Some names seem to carry a certain magic, at least when we’re young. I think back to all the hours I used to haunt St. Louis-area libraries when I was growing up. I would go to the astronomy section and start checking out books until over time I had read the bulk of what was available there. Fred Hoyle’s name was magic because he wrote The Black Cloud, one of the first science fiction novels I ever read. So naturally I followed his work on how stars produce elements and on the steady state theory with great interest.
Willy Ley’s name was magic because he worked with Chesley Bonestell (another magic name) producing The Conquest of Space in 1949, and then the fabulous Rockets, Missiles, and Space Travel in 1957, a truly energizing read. Not to mention the fact that he had a science column in what I thought at the time was the best of the science fiction magazines, the ever-engaging Galaxy. It still stuns me that Ley died less than a month before Apollo 11 landed on the Moon.
My list could go on, but this morning I’ll pick one of the more obscure names, that of Ernst Öpik. Unlike Hoyle and Ley, Öpik (1893-1985) wasn’t famous for popularizing astronomy, but I would occasionally run into his name in the library books I was reading. An Estonian who did his doctoral work at that country’s University of Tartu, Öpik also did work at the University of Moscow but wound up fleeing Estonia in 1944 out of fear of what would happen when the Red Army swept into his country. He spent the productive second half of his career at the Armagh Observatory in Northern Ireland and for a time held a position as well at the University of Maryland.
Image: Ernst Öpik. Credit ESA.
Did I say productive? Consider that by 1970 Öpik had published almost 300 research papers, well over a hundred reviews and 345 articles for the Irish Astronomical Journal, of which he was editor from 1950 to 1981. He remained associate editor there until his death.
I found the references to Öpik in my reading rather fascinating, as I was reminded when Al Jackson mentioned him to me in a recent email. It turns out, as I had already found, that Öpik turns up in the strangest places. Recently I wrote about the so-called ‘manhole’ cover that some have argued is the fastest human object ever sent into space. The object is controversial, as it was actually a heavy cover designed to contain an underground nuclear blast, and rather spectacularly proven unsuccessful at that task. In short, it seems to have lifted off, a kind of mini-Orion. And no one really knows whether it just disintegrated or is still out there beyond the Solar System. See A ‘Manhole Cover’ Beyond the Solar System if this intrigues you.
Öpik’s role in the ‘manhole cover’ story grows out of his book The Physics of Meteor Flight in the Atmosphere, in which he calculated the mass loss of meteors moving through the atmosphere at various velocities. Although he knew nothing about the cover, Öpik’’s work turned out to be useful to Al as he thought about what would have happened to the cover. Because calculations on the potential speed of the explosively driven lid demonstrated that an object moving at six times escape velocity, as this would have been, would vaporize. This seems to put the quietus on the idea that the 4-inch thick iron lid used at the test detonation of Pascal B had been ‘launched’ into hyperbolic orbit.
But this was just a calculation that later became useful. In broader ways, Öpik was a figure that Al describes as much like Fritz Zwicky, meaning a man of highly original thought, often far ahead of this time. He turns out to have played a role in the development of the Oort Cloud concept. This would have utterly escaped my attention in my early library days since I had no access to the journals and wouldn’t have understood much if I did. But in a paper called “Note on Stellar Perturbations of Nearby Parabolic Orbits,” which ran in Proceedings of the American Academy of Arts and Sciences in 1932, the Estonian astronomer had this to say (after page after page of dense mathematics that are to this day far beyond my pay grade):
According to statistics by Jantzen, 395 comets (1909) showed a more or less random distribution of the inclinations, with a slight preponderance of direct motions over retrograde ones, with an age of from 109 to 3.109 years, this would correspond to an average aphelion distance of 1500-2000 a.u., or a period of revolution of 20000-30000 years. For greater aphelion distances the distribution of inclinations should be practically uniform, being smoothed out by perturbations.
