beauty of it is that it doesn't need to stay at 550 AU. Any distance greater than that works just as well (or even better), as long as you stay within the arc where the object you are interested in is magnified.

As for whether it would be useful - certainly debatable, but imho infinitely more useful than a 250 kg slow interstellar probe. You are right about it only being able to look at a single target - there would have to be multiple missions to beyond 550 AU - one for each of the neighboring stars that we decided we wanted to take a closer look at. Keep in mind we are talking about using the Sun itself as a lens - there is absolutely no imaginable way to create a more powerful telescope. The way it is supposed to work is that if you get your position just right, you'll be able to see the sun-magnified system about as well as you can see the solar system itself from that position. I've seen someone calculate that if you put the Hubble telescope there and pointed it in exactly the right direction, you'd be able to resolve a planet the size of Mercury in a system 10 light years away as a single pixel of light. That's either underwhelming or unimaginably awesome, depending on your expectations. Regardless, the Hubble is hardly the pinnacle of space telescope technology. With better technology we could probably expect to get a picture like this, except much less spherical due to distortion:

which is about as much as we could expect to get from a first-generation interstellar probe, for probably a fraction of the cost, and with the ability to observe the target system for more than mere 70 hours (that's how long the Daedalus flyby is calculated to last).

(That picture above is Pluto as seen by the Hubble, getting a picture like this from 550 AU would require 20x better fidelity than what Hubble can provide)

What are you suggesting these telescopes would record? Something other than electromagnetic radiation??

No, I'm suggesting they would record all kinds of electromagnetic radiation (obviously in separate telescopes, not all in one), without particular regard for the tiny band of radiation that is visible light. In other words, it doesn't matter if we can "see" the exoplanets in the conventional sense of "seeing".

Edited September 27, 2014 by Hattivat

[cantab]

Would a space telescope at 550 AU be useful at all? You could really only point it toward a single direction unless you wanted to burn massive amounts of fuel.

The opposite is my concern. It's necessarily orbiting and thus the view is slowly changing. Though it will still be in the same general direction over the lifespan of the telescope, will this not cause issues with making follow-up observations?

Pointing in one direction isn't necessarily a dealbreaker mind. That's what Kepler did and it discovered about as many planets as everything else put together.

[kurja]

No, I'm suggesting they would record all kinds of electromagnetic radiation (obviously in separate telescopes, not all in one), without particular regard for the tiny band of radiation that is visible light. In other words, it doesn't matter if we can "see" the exoplanets in the conventional sense of "seeing".

Well then maybe I was just being overly pedantic, but afair images are data and the nature of all telescopes is that they produce images, regardless of what wavelength they receive; 'data' from an infra red, x-ray or what have you telescopes is presentable as images. What I'm saying is that when a space telescope sends down the signal it has recorded, it's the same thing if it's sending us 'images' or 'data'.

[Beowolf]

And even if our electronics could survive for that long, you'd still need to develop some exotic energy source to keep it going that long, no RTG can keep on working for that long, and solar panels obviously won't cut it in interstellar space.

I was catching up on this whole thread at once, and you stopped me cold here. Been staring at the ceiling for several minutes, and I think you're right. I can't come up with any known or plausible form of energy storage that'll last over 20k years. Not even one that'd work on Earth in a controlled environment, much less in space with ionizing radiation and temps near absolute zero.

A most interesting puzzle!

[Hattivat]

Well then maybe I was just being overly pedantic, but afair images are data and the nature of all telescopes is that they produce images, regardless of what wavelength they receive; 'data' from an infra red, x-ray or what have you telescopes is presentable as images. What I'm saying is that when a space telescope sends down the signal it has recorded, it's the same thing if it's sending us 'images' or 'data'.

When you have an "image" of something which is itself invisible to the human eye, then it's not really an image, but rather a mere graphical representation. The raw "image" from a radiotelescope is by definition invisible; it only becomes presentable after someone more or less arbitrarily assigns colors to particular radio frequencies. In the same way you could graphically represent the soundwaves and claim to have "an image" of a symphony.

@Beowulf: Indeed, these are all fascinating questions.

Edited September 28, 2014 by Hattivat

[Kryten]

That hubble image of pluto was created by very carefully analysing data from transits of it by Charon and vice versa; it's not a good model for hubble resolution at that kind of distance.

[kurja]

When you have an "image" of something which is itself invisible to the human eye, then it's not really an image, but rather a mere graphical representation. The raw "image" from a radiotelescope is by definition invisible; it only becomes presentable after someone more or less arbitrarily assigns colors to particular radio frequencies. In the same way you could graphically represent the soundwaves and claim to have "an image" of a symphony.

Analogy to sound waves isn't very accurate, sound isn't electromagnetic and isn't usually presented visually. Data of 'invisible' electromagnetic radiation on the other hand often is. Also from a technical point of view, there is no difference in generating an image of a visible light recording, or other frequency of em radiation recording. You can even do IR/near-IR photography with an ordinary consumer camera, with only minor modifications.

M109 in x-ray frequency, with and without visible spectrum & ir mixed in

And a visible light / ir comparison

And one more example of invisible spectrum imaging

EDIT - this got a bit sidetracked and hung up on semantics, my point was that you were saying that it would be somehow easier to transmit 'data' from a telescope than 'images' from that same telescope, and I'm pretty sure that you're mistaken because the two are actually the same thing.

Edited September 28, 2014 by kurja
fixed link & image

[metaphor]

The opposite is my concern. It's necessarily orbiting and thus the view is slowly changing. Though it will still be in the same general direction over the lifespan of the telescope, will this not cause issues with making follow-up observations?

Pointing in one direction isn't necessarily a dealbreaker mind. That's what Kepler did and it discovered about as many planets as everything else put together.

But if you're sending a telescope to 550 AU in a reasonable amount of time you have to send it fast. If you want it to get there in 20 years it has to be going about 100 km/s. Therefore if you want to stop at that distance (or orbit) you need another 100 km/s of delta-v. Orbiting is kinda useless, better to just stop it entirely, it's not going to fall a significant distance towards the Sun for hundreds of years.

The fact that you can only look in a single direction seems like a huge downside. Kepler's field of view was 0.25 square degrees, while the size of the Sun from 550 AU is 8*10^-7 square degrees (and the field of view of a telescope there would be about the same). If you wanted to view something 0.1 degrees away, you would have to travel 1 AU.