Catching Up with FOCAL

by Paul Gilster on June 28, 2013

Michael Chorost has written a fine essay on Claudio Maccone’s FOCAL mission concept for The New Yorker blog. Centauri Dreams regulars will know Chorost from several previous posts here, particularly a discussion on SETI that I talked about in On Cosmic Isolation, where he analyzed the hunt for extraterrestrial civilizations in terms of problems of perception, with reference to his own thoughts on deafness, cochlear implants and neurotechnology. Mike is the author of the superb World Wide Mind (Free Press, 2011) that examines the interface between future humans and future machines.

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I also like to remind readers of something Mike wrote on his own blog last year, which refers to a book I deeply admire and issues I’ll be writing about in future essays here. In particular, how do we deal with advanced alien civilizations if we run across them, and would the gap between us and them defeat our attempts at communication? Chorost takes a positive view:

I’d like to be optimistic. I’d like to think we’d be better off than preliterates puzzling over Wikipedia on an iPad. In his book The Beginning of Infinity, David Deutsch argues that humans crossed a crucial threshold with the scientific method. We now know that everything is explainable in principle, if we make the effort to understand it. Arthur C. Clarke famously said that any sufficiently advanced technology will seem like magic. This may be true, but we will not mistake it for magic. We have a postmodern openness to difference, a future-oriented culture, and well-established methodologies for studying the unknown. Our relative horizons are much larger than our ancient ancestors’ were.

More on all these issues in last year’s post SETI: Contact and Enigma. For today, though, some thoughts about the FOCAL mission, which envisions sending a probe out past 550 AU, the distance at which light is bent around the Sun by its gravity to form a gravitational lens. I say ‘beyond 550 AU’ because the focal line goes to infinity, and also because at 550 AU itself, we have severe coronal distortion effects to deal with, so a FOCAL mission, if the technology and the science check out to make it happen, would begin its observations further out and would keep traveling as it made them.

Lensing and Technology

We’ve seen recently –and Chorost writes about this — that there is to be an attempt to examine Proxima Centauri for planets because of an upcoming gravitational lensing event. The star will pass in front of a far more distant star in October of 2014, and an analysis of that event may flag the presence of small planets that have thus far eluded detection. Maccone, of course, has been arguing since the early 1990s that we can stop waiting for chance observational occurrences and start exploiting the gravitational focus with a directed mission. His book Deep Space Flight and Communications (Springer, 2010) is an analysis of mission possibilities.

My conversations with Claudio Maccone about FOCAL go back to one of Ed Belbruno’s New Trends in Astrodynamics and Applications conferences at Princeton in 2005, where Greg Matloff, his wife C and I had breakfast with Claudio and, over orange juice, coffee and a stack of enormous pancakes, talked about the problems and potential solutions for getting the mission to work. Since then the indefatigable Maccone has worked tirelessly on the issues and in his new book discusses, among other things, the kind of antenna deployment such a probe would use, and the prospects for using gravitational lensing not only for astronomical observations but communications.

Chorost had numerous exchanges with Maccone for this piece and does an admirable job at making the basics clear to a lay audience. He’s particularly interested in SETI possibilities, something out of which the initial FOCAL work grew through meetings in Italy in the 1990s that examined the kind of missions that could be run using solar sail technologies. From the essay:

Maccone wants to use the sun as a gravitational lens to make an extraordinarily sensitive radio telescope. He did not invent the idea, which he calls FOCAL, but he has studied it more deeply than anyone else. A radio telescope at a gravitational focal point of the sun would be incredibly sensitive. (Unlike an optical lens, a gravitational lens actually has many focal points that lie along a straight line, called a focal line; imagine a line running through an observer, the center of the lens, and the target.) For one particular frequency that has been proposed as a channel for interstellar communication, a telescope would amplify the signal by a factor of 1.3 quadrillion.

Could we actually build such a craft and, having sent it on its journey to a place roughly five times as far as Voyager 1 has reached to begin observations, untangle the information it sent us? We’re pushing theory and technology hard and have much to do with both before we can be sure it will work. For that matter, when Mike asked me how we could get Maccone’s payload into this kind of trajectory within the lifetime of a human researcher, I had to say that with present-day methods, we’d be limited to a solar sail (perhaps with a close solar pass to boost acceleration) or nuclear-electric technologies that might be used in conjunction with such a sail.