Does this remind you of anything? Öpik was writing eighteen years before Jan Oort used cometary orbits to predict the existence of the cloud that now bears his name. Öpik believed there was a reservoir of comets around the Sun. There had to be, for a few comets were known to take on such eccentric orbits that they periodically entered the inner system and swung by our star, some close enough to throw a sizeable tail. Öpik was interested in how cometary orbits could be nudged by the influence of other stars. In other words, there must be a collection of objects at such a distance that were barely bound to the Sun and could readily be dislodged from their orbits.
I’m told that the Oort Cloud is, at least in some quarters, referred to as the Öpik/Oort Cloud, in much the same way that the Kuiper Belt is sometimes called the Edgeworth/Kuiper Belt because of similar work done at more or less the same time. But such dual naming strategies rarely win out in the end.
Being reminded of all this, I noticed that Öpik had done major work on such topics as visual binary stars (he estimated density in some of these), the distance of the Andromeda Galaxy, the frequency of craters on Mars, and the Yarkovsky Effect, which Öpik more or less put on the map through his discussions of Yarkovsky’s work. Studying him, I have the sense of a far-seeing man whose work was sometimes overlooked, but one whose contributions have in many cases proved to be prescient.
Naturally I was interested to learn whether Öpik had anything to say about our subject on Centauri Dreams, the prospect of interstellar flight. And indeed he did, in such a way that the sometimes glowering photographs we have of him seem to reveal something of his thinking on the matter (to be fair, some of us are simply not photogenic, and I understand that he was a kind and gentle man). Indeed, Armagh Observatory director Eric Lindsay described him thus:
…a “very human person with an understanding of, and sympathy for, our many frailties and, thank goodness, with a keen sense of humour. He will take infinite patience to explain the simplest problem to a person, young or old, with enthusiasm for astronomy but lacking astronomical background and training.”
The interstellar flight paper was written in 1964 for the Irish Astronomical Journal. Here he dismissed interstellar flight out of hand. Antimatter was a problem – remember that at the time he was writing, Öpik had few papers on interstellar flight to respond to, and he doesn’t seem to have been aware of the early work on sail strategies and lasers that Robert Forward and György Marx were exploring. So he focused on two papers he did know, the first being Les Shepherd’s study of interstellar flight via antimatter, seeing huge problems in storage and collection of the needed fuel. Here he quotes Edward Purcell approvingly. Writing in A.G.W. Cameron’s Interstellar Communication in 1963, Purcell said:
The exhaust power of the antimatter rocket would equal the solar energy received by the earth – all in gamma rays. So the problem is not to shield the payload, the problem is to shield the earth.
Having dismissed antimatter entirely, Öpik moves on to Robert Bussard’s highly visible ramjet concept, which had been published in 1960. He describes the ramjet sucking up interstellar gas and using it for fusion and spends most of the paper shredding the concept. I won’t go into the math but his arguments reflect many of the reasons that the ramjet concept has come to be met with disfavor. Here’s his conclusion:
…the ‘ramjet’ mechanism is impossible everywhere, as well as inside the Orion Nebula – one must get there first. “Traveling around the universe in space suits – except for local exploration… belongs back where it came from, on the cereal box.” (E. Purcell, loc. cit.). It is for space fiction, for paper projects – and for ghosts. “The only means of communication between different civilizations thus seems to be electro-magnetic signals” (S. von Hoerner, “The General Limits of Space Travel”, in Interstellar Communication, pp. 144-159). Slower motion (up to 0.01 c is a problem of longevity or hereditary succession of the crew; this we cannot reject because we do not know anything about it.
I always look back on Purcell’s comment and muse that cereal boxes used to be more interesting than they are today. I do wonder what Öpik might have made of sail strategies, and I’m aware of but have not seen a paper from him on interstellar travel by hibernation, written in 1978. So he seems to have maintained an interest in what he elsewhere referred to as “our cosmic destiny.” But like so many, he found interstellar distances too daunting to be attempted other than through excruciatingly long journey times in the kind of generation ship we’re familiar with in science fiction.
