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A Beamed Sail to the Sun’s Gravity Focus

Our recent discussions about Claudio Maccone’s FOCAL mission to the Sun’s gravitational focus, and the ongoing work at the Jet Propulsion Laboratory for NASA’s Innovative Advanced Concepts office, have had Alex Tolley thinking about alternative scenarios. Yes, a spacecraft moving along the focal line extending from the solar gravitational lens (SGL) would be capable of extraordinary imaging, and could serve as a communications relay for interstellar probes, but that tricky Sundiver maneuver suggested by Slava Turyshev and team in their ‘string of pearls’ concept puts huge demands on sail materials. Moreover, we’d ideally like to be able to slow the craft as it moves along the focus, to allow maximum time for observations. To achieve both fast transit and maneuverability at the gravitational focus, Alex advocates beamed propulsion, a method whose advantages and consequences are discussed below. Synergies with the ongoing Breakthrough Starshot effort are apparent.

by Alex Tolley

The huge increase in discovered exoplanets, many in the habitable zones (HZ) of their stars, has increased the push to determine if life exists on any of these worlds. With even our best telescopes, light from these worlds is just a single pixel in extent, which allows spectrographic analysis for biosignatures and techno-signatures. However, a 2D image of an exoplanet would answer many more questions about these worlds. The image makes a lot of difference both to scientists and the public. The astronomy books of my youth showed the moons of the gas giants as points of light, and Pluto was just a star-like object. As late as 2010, the Hubble telescope could only manage a crude blurry image of Pluto showing some light and dark regions. The New Horizon probe flyby with high-resolution images changed our view of the Pluto-Charon binary.

Image: Hubble 1200×1200 pixel image of Pluto in February 2010.

To acquire a megapixel image of even a nearby exoplanet would require a telescope of tens of kilometers in diameter to collect the light of the planet while masking out the far more intense light of its star. A possible solution was proposed by Claudio Maccone, with his proposed FOCAL mission to use our sun’s gravity to focus distant light rays on a telescope [5]. While there are technical issues still to be resolved in how to form a low noise 2D image of 10,000 (100 x 100) to 1 million (1000 x 1000) pixels, the big question is:

“How do we get there in a short enough time for a project , and how do we best collect the data to make the image?”

The Sundiver Problem

The solar gravitational focus (SGF) is a line extending from about 550 AU to infinity. The focus starts out in interstellar space. For comparison, Voyager 1 launched over 40 years ago and has just passed through the heliopause of our sun and into interstellar space. It is only ¼ of the way to the SGF. It will not reach the focus for another 120 years. Clearly, we need a much faster way to reach the focus so that scientists and engineers can reasonably engage in a realistic mission, rather than taking the generations that were required to build a cathedral.

At this point, we can rule out most propulsion technologies, even using gravity assists. The most promising is a solar sail, and as I will argue, a sail augmented with a beam.

The authors of the recent NIAC II report [1, 6] opt for a pure solar sail. They propose a mission that could reach the SGF within a reasonable 25 years using a very advanced solar sail and leveraging a ‘sundiver’ trajectory. This sail could achieve a velocity of 25 AU/yr (about 125 km/s), of which most of that velocity is achieved quickly as the sail leaves perihelion. When I say ‘advanced,’ the mission needs a sail that has an areal density of less than 10 g/m2, and possibly 1 g/m2. Note that this density is not just the sail material, but must include the probe structure, payload, and any auxiliary equipment, such as communications, maneuvering thrusters, etc.

To put this in perspective, the Planetary Society’s CubeSat LightSail has an areal density of about 143 g/m2, a similar density as the upcoming NEA Scout probe. The target areal density of the Breakthrough Starshot beamed sail is 1.4 g/m2, achieved by having just a chip-sized payload on the sail of just 1 m2 [4].

Without such low areal densities, 2 to 4 orders of magnitude lower than currently achieved, there is no hope of reaching the needed velocities. The sail craft must also do a sundiver maneuver to gain maximum thrust at perihelion. Maximum velocity is achieved by the orbit of Saturn. How close the sail can approach the sun depends on the sail materials. To get both the very low sail mass to achieve the needed areal density, and approach the sun to within 0.1 AU (15 million km) requires a strong, low-density material with a high melting point, such as a ceramic.

If such a solar sail is achievable, then the craft with its telescope payload will continue on into interstellar space along the focal line.

While the focal line continues to infinity, is there an optimum “sweet spot” to image the target? Maccone states that although complex, the best position might be fairly close to the 550 AU start of the SGF [1]. If that is correct, it is suboptimal to allow the telescope to continue traveling rapidly away from that position.

The NIAC authors solve this rather cleverly: launch a series of sails, a year apart, forming a “string of pearls” separated by 25 AU, so that each probe stays within 25 AU of the start of the SGF at 550 AU before handing off the data collection to the next probe, or even contributing to the collection of other data as it journeys on into interstellar space.

This solution has several disadvantages, not the least of which is the need to ensure that each sail can align independently and correctly with the target.

Therefore, ideally, one would want the sailcraft and its telescope to effectively stop at the best distance just beyond 550 AU. This has several advantages:

  • The focus remains unchanged – reducing issues with image deconvolution
  • The required coronagraph to isolate the Einstein ring of light is fixed in dimension and position.
  • The tracking of both the target star’s position and the exoplanet in its orbit is simplified as the distance to track relative motion increases as the distance from the sun increases.
  • Only one imaging telescope is needed, as well as the auxiliary equipment. The scale economies of building and flying multiple sails can be used to image multiple targets instead of just one.
  • Data collection can be as long as desired, not fixed for the number of sails sent, allowing better images to be produced, as well as longer-term observations of the target for other purposes. If the payload includes a receiver for a communication bridge to a probe orbiting an exoplanet, the communication link can be established for as long as needed, perhaps many decades.

