Why X-Rays Can’t Push Interstellar Sails

Although solar sails were making their way into the aerospace journals in the late 1950s, Robert Forward was the first scientist to consider using laser beams rather than sunlight to drive a space sail. That concept, which György Marx picked up on in his 1966 paper, opened the door to interstellar mission concepts. Late in life in an unpublished memoir, Forward recalled reading about Theodore Maiman’s work on lasers at Hughes Research Laboratories, and realizing that this was a way to create a starship. His 1962 article (citation below) laid out the idea for the journal Missiles and Rockets and was later reprinted in Science Digest. Marx surely knew the Forward article and his subsequent paper in Nature probed how to achieve this goal.

Image: One of the great figures of interstellar studies, Robert Forward among many other things introduced and explored the principles of beamed propulsion. Credit: UAH Library Robert L. Forward Collection.

Marx was at that time a professor of theoretical physics at Roland Eötvös University in Budapest. He was plugged into the difficulties of interstellar flight through Les Shepherd’s work in Britain, and he cites the latter’s Realities of Space Travel (1957) in the paper as one of many sources highlighting the depth of the problem. His paper “Interstellar Vehicle Propelled by Terrestrial Laser Beam” is a mere two pages built largely around equations, reporting on the “commonly accepted view that, apart from the technical difficulties involved, the laws of conservation of energy and momentum forbid the visiting of other planetary systems in the human lifespan.”

Marx had already explored energy issues for interstellar flight in a 1960 paper for Astronautica Acta (as it was then known) and a second for the same journal in 1963 (citations below). But the idea of laser beaming offered the physicist a glimmer of hope. In the 1966 paper, he cites the advantages of beamed sailing vs conventional rocket propulsion. The paper argues that “a vehicle can be accelerated almost to the speed of light if an emitter on the Earth can accurately project light onto its mirror.”

The ideal focusing mechanism would be the laser, and it is here that he runs into trouble. For Marx worried about the size of the transmitter aperture, which determines the size and initial beam diameter that will emerge. Remember we’re in the Robert Forward era of maxed out engineering, when the idea was simply to establish what was possible even if it required building capabilities far beyond those of the present day. So here’s what Marx comes up with, the best concept he thought feasible:

…the technical conditions are extremely challenging. A range of operation of 0.1 light year would require a coherently radiating surface of the order of 1 km2 which emits hard X-rays, and the vehicle would need an X-ray mirror with an effective cross-sectional area of several km2.

Marx talks about the absorbed energy of his sail ‘mirror’ radiating out isotropically into space, and here we run into serious problems. I took my questions to Jim Benford, CEO of Microwave Sciences and author of High Power Microwaves, a standard text which is about to go into its fourth edition at CRC Press. Jim is also a regular contributor to Centauri Dreams. And he was quick to point out that X-rays reflect from conducting surfaces in ways that defeat Marx’s purpose.

Image: György Marx in his office. Credit: REAL-I, the Image File Collection of the Academic Library.

As Jim told me, incoming X-rays reflect only at very low grazing angles. The efficiency of energy transfer is at stake here. Here’s a bit more of what he said, using one of the most important formulae in all of modern physics:

X-ray photons have far more energy than visible light or microwaves. Remember the relation E=hν, where E is the energy of the photon, h is Planck’s constant, ν the frequency. X-ray photons have energies about a thousand times that of visible light, a million times that of microwaves. If they come in normal to the surface [i.e., striking the sail head on], they ionize atoms, damaging the lattice of the material.

X-ray telescopes, as a matter of fact, work through a series of grazing incidence reflectors. In other words, we can’t direct Marx’s fantastic X-ray beam toward our sail without seriously damaging it, not unless we are willing to bring the beam to it at such a low angle that the intrinsic power of the beam is largely lost. Benford again:

There’s no way to accelerate a sail with X-rays. The cross-section of the sail must be at a slight angle to the beam, not perpendicular to it, for the X-rays to reflect. That’s hugely inefficient. Grazing incidence means that only the slight transverse component of the photon velocity vector is reversed, leaving the far larger axial component almost unchanged. Little energy is transferred to the inclined sail, and that drives it sideways to the beam, not antiparallel to it, as reflected photons do when they incident normally. So the sail is accelerated very little in the direction of the X-ray beam.

This is the coup de grâce for the X-ray sail. It’s interesting to see what Robert Forward thought of Marx’s idea. Here he is, writing in a 1984 paper called “Roundtrip Interstellar Travel Using Laser-Pushed Lightsails,” which is one of the classics of the field:

The concept of laser-pushed interstellar lightsails was reinvented by Marx in 1966. Since Marx was unwilling to consider a laser aperture greater than 1 km2, he was forced to assume the use of hard x-rays in order to obtain the operational ranges needed for interstellar flight. The impossibility of constructing both an x-ray laser and a lightweight sail to reflect those x-rays led to Marx’s highly pessimistic conclusion about the feasibility of the concept. If Marx had been willing to consider a larger transmitter aperture, then his laser frequencies and sail requirements would have been much easier.

