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Developing FOCAL Mission Concepts

In the early summer of 2005, I found myself, thanks to the efforts of Greg Matloff and Princeton’s Ed Belbruno, in Princeton for a conference called New Trends in Astrodynamics and Applications II, which Dr. Belbruno had organized. I was to give a brief talk at the end of the session summarizing what was going on in the interstellar travel community. Two days of chill rain didn’t dampen my enthusiasm at seeing Greg and his wife, the artist C Bangs, as well as Belbruno himself, who had been a great help as I put together my Centauri Dreams book. And on the morning of the first day of the conference, I joined Greg, C and Claudio Maccone for breakfast at the Nassau Inn, Princeton’s lovely colonial era hostelry.

I’ve since had the opportunity to talk with Dr. Maccone many times at conferences, and one year enjoyed memorable meals with him in the Italian Alps, but that first encounter really sticks in my mind. I had been thinking about gravitational lensing for several years, but it was only through studying Maccone’s writings that I began to fathom what the idea represented. Suddenly I found myself having breakfast with the man who had made a mission to the Sun’s gravitational lens a cornerstone of his professional life. What a way to start the Princeton conference!

Here was an opportunity offered up by nature to achieve extraordinary magnifications. It would be a natural lensing available to any civilization that could reach the gravity focus of its star, which for the Sun means about 550 AU. As the focal line extends to infinity, studies at various wavelengths are enabled as the spacecraft continues its departure from the Solar System.

Image: The FOCAL mission as described by Claudio Maccone in his 2009 book Deep Space Flight and Communications (Springer).

Let’s dwell on that a moment. The goal in a mission to the gravitational lens — Maccone calls the concept FOCAL, which is what I will use as we talk about the topic in the next few days — would not be to reach 550 AU and stay there. The gravitational focus is not like an optical lens, where light diverges after the focus. Instead, as light from the other side of the Sun is focused by the Sun’s gravitational field, it remains fixed along the focal axis. Every point along the straight line trajectory beyond 550 AU remains a focal point.

So if you want to image a target, you move toward 550 AU precisely opposite to the target you want to image, putting the Sun between you and it. Think Alpha Centauri, for example, where a close look at Proxima Centauri b would be exceedingly useful as projects like Breakthrough Starshot ponder sending probes there. The gain for optical radiation from the planet would be amplified by a factor of 1011. Now think of the uses not just in imaging but in astronomy at a wide range of wavelengths. FOCAL would be a true interstellar precursor.

Assuming, of course, that you could untangle the image. The planet we want to study is, after all, spread out into an Einstein ring surrounding the Sun. More on this in a moment.

Beyond imaging, the communications potential for the gravity lens is intriguing and enabling. I’ll pause to note that the history of gravitational lens mission ideas even includes a SETI aspect. Back in the 1990s, drawing on work at the Italian aerospace company Alenia Spazio, Greg Matloff examined an inflatable radio telescope called Quasat, which became the subject of regular meetings in that decade. All this ties in with the extensive Italian investigation of solar sail ideas that I’ve discussed before in these pages, work informed by the efforts of physicists like Giovanni Vulpetti, Maccone himself, Giancarlo Genta, and the continuing work of Matloff.

Matloff explored solar sailing to reach the gravitational focus in a 1994 paper (“Solar Sailing for Radio Astronomy and SETI: An Extrasolar Mission to 550 AU,” Journal of the British Interplanetary Society Vol. 47, pp. 476-484), where sails enabled placement of a probe at the gravity lens whose observations ranged through radio astronomy and into SETI.

I mentioned Von Eshleman’s work back in the 1970s, which to my knowledge was the first time anyone proposed an actual mission to 550 AU and beyond to study the Sun’s gravitational lens. It took just over a decade for the concept to take deep root and spawn a mission concept.

