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 Late in life in an unpublished memoir, Forward recalled reading about Theodore Maiman’s work on lasers at Hughes Research Laboratories, and realizing that this was a way to create a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.
References
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.
X rays can be used power sails, it all depends on the relative velocity of the craft. If we used a free electron laser and started with optical we can move it into higher frequencies as the craft gained speed. Also electrons will happily absorb x rays provided there was an huge quantity of them perhaps at the sharp end of a highly charged cyclinders. The electron’s get ejected as propellant and then the cyclinder regains electrons from interstellar space.
@Michael
Perhaps some sort of scoop to harvest the very sparse ISM electrons? You make a good point that X-rays arriving at a craft already at relativistic velocities will appear as lower energy em radiation. Probably not that much lower as 0.1-0.2 c, but certainly an issue for optical lasers at high fractions of c.
If the craft already has electrons for propellant, would it not be better to eject them directly given an energy supply? Would the X-rays be useable as an energy source just as light is in PV panels but using different materials?\
I like your thinking though.
Let’s suppose Planet 9 really is a Neptune mass black hole:
Let it feed, and I imagine a polar jet strong enough to overcome any problems.
While an X-ray laser doesn’t work, there is nothing to stop collimating an X-ray source analogously like Forward’s Fresnel lens approach to collimating visible light. The question then becomes whether the “sail” can have a sufficiently low areal density to absorb the X-rays to transfer momentum to the craft. Metal sails would seem to be required but the tradeoffs may not work.
If both the collimation and sail do work, then ETIs could use X-ray binaries as the strong X-ray source to propel sail craft in the direction of the beam.
As we do create X-rays for imaging and X-ray crystallography, could the X-ray source be increased in size and the beam collimated by a grazing mirror lens like the Chandra X-ray telescope? I do not know the efficiency, which may be very low (let alone the needed heat dissipation from the X-ray-producing target). Still, at least the electron source could be supplied by our sun’s solar wind, concentrated, and used to impact the target to produce the X-rays.
If the X-rays have 1000x the energy of visible light, it does suggest that any absorbing carbon lattice sail could theoretically have an areal density of up to 1000x that of a similar sail and have a net gain in performance. Strictly for robotic craft, as biological crews would suffer from any X-ray leakage from the sail.
[This is all rather Rube-Goldberg and in no way suggests the performance would really be better than other forms of sail propulsion using longer em wavelengths.]
What do you mean the x ray laser does not work? Michael is correct. The problem is that a free electron laser is needed to make an x ray laser which was not invented in the year 1967 and it was feasible 1972, but was not developed until 1976. Google source. A laser is the only thing that will work with a solar sail since we are talking about long distance because laser light is coherent and monochromatic and does not spread out like ordinary light which is made of different wavelengths. X ray machine not using x ray laser would have the same problem. An x ray solar sail is very feasible.
Jim Benford will certainly disagree about the feasibility of X-ray sails. As to laser light, it is not true that it does not spread. It certainly spreads less than ordinary light but Forward had to design his Fresnel lens in the 1984 paper so as to refocus the beam and keep it tight for the interstellar sail he studied. Diffraction is fundamental and apparent at interstellar distances. Varying aperture size and wavelength can reduce but not eliminate it. Scattering in the interstellar medium is another cause of spread.
I agree. Laser does spread out, but much less than ordinary light. We could easily add a lens to focus many laser beams into one for more power and better accuracy. I agree an x ray laser would be a problem for ordinary solar sails. It still might be used to get heat energy if part of the solar sails had a spot in the center with x ray an x ray absorber although there might as usual objections to it’s potential military application as a weapon.
Fresnel lenses for X-rays are tiny things. For 1 KeV X-rays they are about 1 mm or less diameter and diameter is proportional to wavelength. So Fresnel lenses are not the answer.
David, I was not suggesting a Fresnel lens for X-ray beaming. I was responding to a comment that laser beams do not spread, which they do, and this has to be taken account of at interstellar distances. That diffraction was why Forward’s solution for his beamed sail mission in the 1984 paper — which used much different laser frequencies — was a Fresnel lens in the outer solar system.
Maybe I am out of date, but I thought X-ray lasers were just single-shot fast bursts. Useful as weapons maybe, but not as long-running beam generators to accelerate a sail.
Those pulses can be close together. It would certainly be expensive to make one powerful and large enough for beamed propulsion.
X-ray mirrors can only reflect hard X-rays at grazing angles of about .1 degree. One can combine multiple reflections to get effective angles of about 4 degrees.
One doesn’t need reflections to get momentum transfer, perpendicular incidence will give half the transfer of reflection so can be significant.
One might drive such an absorbing sail with nuke explosions as 75% of the output of a nuke is soft X-rays of about 1keV blackbody temperature.