Using sail material that is off-the-shelf, how could we achieve such a mission?

The Benefits of Beamed Propulsion

My proposed solution is to use a beamed sail. The beam would most likely be a phased laser array located on Earth with the capability of pushing a sail in all but the highest latitudes. This is the type of power source that is being studied by the Breakthrough StarShot team. Because the sail need not achieve the fractional c velocities needed for interstellar flight, the size of the sail, payload, and beam intensity on the sail can be matched to the materials and payload requirements. Ideally, the sail velocity would exceed that of 25 AU/yr to shorten the flight time, but no longer restricted to the high-performance sundiver mission needed to reach that velocity. There is also no need to tolerate high temperatures at perihelion, although this will also depend on the laser power and duration.

Moreover, there is a way to stop a beamed sail, which was suggested by Robert Forward [7]. Figure 1 below shows a schematic of the concept for this deceleration maneuver.

The sail would separate into 2 parts with the larger part focussing the beam on the smaller sail to decelerate it. This was proposed by Forward as a way to decelerate a sail and its payload on arriving at its desired destination. The same approach could be used to decelerate a beamed sail when it reaches the 550 AU minimum focus position. When “stopped”, the sail would deploy its imager/receiver.

Image: Schematic of the round-trip interstellar lightsail concept proposed by Forward (not to scale), shown during the deceleration phase. Credit: Geoffrey Landis [2].

The relative sizes of the two sails will depend on the achievable laser strength on the sail over the deceleration distance that ends around 550 AU, and on how well the reflected beam can be targeted to the sail with the payload. The aim is to slow the smaller sail with the telescope payload down towards a dead stop, although any low velocity, like that of Voyager 1, is adequate to ensure a long data collection period and minimal changes to the required maneuvering to track the star and planet. Pointing the laser at the sail will require relatively simple position prediction up to about 160 hours in the future based on the most recent time-stamped location received from the craft. [160 hours is 2x the light travel time to 550 AU; signal sent from the craft that reaches earth 80 hours later and the next 80 hours for the beam to reach the sail.]

The slow movement of the stars relative to the sun will still require some traversal across the focus to track this motion and ensure the craft stays within the best position to receive the communication or visual image. For example, Proxima moves about 3.85 arcseconds per year across the heavens. At 550 AU, this implies that to keep the sun in line with Proxima, the craft must travel a modest 50 m/s to maintain position.

One issue not so far mentioned is the position of the stellar target in the sky. Unlike most of our planetary missions to date, where the planets fairly closely aligned with the ecliptic, the target stars for an SGL mission are spread out over the celestial sphere. For example, Alpha Centauri is in the southern sky with a declination of over -62 degrees. The TRAPPIST 1 planetary system, about 40 ly away, with some possibly very interesting exoplanets in close proximity for imaging, is about -5 degrees of declination when observed from Earth. The sail craft must travel in the opposite direction so that the target is directly behind the sun, so that for Proxima the telescope must be positioned with a declination of over +62 degrees and the appropriate 180 degrees offset to right ascension.

Each sail craft can only image one target star at a time, although if that star has several planets, all these planets may be imaged over time. The stars are far too separated over the celestial sphere for any reasonable time to navigate between them. For the 30 pc (100 ly) volume, the 15,500+ FGK stars would be separated by about 60 AU on average. Therefore each sailcraft could only image one star system. This would therefore require considerable care in selection. However, given the laser infrastructure and the scale economy of manufacturing the sail craft, it might well make sense to send many craft out to their SGF positions.

The benefit of a FOCAL mission to acquire relatively high resolution images of exoplanets far beyond any single telescope we can envision today is offset by the demands of reaching the gravitational focal line starting at 550 AU. Once there, ideally, the craft should not continue its outbound journey. To achieve this, a beamed sail that can decelerate its payload is proposed.


1. Gilster, P. (2020) JPL Work on a Gravitational Lensing Mission, https://www.centauri-dreams.org/2020/12/16/jpl-work-on-a-gravitational-lensing-mission/

2. Landis, G (1989). Optics and Materials Considerations for a Laser-propelled Lightsail” http://www.geoffreylandis.com/lightsail/Lightsail89.html

3. Vulpetti, G., Johnson, L., & Matloff, G. L. (2008). Solar sails: A novel approach to interplanetary travel. New York: Copernicus Books.

4. Montgomery, E, Johnson, L, (2017) Solar Sail Propulsion: A Roadmap from Today’s Technology to Interstellar Sailships. Presentation to Foundations of Interstellar Studies Workshop, New York City College of Technology, Brooklyn, NY

5. Maccone, C. (2009) Deep Space Flight And Communications: Exploiting The Sun As A Gravitational Lens (Springer Praxis Books / Astronautical Engineering)

6. Turyshev et al. (2020), “Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravity Lens Mission.” Final Report. NASA Innovative Advanced Concepts Phase II

7. Forward, R (1984) “Roundtrip interstellar travel using laser-pushed lightsails,” American Institute of Aeronautics and Astronautics, v21 No.2, pp 187-195


Comments on this entry are closed.

  • Michael December 29, 2020, 12:41

    I am surprised tungsten is not used, although very dense it has a very low vapour point in a vacuum even at high temperatures and has a fair reflectivity. Perhaps we could have one side of the sail of this material coated in pu 238 which is a alpha emitter. So we use the alpha emission to slow the sail down to drop into a close pass of the sun. Once at closest approach the sail is flipped from edge on to reflective side and driven out. Once at a fair distance the alpha side is exposed to power the sail onwards. Perhaps little micro paddles could be added to allow station keeping, the alpha emissions could also power the instruments onboard.

    Could FOCUL be change to FOCULine, the i from Claudio’s first name and ne from his surname in honour.