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Image: Claudio Maccone (left), Jill Tarter and myself at the 100 Year Starship conference last year in Houston. What an evening that was, and yes, FOCAL was a major topic when Claudio and I had dinner the next evening. Thanks to Thomas Hair for snapping this.

But then what? Long-time readers will know of my admiration for A. E. van Vogt’s short stories (his novels, alas, are another matter, although I have a lingering fondness for The Weapon Shops of Isher). In particular, the story “Far Centaurus,” which ran in the January, 1944 issue of Astounding, sticks with me because its Centauri-bound crew finds the destination already settled by people who had left long after them on faster ships. So if a 50-year FOCAL mission is enabled by sail or hybrid technologies, can we guarantee that twenty years after launch, we won’t have something better — a fusion option, perhaps — that will pass the initial mission along the way?

It’s always a tradeoff, and the fact is that we could have waited before sending our Voyagers to the edge of the Solar System. If we had, we’d still be waiting, because it’s proving mighty hard to come up with that next big propulsion breakthrough due to funding limitations and perceived lack of public enthusiasm for major space projects. New Horizons is arcing toward Pluto/Charon and at one point was moving a bit faster than Voyager 1, but the Sun’s gravity has slowed it back down, leaving the intrepid Voyager 1 at 17.1 kilometers per second as our fastest moving object.

Or, I should say, ‘our fastest moving object that is departing the Solar System,’ since we’ve been able to get solar probes like Helios up to faster speeds. In any case, FOCAL is one of those mission concepts that, like Innovative Interstellar Explorer, demand innovative thinking and a serious gut-check in terms of what is possible and what we want to do. For me, the scope of the challenge makes it all the more fascinating to study such missions. Pushing hard at our limits awakens creative thought and suggests unexpected options in a future that is anything but foreordained.

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{ 33 comments }

Michael Chorost June 28, 2013 at 13:37

Thank you very much, Paul, for the thoughtful discussion of my piece. I think it’s a fascinating concept and I wanted to help get it into a larger discussion space. (To my disappointment, the comments section is messed up due to some third-party glitch. They’re still trying to fix it. It’s really too bad because I was eager to learn from the discussion about it.)

Two thoughts. At the end of the piece I suggested that a FOCAL radiotelescope might be able to observe an entire galaxy for signals. That would help us get around the extreme unlikeliness of any particular stellar system having a species that is intelligent, technological, and broadcasting at the exact moment that we look at it. My thinking was, if we could look at several hundred billion stars at once, that would improve our odds.

Dr. Maccone was skeptical of this idea when I raised it to him. I believe he felt that unless somebody there was pointing a FOCAL transmitter at us, we would have no chance of hearing anything, given intergalactic distances. I wanted to be clear about his skepticism in the story, but that got lost in deadline pressures. I decided to raise the idea anyway, because I was pretty impressed by that gain of 1.3 quadrillion (at 203 GHz using a 2.7 meter antenna.) And I thought, maybe somebody will figure out a way to do it.

My other thought is, you can’t build the fast ship without the experience you get from building the slow ship. You can’t build a 747 until you’ve built the 707, which you can’t build unless you’ve built the Wright flyer. It’s not just a matter of knowledge, it’s also the infrastructure. Building the slow ship gives you the civilization you need to build the fast ship. I don’t think there’s any shortcut. So, I really hope that the “wait for the fast ship” mentality doesn’t block things like FOCAL missions.

Thanks again for the nice mention and for all your help while I was writing the story.

coacervate June 28, 2013 at 15:01

You can’t build a 747 until you’ve built the 707, which you can’t build unless you’ve built the Wright flyer….

Could we extend this reasoning to propose using smaller, more accessible lenses (Jupiter? the Moon??) to make smaller but easy-to-build telescopes?

coacervate June 28, 2013 at 15:03

Oh darn, I just realized smaller gravity fields have longer focal lengths….nevermind.