Nuclear driven X-ray lasers were proposed for the origin Strategic Defense Initiative withe explosion driving lasing in fully ionized foam rods. The. Aspect ratio of the rods determined the beam divergence. I was told to assume a power density of 4 joules/cm2 of 1 KeV X-rays deposited in 10 nanoseconds from a Soviet system. These could drive an absorbing sail.
BTW, electrically charged surfaces only increase X-ray reflectivity by a tiny amount.
Oh, that 4 watts/cm2 was at a distance of 1000 km
It was implied but not discussed that it might be possible to further collimate these. 1KeV X-rays with a lens. The only possible X-ray lens I could imagine for this was a foam structure of depth varying density that would be fully ionized to produce little absorption and higher refraction than any other surface. I’ve never calculated a refraction angle because I never saw a good application till now
A synchrotron based X-ray source can be both monochromatic and of low divergence. I’d suggest an X-ray energy of 3.4 KeV because this will diffract at a perpendicular angle from a thin film of Highly Oriented Pyrolytic Graphite giving the same momentum increase as reflection.
If we use x rays which say are a 1000 times lower in wavelength than optical light the area of the sail drops by 1000 000 as the diffraction limit scales linearly and area is to the squared. Although we cant effectively reflect xrays they can be absorbed and emitted as higher wavelengths say as UV light with say tungsten. Not sure if there is a good enough trade off to use them though.
What intrigues me in this beautiful project is: have the power to operate a powerful laser towards these sails for several minutes without making everything fart on earth? What about the Joules effect of this type of laser? If I’m not mistaken, Forward developed the idea, but based on the idea of a solar energy source. Correct me if I’m wrong…
Ok, I totally misremembered the 2-d spacing of HOPG. The correct X-ray energy for perpendicular diffraction from HOPG is .19 KeV.
David Ohara suggests a 3.4 keV X-ray beam on carbon. Such an energy means that a single photon with such energy is more than enough to fully ionize a carbon atom. So that photon doesn’t reflect, it ionizes the atom and damages the lattice of the material. As he points out, the short wavelength allows the sail to be much smaller. An intense beam of such photons, incident on a much smaller sail area, will destroy the integrity of the sail.
For some reason I can’t calculate the most basic things on my phone. The correct energy for perpendicular diffraction from HOPG IS 1.85 KeV.
Now, descriptions of Orion always say the impulse is provided by the plasma expanding against the pusher plate. WELL, 75% of the output of a nuke is low energy black body X-rays so maybe the X-rays could do better pushing. Has anybody considered this? Yes, the pusher plate would wear but you let some liquid cover it between impulses. Is this worth thinking about?
The impulse of the nuclear explosions driving a sail was discussed in
https://www.centauri-dreams.org/?s=three+body
A very exciting idea. Is there any basis for thinking that a sail could be engineered to stand up to multiple x-ray impulses from a close micro-nuke?
BTW watched the 30 episode Chinese version of the Three Body series. Has all the same major plot points seen in the western version (but not the interstellar pulse sail). Really enjoyed the Chinese series just a much. Mostly different characters – still has the scientist who survived the cultural revolution, the oil billionaire, and the cigarette smoking detective though.
You have a typo: instead of “Roland Eötvös University in Budapest” it is Eötvös Loránd University.
https://www.elte.hu/
https://en.wikipedia.org/wiki/E%C3%B6tv%C3%B6s_Lor%C3%A1nd_University
Thank you. I’ve repeatedly run into “Roland Eötvös University” as well online, but I assume your formulation is what the university prefers.
This article looks to have xray reflectivity a lot higher with diamond.
https://www.nature.com/articles/nphoton.2011.197
X rays have much shorter wavelengths than visible light – so they would need a smaller collimating lense
Forward – in his fiction – did not use X rays – but he did have the collimating lens hundreds of km across
Which seems reasonable to me – for a future civilisation
Some info from CERN about x ray reflectivity.
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwi3o4Cp586GAxVdWEEAHfJhBkoQFnoECBIQAQ&url=https%3A%2F%2Faccelconf.web.cern.ch%2Fipac2022%2Ftalks%2Ftuiysp3_talk.pdf&usg=AOvVaw2hpMz102wsS-uK0LgD94Dj&opi=89978449
Michael: Good articles as I had not kept up with diamond optics. I’ll look over that CERN article and calculate the beam divergence and maybe a maximum power level for the free electron laser
So after looking at the data it is possible to have an x-ray mirror if say in a retroflective configuration. Diamonds can also be used across the spectrum including the laser wavelengths Starchip looks to need.
From that CERN paper and using their FWHM FOR 9831 eV X-rays, I get a divergence angle of .01 degree or .00017 radian. At a distance of 1 AU the beam is. 16182 miles wide.
I’m not feeling too optimistic about this.
Its the waist size that is important and using it in a phased array design would help and also upping the g’s reduces the distance before divergence becomes too much. Even so it may help with other types of propulsion such as fusion or fission implosions on a much larger craft, simply reflect the x-rays to compress a fuel pellet.