The Fiery Phantom

Gravity is the force that binds our solar system together and keeps it in motion. Fully understanding it was one of the greatest aims of early modern astronomy. Measuring the positions of the planets, tracking their orbits and measuring their properties was a preoccupation for scientists for whom Newton’s law of gravity was still relatively new and Kepler’s laws (which dictate the orbits of planets) a novelty. Unexpected variations in the orbit of Uranus led in 1846 to the identification of Neptune - the first planet to be discovered since antiquity. When Mercury was also shown to be defying gravity, therefore, there seemed to be just one likely explanation: another planet must be present within its orbit, close in to the sun. French astronomer Urbain Le Verrier, who played a leading role in the discovery of Neptune, calculated the possible properties of the new world in 1859 and even gave it a name: Vulcan.

Over the next half-century, astronomers peered hard towards the Sun, studying the horizon just before or after dawn, looking for tiny specks of darkness transiting the solar disk, and trying to make the most of the few minutes of totality during rare solar eclipses. They met with mixed success, some sighting the tentative planet and others failing to do so. As time passed, the certainty grew that no planet capable of explaining Mercury’s orbit could have been missed so often and by so many. By the first decade of the 1900s, a series of photographic surveys during total eclipses rules out any plausible case, and the trickle of academic publications on the topic of intra-Mercurial planets dried up [1]. The theoretical motivation for Vulcan was soon undermined by a new theory, and the supposed planet vanished into obscurity.

None of this prevented Vulcan from making frequent appearances in science fiction of the pulp era.

Vulcan - the undiscovered world

The genre of planetary romance saw (mostly male) human adventurers set out from Earth to explore a range of planetary surfaces, most implausibly habitable (even insofar as was known at the time) and many hosting primitive tribes and surprisingly attractive young women. Stories were set on every available surface (solid or otherwise) in the Solar System, and it is small wonder that Vulcan was amongst them.

A prolific author in this tradition was Ray Cumming, whose short story The Flame Breathers appeared in Planet Stories in March 1943. This described a small, three person expedition which sets out to explore Vulcan after a previous mission failed to return. They find that “Vulcan was a world of some eight hundred miles diameter, with an orbit approximately eighteen million miles from the Sun” (i.e. half way between the Sun and Mercury, and a quarter of its size). This Vulcan is tidally-locked, always showing one face to the Sun, as expected for an object so close, and as both Mercury and Venus were believed likely to be at the time, and so has a hot side and a dark side. However despite its tiny size, this world is described as a metal desert, with an extremely high density which gives it both a dense atmosphere and an Earth-like gravity.

These conditions also permit the existence of intelligent (although not technologically advanced) natives. These exist in at least two tribes - a savage, ape-like tribe who captures the explorers and an apparently more peaceful group represented in the story only by an attractive young woman, captured by the savages and their flame-animal pets. These indigenous people use a mineral, rare on Earth but common on Vulcan, called allurite for tools and weapons. And this allurite lies at the root of conflict amongst the human crew.

A similar tale of human greed and exploitation of native resources can be found in Hell Planet, by Leslie F Stone (Wonder Stories, 1932, recently reprinted in Mike Ashley’s anthology Born of the Sun). Again this involves a second expedition following up after a disastrous first. In this case, the first mission fell prey to acute radiation sickness after consuming Vulcan’s radium-riddled food and drink. This is a world of rich, abundant forests of vegetation, much of it resembling Earth’s desert dwellers:

“It’s the sun,” explained Wendell. “Were the leaves of the trees broad they would absorb too much vitality beside that already partaken of from the radioactive soil, hence they would shrivel and die under the glare of the white-hot sun.”

The second mission follows to this dangerous world largely because the planet is an abundance source of cosmicite, known to the locals as dasie, a rare mineral essential for protecting humans from the hard radiation of space, and thus a bottleneck in the colonisation of the Solar System. Like Cummings’ investigators, the crew of Stone’s explorer ship find a complex tribal culture, and use it to try to get access to the world’s natural resources. In this case, the approach to the tribe is a little less aggressive, but still exploitative: the humans pose as gods and demand ever more of the metal. However the crew pays dearly for its greed, and it is not hard to see that others will follow in their tracks.

A more cerebral (if equally fantastical) view of Vulcan is found in Child of the Sun by Leigh Brackett (Planet Stories, Spring 1942). This story describes a future world in which Earth and inner planets are ruled by a tyrannical dictatorship under Gantry Hilton. The handful of people who dare dissent (known as Unregenerates) are driven off-world and forced to scrape an existence in the dark and resource-scarce outer solar system. In the story, the dissident leader called Eric Falken and two Unregenerates he’s trying to sneak away from Earth are pursued and forced in towards the Sun, until their spaceship stumbles across the hitherto-undiscovered Vulcan.