    • Robin Datta December 29, 2020, 23:33

      I had the impression that one would want to swoop around the sun as fast & furious, and as sheer & near as possible: slowing down while approaching the sun would negate the benefit of the manoeuvre.

      • Michael December 30, 2020, 8:01

        Slowing down from earth orbit allows you to fall towards the sun, negate our 36 km/s orbital velocity and you fall into the sun or can skim it.

      • Robin Datta December 31, 2020, 13:30

        Would the escape velocity (from Earth) vector pointed just off from the sun make a dference?

        • Michael December 31, 2020, 20:09

          Yes correct you would need the escape velocity, I was just giving a simple example.

  • Michael December 29, 2020, 12:50

    I use this sometimes to see stars relative to each other.


    Some stars are grouped together so moving from one to the other is not that great a change in distance.

  • Christian G December 29, 2020, 18:02

    Has there been any discussion of the primary non-technical issue with the laser array? To wit: Any system capable of delivering X amount of energy to 550 AU is also capable of delivering it to a point on the Earth’s surface.

    • Paul Gilster December 29, 2020, 21:20

      Yes, it’s been a matter of serious discussion. It’s one reason that an array on Earth seems less threatening than one with the capability of beaming at the Earth from space.

      • Robert December 30, 2020, 12:38

        How hard would it be to bounce a beam from the surface around the earth to any point with space based mirrors? It seems that take less effort than to build a space based laser array.

        • Paul Gilster December 30, 2020, 13:38

          You mean if someone were trying to weaponize the array? I suspect a beam as strong as Starshot talks about would wreak havoc on any mirror it was directed toward.

          • Brett P Bellmore December 31, 2020, 9:47

            You’d have to deliberately defocus it a bit, and then use the redirecting mirror to focus it back down.

            A better location would probably be the back side of the Moon, less concerns about toasting satellites, and the whole “firing an enormously powerful laser through the atmosphere” thing.

          • Andrei January 1, 2021, 3:46

            Not only that, you could turn an entire population blind. You guys will hate this but if anything like this is build it will have to be under very strict control and surveillance.

            • Michael January 1, 2021, 6:38

              Surprising it’s not at a frequency that causes damage to the eye because it’s absorbed by water but a vast amount may cause irritation and skin burns.

  • charlie December 29, 2020, 20:22

    Given the fact that one of these spacecraft arrives at their station keeping orbit, for example the one that is keeping an eye on Alpha Centauri-how does one go about maintaining a station keeping position on the focal line (i.e. not wandering off the focal line) at the 550 AU point? The spacecraft has to continue in its orbit about the sun which I assume would mean that it would lose the focal point line, am I correct or am I missing something ?

    • Alex Tolley December 29, 2020, 22:40

      550 Au is already in interstellar space. The orbital velocity is extremely low. If you stopped, there would be n almost imperceptible drift back towards to sun. The more important issue is that the target star has a relative motion to the sun, and the target planet a relative motion on top of that. To maintain the target on the exact focal line that you want requires tracking it to keep it exactly opposite to the sun, and doing so without being able to observe it directly except via the focal point. So yes you do need to navigate laterally, but the orbital issue at that distance is relatively minor – at least according to my basic calculations.

      [I recommend using the orbital formula, and calculating G*M from known examples like Earth and Jupiter, and then doing the calculation for 550 AU – about 100x that of Jupiter’s distance from the sun.]

      • charlie December 30, 2020, 14:39

        Actually at the risk of sounding like I actually knew it all before, I had considered that the accelerations at that distance might be extremely low and therefore you might be able to stay on the focal line for at least a reasonable period of time – but quite frankly I was actually far too lazy to sit down and do even a simple calculation (that’s what you become when you become an old man).

        As for the commenter who said that the orbital velocity might be as small as measly 1.27 km/sec, and I’m more than willing to accept that person’s figure, whether right or wrong that still seems pretty fast for that distance. The only important thing that I was thinking about was, as we are almost always are talking about with regards to any spacecraft that’s worthy of its name is something that is usually fantastically expensive. It’s not like I harp on the money because I’m a cheapskate, it’s just that if you are doing something and expecting to get a scientific return you have to be able to actually accomplish what you set out to do even if the expected return is fantastic. I confess I’m starting to warm up a little bit to this type of mission, but I can’t help but wonder whether or not the expectation is going to fall a little bit short of the reality for many reasons. But I’m a natural skeptic.

        On musing a bit further on this issue I wondered whether or not periodic bursts of laser light from Earth (that’s where I expect the laser array to be anyway) might be enough, at least for a reasonable fraction of the life’s mission, might be enough to maintain a approximate station keeping position-to allow the life of the mission to be extended. Could that be accomplished? What was amazing to me, finally, was the fact that they were talking about a little under seven days time required to perform a bit of station keeping for the nearly stopped craft. Getting to be a fairly fantastical amount of time required to keep track of our little children out there.

        • jonW December 31, 2020, 16:23

          charlie, I think you’re right that 1.27 km/s is not so slow after all. Due to the ratio of distances to the sun, it’s 1/sqrt(550) times Earth’s orbital velocity, or roughly 1/23rd Earth’s orbital velocity. Maybe someone could figure out how long the Proxima system would be in focus if the craft slowed to that speed. If the craft arrived at the focal point from just the right orientation and slowed to orbital speed, I wonder how long after Proxima exited its field of vision we would see the A/B system swim into it?

          • Alex Tolley December 31, 2020, 20:05

            I estimate that Proxima moves across the sky such that at 550 AU, it traverses at about 0.5 km/s. If the probe was moving in an orbit at about 1.27 km/s, it would “see” Proxima for barely 1/3 second. As it takes time to collect and deconvolve the light, the craft really does have to track Proxima (or any target star) at about the velocity it moves.
            [Please check my calculations to verify this.]

