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.