J. L. Redding, then at Bishop’s University in Quebec, saw Marx’s paper and responded to it in the same year, offering corrections to Marx’s equations without challenging the X-ray concept. His telling remark that “…one does not need to consider the difficulties of arranging suitable deceleration and landing facilities” refers to what he saw as the overwhelming problems in making a beamed energy propulsion system work at all. Marx had commented on the deceleration problem and Forward would go on to offer a potential solution in his 1984 paper, one so baroque that it deserves a future post of its own.

I should also mention a little referenced paper by W. E. Moeckel, “Propulsion by Impinging Laser Beams,” which ran in the Journal of Spacecraft and Rockets in 1972. Moeckel (working at what was then NASA’s Lewis Research Center in Cleveland) analyzed laser beaming to 100 ton relativistic flyby probes, each of which would require 1012 watts of X-ray energy. Making specific reference to Marx, Moeckel found X-ray beaming promising but did not know if it was feasible. His conclusion would have warmed the hearts of science fiction writers of the time:

…some future generations of mankind, with a somewhat different ordering of priorities than ours and much more available power, could conceivably explore other stars and other solar systems with highly sophisticated unmanned spacecraft capable of relaying information in elapsed times of the order of decades.

If only it worked! Fortunately, we’re not restricted to X-rays when it comes to beamed propulsion.


The early Forward paper is “Pluto-Gateway to the Stars,” Missiles and Rockets 10, 26 ff. (2 April 1962); reprinted in Science Digest 52, 70-75 (August 1962). Forward’s “Roundtrip Interstellar Travel Using Laser-Pushed Lightsails” appeared in the Journal of Spacecraft and Rockets 21 (1984), pp. 187-195 (abstract).

György Marx’s paper on X-ray beaming is “Interstellar Vehicle Propelled by Terrestrial Laser Beam,” which ran in Nature on July 2, 1966 (abstract). His two other interstellar papers are “The mechanical efficiency of interstellar vehicles,” Astronautica Acta 9 (1963) 131–139, and “Über Energieprobleme der Interstellaren Raumfahrt,” Astronautica Acta 6 (1960) 366–372.

The Redding paper in response to Marx has the same title, “Interstellar Vehicle Propelled by Terrestrial Laser Beam,” Nature February 11, 1967 (abstract). W. E. Moeckel’s paper “Propulsion by Impinging Laser Beams” appeared in the Journal of Spacecraft and Rockets Vol. 9 (1972), 942-944 (abstract).

My thanks to Jim Benford, Greg Matloff and Al Jackson for invaluable references and commentary.

A Shifting, Seething Solar Wind

In search of ever-higher velocities leaving the Solar System, we need to keep in mind the options offered by the solar wind. This stream of charged plasma particles flowing outward from the Sun carves out the protective bubble of the heliosphere, and in doing so can generate ‘winds’ of more than 500 kilometers per second. Not bad if we’re thinking in terms of harnessing the effect, perhaps by a magnetic sail that can create the field needed to interact with the wind, or an electric sail whose myriad tethers, held taut by rotation, create an electric field that repels protons and produces thrust.

But like the winds that drove the great age of sail on Earth, the solar version is treacherous, as likely to becalm the ship as to cause its sails to billow. It’s a gusty, turbulent medium, one where those velocities of 500 kilometers and more per second can as likely fall well below that figure. Exactly how it produces squalls in the form of coronal mass ejections or calmer flows is a topic under active study, which is where missions like Solar Orbiter come into play. Studying the solar surface to pin down the origin of the wind and the mechanism that drives it is at the heart of the mission.

Launched in 2020, Solar Orbiter carries a panoply of instruments, ten in all, for the analysis, including an Electron Analyzer System (EAS), a Proton-Alpha Sensor (PAS) for measuring the speed of the wind, and a Heavy Ion Sensor (HIS) designed to measure the heavy ion flow. Critical to the analysis of this paper is the Spectral Imaging of the Coronal Environment (SPICE) instrument, as we’ll see below. Steph Yardley (Northumbria University) is lead author of the paper on this work, which has just appeared on Nature Astronomy:

“The variability of solar wind streams measured in situ at a spacecraft close to the Sun provide us with a lot of information on their sources, and although past studies have traced the origins of the solar wind, this was done much closer to Earth, by which time this variability is lost. Because Solar Orbiter travels so close to the Sun, we can capture the complex nature of the solar wind to get a much clearer picture of its origins and how this complexity is driven by the changes in different source regions.”

What the work is analyzing is a theory that the process of magnetic field lines breaking and reconnecting is critical to producing the slower solar wind. Different areas of the Sun’s corona are implicated in the origin of both the fast and slow winds, with the ‘open corona’ being those regions where magnetic field lines extend from the Sun into space, tethered to it at one end only and creating the pathway for solar material to flow out in the form of the fast solar wind. Closed coronal regions, on the other hand, are those where the magnetic field lines connect to the surface at both ends, forming loops.