            Clearly, as the orbital distance increases, the velocity of the probe must increase to track the radial movement of the star, but at the same time, the orbital velocity will decrease.
            At around 1000 AU, the orbital velocity and the needed tracking velocity are both about 1 km/s, suggesting that for any star, there is an orbit where the probe and the star move at the same velocity and therefore stay stationary. The more distant the star, and the slower its movement across the sky, the more distant the needed orbital radius needs to be. For most stars, this will exceed 1000 AU, and hence it is better to stay closer to the 550 AU position and try to stop any orbital velocity if possible. A further complication is that we don’t want to track the star, but rather an exoplanet orbiting its star and therefore the probe must match the net movement and direction of the planet relative to our sun.

    • jonW December 30, 2020, 0:24

      charlie, I think it’s the fact that orbiting the sun at 550 AU means the spacecraft will only be travelling at a measly 1.27 km/s, if I’ve got my calculations right. Presumably this is slow enough that it will enjoy a long enough period where the target system is aligned with the sun, that it can get all the data that is needed. But yes eventually it will drift out of the line of focus for the target system. I’m sure someone will correct me if I’m wrong.

    • Michael December 30, 2020, 1:16

      Once you are passed the escape velocity we are pretty much going in a straight line. I think we should look into decay fragment materials on microactuators to station keep.

    • Robin Datta December 30, 2020, 6:50
      • charlie December 30, 2020, 17:46

        I watched the video on static satellites and at first I was entranced and thought: Problem Solved ! However as I thought about it a little further I began to realize that at 550 AU the sunlight (as well as a gravity) would be quite minuscule. I suspect that you would need inordinately large sail to permit you to station keep using sunlight only.

        The more thought that I gave to it, the more I began to realize that perhaps the better way would be simply to use the same laser that sent the craft to that point as a station keeping radiation source. That way you can have a much smaller sail and at the same time maintain your focal point line. It seems like that aside from small perturbations the point would be stable over a highly extended period of time.

        • charlie December 30, 2020, 22:04

          Well I did it! I really did it ! I put my foot in my mouth and it certainly turned out to be a comfortable fit ! Also I gave myself a good swift kick (and I want to tell you it actually hurt badly!). What am I talking amount ?
          The huge, huge GOOF that I made in pronouncing the idea that Robin Datta had put forth as a possible way to keep yourself in the focal line.

          It won’t work! And I suddenly realized that even if you achieve station keeping on the focal line, your Helio stationary orbit would only be stationary with respect to a point directly on the surface of the Sun ! Not with respect to the necessary focal line that aligns itself with the alpha Centauri – solar axis ! None of these objects that are called Statites would in fact NOT possess no tangential velocity, in fact they would they would simply appear to be quasi-stationary with respect to a point on the surface of a given body (in this case, the sun).

          So perhaps the use of a laser with a proper ability to produce a drag (so to speak) on the sailcraft might be enough to maintain it on the star-son axis.

        • Brett P Bellmore December 31, 2020, 9:52

          “I suspect that you would need inordinately large sail to permit you to station keep using sunlight only.”

          Nah, that’s the beauty of the concept, both gravity AND light are inverse square, neglecting issues arising due to the Sun not being a point source, the same sail that allows station keeping at the orbit of Mercury allows it at the focal line, because the ratio of sunlight to gravity stays constant.

          The real issue is that to get there in the first place, you need enormous delta V, and about the same delta V to stop again. If you did come to a stop, you wouldn’t be worried about falling back out of the focus in a humanly meaningful time, you delta V requirement is actually for moving around normal to the line through the Sun, to scan for imaging.

  • Robin Datta December 30, 2020, 0:02

    A long term investment could perhaps prove worthwhile. A Chinese saying geos “For a return on investment: in one year – plant rice; in ten years – plant fruit trees; in a hundred years – educate people”. Sending off the equivalent of a Hubble or a James Webb or some combination thereof towards the focal point would be a sunk investment with no early return (unless a creative trajectory to observe points of interest close-up en route is planned).

    Such an instrument would have to be a part of an autonomous observatory run by on-board artificial inelligence with maintenance, repair, course correction, attitude control et cetera built-in. With a power supply independent of Sol’s radiance.

  • Brett P Bellmore December 30, 2020, 10:06

    Forward’s split sail scheme requires maintaining a fractional wavelength figure on a low areal density sail, AND the sail needs to be refocused, too. I mean, it was clever as SF, but doesn’t strike me as a plausible engineering solution. It’s not technology we’re close to having.

    I know that nuclear is politically disfavored, but this really IS a job for fission fragment propulsion. I’ve never seen any (non-political) show stoppers for dusty plasma fission fragment rockets, and such an engine can provide the probe with both thrust and power.

    • Alex Tolley December 30, 2020, 15:39

      I expected a lot more pushback on the use of such a technique and the likely power requirements for the laser. I am glad you raised this issue. While I think the focussing is not that difficult – slow the spin of the sail and use “parachute” lines to the smaller sail to create the needed curvature – there are issues with the beamed laser power. I don’t have the expertise to do the necessary calculations for a phased laser array, but clearly beam divergence over 100s of AUs is going to be an issue, and hence the needed power.

      • Brett P Bellmore December 31, 2020, 10:00

        The “parachute” lines actually hurt rather than help: They dimple the sail. Not a serious issue if you’re using it for propulsion, but a big deal if you’re trying to use it as a precision mirror.

        You’d have to jettison the lines to maximize the uniformity of the sail, and then manipulate the figure some other way, maybe by locally altering the reflectivity and thus thrust from the light beam.

        Mind, you might want some such capability built in anyway, it would be a good way for the sail to actively manage stresses at initial high accelerations.