For it was in 1992 that Claudio Maccone began discussing a FOCAL mission at a conference in Torino, and in 1993 he went on to submit a formal proposal to the European Space Agency for the funding needed to firm up a mission design. Meanwhile, he was identifying issues that needed to be addressed. The 550 AU distance may be a bare minimum, given the problem of distortion caused by the Sun’s corona. But the farther the probe travels from the Sun, the less the coronal effect, and with a focal line extending to infinity, we have room to maneuver.

A FOCAL mission is quite an interesting problem for spacecraft design, one that Maccone subsequently addressed in his Deep Space Flight and Communications (Springer, 2009). There he advocated using tethers to deploy two antennae, each tether several kilometers in length, allowing a larger, more detailed field of view than one provided by a single antenna. If these methods work, we still have a very difficult image to untangle. Potentially, we could look at details on a planetary surface even at the distance of Proxima b, studying these with a mission that is 278 times closer than the actual distance to the target. But about that image…

Geoffrey Landis has analyzed the problems we’ll face in using the Sun as a ‘gravitational telescope’ when trying to image the surface of an exoplanet. The issues are manifold, and include pointing and focal length at these magnifications, signal-to-noise ratio involving the Sun’s corona, and what he calls ‘focal blur,’ which seems inherent in the nature of gravitational lensing. All of this in a significant paper titled “Mission to the Gravitational Focus of the Sun: A Critical Analysis,” which addresses matters that are also being examined in NASA’s latest NIAC study. JPL’s Slava Turyshev is principal investigator on that one, an unusual Phase III grant called “Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravitational Lens Mission.” The concept has already gone through two prior rounds of development.

Image: Artist’s depiction of a possible image from a Solar Gravitational Lens (SGL) telescope. Credit: Slava Turyshev.

But I’m getting ahead of myself, and deep into unknowns under current investigation. Let’s talk in the next post, then, about using the gravitational lens for communications. It’s a fascinating option for efforts like Breakthrough Starshot, in which getting a signal back to Earth from tiny payloads is an obvious concern. Just what, then, is a radio bridge, and how does Claudio Maccone hope to use one to put a significantly different spin on missions like Starshot? I’ll also have thoughts on the NASA effort and the continuing mission design within NIAC.

Note: Geoffrey Landis also presented a later version of his work on the lens in a 2017 conference paper for the Tennessee Valley Interstellar Workshop, “A Telescope at the Solar Gravitational Lens: Problems and Solutions” (full text). This paper, along with another by Slava Turyshev on imaging issues, is also available in the conference’s proceedings volume.


Comments on this entry are closed.

  • Thomas R Mazanec December 10, 2020, 21:28

    The gain for optical radiation from the planet would be amplified by a factor of 1011
    Is that true for the star as well? If so, it would appear at a magnitude of about -27.5, at least as bright as the sun from Earth.

    I mentioned Von Eshleman’s work back in the 1970s, which to my knowledge was the first time anyone proposed an actual mission to 550 AU and beyond to study the Sun’s gravitational lens.

    I first heard about this in one of David Edward Hugh “Daedalus” Jones’ columns back in the Seventies. I think this was before Von Eshleman.

    • Paul Gilster December 10, 2020, 21:36

      Not familiar with these columns, but if this ran before Von Eshleman, that’s interesting! Thanks for the tip, which I’ll try to confirm.

    • Patient Observer December 14, 2020, 17:09

      That is a very interesting observation. Could the level of radiant energy be sufficient to power the probe once it reaches the focal point?

      As a SciFi plot driver, could the gravitational lens be used to launch an attack on an unfriendly planet (they don’t like our television shows?) using a powerful laser of some sort with the necessary optics to end up with a tightly focused beam at the other end?

      Or detonate a nuclear device at the focal point; only a fraction of the energy would be delivered to the other end but could be quite a light show at the least.

  • Robin Datta December 11, 2020, 6:07

    “as light from the other side of the Sun is focused by the Sun’s gravitational field, it remains fixed along the focal axis. Every point along the straight line trajectory beyond 550 AU remains a focal point.”

    The questions immediately arising are: how far out does this remain true, and how and how much is the image degrraded by increasing distance along the focal axis.