As you would imagine, the process is wildly turbulent and marked by the frequent breakage of these closed magnetic loops and their subsequent reconnection. The researchers have probed the theory that the slow solar wind originates in the closed corona during these periods of breakage and reconnection by studying the composition of solar wind streams, for the heavy ions emitted vary depending on their origins in either the closed or open corona. Solar Orbiter’s Heavy Ion Sensor (HIS) is able to take the needed measurements to relate the effects of this activity on the surrounding plasma.

The image below is from the Solar Dynamics Observatory spacecraft rather than Solar Orbiter, reminding us of the different views we are gaining by our various missions to our star. The comparison of key datasets tells the story.

Image: This is part of Figure 1 from the paper. The caption reads: SDO/AIA [Solar Dynamics Observatory data using its Atmospheric Imaging Assembly] 193 Å image showing the source region from the perspective of an Earth observer. Open magnetic field lines that are constructed from the coronal potential field model are overplotted, coloured by their associated expansion factor F. The large equatorial CH [Coronal Hole] and AR [Active Region] complex are labeled in white. The FOVs [fields of view] of SO EUI/HRI and PHI/HRT [references to instruments aboard Solar Orbiter] are shown in cyan and pink, respectively. The back-projected trajectory of SO [Solar Orbiter] from 1 March 2022 until 9 March 2022 is shown by the olive dotted line (from right to left).

So because we have Solar Orbiter, we can now combine observations of the Sun from various sources including other space missions, like the Solar Dynamics Observatory, with the measurements of the solar wind actually flowing past the spacecraft. Susan Lepri (University of Michigan) is deputy principal investigator on the HIS system:

“Each region of the Sun can have a unique combination of heavy ions, which determines the chemical composition of a stream of solar wind. Because the chemical composition of the solar wind remains constant as it travels out into the solar system, we can use these ions as a fingerprint to determine the origin of a specific stream of the solar wind in the lower part of the Sun’s atmosphere.”

The results have been productive. The analysis gives us a precise breakdown of just what Solar Orbiter has encountered during the period studied. This is a thorny quotation but it includes a key finding. From the paper:

Combining the SO [Solar Orbiter] trajectory, coronal field model, magnetic connectivity tool, the SPICE composition analysis of the AR [Active Region] complex, and the in situ plasma and magnetic field parameters, we suggest that SO was immersed in three fast wind streams… originating from the three linked sections of the large equatorial CH [Coronal Hole]… These were followed by two slower streams associated with the negative polarities of the AR complex… The decrease of the solar wind speed can be explained by the expansion of the open magnetic field associated with the CH-AR complex, as the connectivity of SO transitioned across these regions. Credit: Yardley et al.

The findings described here are significant. We learn from this work just how complex the solar wind flow is, in this case involving three fast streams and two slower ones, all involving changes in magnetic field connectivity. Matching the composition of the solar wind streams to different areas on the corona gives us new insights into the turbulent mix found where the open and closed corona meet. The slow solar wind’s ‘breakout’ from closed magnetic field lines is demonstrated. The phenomenon of magnetic reconnection proves critical to the wind’s variability.

Demonstrating these linkages means that we can now use the findings to probe further into the origins of the solar wind. But this is a variability that is in no way predictable, making the prospect of riding the solar wind via electric or magnetic sail a daunting one. We’ll continue to learn more, though, as we bring in data from missions like the Parker Solar Probe. It will be fascinating to see one day how we use the solar wind to test out possible spacecraft designs in search of a faster route to the outer Solar System.

Addendum: In an earlier draft, I mistakenly criticized the authors for not initially clarifying some of the acronyms in this paper. I’ve removed that comment because a later reading showed I was mistaken about the two examples I cited.

The paper is Yardley et al., “Multi-source connectivity as the driver of solar wind variability in the heliosphere,” Nature Astronomy 28 May 2024 (full text).

To the Stars with Human Crews?

How long before we can send humans to another star system? Ask people active in the interstellar community and you’ll get answers ranging from ‘at least a century’ to ‘never.’ I’m inclined toward a view nudging into the ‘never’ camp but not quite getting there. In other words, I think the advantages of highly intelligent instrumented payloads will always be apparent for missions of this duration, but I know human nature well enough to believe that somehow, sometime, a few hardy adventurers will find a way to make the journey. I do doubt that it will ever become commonplace.

You may well disagree, and I hope you’re right, as the scenarios open to humans with a galaxy stuffed with planets to experience are stunning. Having come into the field steeped in the papers and books of Robert Forward, I’ve always been partial to sail technologies and love the brazen, crazy extrapolation of Forward’s “Flight of the Dragonfly,” which appeared in Analog in 1982 and which would later be turned into the novel Rocheworld (Baen, 1990). This is the novel where Forward not only finds a bizarre way to keep a human crew sane through a multi-decade journey but also posits a segmented lightsail to get the crew home.

Image: The extraordinary Robert Forward, whose first edition of Flight of the Dragonfly was expanded a bit from the magazine serial and offered in book form in 1984. The book would later be revised and expanded further into the 1990 Baen title Rocheworld. The publishing history of this volume is almost as complex as the methods Forward used to get his crew back from Barnard’s Star!