Vulcan was smaller even than Mercury. There was no atmosphere. Peaks like splinters of black glass bristled upward, revolving slowly in the Sun's tremendous blaze.

Exploring the surface they find the glass-like material ubiquitous and this is used to explain Vulcan’s previous non-detections:

“Whatever it is," said Avery, "it sucks up light. That's why it's never been seen. Only little glimmers seep through, too feeble for telescopes even on Mercury to pick up against the Sun. Its mass is too tiny for its transits to be visible, and it doesn't reflect.”

A shaft takes them into the hollow centre of the planet, where they find a bewildering array of creatures and apparent structures. However this proves to be the fantasy landscape created for its own entertainment by a juvenile but powerful entity which describes itself as a child of the Sun. A splinter of light and energy, “living on the conversion of our own atoms”, it built a shell around itself to stop its energy dissipating, hence creating Vulcan. It is capable of creating and dissipating matter, including structures and living beings, as it wishes.

The capricious creature is persuaded to create a human-habitable environment within Vulcan, but then threatens to destroy its new playthings. Falken must use all his persuasion to show the sun-child the error of its ways. Eventually it sheds its shell and departs, leaving Falken with a new underground world, close to the warmth of the Sun, to house his Unregenerate people.

A hollow Vulcan seemed to be something of a feature of the times. In Outlaw World, one of his series of novels featuring scientist-adventurer Captain Future, Edward Hamilton made use of a similar idea. In the novel, the solar system is being ravaged by a band of space pirates stealing radium. After several encounters, Captain Future, a.k.a. Curt Newton, and his team track the villains in towards

Vulcan, the little world that circled the Sim just outside the burning corona, was never reckoned as one of the System’s worlds. For it lay in such terrific solar heat that no one had yet been able to visit it.

In fact, after a perilous journey which threatens to overload their “anti-heaters” the Futuremen, in their ship the Comet, discover a reason for the low specific gravity of this little world:

The inner world of Vulcan was like the interior of a hollow ball, hundreds of miles across. It was illuminated to greenish daylight by the dazzling brilliance of the great Beam that struck straight across it.

The whole concave inner surface was blanketed by dense, pale green jungles. At one point glimmered the waters of a large yellow lake, into which ran several small, glittering rivers. Wispy clouds floating in the inner vault, and the whistle of air outside the falling Comet, told them that this hollow world had an atmosphere.

Like Leigh Brackett’s world, the crust of this Vulcan provides a protective shell that keeps its interior relatively cool, while also supplying it with energy. In fact the pirate’s radium thefts have had the goal of turning this habitable world into a controllable spacecraft - a goal with potentially dire consequences for the whole system. It’s up to Captain Future to foil this dastardly plot (and incidentally rescue the young woman he loves).

Vulcan as mine and prison

A recurring feature in the planetary romances is the idea that planets close to the Sun, including Mercury and Vulcan must be rich in heavy metals. Allurite in The Flame Breathers and Cosmicite in Hell Planet helped motivate the discovery missions in those stories. In other narratives, we see a later stage where the planet is exploited through mining, often by prisoners.

In Vulcan’s Workshop by Harl Vincent (appearing in Astounding, June 1932) commercial mining and prison colonies have been established on the terminator of a tidally-locked Vulcan which was rediscovered (after being ridiculed by astronomers) when spaceships became common. This tiny planet has a core of neutronium and so the planet has a surface gravity six times that of Earth and humans must wear gravity-insulating clothes or stand on insulating flooring. Amongst the commercial mining sites, which have a maximum two month term of duty for their staff, is Vulcan’s Workshop, a prison for the worst offenders in the Solar System.

The protagonist is a convict sentenced on Mars to radium mining on Vulcan. This amounts to a death sentence in a matter of months, due to the effects of the gravity and radiation sickness. The regime is harsh, with gravity itself used as a means of control by the guards, since the prisoner’s nullifiers can be selectively deactivated at will. Perhaps understandably, the protagonist becomes involved in an escape attempt, and eventually learns that the prison regime itself is under investigation by the authorities on Earth. 

An interesting aside in this story is the discussion of libration. This is wobble caused by a tilt in the axis of a world, combined with any ellipticity in its orbit, which results in the terminator shifting backwards and forwards. For example, while the Earth’s Moon always turns one face towards us, its libration permits us to observe 59% of its surface area rather than 50%. In the context of Vulcan’s Workshop this results in a variable climate on the surface at Vulcan’s terminator, with temperature variations causing mists and other problems.