Paul Gilster June 28, 2013 at 16:04

One of Claudio’s papers lists the various focal distances for the planets, and you’re right, they’re a lot further out, although a departing FOCAL probe would pass through the required distances and could theoretically use planetary focusing in some way. Here’s more on focal spheres around planets:

http://www.centauri-dreams.org/?p=15290

Alex Tolley June 28, 2013 at 20:22

@coacervate – looking at Paul’s reference chart, Venus has a focal line starting at 17k AU, which looks like about 1/4 ly. Obviously far off and difficult to get to. But, do we have to use bodies in the solar system? If interstellar space has large bodies (rogue Jupiters, brown dwarfs), we may be able to use those as a test in our nearby space. I don’t know how good they would be, what magnification is possible, but it might be a possible near term approach. I’m guessing that the trade off is that with these smaller, interstellar bodies, what you gain by the focal line being in near earth space, you lose by having to travel a long way to shift the view to a [new] target.

Anthony Mugan June 29, 2013 at 4:20

I would agree with the idea that, in any large project, you have to draw a line and say ‘that is the specification’. Waiting for ‘the next big thing’ tends to lead to procrastination.
Just a thought on the opening remarks concerning the ability of our society to absorb any culture shock arising from contact. I would tentatively suggest that this may vary both within and between cultures. The optimistic scenario discussed above may well be right for a technological and scientific elite, used to thinking in that sort of way. Educational levels vary considerably and a large portion of the population seem vulnerable to superstitious beliefs and respond largely emotionally to issues. Variations in world view between cultures can be profound with some still based on religiously derived models of political authority. The response to contact may therefore differ significantly between individuals and groups.
Experimental data is lacking but it may be worth noting that the famous ‘War of the Worlds’ broadcast had its impact on a society in which the scientific model was well established for the elite, but had its impact on a wider cross section of the population.

David Cummings June 29, 2013 at 7:23

“…the sun’s optimal focal point is seventy billion miles from its surface… Maccone theorizes that, travelling along the sun’s focal line, it would continue to be effective up to a distance of about ninety billion miles.”

My concern is that after that very long journey, how long would the device be effective. Certainly it would be effective at 70B miles, but what does it mean that “Maccone theorizes” it will be good for another 20B miles? Does it mean that its reception gets increasingly blurry every mile past the 70B mile marker?

I guess if it turns out to be good for “only” another 100M miles or so, that’s still a lot of observation time.

In the meantime, assuming these can be produced cheaply enough, we could have a steady stream of followups on the same path out from earth. With the newer ones traveling even faster.

Anyway, it’s a fascinating concept, using the mass of the Sun to amplify signals quadrillions of times.

Michael Chorost June 29, 2013 at 10:34

David Cummings, it’s my understanding that it actually gets better the further out you go, up to about 90B miles. Light rays passing further from the Sun’s surface get bent less, so meet at a focal point that’s further out. That (I think) translates into more light-gathering capacity.

Tulse June 29, 2013 at 10:41

The thing I find so tantalizing about FOCAL is it seems right at the edge of what we’re currently able to do, and thus an excellent aspirational mission. Are there any technological advancements needed, or could we launch FOCAL with today’s capabilities?

The major downside I see for FOCAL is that unlike conventional telescopes, its field-of-view is so narrow and fixed. You need to be darned sure that there’s something interesting to “see” on the opposite side of the Sun, or all that travel time is wasted, since you can’t really point FOCAL elsewhere using the Sun.

Using planets as foci is a really intriguing idea, since that greatly increases the flexibility of the mission. I could see sending FOCAL missions above and below the ecliptic to maximize the areas that could be viewed as the various planets swept through their orbits.

Paul Gilster June 29, 2013 at 12:23

Tulse writes:

The major downside I see for FOCAL is that unlike conventional telescopes, its field-of-view is so narrow and fixed. You need to be darned sure that there’s something interesting to “see” on the opposite side of the Sun, or all that travel time is wasted, since you can’t really point FOCAL elsewhere using the Sun.

This is one reason I think FOCAL would make the ultimate observational platform for studying the Cosmic Microwave Background. Pointing problems are irrelevant for the CMB.