Forward was a treasure. Like Freeman Dyson, his imagination was boundless. Whether we would ever choose to build the vast Fresnel lens he posited in the outer Solar System as a way of collimating a laser beam from near-Sol orbit, and whether we could ever use that beam to reflect off detached segments of the sail upon arrival to slow it down are matters that challenge all boundaries of engineering. I can hear Forward chuckling. Here’s the basic idea, as drawn from his original paper on the concept.

Image: Forward’s separable sail concept used for deceleration, from his paper “Roundtrip Interstellar Travel Using Laser-Pushed Lightsails,” Journal of Spacecraft and Rockets 21 (1984), pp. 187-195. The ‘paralens’ in the image is a huge Fresnel lens made of concentric rings of lightweight, transparent material, with free space between the rings and spars to hold the vast structure together, all of this located between the orbits of Saturn and Uranus. Study the diagram and you’ll see that the sail has three ring segments, each of them separating to provide a separate source of braking or acceleration for the arrival, respectively, and departure of the crew. Imagine the laser targeting this would require. Credit: Robert Forward.

I tend to think that Les Johnson is right about sails as they fit into the interstellar picture. In a recent interview with a publication called The National, Johnson (NASA MSFC) made the case that we might well reach another star with a sail driven by a laser. Breakthrough Starshot, indeed, continues to study exactly that concept, using a robotic payload miniaturized for the journey and sent in swarms of relatively small sails driven by an Earth-based laser. But when it comes to human missions to even nearby stars, Johnson is more circumspect. Let me quote him on this from the article:

“As for humans, that’s a lot more complicated because it takes a lot of mass to keep a group of humans alive for a decade-to centuries-long space journey and that means a massive ship. For a human crewed ship, we will need fusion propulsion at a minimum and antimatter as the ideal. While we know these are physically possible, the technology level needed for interstellar travel seems very far away – perhaps 100 to 200 years in the future.”

Johnson’s background in sail technologies for both near and deep space at Marshall Space Flight Center is extensive. Indeed, there was a time when his business card described him as ‘Manager of Interstellar Propulsion Technology Research’ (he once told me it was “the coolest business card ever”). He has also authored (with Gregory Matloff and Giovanni Vulpetti), books like Solar Sails: A Novel Approach to Interstellar Travel (Springer, 2014) and A Traveler’s Guide to the Stars (Princeton University Press, 2022), as well as editing the recent Interstellar Travel: Purpose and Motivations (Elsevier, 2023). In addition to that, his science fiction novels have explored numerous deep space scenarios.

Image: NASA’s Les Johnson, a prolific author and specialist in sail technologies. Credit: NASA.

So there’s a much more optimistic take on the human interstellar guideline than the one I gave in my first paragraph, and of course I hope it’s on target. We’re probably not going to be going to what is sometimes called an “Earth 2.0,” in Johnson’s view, because he doubts there are any such reasonably close to us. That’s something we’ll be learning a great deal more about as future space instrumentation comes online, but we can bear in mind that the explorers who tackled the Pacific in the great era of sail didn’t set out thinking they were going to find another Europe, either. The point is to explore and to learn what you can, with all the unexpected benefits that brings.

Johnson’s early interest in sails, by the way, was fired not so much by Forward’s Rocheworld as by Larry Niven and Jerry Pournelle’s novel The Mote in God’s Eye (Simon & Schuster, 1974), where an incoming laser sail from another civilization is detected. The realization that unusual astronomical observations point to a technology, and a laser-beaming one at that, is an exciting part of the book. Here the authors’ human starship crew describes the detection of a strange light emanating from a smaller star (the Mote) in front of a much larger supergiant (the Eye):

“…I checked with Commander Sinclair. He says his grandfather told him the Mote was once brighter than Murcheson’s Eye, and bright green. And the way Gavin’s describing that holo – well, sir, stars don’t radiate all one color. So -”

“All the more reason to think the holo was retouched. But it is funny, with that intruder coming straight out of the Mote…”

“Light,” Potter said firmly.

“Light sail!” Rod shouted in sudden realization…”

For more on all this, see my Our View of a Decelerating Magsail in these pages. It’s not surprising that Niven and Pournelle ran their lightsail concept past Robert Forward at a time when the idea was just gaining traction. We all have career-changing literary experiences. I can remember how a childhood reading of Poul Anderson’s The Enemy Stars (J.B. Lippincott, 1959) utterly fired my imagination toward the idea of leaving the Solar System entirely. It was a finalist for the Hugo Award that year following serialization in Astounding, though I didn’t encounter it until later.

Johnson’s work at Marshall Space Flight Center takes in the deployment of a large solar sail quadrant for the Solar Cruiser mission that was first unfurled in 2022 to demonstrate TRL 5 capability, and has just been deployed at contractor Redwire Corp.’s facility in Longmont, Colorado to demonstrate TRL 6. In NASA’s terms, that means going from “Component or breadboard validation in relevant environment” (TRL 5) to “System or subsystem model or prototype demonstration in a relevant environment (ground or space).” In other words, this is progress. At TRL 6, a system is considered “a fully functional prototype or representational model.” Says Johnson in a recent email:

“25 years ago, when I first met Dr. Forward, he inspired me to plan a development program for solar sails that would eventually lead us to the stars. With Bob’s help, I laid out a milestone driven roadmap that began with the space flight of a 10 m² solar sail, which we did in 2010 with NanoSail-D.