Since Öpik’s day a much broader community of scientists willing to study interstellar flight has emerged, even if for most it is a sideline rather than a dedicated project. We have begun to explore the laser lightsail as an option, but are only beginning the kind of laboratory work needed, even if a recent paper out of Harry Atwater’s team at Caltech shows progress. An unmanned flyby of a nearby star no longer seems to belong on a cereal box, but it’s a bit sobering to realize that even with sail strategies now under consideration by interstellar theorists, we’re still a long, long way from a mission.
Öpik’s paper on what would come to be known as the Oort Cloud is “Note on Stellar Perturbations of Nearby Parabolic Orbits,” Proceedings of the American Academy of Arts and Sciences, vol. 67 (1932), p. 169. The paper on interstellar travel is “Is Interstellar Travel Possible?” Irish Astronomical Journal Vol. 6(8) (1964), p. 299 (full text). The Irish Astronomical Journal put together a bibliography covering 1972 until his death in 1985, which students of Öpik can find here. The Atwater paper on sail technologies is Michaeli et al., “Direct radiation pressure measurements for lightsail membranes,” Nature Photonics 30 April 2025 (abstract). More on this one shortly.
In Willy Ley’s “Beyond the Solar System” (1964) illustrated by Chesley Bonestell, Von Braun’s forward indicates that a photon drive could potentially have a starship reach Alpha Centauri in 30 years. Unfortunately, this starship is not illustrated in the text. Instead, there is an ion drive. The ship is shown being prepared in LEO and the drive is turned on when departing.
A minor nit. If that is the mental image that Öpik has when he suggests that Earth needs to be shielded from the gamma-ray exhaust of photon propulsion, it seems like a strawman argument to dismiss the concept. Why couldn’t the ship use “conventional” propulsion to reach a safe distance from Earth before the photon drive is used to accelerate the ship slowly to the needed velocity to reach the stars within a human lifetime? Von Braun suggests that an antimatter drive might be unrealistic and that a generation ship is a more likely approach, although he speculates that the doubling time of knowledge might change the situation in 50 years. We are still trying to design fusion drives with anti-matter propulsion, which is still a speculative idea. At least we have ideas about where anti-matter might be harvested rather than manufactured.
Do you know if anyone has calculated a safe distance, either directly in the line of exhaust or somewhat off the side? Of course the engine may suffer spontaneous disassembly regardless!
No, but I assume an R-squared intensity relationship and that travel opposite the sun provides a decent gamma ray shield. A 10,000 MT ship with about 1/2 its mass as antimatter would certainly make a big BOOM! if it exploded all at once.
However, I also think that ships, even robotic ones, won’t be that unreliable. Even in Star Trek, a Federation ship that is crippled doesn’t undergo a RUD and complete disappearance in a huge burst of radiation despite its supply of antimatter. Either the technology to use antimatter is reliable and “safe”, or it will be used very carefully in ways that cannot harm Earth and other planets, or not at all in amounts that cannot be contained.
We usually assume a starship using antimatter would have it onboard to mix with propellant in various ratios up to 50%. But what if instead a beam is produced that then is sent to the ship where its rear shield is the “propellant”? A beamed version of an Orion-type ship. The beam would be neutral and not be very divergent. Its intensity is controlled by the production rate at the source, and any problems can be halted by stopping the beam. Storage problems solved. Even the “engine” is simple and not unlike the proposed nuclear or fusion bomb approach, with the benefit of no shock absorbers needed. The ship design would be something like a photon sail ship – a vast plate behind the payload that is slowly destroyed by the beam. Or perhaps the beam is anti-ferrous metal atoms/tiny slugs, which are maneuvered by magnetic fields to interact with a propellant as the exhaust. Again, the antimatter amount reaching the ship in any moment is small, and problems can be averted by the ship steering away from the beam or turning off its collection fields. The ship still suffers from all the problems of beamed sails, but it potentially gains from more powerful energy transfer, higher acceleration, and therefore shorter beaming times to reach cruise velocity.
If creating antimatter becomes much easier and more efficient, then the ship could make its own antimatter instead using the energy from conventional beams, again making it in small enough amounts that are safe and controllable. (This would be a highly inefficient use of an energy beam.)
All suggestions to deal with Öpik’s offhand dismissal of antimatter propulsion.