Tulse June 29, 2013 at 14:35

FOCAL is great for studying the CMB, but I don’t know if that is sexy enough to get the mission funded, especially compared to the potential for imaging exoplanets (however unrealistic that might be).

Astronist June 29, 2013 at 18:38

Voyager is our object with the greatest specific energy. When going to the outer Solar System or the stars, energy per unit mass is everything.

Stephen
Oxford, UK

JD June 29, 2013 at 22:15

Given that an exotic propulsion mode is needed to get FOCAL out there relatively fast, may be DoD has made a few discrete discoveries related to fusion that could do the job, if applied to space propulsion:

http://www.sandia.gov/z-machine/?page_id=140

Lionel June 30, 2013 at 3:24

a- Seems to me that exoplanet science is progressing at a pace that by the time we’d be ready to launch a FOCAL mission we *will* have found something that we’re darned sure is interesting

b- How much of the target system do we get to see in our field of view? Just one planet? The whole system including the outer planets?
Actually I saw Claudio Maccone talk about FOCAL in the UK space conference four years ago and asked him this question, if I remember correctly he said that to see whole exoplanets within the system, the FOCAL probe needs to be in two halves connected by a tether and spun about the axis. But how long does the tether has to be to observe all habitable zone planets?

c- Michael or Paul, could you tell us why Claudio theorizes FOCAL no longer works after 90B miles out? The lensing events we view from earth are brought about by bodies many orders of magnitude further away

d- Great point about the CMB. So FOCUS actually becomes a CMB mission as well as an exoplanets mission – presumably it will find other stuff of great scientific value too, in the pinprick of sky which it peers into

Brett Bellmore June 30, 2013 at 8:35

“So if a 50-year FOCAL mission is enabled by sail or hybrid technologies, can we guarantee that twenty years after launch, we won’t have something better — a fusion option, perhaps — that will pass the initial mission along the way?”

Less of an issue with an unmanned mission. Presumably the later missions would be launched in different directions, the first mission would not be wasted.

I’m reminded of a old saying, “There comes a point in the life of every program, when it’s time to shoot the engineers, and ship the product.” Being an engineer myself I wouldn’t take that too literally, but there’s a certain truth to it.

Paul Gilster June 30, 2013 at 10:16

Lionel writes:

Michael or Paul, could you tell us why Claudio theorizes FOCAL no longer works after 90B miles out? The lensing events we view from earth are brought about by bodies many orders of magnitude further away.

Let me see if I can get an answer from Claudio about this. I’ll post his answer.

Brasidas June 30, 2013 at 10:55

Michael Chorost,

Would designing the mission so that it could view the center of our own galaxy, where the starfield is dense, solve some of the issues with “intergalactic distances” while still allowing a large number of stars to be in the observation field?

Tulse June 30, 2013 at 11:28

Do we need very exotic propulsion systems to get FOCAL to its location in a reasonable time, or would some sort of existing electric propulsion (coupled with a power source like a fission reactor) do the job? Stated another way, how fast could we get it out there with existing or very near term technologies? And are there other meaningful missions the craft could carry out while in transit?

Michael June 30, 2013 at 12:53

I had nothing to do this weekend and was thinking about using back ground stars as light sources to probe stars systems that are close by moving them in line with each other . I derived the formulae as approx,

X (perpendicular distance movement required) = (Tan (apparent degree angle between stars) x D (back ground star distance) x D (fore ground target star distance)) /(D (back ground star distance) x D (fore ground target star distance))

For nearby stars movment distances of around 90 – 200 billion km’s are required, proper motion has been ingnored.

Eniac June 30, 2013 at 20:32

@Lionel:

How much of the target system do we get to see in our field of view? Just one planet? The whole system including the outer planets?

You can only “see” one planet at a time. However, it should be possible to observe an entire system, and track any of the planets, by moving the craft. An easy way to see this is as follows:

The ratio between the distances craft-sun (focal length) and sun-target (target distance) is about 1:1000, given a target at 5-20 ly. Thus, in order to track the target, for every km it moves, the craft will have to move 1 m. The Earth, in its circular orbit, changes direction (or “accelerates”) at a rate of ~200 km/s/year, or less than ~ 0.01 m/s^2. The craft will need to accelerate at the even much lower rate of a thousandth of that to keep focussed on an Earth-like planet. Seems easily feasible, so we can check off tracking planets and exploring entire systems.