        • Alex Tolley December 31, 2020, 14:27

          AFAIK the sail does not have to be a “precision mirror” as it is not trying to create an image. I am sure you have seen simple solar concentrators for backyard solar cookers that used aluminum foil pasted to wood or cardboard to focus the sun onto an oven. As long as the net intensity is increased at the focus, a wrinkled reflector will work.

          • Brett P Bellmore January 2, 2021, 10:09

            No, actually it does need to be precision: Consider how rapidly the reflected light intensity would drop off otherwise. (And the beam intensity is already low due to the distance from the transmitter!)

            You’d never get enough delta V out of the reflected light to be worth bothering with, unless it was precision. The acceleration is low, it has to be maintained for a long while to add up to anything worth doing.

    • Mike Serfas December 31, 2020, 8:35

      So far as I can see, the portion of the sail sent forward is essentially used as propellant. It seems like there should be more efficient ways to absorb the light and emit propellant. The momentum change from bouncing the light is E/c (can be doubled for reflections) while the momentum change from absorbing that energy and shooting propellant, in a non-relativistic approximation, is mv = (2m*0.5mv^2)^0.5 = (m*E)^0.5 = E’/c, where E’ is the geometric mean of the energy of the absorbed energy and the propellant mass if it were converted to energy. (In the relativistic version you would use the relativistic mass of the propellant particles, which depends on how much energy they absorbed) So long as the maintainers of the massive laser that launched the ship find no more compelling targets for their fire, it should certainly have no need of local power production, but if it absorbs the energy instead of reflecting it (incidentally cutting the outbound momentum change in half) and then operates any run-of-the-mill drive with the received energy, it should be able to reverse far more quickly than it accelerated. The amount of reverse thrust is clearly limited, but given the change of operation there seem to be many engineering possibilities, all terribly difficult, at this point – for example reuse the mirror to concentrate sunlight to heat propellant, or recycle it as propellant mass, or have it so precise and preternaturally durable under fire that it can function in the telescope itself and use much of the payload capacity for propellant instead. But sending the mirror off into the sunset, relying on that far-travelled and much-abused mirror to perfectly reflect light onto a distant spacecraft that needs its constant light from afar, seems one of the less probable options.

      • Alex Tolley December 31, 2020, 14:12

        So far as I can see, the portion of the sail sent forward is essentially used as a propellant.

        The outer sail is not in any way used as a propellant. It is simply a method to move the source of light from behind the sail at sunward, to in front of the secondary sail, to decelerate it. Just think of the inner sail becoming non-reflective for a while so that the outer sail, now detached, can accelerate away from the inner sail. There is no transfer of momentum. Once the outer sail can focus the light onto the front surface of the inner sail, the inner sail can now be decelerated with the light reflected off its front face.

        The engineering and operational difficulties are obvious. However, (here I sprinkle some magic pixie dust to deceive the eye from seeing the handwaving ) I think that advanced computation with perhaps some added AI would help to ensure that the outer sail curvature and focus point remains optimal to decelerate the inner sail, as well as the pointing of the laser source.

        • Mike Serfas December 31, 2020, 23:25

          The way I’m thinking of it is that the light moving outward from Earth carries a momentum E/c. If the Forward scheme operates perfectly, without absorption or deflection of any light, the light continues outward with its momentum unchanged. (This is true even if you change the frequency of the light before reemitting it, since the momentum depends only on the energy). This means that the probe’s overall momentum cannot be changed by this means; however, the outer ring is pushed forward by the light while the inner ring is pushed backward. The energy to separate the two portions of the probe comes from the red shift of the light (and to the degree some energy is consumed, the center of mass is accelerated outward I think, but it shouldn’t be much). Anyway, the net result is that energy from the light beam is being used to push one part of the probe backward by propelling another part forward, so in principle I would call the outer ring a propellant. The main advantage of the scheme is that if the beam can be kept focused a long time, the outer ring might end up at a high specific velocity. Still, I’m skeptical that the same specific velocity can’t be reached more reliably by shooting out propellant one ion at a time.

  • Brett P Bellmore December 30, 2020, 11:07

    “A 10 year mission to the 550AU gravitational lens point would require only 180kg of nuclear fuel, and a 350MW reactor power, well within the calculated thermal limit of 1GW. A 30 year trip to the Oort cloud at 0.5 Ly is more strenuous, requiring a 5.6 GW reactor. And a 50 year trip to Alpha Centauri, 4 Ly distant, is probably not feasible, requiring a 208 GW reactor, and consuming 240 tons of fission fuel.”

    Dusty Plasma Based Fission Fragment Nuclear Reactor

    • Alex Tolley December 30, 2020, 15:33

      One issue with using fission rockets with this amount of fissile material (100s kg) is that you may well fall afoul of political resistance to them, especially at the launch phase. But in principle, the very high Isp is very attractive for such deep space missions.

    • Adam January 1, 2021, 0:34

      Hi Brett
      Unfortunately the excited fission fragments lose a lot of energy as x-rays before they escape the thrust chamber. This will heat the structure significantly, reducing the maximum sustainable thrust. Getting the dusty plasma to fission reliably is also an unanswered question. Magnetically collimated fission fragment reactors might be better systems, but they require further study too.

      • Brett P Bellmore January 3, 2021, 12:43

        Well, sure, you’re not going to take off from a planetary surface with a fission fragment rocket, but the thrust should be high enough for a reasonably fast trip the the focal distance.

        The main issue I see is that you probably can’t test a dusty plasma fission fragment engine on Earth, you’d really need to do the work out in space, and beyond the Earth’s magnetosphere, too.

    • anon January 2, 2021, 21:22

      Yeah, my takeaway from this thread so far is that nuclear looks needed. Turyshev considered nuclear but wrote it off, saying current nuclear options are inadequate. It would be great to see a more in-depth look at the benefits you would get to this specific mission from more advanced & speculative fission engines. I guess Brett’s link is to one of those.