“Next on the plan was the development of something an order of magnitude larger. This was achieved with the development and launch of the 86 m² Near Earth Asteroid Scout solar sail in 2022 and the soon to be launched ACS-3 sail. The Solar Cruiser sail is an order of magnitude larger still at 1653 Square meters. The next step is 10,000!”

Image: NASA and industry partners used two 100-foot lightweight composite booms to unfurl the 4,300-square-foot sail quadrant for the first time Oct. 13, 2022, at Marshall Space Flight Center, making it the largest solar sail quadrant ever deployed at the time. On Jan. 30, 2024, NASA cleared a key technology milestone, demonstrating TRL6 capability at Redwire’s new facility in Longmont, Colorado, with the successful deployment of one of four identical solar sail quadrants. Credit: NASA (although I’ve edited the caption slightly to reflect the TRL level reached).

Solar sails are becoming viable choices for space missions, and the Breakthrough Starshot investigations remind us that sails driven not just by sunlight but by lasers are within the bounds of physics. A key question that will be informed by our experience with solar sails is how laser-driven techniques scale. Theoretically, they seem to scale quite well. Are the huge structures Forward once wrote about remotely feasible (perhaps via nanotech construction methods), or is Johnson right that fusion and one day antimatter may be necessary for craft large enough (and fast enough) to carry human crews?

A Novel Strategy for Catching Up to an Interstellar Object

Reaching ‘Oumuamua through some kind of statite technology, an idea we’ve been kicking around recently, brings up the interesting work of Richard Linares at MIT, who has been working on a “dynamic orbital slingshot” for rendezvous with future objects from the interstellar depths (ISOs). Linares received a Phase I grant from the NASA Innovative Advanced Concepts (NIAC) Program to pursue the idea of a network of statites on sentry duty, any one of which could release the stored energy of the sail to enter a trajectory that would take it to a flyby of an object entering our system on a hyperbolic orbit.

The concept is simplicity itself, once you realize that a statite balances the pressure of solar photons against the Sun’s gravitational pull, and essentially hovers in place. As I mentioned when covering Greg Matloff and Les Johnson’s paper on using statites to achieve fast rectilinear trajectories to reach interstellar interlopers, Robert Forward was the one who came up with the idea and practical uses for it. He could envision, for example, communications satellites in polar position to cover high latitudes on Earth.

Here’s what Forward said about the statite concept in his wonderful essay collection Indistinguishable from Magic (1995):

…I have the patent on it — U.S. Patent 5,183,225 “Statite: Spacecraft That Utilizes Light Pressure and Method of Use”… The unique concept described in the patent is to attach a television broadcast or weather surveillance spacecraft to a large highly reflective lightsail, and place the spacecraft over the polar regions of the Earth with the sail tilted so the light pressure from the sunlight reflecting off the lightsail is exactly equal and opposite to the gravity pull of the Earth.

You can see why we need a new term here. If you deploy a sail in the configuration Forward describes, it essentially sits over the polar region while the Earth rotates below it. In other words, technically it is not a satellite. ‘Statite’ is a Forward coinage to describe such a hovering object in space. He wrote of a statite he dubbed the ‘Hovering Hawke’ in one of his short stories. It would be placed too far from the surface to be effective as a communications satellite, but could offer direct broadcasting to places on Earth that are without that capability. Weather surveillance is another use.

Polesitters become interesting when we consider the nature of a geostationary orbit. Put a satellite directly over the equator at 35,786 kilometers altitude and it will appear stationary over the Earth, a useful trait for communications. But the satellite must be positioned directly above the equator, matching Earth’s rotation, to maintain its position relative to the surface.

If we put our satellite at an angle relative to the equator, its apparent motion on Earth will be a figure eight, in what is called an inclined geosynchronous (not geostationary) orbit. That’s useful for areas not covered by geostationary satellites but not good enough for continuous coverage of a specific area, especially the more latitudinally challenged regions like the poles, and that’s why the polesitter is attractive. It can give us continuous coverage even when the region it sits above is far from the equator.

Image: Analog‘s December, 1990 issue contained an article by Robert Forward describing the ‘polesitter’ concept, one of many innovative ideas the scientist introduced to a broad audience. Credit: Condé Nast.

There’s always a catch, and here’s the catch with polesitters, as Forward explained it in his article. When the summer months arrive and the polar regions are in sunlight, keeping the statite precisely balanced (to maintain the hover) becomes quite tricky. He saw that such seasonal instability demanded that a statite be relatively far from Earth, and calculated that it cannot get any closer than 250 Earth radii to the surface.