    If degradation is not significant or can be overcome, then every star and galaxy out there might be a “telescope”.

    • Paul Gilster December 11, 2020, 9:28

      The focal line extends to infinity. Dr. Maccone’s equations show that the gain is constant along the focal axis but is wavelength-dependent. Bear in mind, too, that we do better in an important respect in going beyond 550 AU because radio waves impinging on the spacecraft at distances higher than this will have to cross less and less dense layers of the solar corona.

      • Alex Tolley December 11, 2020, 12:30

        The video presentation has an equation that shows that the signal gain is proportional to the sqrt(1/dist).

        IOW, the gain at 1000 AU vs 550 AU ~= 0.74
        at 10,000 AU vs 550 AU ~= 0.23

        So while the focus does go to infinity, the signal intensity declines compared to the closer focus. What was not discussed was where a “sweet spot” might be where the interference of the solar corona and any other imperfections is reduced compared to the loss of signal strength, especially for communication purposes. This may be hard to compute directly, but possibly a simulation might help provide an answer.

        • Paul Gilster December 11, 2020, 13:11

          Yes, and I’ve been hunting around for such a ‘sweet spot’ in the literature, but haven’t come up with where this would be. I need to dig back into the 2009 book, but let me see what Claudio says directly.

          • Dimjo December 15, 2020, 18:46

            Pardon my ignorance, but is it possible the ‘wow’ episode was a consequence of us passing thru a stars’ axis?
            Thank you.

            • Paul Gilster December 15, 2020, 21:33

              Interesting you should mention the Wow! signal; I’ve just been looking at an about to be published paper on it with some interesting new ideas. But no, the Wow! signal was incredibly strong and rotated out of view along with the background stars in such a way that an accidental microlensing — and I’m not sure how to imagine one here — doesn’t seem like a feasible explanation.

  • Hal December 11, 2020, 12:32

    Suddenly, getting to Sirius B becomes more interesting.

    What happens if you focus a 550 AU telescope AT the focal point of Sirius B? Can you create a 2 lens “super scope”?

    • Michael December 11, 2020, 16:15

      The gravilens around Sirus B need not be anywhere near 550 AU it can be less than 1 AU from that star. Imagine the destructive power for anything moving in the focal line as it goes around Sirus !

      • Martin December 11, 2020, 18:06

        Hal, Michael:
        Some time ago, just for fun and using a somewhat simplified equation, I calculated the focal point distance for a number of nearby white dwarfs. Here they are:
        Sirius B: 3.7 million miles (0.039 AU)
        Procyon B: 12.9 million miles (0.14 AU)
        Van Maanen’s Star: 9.05 million miles (0.097 AU)
        40 Eridani C: 19.9 million miles (0.21 AU)
        note: I used the Wikipedia values for the radius and mass of the white dwarfs.
        A typical neutron star gives a value of 10 miles!

      • Hal December 11, 2020, 22:20

        -“The gravilens around Sirus B need not be anywhere near 550 AU”- Understood. I’m asking something a bit different – can you create a dual lens gravi-scope by pointing a 550 AU Sol-scope at the Sirius B white dwarf’s focal point? You won’t get a huge field of view, but the magnification should be magnificent!

        • Michael December 12, 2020, 14:07

          The white dwarf would focus it and then it would start to diverge again so of little use.

    • Harold Shaw December 12, 2020, 15:23

      Constructing and image from one Einstein ring is challenging. We would need to untangle and construct and image from 2 overlapping Einstein rings. That may not be possible or well beyond what existing computers can manage. The second focal may just add noise, making it more difficult to construct an image from the first focal.

      Keeping the second focal and the “telescopes” target in view is another problem to overcome. I am sure the next columns will deal with this issue. The focal point might continue into infinity but the target is always moving. In the case of one focal, the telescope will only have a small window of time to view the target. There may be no way to line up 2 focals with a target.