    • anon January 13, 2021, 17:12

      Questions, if anyone is still here:

      What is the advantage of a fission fragment rocket over Orion? Are there any other types of fission rocket that are comparable in feasibility? And I am thinking that fission fragment is more feasible than an outright fusion drive, would you agree?

  • Michael December 30, 2020, 18:45

    After rough calculations it looks like that once at the focal point the star light from the other star system can be sufficient to hover a light weight system indefinitely at the focal point due to the powerful magnification, against our stars gravity (star distance and luminosity dependant). It can also provide enough power to run the system indefinitely ! If an alien wishes to hide there would be a very nice place to hang out and communicate back to and fro with their home system.

    • Michael December 31, 2020, 5:19

      If we used alpha centauri as the source of power we could levitate around 30 tons at the solar focal point by its starlight alone, it would be there for ages, depending on spectrum dispersion. The power would also be substantial at over 100 kilowatts just for using a metre of the sun’s bending power, a star shade would be needed for both our sun and importantly the target stars. It may though disturb the station keeping of the observatory, that’s a lot of power in a small area.

      • Michael December 31, 2020, 11:50

        Oops got my zeros wrong, it’s about 70 watts and can hold up around 25 kgs at 550 au’s. Still a fair amount.

    • Alex Tolley December 31, 2020, 14:22

      I think this is fallacious. The solar flux at the SGL is far higher than the flux of starlight from the Einstein ring to the focal point. If this was not so, there would be no need to use a coronagraph to block the sun’s light when forming the image. The intensity of the light from a distant star is increased at the SGL, but it remains very weak by comparison to the sun’s energy.

      As light intensity and gravity decline at the same rate (proportional to 1/r^2), an ultralight sail where the payload and sail areal density is low enough could remain stationary as a statite whatever the distance from the sun, once the craft had stopped.

      • Michael December 31, 2020, 20:07

        Try the equation yourself, starlight from a similar sun as ours delivers about 1 / 54 000 000 of a watt per meter squared at the circumference of the sun at 4.3 lyrs. The sun is pretty big and if you take 1 m all the way around the sun and focus it at around 550 AU you get around 70 to 80 watts. 80 / 1350 watts times the light pressure at 1 AU gets you 0.000 000 53 N for a perfect reflector. Now Via the gravity equation, I was a bit lazy…


        You can calculate the force required to resist gravity at 550 AU which is around 27 kg. At a proxima type sun for its distance it’s 0.5 grams, a lighter starchip version. The power per meter squared from the sun is only a tiny 0.004 watts or so at 550 AU and you need 2 star shades one our sun and one for the targets. Unless I missed something…

        • Alex Tolley January 1, 2021, 12:11

          I think you are “double counting” the intensity. If your calculation was correct, then the Einstein ring would appear to be many orders of magnitude brighter than the sun at 550 AU. IOW, as the sun passed in front of a similar star 4.3 ly away, it would appear to become very bright, rather like a nova. This seems counter-intuitive to me. As the distance from the sun increases, the relative intensity of the star’s light over the sun would increase. As the focal line reaches infinity, the sun’s energy falls to zero, but the target star’s energy focused from the Einstein ring would remain constant. We would see constant bright flashes of distant stars and galaxies
          as nearer stars occulted them.

          I think we know this does not happen as galaxies that are occulted do not appear much brighter than the galaxy focusing their light.

          Logically, the light from the Einstein ring must diminish in intensity as the observation point recedes from the gravity lens (the sun). IOW, you need to reduce its intensity by the distance squared, just as you did for the sun.

          If I have not thought through this correctly, can an astronomer please correct me?.

        • Alex Tolley January 1, 2021, 13:59

          Another way to understand the issue is that while we are calling the solar gravitational focal line a “focus”, it is not focusing the rays like a lens. (It actually is more like a concave, diverging lens that is “unrolled into a torus, with the thickest part towards the center.) A particular focal point is only capturing a unique single ray from the width” of the Einstein ring. Rays on either side of it focus at points on either side too. We see a ring with a thickness rather than an infinitely narrow line because of coronal disturbances (noise).

          So rather than capturing a meter of ring thickness and multiplying it by the ring circumference, you should be calculating the number of photons passing through just one very thin circumference (again modulated by noise). This will be far less than your calculation of energy per sq m that you compare with the solar energy at 550 AU.

          • Michael January 1, 2021, 15:37

            The intensity of the focus becomes less quite quickly, a light ray another radii away only comes into relief around 2200 AU. That’s why I chose 1 m but 10 or 100 would have been fine as well because the radius is so large. The reas on some of the lens we see are not so great is because we are not at the exact point of maximum magnification but somewhat off as can be seen by the distance of the ring from the lensing object. The corona is also less of a problem because we want power but for a better image we need to go out a bit more which would reduce the gain. Perhaps we could have the power generation unit close to 550 AU and then laser power from it to an observer unit further away. In fact the power and laser unit could send power to a group of these observation units for station keeping and comms.

  • Kamal Ali December 31, 2020, 4:09

    Assume a reflector of radius 10m. Does anyone know how much power would need to be emitted and with what pointing accuracy so that a sufficient fraction of its light cone would hit the reflector? [Also I assume the laser would have to be at the Atacama site because of its high altitude and low humidity]

  • torque_xtr December 31, 2020, 6:36

    Splitted sail deceleration is an elegant concept, but it may provide tricky. The attitude control issues are multiplied many times. And it severely limits sail design by adding optical quality reflection requirement. Excluding dielectric sails, chute sails, and many other (comparatively) simple and efficient types.

    I think since nuclear power is a must at gravitational focus, and positioning thrusters – too, then maybe advanced nuclear electric propulsion powered by a compact reactor, and possibly combined with deceleration magsail, would be enough to decelerate.