But Linares and team are not thinking about statites supplying services to Earth. The NIAC work explores using statites to set up an early warning system for interstellar objects, one that will allow fast intercepts before these interlopers blow through our system and return to interstellar space. Consider what happens when we ‘turn off’ the statite capability on our satellite (as from rotating the sail to an edge-on position, for example, or simply releasing a CubeSat). At this point, the released object has no forces impinging upon it but gravity. Let me quote Linares from a white paper on the subject:

…a statite at 1 AU has a free-fall trajectory of about 64 days. This fast response time to a potential ISO can be thought of as a slingshot effect, since the solar sail is used to “store energy” that is released when desired. Additionally, to achieve a flyby some Delta-V is required to adjust from the free-fall path to a flyby trajectory. The proposed mission for the statite concept is to utilize a constellation of such devices to achieve wider coverage over a spherical region of 1 AU for potential ISO missions. Additionally, the orbital plane can be adjusted with relatively low Delta-V.

Image; A constellation of statites as envisioned by Richard Linares for intercepting a future interstellar interloper. Credit: MIT/Richard Linares.

The levitating sail has an inertial velocity of zero, and when released from ‘hover,’ it enters a Keplerian orbit. So as Linares points out, we can turn any one of the statites in our constellation of statites into a ‘sundiver,’ hurtling toward the Sun before its trajectory is adjusted by use of the sail (or perhaps other propulsion). Which statite is deployed simply depends upon the optimum trajectory to the incoming ISO.

We are now on a fast track toward reaching the interstellar object with at least a flyby. Linares calls this a “dynamic orbital slingshot for rendezvous with interstellar objects.” And the idea is to have a constellation of these statites always at the ready for the next ‘Oumuamua. Or, considering how odd ‘Oumuamua seems to be, perhaps I should say “the next Borisov.” Even so, with this net, who knows what we might catch?

The paper makes the case that a statite free-falling toward the Sun from an initial position at 1 AU and then deploying its sail away from the Sun at perihelion can achieve speeds of up to 25 AU/year, making it possible to deliver payloads to the outer Solar System. Now we’re in Matloff/Johnson ‘sundiver’ territory. Voyager 1 has reached 3.6 AU per year by comparison, making the statite concept attractive beyond its value as a station-keeper for quick response missions to interstellar comets/asteroids.

For more on Richard Linares’ work, see “Rendezvous Mission for Interstellar Objects Using a Solar Sail-based Statite Concept,” a white paper available on arXiv.

Interstellar Precursor? The Statite Solution

What an interesting object Methone is. Discovered by the Cassini imaging team in 2004 along with the nearby Pallene, this moon of Saturn is a scant 1.6 kilometers in radius, orbiting between Mimas and Enceladus. In fact, Methone, Pallene and another moon called Anthe all orbit at similar distances from Saturn and are dynamically jostled by Mimas. What stands out about Methone is first of all its shape and, perhaps even more strikingly, the smoothness of its surface. We’d like to know what produces this kind of object and would also like to retrieve imagery of both Pallene and Anthe. If something this strange has equally odd companions, is there something about its relationship with both nearby moons and Saturn’s rings that can produce this kind of surface?

Image: It’s difficult not to think of an egg when looking at Saturn’s moon Methone, seen here during a Cassini flyby of the small moon. The relatively smooth surface adds to the effect created by the oblong shape. NASA/JPL-Caltech/Space Science Institute.

Our path to interstellar missions will see us ramp up the velocities of our probes to objects in our own system, made more accessible by shorter mission times, sail technologies and miniaturization. There is no shortage of targets between high-interest moons like Europa, Titan and Enceladus and Kuiper Belt Objects like Arrokoth. For that matter, the interstellar interloper ‘Oumuamua may yet be within range of faster missions (and in fact we’ll be examining ‘Oumuamua prospects in at least one upcoming article). But the point is that intermediate steps to interstellar will enhance exploration of objects we’ve already visited and take us to numerous others.

One way to proceed is discussed by Greg Matloff and Les Johnson in a recent paper for the Journal of the British Interplanetary Society that grew out of a presentation at the 6th International Space Sailing Symposium this summer. Here the idea is to adjust the parameters of a solar sail so that a balance is achieved between the gravitational force of the Sun and the solar photon radiation impinging upon it. The parameters are clear enough: We need a sail of a specific thickness (areal density), and tightly constrained figures for its reflectance and absorbance. We want to cancel out the gravitational acceleration imposed by the Sun through the propulsive effects of solar photons, allowing us to effectively ‘hover’ in place.

Hovering isn’t traveling, but bear with me. We’ve looked at this kind of sail configuration before and discussed its development in the hands of Robert Forward. It was Forward who dubbed the configuration a ‘statite,’ implying that when the force on the sail from solar radiation exactly balances the gravitational force acting upon it, the spacecraft is effectively in what the paper calls a ‘force-free environment.’

This gets interesting in terms of fast probes because while the statite is normally considered to remain stationary (and it will do so when the sail is stationary relative to the Sun during sail deployment), something else happens when the craft is orbiting the Sun when the sail is deployed. The sail now moves in a straight line at its orbital velocity at the time of deployment. The authors style this ‘rectilinear sun-diving.’ As Matloff noted in an email the other day:

“To do this operationally, it is necessary to maintain the sail normal to the Sun – broadside facing the Sun – during the acceleration process. The sail moves off at its velocity relative to the Sun at sail deployment because radiation pressure force on the sail balances solar gravitational attraction. This is a consequence of Newton’s First Law.”