On the ‘destructive’ beam of an antimatter rocket. I don’t know if Öpik’ had read it, but in Handbook of Astronautical Engineering
(Heinz Hermann Koelle, editor, McGraw-Hill, 1961, 1867 pages)…there is an entry on Photon Propulsion by Eugen Sänger. (Three years before he died.)
Towards the end Sänger muses about the propulsion ‘beam’.
” The reflector cone of a 100-ton light-pressure rocket, having a total vertex angle of 10°, will generate the terrestrial radiative intensity of the sun at a distance of 3000 miles. At a distance of 600 miles, forests, fields, and housing areas would be ignited by this beam on an area of about 3000 square miles. At a distance of 300 miles, all life would be destroyed instantly; at 30 miles, metal slabs would be melted in a few seconds, and even the best silver reflector would melt at a distance of 6 miles. It is obvious that many geophysical conditions could, thereby, be affected in particular climatological and meteorological processes. This undesirable aspect of photon rockets entails some limitations for the application of this propulsion system. It is obvious that the photon beam of such a rocket must never hit the terrestrial surface or any other surface, or any flying object, except from a very great distance. Vertical take-off from the surfaces of celestial bodies does not seem to be feasible at the present time except in emergencies.”
Why Sänger , such a visionary, emphasize more starting the ship way away for the Earth is a bit odd.
Sänger’s antimatter propulsion system is the first, that I know of, with technical engineering details. He presented this 1953, tho he had been interested in interstellar flight since the 1930s.
Sänger , only focused on electron-positron annihilation, tho the antiproton was discovered in 1955. His propulsion system was by gamma rays! He struggled with focusing gammas. He proposed ‘plasma mirrors’ , and goes on a riff about the idea, even to the point of imagining uranium gas to produced electron mirrors!
Tell ya, Sänger was not a closed box thinker , he came up with astronautical concepts years before others.
The Earth would not needed to be shielded from gamma ray radiation, but only the passenger compartment which is much closer. Radiation follows an inverse square law that attenuates with the square of the distance from the source. The main reason is the upper atmosphere shields our Earth and protects us on the ground from the short wave higher energy wavelength EMR Ozone blocks the ultra violet wavelength due to it’s specific molecular energy level absorption of the electron at the ozone molecule’s electron ground state in the electron jump from zero to one. Oxygen and Nitrogen absorb x rays and gamma rays only by electrons in the inner shells and not the ground state since the inner shells absorb energy at a high level than the ground state x rays and gamma rays knock out electrons in the inner shells of oxygen and nitrogen. Consequently if we want to see the ultra violet, x rays and gamma ray sky, and even in some wavelength in the infra red the invisible universe we need to get above the atmosphere into space. I agree with the idea that conventional rockets should be used to get the space craft out of orbit for safety.
Most of the kinetic energy from the anti matter annihilation drives the spacecraft with the gamma ray radiation being secondary.
The Earth’s atmosphere is blocking many wavelengths, but not completely. After all, some UV does reach the ground. Gamma-ray astronomy does need to send instruments into the high stratosphere and beyond because the distant sources are so weak. It has long been suggested that a supernova close enough to our system could sterilize the surface of the Earth. It all depends on intensity. I believe it when an astronomer says that a direct blast of gamma rays of an intensity needed to move a starship would be a problem for Earth, even without an accidental blow-up.
Quote by Alex Tolley: “After all, some UV does reach the ground. Gamma-ray astronomy does need to send instruments into the high stratosphere and beyond because the distant sources are so weak” This statement is false. The gamma ray energy reaching our Earth from distant sources like supernova explosions and active galactic nucleus are much higher than the Sun. AI Google source. Our atmosphere protects us from harmful radiation which is fact.