Actually I saw Claudio Maccone talk about FOCAL in the UK space conference four years ago and asked him this question, if I remember correctly he said that to see whole exoplanets within the system, the FOCAL probe needs to be in two halves connected by a tether and spun about the axis. But how long does the tether has to be to observe all habitable zone planets?

Here, again the 1:1000 ratio of focal length vs. distance to target allows us an easy estimate: If the planet has a diameter of 12,000 km, in order to image its entire surface we need to scan an area in the image plane that is 12 km across. That would be the length of tether needed.

Arrayed along the tether would be small telescopes, coronagraphs, really, the task of which would be to focus on the Einstein ring around the sun. This is the only place where light from the target is coming from, all other light needs to be blocked out, especially that coming from the sun itself. Each such coronagraph would not acquire an image, but rather a single pixel: the intensity of the Einstein ring. An image would be produced by scanning the image plane. This is best achieved by rotating the tether and slowly shifting the single-pixel “cameras” around so they acquire spiral shaped scan lines. As discussed earlier, there also must be active propulsion to keep the whole thing aligned while tracking stellar and orbital motions.

Besides exoplanets and the CMB, it would also be interesting to observe the central star itself. Lack of light might make observation of planets difficult. this is less of a problem with the stellar surface. Imagine seeing a nearby star with the same resolution as we can now see our sun. Solar/stellar scientists will go nuts over it.

So this is a wonderful strength of FOCAL: It has tasty morsels for at least 3 constituencies: cosmologists (CMB), stellar astrophysicists, and exoplanet observers. A properly targeted mission could conceivably target all aspects at once, providing mission safety and a maximum of political appeal.

Joy June 30, 2013 at 22:51

“it’s proving mighty hard to come up with that next big propulsion breakthrough due to funding limitations and perceived lack of public enthusiasm for major space projects”

I would have said “due to those pesky laws of physics”. Thermodynamics and inertia in particular. Perhaps we should start a petition to outlaw the Higgs field?

torque_xtr July 1, 2013 at 11:51

A sudden realization: if the direction doesn’t matter at all to study CMB, then it’s possible to study it “from home”, looking at the stars with suitable coronagraph,and having as many points in the sky to look at as many suitable stars we have in our skies. But matching the proper motion would be the issue, so the heliocentric probe with solar-powered ion drives will be needed, because te needed maneuvers in LEO steep gravitational gradients are much trickier..Also this would be great possibility to learn how to work with such great focal distances, to manipulate tethered probes and to extract the information convoluted into the shape and brightness of the Einstein rings before actually going to focal sphere of the Sun.

Michael Chorost July 1, 2013 at 14:15

Brasidas,

“Would designing the mission so that it could view the center of our own galaxy, where the starfield is dense, solve some of the issues with “intergalactic distances” while still allowing a large number of stars to be in the observation field?”

That’s a very good question. It depends on the field of view of a FOCAL system, which I don’t know anything about. I think Eniac’s answer above is helpful. From his response it looks like you just have to wiggle the spacecraft, so to speak, to survey different planets in a single system. But stars are further apart. I imagine that you would have to alter the trajectory of the probe to bring each one into view. Could you view a mass of stars (in radio frequencies) at one time? I don’t know. Anyone have any thoughts?

Paul Gilster July 1, 2013 at 14:33

Well, I’ve written Claudio about several questions readers have raised, but he’s in the midst of an IAA conference and too engaged to answer but briefly. I hope when the conference is over we can get his thoughts on this.

Brett Bellmore July 1, 2013 at 17:53

“I would have said “due to those pesky laws of physics”.”

I would have said, due to those pesky laws of governments. It’s fairly clear that almost all avenues for high performance propulsion involve exploiting nuclear energy, and nuclear energy is perhaps the first technological development that’s been effectively regulated from it’s birth. This has seriously stunted it’s development.

Can you imagine what Edison or Tesla would have done with nuclear energy? Not a chance in the world they wouldn’t have gone ahead with Orion.