Using this method we can fling the sail and payload outward. What is known as the sail’s lightness factor is the ratio of solar radiation forces divided by the solar gravitational force, and in the case of the rectilinear trajectory described above, the lightness factor is 1. So consider a sail being deployed from a circular orbit of the Sun at 1 AU. The statite, free of other forces, now moves out on a rectilinear trajectory at 30 kilometers per second, which is the Earth’s orbital velocity. The number is noteworthy because it practically doubles the interstellar velocity of Voyager 1. Matloff and Johnson point out that at this velocity, the Sun’s gravitational focus at 550 AU is reachable in 87 years.

Moving at the same pace gets us to Saturn (and the interesting Methone) in 1.5 years. I’m going to run through the other two scenarios the scientists consider to show the range of possibilities. Assume an orbit that is not circular but rather one having a perihelion of 0.7 AU and aphelion at 1 AU. Deploying the sail at perihelion allows the spacecraft to reach 38 kilometers per second, getting to the inner gravitational focus in about 66 years. Finally, with an aphelion at 1 AU and perihelion at 0.3, our craft achieves a velocity after sail deployment of 66 km/sec, reaching the focus in 38 years.

As regards to ‘Oumuamua, the third scenario, with sail deployment at perihelion some 0.3 AU out from the Sun, achieves enough interstellar cruise velocity to catch the object roughly around 2045, when it will be some 220 AU from the Sun. To these times, of course, must be added the time needed to move the sail from aphelion to the sail deployment point at perihelion, but the numbers are still quite satisfactory.

This is especially true given that we are talking about relatively near-term technologies that are under active development. Matloff and Johnson calculate using an areal mass thickness of 1.46 X 10-3kg/m2 for the proposed missions. They show current state of the art solar sail film as 1.54 X 10-3kg/m2 (this does not include deployment mechanisms, structure, etc). The point is clear, however: Achieving 30 km/sec or more offers us fast passage to targets within the outer Solar System as we analyze options for missions beyond it, using technologies that are not far removed from present capability.

The authors note that we can’t assume a constant value for solar radiation; the solar constant actually varies by about 0.1% in response to the Sun’s activity cycle. Hence the need to explore options like adjusting the curvature of the sail or using reflective vanes for fine-tuning. Controlling the sail will obviously be critical. The paper continues:

Control of the sail depends upon the ability of the system to dynamically adjust the center of mass (CM) versus the center of (photon) pressure (CP). Any misalignment of the CM versus the CP will induce torques in the sail system that have to be actively managed lest the offset result in an eventual loss of control. The sail will encounter micrometeorites and interplanetary dust during flight that will create small holes in the fabric, changing its reflectivity asymmetrically and inducing unwanted torques. Depending upon how the sail is packaged and deployed, there may also be fold lines, wrinkles, and small tears that occur with similar end results.

Hence the need for a momentum management system, which could involve possibilities like reflective control devices for roll or diffractive sail materials that manipulate the exit direction of incoming photons as needed to counter these effects. The authors point out that the solar sail propulsion systems for this kind of mission are at TRL-6 despite recent failures such as the loss of the Near-Earth Asteroid Scout Cubesat mission, which carried an 86 square meter solar sail that was lost after launch in late November 2022. With solar sails under active development, however, the prospect for exploring rectilinear sundiver missions in the near term seems quite plausible.

The paper is Matloff & Johnson, “Breakthrough Sun Diving: The Rectilinear Option,” Journal of the British Interplanetary Society Vol. 76 (2023), 283-287.

A ‘Pinched’ Beam for Interstellar Flight

Take a look at the image below. It’s a jet coming off the quasar 3C273. I call your attention to the length of this jet, some 100,000 light years, which is roughly the distance across the Milky Way. Jeff Greason pointed out at the Montreal symposium of the Interstellar Research Group that images like this suggest it may be possible for humans to produce ‘pinched’ relativistic electron jets over the much smaller distances needed to propel a spacecraft out of the Solar System. This is an intriguing image if you’re interested in high-energy beams pushing payloads to nearby stars.

Greason is a self-described ‘serial entrepreneur,’ the holder of some 29 patents and chief technologist of Electric Sky, which is all about beaming energy to craft much closer to home. But he moonlights as chairman of the Tau Zero Foundation and is a well known figure in interstellar studies. Placing beaming into context is a useful exercise, as it suggests alternative ways to generate and use a beam. In all of these, we want to carry little or no fuel aboard the craft, drawing our propulsion from the home system.

Image: Composite false-color image of the quasar jet 3C273, with emission from radio waves to X-rays extending over more than 100,000 light years. The black hole itself is to the left of the image. Colors indicate the wavelength region where energetic particles give off most of their energy: yellow contours show the radio emission, with denser contours for brighter emission (data from VLA); blue is for X-rays (Chandra); green for optical light (Hubble); and red is for infrared emission (Spitzer). Credit: Y. Uchiyama, M. Urry, H.-J. Röser, R. Perley, S. Jester.