The history of our invisible universe is interesting. Astrophysicists thought that the Moon would emit x rays reflected from the Sun and it does, but when they first sent of the V 2 rocket with x ray detectors the entire sky was filled with x rays and the Moon was a dark area in the x ray sky literally a hole in the x ray sky. Our atmosphere and magnetic field also shield us from cosmic rays which are the high energy, short wavelength EMR and energetic, and relativistic particles which are mostly protons, electrons and heavy element ions. These relativistic particles hit our atmosphere with energies that are exponentially higher than the protons collided in the LHC. Comics rays hit the surface of Mars because it’s atmosphere is so thin. Mars also has no ozone layer thick enough to block all UV, UV-A which reaches our ground, but not UV-B, and UV-C so microbes like viruses and bacteria die on the surface of Mars.
A Wolf-Rayet star super nova explosion can create a narrow gamma ray burst which can destroy our ozone layer, but these are rare and the beam has to be in the right direction at the right distance.
Wavelength tells us more than intensity when it comes to radiation. First UV-A can still hurt us with long exposure, but UV-C of less intensity is still more energetic. For example for a particle to be ionizing, it has to be at the x rays wavelength which means it can strip or knock the electron right of the atom or molecule. Oxygen and Nitrogen absorb everything above the ultra violet wavelength in the inner shells. Gamma and x rays don’t reach the surface Mars unless they are really energy gamma rays.
Even if the radiation is not ionizing, it still can destroy cells like ultra violet which is why we can’t afford to loose our ozone layer. Space is definitely an inhospitable place for unprotected life.
I am not clear why this has any relevance. The issue is the relative strength of galactic gamma-ray sources (which are blocked by our atmosphere) vs. a gamma-ray photon rocket in LEO which may be orders of magnitude more luminous.
Wavelength always tells us the energy since wavelength is inversely proportional to energy. The shorter the wavelength, the higher the energy of the particle. We see this with the photo electric effect which proves that light or photons with shorter wavelength knocks out electrons with more kinetic energy than longer wavelength light. Higher intensity knocks out more electrons, but wavelength determines the energy.
The whole idea of Kirchhoff’s law of black body radiation is based on wavelength. Cold bodies emit radio waves and microwaves, and cause rotational transitions of molecules. Infra red is associated with vibrational transitions and electron transitions emit visible light. Also radiation and temperature. A hot body emits ultra violet from 5400 F up to several million degrees it emits x rays and a body emits gamma rays at about one hundred million K. The temperature at the center of the Sun, 15 million kelvins. Google AI source.
This is a great review for many reasons. For one, from personal but perhaps widely shared experience, Opik’s presence is much as described. It sent me looking all over the house searching perhaps for a used bookstore copy of one of his “stellar” works. Perhaps they actually were all on loan from one library or another.
But on matters such as stars, he was among the many background sources which were supplementary, not necessarily among the reserved books or required reading. And at the time, say, in graduate school, there was hardly time to contemplate how forward thinking or what stakeouts his studies represented.
On the matter of an Oort Cloud, it is also interesting to consider how we have come to accept the notion of a vast (“condensated)” cloud of matter that we can hardly see. Especially close to a century back. Pre-occupied with the visible stars and planets, it took some time growing up to distinguish between what was Kuiper and what was Oort. And I suppose details of the definitions or boundaries would still be arguable now. Pictures of many young stars appear to illuminate such structures too. But at the time of Opik’s writing, the larger body was an inference more than an observation. And so much of observation pointed so long to void.
Opik’s pessimism about human interstellar travel is understandable. Even now, contemplating it seems like the most excruciating possible example of being a child in the back seat of a car screaming, “When are we going to get there?”
Not much consolation to be on the front seat either guiding a vehicle down such a lightless highway with no turnoffs for rest.
It seems like since Opik’s time the most encouraging elements about interstellar travel are, for one, the verification that there are destinations beside stars, being exoplanets. Looking at his books over the years with uneven interest or study mission, I don’t recall him addressing exoplanets specifically. But owing to the prevailing planetary origin theories, that exoplanets were not necessarily a given.
But beside their existence validating a less accepted theory among stellar astronomers, their detection illustrates an enormously increased ability to survey star systems from afar.
Still, if say we achieved a solar system escape velocity for pioneers with a velocity of 1 percent of light’s ( 3,000 km/sec), there is still the problem of decelerating the vehicle once it reached the vicinity of its stellar port of call, say four centuries later. If that were achieved, I suppose there would be crash programs to continue or return… Maybe Oort Clouds here and “there” could alleviate this problem. …Somehow?