  • Michael December 11, 2020, 16:20

    I think a radioactive decay type mission would do well here, we could put radiative decay material on actuators to propel and stear the craft been largely ejected as it neared the focal line.

  • FrankH December 11, 2020, 17:39

    We’re used to talking about missions to Proxima Centauri – a “mere” 4.2 LY away – maybe because the technology to make that mission possible (in a human lifetime) is still lifetimes away from reality.

    In comparison, 550AU is almost within reach yet it’s a staggering distance. The Parker Solar Probe (fastest spacecraft at 129.609 km/sec – at least for a brief period) would take * 20 years * to reach 550AU, if it could sustain a constant 130km/sec.

    If we could get a spacecraft to reach “only” 1/1000 C – more than 2x faster than the Parker probe but still in the realm of what we could do in the next few decades – it would take a little over 8 years.

    550AU is far, far away.

  • Mikelorrey December 11, 2020, 18:26

    I have difficulty believing that politicians will spring for the billions to fund looking at only one star.

    • Michael December 11, 2020, 21:59

      It does not just mean stars for planets but even the BH at the centre of the Galaxy and perhaps use its gravilens to see even further deep field objects.

  • Robert Flores December 12, 2020, 13:11

    This mission should be in our pocket before we even think of
    a extra-solar colonization effort.
    In that vein is see some difficulties in orienting the spacecraft.
    Assuming we want to look at G and hotter K type stars within
    200 LY, There are targets . As an exercise let see how many
    of these type stars would be in that volume.
    Stellar density in our neighborhood = .004 cu/ly
    Volume of 2o0Ly x Density = 134,016 x .o5(only G, hot Ks) bout 6700
    but our Scope can only look in one direction so more like 3,350 stars.

    What is the average distance between them This is crux of the problem.
    You must move the spacecraft the focal point of EACH STAR. How long would that take?. So if you used your main propulsion to the 1st position on your “watch list”, what kind of propulsion do you use to
    re-orient the space craft (not the station keeping one that is liable to be
    a small Ion thruster.) So a third Propulsion system that allow repositioning of say 1 AU per month.

    It means the survey wont be complete until 279 years. and that will survey only Half of the sky.

    This craft is going to be VERY heavy, and will have to be self-repairing.
    This is a mission for the next century, But overall it helps to line up our Ducks and find out what the capabilities are need in such a craft.

    • Ivan Vuletich December 13, 2020, 20:15

      The geometry is such that you effectively have to dedicate one space craft to one target. Great for a dedicated exoplanet mission or comms relay, but pretty much useless for anything else.

  • Mike Serfas December 12, 2020, 14:47

    Could we really see clouds like that? I started thinking the path lengths around the Sun could be quite different – would we see several hours of data smeared together? (Unless there’s a 4D general relativity issue that cancels out what the path length really is??) More importantly, would we see the planet itself smeared out along its direction of proper motion? (I looked in the Landis critique and it has one brief sentence about “motion blur” that I can’t take either way with confidence)

    • Alex Tolley December 12, 2020, 18:44

      If there is that much issue with different pathlengths, then this would naturally limit the communication bandwidth. The video presentation had an equation for the bit rate for any given bandwidth, but I did not see any obvious handling of path length issues in the error rates and therefore the achievable channel capacity.

      • Mike Serfas December 13, 2020, 5:35

        On second glance I really overstated the issue there, since the light certainly is not travelling the legs of a right triangle, and also because .875 seconds of arc (which is a larger figure than I should use) turns out to only be about a light-second away from the Sun in the first place, even at 500 AU! Arc seconds are the nemesis of intuition…

  • Harold Shaw December 12, 2020, 16:47

    Looking forward to the next columns. Focal length missions are fascinating. Imo, they are our most worthy first cathedral project. They are within our reach but mastering them will push us to grow. Targets will have relative motion. For any craft there is only a window of observation as it moves through the focal point. Even this possibly tiny window will produce a data set that will yield new discoveries over generations.