    Since focal line extends to infinity, the craft would have plenty time to shed velocity. Arriving at 550 AU at 25-50 AU/yr and getting to 1000 AU at 10-20 AU/yr still does not prevent the mission from reaching any of the objectives. That’s 0.1 mm/s2 deceleration and 100 mN of thrust for 1-ton mission – highest Isp electric propulsion designs could be used without any concerns about low thrust.

    The initial acceleration is trickier because rocket equation reaches it’s steep side even with the highest ion/plasma propulsion Isp, but without any sail, a sundiver maneuver becomes easier. And acceleration burn at the perihelion, using solar thermal propulsion with something like lithium hydride as working fluid, could provide a good percentage of the needed delta-V.

  • Michael December 31, 2020, 8:04

    We could use the Landis concept of two sails but instead use the power of the other stars light amplified by the focus to slow it down, no need for a laser then.

    • Paul Gilster December 31, 2020, 11:13

      The two-sail deceleration idea is Forward’s, but I suspect Geoff Landis has indeed written about it somewhere as well.

      • Alex Tolley December 31, 2020, 13:40

        See my reference #2

  • Michael December 31, 2020, 11:31

    Apologies Paul, I vaguely remembered Robert first but thought Landis improved on the idea.

    • Paul Gilster December 31, 2020, 11:33

      You may be right, Michael. I haven’t checked to see what Geoff has said about the notion, which gives me a task for my to-do list!

  • Alex Tolley December 31, 2020, 14:00

    25 AU/yr is around 120 km/s. Whatever the power source and rocket propulsion system, have you run the rocket equation for realistic fuel loads and Isp to slow the probe to a halt?

    • Michael December 31, 2020, 20:35

      I would rather use a alpha emitter to slow it down, Pu 238 and Radium look good for a very light weight sail craft. Alpha emission has a very high exhaust velocity of 1.91 x 10^7 m/s and with Radium the decay product is radon which is a gas and leaks out so you don’t have to carry it. As for the amplified starlight a larger braking sail will collect more power but it will still take time to slow it down, I will run some calculations after the festive period, wine and the brain have a strange working relationship !


  • wdk January 1, 2021, 23:40

    “To acquire a megapixel image of even a nearby exoplanet would require a telescope of tens of kilometers in diameter to collect the light of the planet while masking out the far more intense light of its star. ”

    Becoming more familiar with the gravity lens proposal and the trades, I am beginning to wonder if the 1 kilometer lens in near-space has some significant merits after all. Granted, let’s say that Trappist-1 shows so much promise that it might make sense to deploy a gravity lens device out 550 AU to observe it. But what about the next high value prospect and the next scattered all over the celestial sphere? And what if most of these turn out to be dry wells?
    Let’s quantify the test case a little more. If a mega pixel image would require a tens of kilometers aperture, for sake of argument, let us say 36 kilometers provides the equivalent of 4 megapixels, a two thousand by two thousand array. That would be an excellent monitor image, high definition these days. Early days of desk top computers were about 400 by 400 which would give 160,000 pixel elements. This would require about 4 percent of the area or 20 percent of the aperture. Eight kilometers.

    OK, we are still dealing with large dimensions, but a reflector telescope and a solar sail have very similar features. What’s more, there probably will be an industry for solar reflectors for shades in equatorial regions and collectors in polar ones owing to climate concerns. The requirements for local use ( Earth-Moon or Earth-Sun libration points) will not be as stringent as deep space. Such devices can slew between, say Trappist 1 and Proxima or Alpha Centauri much more easily than objects placed 550 AUs from the sun. And perhaps, examining the power output for placement and the number of reflectors dedicated to transport or observing – and the number of planetary systems covered…
    we might have a case for the alternate.

    Also, in these days of conspiracy theories, it is hard not to fail to imagine that behind every star in our local group of 10 parsecs or so there are gravity lens telescopes looking at us, But we can’t see them, because various stars are in the way.

    Our only hope is to spot them at some proportionate standoff of 550 AUs, looking at each other…

    • Brett P Bellmore January 3, 2021, 13:03

      I’ve proposed building a large baseline telescope in space, using free flying mirror segments. It would have to be stationed in a planet/Sun Lagrange point, to keep station keeping thrust requirements down, but it should in principle be possible to station-keep precisely enough to maintain a common focal point between many individual mirror segments.

      The nice thing is that the light gathering capacity could be increased over time by adding new segments, and you could have several free floating objectives, too.

      By mass producing modest sized mirror segments of identical design, costs could be reduced.

      Amusingly, given a long enough focal length, the individual mirror segments could be optical flats, and still be within a fractional wavelength of an ideal parabolic mirror, reducing the cost of manufacture even further. (That last idea isn’t original to me, though; Read it in an SF story years ago.)

  • wdk January 2, 2021, 11:28

    Reflecting further on local wide aperture telescopes vs. at gravity lens focal point, re-examined the Hubble Pluto illustration. And I hope other people can do this as well. To some degree, it has to argue for higher resolution considering the difference in feature identification between the New Horizons mission and Hubble. Yet I remember in the 1980s how science advisers in the Reagan administration argued that Hubble would reduce the need for deep space probes… Disingenuous perhaps, but they were physicists with optical laser backgrounds. There could be more to the image processing issue that we are not considering, issues of contrast and brightness – or reasons why screensavers would even be desirable on old cathode ray tube (CRT) monitors, presuming that the image resolution described is so bad on a better equivalent. Advances in imaging software revolutionized images transmitted from the lunar surface between Apollo 11 and, say Apollo 15. I note that Pluto in Hubble image is given a very sharp circular edge.
    The wrong or inappropriate Bayesian statistical reductions perhaps applied to everything within?