Laser beaming to a starship comes first to mind, going back as it does to the days of Robert Forward and György Marx, who explored options in the infancy of the technology. Later work on laser ad well as microwave beaming has included such luminaries as Geoffrey Landis, Gregory Matloff and James Benford, not to mention today’s intense laser effort via Breakthrough Starshot and the ongoing work at UC-Santa Barbara under Philip Lubin. A separate track has followed beamed options using elementary particles or, indeed, larger particles; the name Clifford Singer comes first to mind here, though Landis has done key work. A major problem: Beam power is inversely proportional to effective range. If we’re after faster, bigger ships, we need to find a way to extend the range of whatever kind of beam we’re sending.

We’ve lost some of the scientists who have dug deeply into these matters. Dana Andrews died last January, and Jordin Kare left us some six years ago (I will have more to say about Dr. Andrews in a future post). Kare developed ‘sailbeam,’ which was a string of micro-sails sent as fuel fodder to a larger starship. Pushing neutral particles to the long ranges we need faces problems of beam divergence, and charged particle beams are even more tricky, because like charges cause the beam to diverge.

Greason outlined another possibility at Montreal, one he described as ‘no more than half of an idea,’ but one he’s hoping to provoke colleagues to explore. This beaming option uses the ‘pinch’ phenomenon, in which charged beams in a low-density plasma can confine themselves over long distances. The mechanism: A beam carrying a current creates a circular axial magnetic field which in turn confines the beam. ‘Pinching’ is a means of self-confinement of the beam that has been studied since the 1930s. A pinch forming a jet explains why solar proton events can strike the Earth despite the 1 AU distance, and why galaxy-spanning jets like that in the image above can form.

Image: Jeff Greason, chief technologist and co-founder of Electric Sky.

We normally hear about a ‘pinch’ in the context of fusion research, but here we’re more interested in the beam’s persistence than its ability to compress and heat a plasma. The beam persists until it loses energy by collisions, which causes the current sustaining it to weaken and lose confinement. Although Greason said that ion beams may prove feasible, he noted that we’re getting into territory where we simply lack data to know what will work. Issues of charge neutralization and return currents from the beam come into play, as do long-range oscillations that can affect the beam. But the idea of applying a magnetic field to a stream of electrons along a specific axis to create the z-pinch is well established. If we can create an electron beam using this method, we can resurrect the idea of using charged particle beams to push our starship.

How to use power beamed in this fashion once it arrives at the target craft is a significant question. Greason spoke of the beam striking a plasma-filled waveguide which can ‘couple to backwards plasma wave modes,’ in effect launching plasma in the opposite direction as reaction mass. This keys to existing work on plasma accelerators (so-called “wakefield” accelerators), which use similar physics. How much of the beamed energy can be returned in this way remains up for investigation.

The consequences of mastering pinched beaming technologies would be immense. If we can increase the range of a beam from 0.1 AU to 1000 AU, we open up the possibility of sending much larger spacecraft, up to 105 larger, at the same power levels. We go from a gram-sized spacecraft as contemplated by Breakthrough Starshot’s laser methods to one of 10 kilograms. In doing this we have also changed the acceleration time from minutes to months. That increased payload size is particularly useful when it allows a braking system aboard for long-term study of the target.

This method demands a space-based platform – these ideas are inapplicable when applied to a ground installation and a beam through the atmosphere. Beaming from a location near the Sun offers obvious access to power and could be made possible through a near-Solar statite; i.e., an installation that ‘hovers’ over the Sun at Parker Solar Probe distances. Greason adds that to add maximum stability to the beam, the statite would have to transmit from a location between the Sun and the target star; i.e, the flow should be with the current of the solar wind as opposed to across the stream.

Image: Can we operate a statite at 0.05 AU from the Sun? This NASA visualization of the Parker Solar Probe highlights the kind of conditions the craft would be operating in.

The operative statite technology is thermionics, where electrons ‘boil’ out of a hot cathode and collect on a cold anode. Greason’s statite winds up with approximately 50 kilowatts per square meter of useful power; factoring in the thickness of the foils used in the installation, he calculates 150 kilowatts per square kilogram. A 1 gigawatt electron beam results. So operating at about 11 solar radii, we can produce the beam we need while also being forced to tackle the issues involved in maintaining a statite in position. One possibility is a plasma magnet sail to make use of the supersonic solar wind, a notion Greason has been exploring for years. See Alex Tolley’s The Plasma Magnet Drive: A Simple, Cheap Drive for the Solar System and Beyond for more.

Greason’s tightly reasoned, no-nonsense approach makes him a hugely appealing speaker. He’s offering a concept that opens out into all kinds of research questions, and spurring interested parties to advance the construct. A symposium of like-minded scientists and engineers like that in Montreal provides the kind of venue to gin up that support. The implication of being able to reach 20 percent of lightspeed with a multi-kilogram spacecraft is driver enough. A craft like that could begin exploration of nearby stars in stellar orbit there, rather than blowing through the destination system within a matter of minutes. What smaller beam installations near Earth could do for interplanetary exploration is left to the imagination of the reader.