Another story which features a metal-rich Vulcan being mined is Alcatraz of the Starways by Albert diPina and Henry Hass (appearing in Planet Stories, May 1943). This imagines a future in which Earth has dominated the native races of the other worlds, and runs a prison colony in the swamps of Venus. Rescued from there by what seem to be allies, a deep-cover Earth agent is taken along with other escaping prisoners to the hitherto-unexplored world of Vulcan.

Here they are imprisoned by their apparent rescuers - a collaboration of pirates and the oppressed peoples of Venus and Mars, led by an embittered former Earthman whose wife had been killed decades before by the Earth corporations when he challenged their monopolies in the outer Solar System. The agent is amongst a number of prisoners sent from the pirate base on the dark side of tidally-locked Vulcan to mine the twilight strip for “a new allotropic form of beryllium” [2]. This is essential for construction of the rebels’ new super spacecraft - and these are essential in turn for the freedom fighters on the other worlds of the Solar System to reach their goal.

The human settlement on this Vulcan is confined to a large grotto on the dark side, but it is clear that this Vulcan, while hot, barren and dense, hosts intelligent life. This life is metallic, but nonetheless organised. Indeed, the pirates appease the primitive natives with gifts of the metal they consume. It is clear, however, that the Vulcs interest in the humans is limited.

Vulcan Enthusiast

For some authors, Vulcan seemed to have an abiding fascination. Science fiction author Ross Rocklynne wrote a number of short stories featuring the planet.

At the Centre of Gravity first appeared in Astounding SF in June 1936. In the story an interplanetary police agent, Lieutenant Jack Colbie, pursues a Martian canal pirate, Deverel, into the inner Solar System. Skirting dangerously close to the Sun, they both land on the known but hitherto unexplored, tidally-locked planet Vulcan. Unfortunately both pursued and pursuer fall down a large hole which seems to have no end:

“About seven hours down,” he continued, “I began to suspect the truth — that Vulcan is as hollow as a bubble, probably is one, the result of a huge, internal explosion, just before it cooled, ages ago. Some other explosion pushed a hole through the crust.

“At first I thought I’d stop when even with the inner surface. Second thought showed otherwise. If the planet was actually hollow. I’d drop to the center”

This is, in fact, what happens - both men come to rest at the exact gravitational centre of the hollow planet, with only a thin gas atmosphere filling the space around them. While their space suits will sustain them for some time - particularly in deep hibernation sleep - they do not have the resources to reach the surface. Certainly any form of reaction (i.e. throwing or pushing) available to them is insufficient. 

“Consider. Vulcan is eight hundred and ninety miles in diameter, and hollow. Probably the crust is a hundred miles in thickness — a thinner one would crack up under the attraction of the Sun. That would give us three hundred and forty-five miles to travel by reaction — to the inner surface.”

While I won’t give away the solution to this puzzle story, it cunningly involves Vulcan’s slightly elliptical orbit and is based on solid physics. What’s a little less obviously convincing is the explanation for how a hollow planet formed, and the way the two men settled smoothly to the centre of the void. Anyone falling in towards the centre would pick up speed as they fell. When they reached the centre, there would be no further force acting on them, but they’d still be moving and would overshoot and rise back towards the opposite side. They would eventually settle to the centre through friction against any gas present, but not until several oscillations had passed [3].

Another of Rocklynne’s stories, written a full decade later, returned to Vulcan with another police/criminal pair (who are effectively Colbie and Deverel by another name). In The Bottled Men (Astounding, June 1946), Second Lieutenant Marc Sturm pursues maverick Gull Norse and attempts to capture him using a magnetic field trap. An escape attempt by Norse results in the ship being wrecked and crash landing on the known-but-neglected planet Vulcan. As we’re told,

“It keeps one face to the Sun, but it wobbles sharply back and forth like a perfect pendulum. That creates a ‘twilight’ zone about a hundred miles wide, where it’s hot sometimes, cold others. No atmosphere, but we can land and repair the ship.” 

The dense planet has a surface gravity of 0.2g despite its tiny size. However when the men fall into a pool of mercury (the liquid quicksilver rather than the planet!) and become trapped in a cave, that libration (the wobble in the twilight zone) again becomes a crucial factor in their escape [4].

