Though photo voltaic sails have been making their manner into the aerospace journals within the late Nineteen Fifties, Robert Ahead was the primary scientist to think about using laser beams relatively than daylight to drive an area sail. That idea, which György Marx picked up on in his 1966 paper, opened the door to interstellar mission ideas. Late in life in an unpublished memoir, Ahead recalled studying about Theodore Maiman’s work on lasers at Hughes Analysis Laboratories, and realizing that this was a solution to create a starship. His 1962 article (quotation beneath) laid out the thought for the journal Missiles and Rockets and was later reprinted in Science Digest. Marx certainly knew the Ahead article and his subsequent paper in Nature probed find out how to obtain this purpose.
Picture: One of many nice figures of interstellar research, Robert Ahead amongst many different issues launched and explored the rules of beamed propulsion. Credit score: UAH Library Robert L. Ahead Assortment.
Marx was at the moment a professor of theoretical physics at Roland Eötvös College in Budapest. He was plugged into the difficulties of interstellar flight via Les Shepherd’s work in Britain, and he cites the latter’s Realities of House Journey (1957) within the paper as one among many sources highlighting the depth of the issue. His paper “Interstellar Car Propelled by Terrestrial Laser Beam” is a mere two pages constructed largely round equations, reporting on the “generally accepted view that, other than the technical difficulties concerned, the legal guidelines of conservation of vitality and momentum forbid the visiting of different planetary methods within the human lifespan.”
Marx had already explored vitality points for interstellar flight in a 1960 paper for Astronautica Acta (because it was then recognized) and a second for a similar journal in 1963 (citations beneath). However the concept of laser beaming provided the physicist a glimmer of hope. Within the 1966 paper, he cites the benefits of beamed crusing vs standard rocket propulsion. The paper argues that “a automobile might be accelerated virtually to the velocity of sunshine if an emitter on the Earth can precisely venture gentle onto its mirror.”
The perfect focusing mechanism can be the laser, and it’s right here that he runs into bother. For Marx frightened in regards to the measurement of the transmitter aperture, which determines the scale and preliminary beam diameter that can emerge. Bear in mind we’re within the Robert Ahead period of maxed out engineering, when the thought was merely to determine what was doable even when it required constructing capabilities far past these of the current day. So right here’s what Marx comes up with, the perfect idea he thought possible:
…the technical situations are extraordinarily difficult. A variety of operation of 0.1 gentle yr would require a coherently radiating floor of the order of 1 km2 which emits onerous X-rays, and the automobile would wish an X-ray mirror with an efficient cross-sectional space of a number of km2.
Marx talks in regards to the absorbed vitality of his sail ‘mirror’ radiating out isotropically into house, and right here we run into severe issues. I took my inquiries to Jim Benford, CEO of Microwave Sciences and creator of Excessive Energy Microwaves, an ordinary textual content which is about to enter its fourth version at CRC Press. Jim can also be an everyday contributor to Centauri Desires. And he was fast to level out that X-rays replicate from conducting surfaces in ways in which defeat Marx’s objective.
Picture: György Marx in his workplace. Credit score: REAL-I, the Picture File Assortment of the Tutorial Library.
As Jim instructed me, incoming X-rays replicate solely at very low grazing angles. The effectivity of vitality switch is at stake right here. Right here’s a bit extra of what he mentioned, utilizing one of the crucial essential formulae in all of contemporary physics:
X-ray photons have much more vitality than seen gentle or microwaves. Bear in mind the relation E=hν, the place E is the vitality of the photon, h is Planck’s fixed, ν the frequency. X-ray photons have energies a couple of thousand occasions that of seen gentle, one million occasions that of microwaves. If they arrive in regular to the floor [i.e., striking the sail head on], they ionize atoms, damaging the lattice of the fabric.
X-ray telescopes, as a matter of reality, work via a collection of grazing incidence reflectors. In different phrases, we will’t direct Marx’s improbable X-ray beam towards our sail with out significantly damaging it, not until we’re keen to convey the beam to it at such a low angle that the intrinsic energy of the beam is essentially misplaced. Benford once more:
There’s no solution to speed up a sail with X-rays. The cross-section of the sail have to be at a slight angle to the beam, not perpendicular to it, for the X-rays to replicate. That’s vastly inefficient. Grazing incidence signifies that solely the slight transverse part of the photon velocity vector is reversed, leaving the far bigger axial part virtually unchanged. Little vitality is transferred to the inclined sail, and that drives it sideways to the beam, not antiparallel to it, as mirrored photons do once they incident usually. So the sail is accelerated little or no within the route of the X-ray beam.
That is the coup de grâce for the X-ray sail. It’s attention-grabbing to see what Robert Ahead considered Marx’s concept. Right here he’s, writing in a 1984 paper known as “Roundtrip Interstellar Journey Utilizing Laser-Pushed Lightsails,” which is without doubt one of the classics of the sphere:
The idea of laser-pushed interstellar lightsails was reinvented by Marx in 1966. Since Marx was unwilling to contemplate a laser aperture higher than 1 km2, he was pressured to imagine using onerous x-rays so as to get hold of the operational ranges wanted for interstellar flight. The impossibility of establishing each an x-ray laser and a light-weight sail to replicate these x-rays led to Marx’s extremely pessimistic conclusion in regards to the feasibility of the idea. If Marx had been keen to contemplate a bigger transmitter aperture, then his laser frequencies and sail necessities would have been a lot simpler.
J. L. Redding, then at Bishop’s College in Quebec, noticed Marx’s paper and responded to it in the identical yr, providing corrections to Marx’s equations with out difficult the X-ray idea. His telling comment that “…one doesn’t want to contemplate the difficulties of arranging appropriate deceleration and touchdown services” refers to what he noticed because the overwhelming issues in making a beamed vitality propulsion system work in any respect. Marx had commented on the deceleration downside and Ahead would go on to supply a possible resolution in his 1984 paper, one so baroque that it deserves a future submit of its personal.
I must also point out a little bit referenced paper by W. E. Moeckel, “Propulsion by Impinging Laser Beams,” which ran within the Journal of Spacecraft and Rockets in 1972. Moeckel (working at what was then NASA’s Lewis Analysis Middle in Cleveland) analyzed laser beaming to 100 ton relativistic flyby probes, every of which might require 1012 watts of X-ray vitality. Making particular reference to Marx, Moeckel discovered X-ray beaming promising however didn’t know if it was possible. His conclusion would have warmed the hearts of science fiction writers of the time:
…some future generations of mankind, with a considerably totally different ordering of priorities than ours and rather more obtainable energy, may conceivably discover different stars and different photo voltaic methods with extremely subtle unmanned spacecraft able to relaying info in elapsed occasions of the order of a long time.
If solely it labored! Fortuitously, we’re not restricted to X-rays in the case of beamed propulsion.
References
The early Ahead 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). Ahead’s “Roundtrip Interstellar Journey Utilizing Laser-Pushed Lightsails” appeared within the Journal of Spacecraft and Rockets 21 (1984), pp. 187-195 (summary).
György Marx’s paper on X-ray beaming is “Interstellar Car Propelled by Terrestrial Laser Beam,” which ran in Nature on July 2, 1966 (summary). His two different interstellar papers are “The mechanical effectivity of interstellar autos,” 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 identical title, “Interstellar Car Propelled by Terrestrial Laser Beam,” Nature February 11, 1967 (summary). W. E. Moeckel’s paper “Propulsion by Impinging Laser Beams” appeared within the Journal of Spacecraft and Rockets Vol. 9 (1972), 942-944 (summary).
My due to Jim Benford, Greg Matloff and Al Jackson for invaluable references and commentary.