Öpik is an engaging writer. His papers in the “Irish Astronomical Journal” are educational for anyone who doesn’t mind a bit of maths. He took weird stands on issues of the day, for example arguing that Venus’s observed high temperatures in the late 1950’s came from global sand-storms and friction. Definitely well worth browsing through his Opus in the Astrophysical Database Server.
One of his paper about space travel which allows us to understand his thoughts, which are very much imbued with Relativity :
http://www.bigear.org/CSMO/PDF/CS11/cs11p10.pdf
“Therefore, time inside a photon stands still—at exactly
the velocity of light the foreshortening is absolute”
the kind of idea I like :)
Paul, you might want to take a look at this; it presents some interesting ideas on addressing issues related to lightsails.
Photonic Lightsails: Fast and Stable Propulsion for Interstellar Travel.
“Simple stacked slabs or parachute-like geometric designs
demonstrated modest potential for satisfying
the mission’s high-thrust and stablebeam-
riding requirements, but are ultimately
outclassed by nanostructured designs such as
diffraction gratings and metasurfaces. The substantial
advantage of nanophotonic architectures
is the extensive design parameter space,
in principle, allowing multiple mission requirements
such as thermal control, propulsion, and
stability to be addressed concurrently.”
https://arxiv.org/abs/2502.17828
Good tip, Mike. Thanks.
The issue with gamma ray impingement on the earth could be reduced by have the ship go in a highly elliptical orbit and then on the inward plunge fire up the gamma ray engine until we go passed the earth. Also the drive could still use antimatter on a lot more normal matter and behave more like a fusion/fission drive or chemical engine reducing gamma rays, perhaps using runway pellets mass.
An interesting way of storing the antimatter is perhaps via torus’s around the waist of the craft. The charged antimatter wizzes around been magnetically confined, however the outer side is open to space. In the event of a loss of containment the magnetic field is switched off and the antimatter ejected into space via centrifugal action.
Crazy as it sounds, there’s a chance that humans were outside the heliopause two million years ago. If H. erectus can get to interstellar space, why can’t we? :) There is also a prediction that the inner Oort cloud should be condensed into spiral arms – if true, that means that the “stepping stones” to the nearest stars may form a more concentrated grouping than we thought.
That is very interesting. From the paper: “The encounter and related increased radiation from Galactic cosmic rays might have had a substantial impact on the Earth’s system and climate.”
I wonder what impact this had on Earth’s organisms? More or less extensive than during magnetic reversals? There was also a magnetic reversal around 2.5 mya. The appearance of human genus Homo 2.8 mya? No monoliths required.
Dismissing interstellar travel was once one of the staples of the mainstream SETI community. They did this not only to reinforce listening (and later looking) for signals from the stars, but also to distance themselves even further from the UFO fringe: After all, if aliens could get here in starships, why bother searching for transmissions from them?
Yes, electromagnetic signals are easier, faster, and cheaper than starships for crossing the galaxy, at least the kinds we can conceive that are not purely fantasy ala Star Trek and Star Wars. However, interstellar travel in vessels is neither impossible nor implausible, especially if you are not in a big hurry, which science fiction plots often have to be, to say nothing of impatient and short-lived humans.
It is a disservice to SETI and science in general to dismiss interstellar travel and smacks of those from antiquity (read a century or two ago) who said air flight was impossible because it was out the realm of their very limited experiences and comprehension.
I am reminded of a SETI lecture I attended at Harvard in 2000: The lecturers, all prominent scientists in the field, started off their talk by presenting an overloaded antimatter starship concept and explaining how difficult and expensive it would be to build and launch. They explored no other concepts, not even Orion which IS feasible by modern technology, and immediately went on to why radio was the best way for intelligent beings with technology to signal each other across the stars.
Thankfully, 25 years later, we have expanded our thinking in these fields and become less paranoid about once fringe concepts – a bit. We still have a long way to go.