Ross Rocklynne also returned to Vulcan with a story that posed a very different kind of puzzle. Appearing in Startling Stories in December 1943, Beyond the Boiling Zone followed the crew of the spaceship Venture, who believe they are being driven to their deaths in the Sun. However the astronomer who commissioned the mission, Captain Trask, has calculated that they will land on the planet Vulcan - as indeed they do (rather forcefully). Puzzlingly, Trask asserts that it has a seven year orbit, rather than the twenty days that would be expected in its location. Even more strangely, this world is entirely habitable with a breathable atmosphere, and even cities:

“A planet, green with lush vegetation, rolling in hills and mountains met his eyes. It was a scarred planet for all that, however. There were deep rifts and gorges to be seen on second glance. The hills were shattered mounds of churned up rock and earth. Great blocks of stone lay canted this way and that, as if they had been tossed through the air by giants.”

These facts are not unconnected. The solar irradiation that would turn Vulcan into a furnace over the course of twenty days is instead enough to support a temperate climate when distributed across more than two and a half thousand. The explanation for this oddity leans into the idea that the mass of the sun is sufficient to distort space:

“What has actually happened to Vulcan is that the sheer brutal power of the Sun’s gravity has sheared one of the ‘normal’ dimensions from' the planet, and substituted—a fourth dimension”

The escape of the damaged ship, its crew and the earlier explorer they rescue depends on the brief few minutes in each orbit when Vulcan is far enough from the Sun to shift back into our own three dimensions - although this dimensional shift has violent consequences which explain the scattered blocks the explorers saw on their arrival.

Moving On

However Ross Rocklynne and the other writers of the 1940s were searching for a world which had already vanished from the Solar System as known to both science and science fiction. By 1956, the choice of Vulcan as a location was enough of a science fiction stereotype to be parodied in I, Claude by Charles Beaumont and Chad Oliver (F&SF, Feb 1956, where it is cast as a counterweight planet rather than intra-mercurial [5]). Curiously, amongst the later examples of science fiction to feature Vulcan as an inner planet were narratives aimed at juvenile audiences.

A relatively early example of this genre is The Flat Folk of Vulcan by Denis Clive (aka John Russell Fearn). This was published in Future Fiction (Nov, 1940) and is very much in the tradition of SF aimed at young audiences, with a teenage protagonist accompanying an older relative into space. One of a series of stories featuring Professor Jeffords and his nephew Paul on the spaceship Hope exploring the solar system, the story has the pair discover Vulcan when falling toward the Sun after visiting Venus. The tidally-locked world is only 20 miles across but sufficiently dense to have a surface gravity an eighth of Earth’s; as we’re told, a handful would weigh a ton or more and that its surface is harder than a diamond probe. The Professor and Paul find intelligent two dimensional beings who perceive the humans only by the sole of their feet. However, while this tiny world could plausibly have been missed by astronomers, the reason for its density, or the evolution of its flat folk are never explored. 

Vulcan was mentioned as a part of the Solar System in the marionette puppet television series Space Patrol (1963), written and produced by Roberta Leigh. Known as Planet Patrol in the United States, this followed the crew of a space craft (or Galasphere) crewed by Larry Dart, Slim and Husky, from Earth, Venus and Mars respectively. The governments of the Solar System worlds are autonomous but recognise a common overall organisation. In the episode “The Telepathic Robot”, an astronomer at the Martian Observatory spots a planet close in to the Sun. The planet is not named, but the Galasphere’s mission to investigate is complicated by a colony of malevolent Neptunians and a robot which becomes confused by telepathic signals from a feverish commander. 

Space Patrol as a whole, while having some interesting science-driven plots, represents a vision of Solar System habitability that was deeply outdated by the time it was broadcast, with surfaces more akin to speculations at the turn of the twentieth century. It remains an entertaining romp around our neighbouring worlds.

Hugh Walters also mentioned Vulcan in his Chris Godfrey of UNEXA series of novels aimed at teenage boys. Mission to Mercury (novel, 1965) describes one of the young crew’s frequent exploration missions to different locations in the Solar System. On their return from the titular planet, they are forced to take off in a hurry and set off in a random direction. Before recovering contact with Earth, they come across a planet speeding by close to the Earth, and realise that they have located the long-dismissed Vulcan. They do not land on the planet, but are nonetheless able to claim the discovery when returning home.

As I’ve observed before, children’s science fiction often appears to lag behind the development of astronomical knowledge. The reasons for this are unclear. However, while most science fiction authors are careful fact checkers, regardless of audience, it’s possible that the lower budgets and perceived lack of critical review associated with juvenile fiction lead some writers to repeat the state of the field as they learned it decades earlier, rather than research the cutting edge of knowledge.