Rob Henry July 1, 2013 at 20:19

Eniac, I love your model, but that probe acceleration in it is the simple harmonic motion equivalent of that circular orbital motion, thus your probe acceleration estimate is a maximum, not typical.

Eniac July 1, 2013 at 22:23
Would designing the mission so that it could view the center of our own galaxy, where the starfield is dense, solve some of the issues with “intergalactic distances” while still allowing a large number of stars to be in the observation field?

That’s a very good question. It depends on the field of view of a FOCAL system, which I don’t know anything about. I think Eniac’s answer above is helpful. From his response it looks like you just have to wiggle the spacecraft, so to speak, to survey different planets in a single system. But stars are further apart. I imagine that you would have to alter the trajectory of the probe to bring each one into view. Could you view a mass of stars (in radio frequencies) at one time? I don’t know. Anyone have any thoughts?

I am pretty sure that when targeting the center of the galaxy, you could easily observe many stars by moving the probe around slightly. However, they will also be very far away, and their planets are probably too dim to see, even with the solar-sized aperture. The same goes for ETI radio signals. Remember that light is only gathered by a very narrow ring around the sun, so light gathering power is not as great as it would first seem. The “trillions” that make the amplification seem high look a lot less spectacular when the equally impressive magnification factor is accounted for. My feeling is that only high intensity targets like stars, active galactic nuclei, and intentionally directed beams may provide enough photons to get good reception. I could be wrong, of course. To my knowledge, it is an open question whether exoplanets provide enough light to be effectively imaged this way.

coolstar July 1, 2013 at 22:34

Off hand, I’d say that a FOCAL mission’s usefulness for studying the CMB is not very high: almost all the useful science being done there is on large scale angular correlations, exactly the opposite of a FOCAL mission.

Lionel July 1, 2013 at 22:51

Thanks Eniac that was great.

When I heard Claudio speak, he said something along the lines that FOCAL would be able to make out “towns and cities” on the observed planet.

This to me sounds like a claim that FOCAL data should be capable, over the full course of observations, of producing us Voyager-quality images of the planets and moons of a star 5-20 ly away (but we only need to go 1000th of that distance). Is FOCAL really this good?

Brett Bellmore July 2, 2013 at 17:53

I would expect the problem is not so much focal power, as light gathering ability. By the time you’re 5 lightyears out from an object the size of a city, even well illuminated, even a telescope the size of Earth’s orbit isn’t going to be intercepting many photons. (A milliwatt or so?) At least, not many photons compared to the background level. It would probably be that good if there weren’t all those stars out there, but then, what would you have to look at in that case?

Eniac July 2, 2013 at 23:09

Lionel:

Is FOCAL really this good?

The diffraction limit is very easy to calculate, As I remember the resulting maximum resolution is at worst a few meters. So, not only cities, but also cars and whales and such. Light gathering ability is a little harder to evaluate (should not be real hard, though), and like Brett I am not entirely optimistic that this will come out well. If anyone knows of a quantitative treatment of this aspect, please speak up.

Another issue is that of the unnatural point spread function, which makes imaging difficult. This is explained very well here: http://www.cesr.fr/~pvb/gamma_wave_2005/presentations/optics/Koechlin.pdf. It may be surmountable by clever image processing and perhaps by analyzing the brightness profile along the perimeter of the Einstein ring, in addition to its overall brightness.

Ronald July 11, 2013 at 10:06

For the downside reason that Tulse mentioned: “its field-of-view is so narrow and fixed. You need to be darned sure that there’s something interesting to “see” on the opposite side of the Sun”, and the clarification that Eniac offers, I think that an instrument like FOCAL would be most suitable for a distant but very rich target, where small adjustments in the instrument’s position lead from one interesting target to the next.

For this reason I would suggest the Andromeda galaxy as a prime target: some 1 trillion stars within about 3 degrees of sky.

Andrew Palfreyman August 30, 2013 at 18:29

It would be handy to know what resolution we get for, say, the nearest 10 solar systems, and what we could expect to see, and how long the spiral scan might take. This would help in determining our first choice for an interstellar mission (instead of just rushing off to Alpha Centauri because it’s closest).

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