  • wdk January 2, 2021, 12:18

    More matters coming to mind.
    A 550 AU venture to set up a telescope differs considerably from proposals to set up lunar observatories in the sense that the pipeline is much, much longer and involves numerous time lags.

    For one, the demonstration or proof of concept could involve a miniature device, but you are still faced with locating a gravity lens site with which to test it. The solar one at 550 AU is the only one that comes to mind unless I missed an alternate suggested. Then assuming an operating telescope on station, it seems like the easiest cases for it to track would be already identified transit planets. We have good ideas where they will show up ( e.g. in terms of milli- or micro-arcseconds.
    Which raises the question of what is an effective field of view?
    But if we are not dealing with transiting exoplanets, then doppler detected, astrometric become more difficult to tie down with their inherent uncertainties. But directed search for planetary images will have a significant time lag. About 500 seconds for every AU and about 3.175 days for 550 AU. You get an image and send it home and then await a directive: What to do next? Since a gravity lens allows only
    one star per lens site, I would imagine a systematic search to identify
    candidate planets in the system and then determination of motion with updates. Thereafter, campaigns of observing particular planets could be conducted. But with very narrow FOV, determining guidestars for
    position and velocity could be problematic.

    Then, Fermi’s paradox, since there are so many stars , all with their focal lines extending away into space, then is that what we are seeing
    with microlensing planet detections? My understanding of these events lags with respect to doppler, transit or astrometric. I suspect that statistically, we see very distant events toward the center of the galaxy, but that their occurrence gives us a statistical sampling of planetary frequency rather than dimensions or other characteristics. Still, for a transiting planet and a gravity lens focus stretching back from a star indefinitely ( starting at some offset similar to the 550 AU of the sun), there might be an object that will pass into the field of view. Rather than a G star, a white dwarf would be more advantageous, assuming the denser object would shorten the focal length.

  • Michael January 2, 2021, 15:27

    That’s some impressive gains, 112 dB for visible red, gulp !


    • wdk January 4, 2021, 12:39

      Once again, I am slow on the uptake here. But perhaps for others as well, I would like to ask: If the red band is associated with 550 AU, and the Blue is associated with double the distance, does that mean the
      spacecraft needs to cruise between 550 and 1100 AUs to get the fully balanced visual effect – or something else? We still might be talking about an image with the same defects as that of Hubble focused on Pluto. Also, the object observed is not standing still either. One hundred AUs is not an over-nighter. If there are any atmospheric effects, well, imagine if Centaurans used the same techniques to observe the Great Red Spot.

      • Alexander Tolley January 4, 2021, 22:30

        There is not prism effect separating the wavelengths. The dispersion is purely based on the distance from the center of gravitational attraction. 550 Au is the minimal distance from the sun where the gravity focus will occur. The focal line extends to infinity, but the intensity of the photons decreases as the distance increases. The further the distance from the sun, the longer the telescope takes to reach the focus, and the fewer photons captured for the image. Any angular movement of the target across the sky will require a greater absolute distance needed to traverse to track the target..

  • wdk January 5, 2021, 22:43


    Thanks. This is a different type of machine and it will take some getting use to. But it does sound like there would be some interesting operational details, whether it is dedicated to Trappist-1 or Alpha and Proxima Centauri. We could suppose that planetary orbital elements are identified and then the focus is offset from a nominal target such
    as Alpha Cen A by a parsec to latch onto a planet like ours. Or for
    Trappist one, there would be a tighter cluster of several. Assuming the spacecraft are both in position to observe, would they have much the
    same strategy? Wait for an intercept in the line of sight or shift the
    line of sight somehow over a fraction of an arcsecond ( e.g., milli-…)?
    I realize you have a high gain for magnification, but have not grasped
    the aperture width. Imagine finally getting a picture from out there for a moment and determining it’s a picture of an empty parking space.

  • Michael January 9, 2021, 16:43

    Here is another angle at the idea of the gravity lense. I am still amazed at the gain.


    • wdk January 11, 2021, 11:40

      That is a very nice survey presentation on gravitational lenses and space telescopes in general. If I am not mistaken, the first author is a
      contributor to this forum. Anyway, thanks to everyone involved
      for bringing all this up.

      • Michael January 12, 2021, 8:48

        I found it presented the concept very well.

        Something you will notice is the amount of power that flows down that focal line, its massive, depending on the luminosity and distance from the target star. The power beam is spread over the focal line and can be intercepted easily enough. If you take up to 1 radii away from the sun and concentrate that light from say Sirus its terrawatts over say 2500 to 3000 AU from the focal point, that’s a lot for power for any craft in fact a civilisation ! It is enough power to slow down a craft to a stop, power them and later once infrastructure is in place can accelerate craft towards the sun at very, very high velocities onwards to the target star. Perhaps we could use the lens of the other star to slow it down, no need for really big lasers. These conduits of power are extremely valuable to a space faring species and should be a deep time project for our species.

        • Michael January 13, 2021, 7:38

          I need glasses or a better calculator, I got mixed up with my powers and exponents, it should be around hundreds of Giga watts not Terra watts. Still a lot though, very useful out there.

  • ljk January 20, 2021, 15:28

    NASA is finally getting into the light sail business:


    They had plans and a full-scale model in the 1960s, but it literally never got off the ground.

    There was also a plan in the 1980s to send a light sail probe to Comet Halley, but it, like so many other space projects in that decade, fell through. As a result, the USA was the only major spacefaring nation not to send a probe to that famous ice ball when it made its latest visit to the vicinity of Earth in 1985-1986:





    This is why I was pessimistic about our future in space (at least in the relatively short term) starting from the 1970s until just a few years ago when the private sector rose up to the task, just as they did with the computer industry when the original giants like IBM and Digital could no longer monopolize the smaller but more numerous and cheaper PC makers.