A Phantom in the Data

The persistent failure of astronomers to locate Vulcan during the early decades of the twentieth century is reflected in the characteristics ascribed to it by science fiction writers. Observations required that it had to be extremely small, but nonetheless massive in order to have a measurable effect on Mercury. Hence writers such as Cumming and Vincent wrote of extraordinary densities and the rare metals (whether allurite, cosmicite, radium or other) which so often required prisoners to mine (although some writers of Hollow Vulcans, such as Edward Hamilton, appear to have missed this point).

The recognition of so many authors that Vulcan (as a small mass so close to the Sun) would inevitably be tidally locked also represents an engagement of authors, even during the planetary romance era, with scientific plausibility. This is also reflected in the recurrent appearance of libration (the slight wavering back and forth of the day/night terminator due to orbit and spin misalignments) in the solution of Vulcan-based puzzle stories.

However Vulcan vanished from the scientific literature not just because people had looked and failed to find it, but because a new paradigm for our Universe removed the need for it. Under Newton’s Laws, the perturbations in Mercury’s orbit, the precession of its perihelion, required Vulcan’s existence. But Einstein’s General Relativity, proposed in 1915, and tested and popularised by observations of light distortion during the total solar eclipse of 1919, provided an alternate explanation. Einstein recognised that the Sun is massive enough to distort the space around it - all the way out to the orbit of Mercury. This distortion in space affects the distance the planet travels and hence the precession of its perihelion to precisely the extent required to explain the observations of Le Verrier and others.

In this regard, Ross Rocklynne’s Beyond the Boiling Zone is an interesting story. It clings to the romantic prospect of a large and habitable Vulcan, while simultaneously invoking the very Einsteinian distortion of space by mass to finesse the obvious scientific objections to any such world.

Science fiction as a genre can as often indulge in nostalgia as excessive extrapolation. By the end of the 1940s, almost thirty years after the astronomical community, science fiction writers (outside of children’s writers) were forced to accept the planet’s non-existence, but this doesn’t stop the name continuing to appear. 

In Clifford D Simak’s 1951 novel Empire, the Vulcan Space Fleet is a group of prison ships in orbit in the harsh conditions close to the sun. Television series Doctor Who used Vulcan as the name of a planet in its 1966 serial “Power of the Daleks”, and as late as 1983, Poul Anderson’s short story Vulcan’s Forge described an AI-driven probe ship attempting to reach a tiny molten asteroid dubbed Vulcan in a near-Sun orbit [6]. Vulcan also appeared as a name for artificial intelligences in Philip K Dick’s novel Vulcan’s Hammer (1960), an industrialist in the first episode of SF-adjacent spy-fi television series The Man from UNCLE (episode “The Vulcan Affair”, 1964), a large number of spaceships, and a company or organisation in a range of other fictions.

However, by far the most famous Vulcan was introduced in 1966 with the television series Star Trek. Mr Spock’s home planet, Vulcan, was hot and arid, but relocated from the inner Solar System to the safely distant (and slightly hotter than the Sun) star 40 Eridani. This reshaped the name in the popular imaginary and ended forever its association with intra-mercurial planets.

Vulcan has never ceased to fascinate. Science fiction writer and science populariser Isaac Asimov discussed the world in his May 1975 essay The Planet That Wasn’t, later reprinted as the title essay in a collection of non-fiction writing. More recent histories of the planet’s brief scientific existence and fall from grace include The Hunt for Vulcan by Thomas Levenson (non-fiction, 2015). Although many have commented on the time it took for observational astronomers to accept the non-existence of Le Verrier’s world, recent work has framed the Vulcan affair as a prominent example of the scientific method. An observation (the precession of the perihelion of Mercury) in the context of a model for the Universe (Newton’s gravity) led to a prediction (the existence of Vulcan). Tests (further observations) proved the prediction wrong. Rather than clinging to a broken model, scientists developed a new picture of the Universe (Einstein’s gravity) which could explain everything that had seen before and the new observations as well. That theory made further predictions (the orbit decay of binary pulsars, gravitational waves, black holes) which were confirmed by further tests (observations). Thus it stands as our current (tested, quantified and robust) theory of the Universe until future observations lead to the development of a better model (which explains everything it does and more). Testing to destruction, revision and improvement is central to western science.

Constant change and innovation is also central to science fiction - although subject to longer delays than those in the scientific literature. The stories of Vulcan provide examples both of how science fiction audiences cling to attractive ideas, and of how the scientific literacy of writers and audiences brings the genre (albeit reluctantly) into line.

Vulcan, like the hypothesised planet which gave rise to the asteroids, never existed - despite numerous works from professional astronomers claiming confidently that it must. It is a phantom in the history of astronomy, and so in the history of science fiction. It tested the ingenuity of both astronomers and science fiction writers, and it opened the eyes of both groups to new possibilities, and new understandings.