    Algorithmic optics will be a crucial aspect of solar focal missions. With multiple telescopes, algorithmic optical interferometry would be possible. Thinking in terms of the long term value of a data set, perhaps a swarm of low vehicles is more profitable than one fast vehicle.

    • Michael December 13, 2020, 2:22

      It’s a focal line, we only need stay on that line. I am thinking of a fission fragment sail with the fission fragments on actuators around the edges for propulsion and steering, they can also be used to slow the craft down to stay in the focal line longer and move around for other targets. Perhaps we could go into orbit around 550 AU to start scanning the sky deep field.

      • Harold Shaw December 13, 2020, 12:06

        From Geoffrey Landis:

        “Alternatively, with sufficiently good knowledge of the target planet’s orbit, the telescope could move to track the planet. Since an orbital velocity of 30 km/s corresponds to a velocity of 30 m/s at the image plane, the ∆V to reacquire the planet will be about 30 m/s.Over the course of a year, about 200 m/s would be required to track the planet, with the telescope moving around at the focal plane in an ellipse with a semimajor axis of ~150,000 km.This could be done using some form of high specific-impulse propulsion.”

        These figures are for a planet ten light years from Earth and cover the delta V just for tracking a planet as it orbits its star. I’m trying to track down a paper I didn’t save that addressed the requirements for staying on the focal line.

  • Andrew Palfreyman December 12, 2020, 18:31

    I found this a useful engineering description.

  • Michael December 13, 2020, 8:31

    Strangely you don’t just need a huge mass to bend light. Have a thought experiment and let’s say you spaghettify the earth into a long thin form. The same mass is there but light can get much closer to the mass, in fact millions of times closer and therefore bend much more at the cost of resolution.

  • Harold Shaw December 14, 2020, 10:55

    I could use some help understanding how long a vehicle could stay on the focal line. The Sun’s and target’s motion cause the focal line to swing and I don’t trust my math skills. Should we expect to find targets with slow moving focal lines that allow decades of observation?

  • Alex Tolley December 14, 2020, 12:49

    One argument against this idea is that it is a single target mission. That makes it very expensive and there would have to be solid evidence pre-mission that the target was a worthy one, perhaps with unambiguous biosignatures or even…unambiguous techno-signatures.

    However, what if the assumption that it is a single target mission is wrong? Has anyone looked at the possibility of serially looking at a number of stars’ exoplanets, rather like the Voyager Grand Tour of the planets? Is there a viable, low-cost orbital path that could, with some maneuvering allow imaging of a number of exoplanets with minimal propellant (or beamed energy)?

    A single mission is OK if the aim is for a communication bridge for an interstellar probe mission.

    Lastly, while there is a focal line, there is the issue of needing a craft to both be in an orbit as well as staying in line with the target star as it moves in relation to our sun. It strikes me that a sail that allows a “hover” might be the best way to maintain a position at a “sweet spot” yet also track the target as it moves.

    • Paul Gilster December 14, 2020, 12:52

      Re multiple missions, this is what the first two Phases of the JPL study for NIAC have been developing. More on this tomorrow. We’re talking about a number of separate missions to explore various exoplanets. The NIAC study continues in Phase III.

  • Michael Fidler December 15, 2020, 8:58

    I think there may be a solution that takes the problem away from 550au and beyond and puts it in our backyard. Take the super light weight carbon foam material that was discussed awhile ago in the form of a solar sail and fly it very close to the sun. This would send out to 550au in no time but it would be used there as an optical reflector to send the signal back towards Earth and space telescopes. The telescopes near Earth would be like the eyepiece and the carbon foam would have a high reflective coating like a secondary mirror in a telescopes. This get rid of the problem of sending a heavy complex probe out to beyond 550au and puts the main instruments near Earth. The only problem is in the details but hopefully this gets rid of waiting 50 years or longer and could be done relatively quickly. The carbon foam is cheap and many could be sent to many locations to study different star systems in detail by just turning the carbon foam reflector. Maybe a thousand sent to the focal point for Proxima Centauri.