The Moon is an imposing and endlessly fascinating presence in Earth’s skies. Perhaps less familiar to most modern readers is the idea that Earth may have, or once have had, a less well-known and more elusive second moon. While the mid-twentieth century marked the heyday of science fiction featuring Earth’s second satellite, the idea continues to intrigue authors well into recent times.

In the years before spaceflight breached the atmosphere, and in particular as astronomical discoveries poured in during the late nineteenth and early twentieth century, the idea of another world within our reach had an undeniable allure. As early as 1869, Jules Verne’s novel Around the Moon (originally Autour la Lune) introduced an incident in which his pioneering lunar explorers (launched from Florida in From the Earth to the Moon, 1865) encounter a large body not long after leaving Earth’s atmosphere, barely missing a collision with it. Verne allows his characters to voice both his explanation of this unnamed body, and the objections to it:

"It is a simple bolide, but one of such enormous dimensions that the Earth's attraction has made it a satellite."

"What!" cried Ardan, "another satellite besides the Moon? I hope there are no more of them!"

"They are pretty numerous," replied Barbican; "but they are so small and they move with such enormous velocity that they are very seldom seen. Petit, the Director of the Observatory of Toulouse, who these last years has devoted much time and care to the observation of bolides, has calculated that the very one we have just encountered moves with such astonishing swiftness that it accomplishes its revolution around the Earth in about 3 hours and 20 minutes!"

"Whew!" whistled Ardan, "where should we be now if it had struck us!"

"You don't mean to say, Barbican," observed M. Nicholl, "that Petit has seen this very one?"

"So it appears," replied Barbican.

"And do all astronomers admit its existence?" asked the Captain.

"Well, some of them have their doubts," replied Barbican
- All Around the Moon, translated Edward Roth, as on Gutenberg.org.

The hazard safely negotiated (more by luck than judgement) the expedition moves on. As Verne informs his readers, the idea that Earth-orbiting asteroids might have been captured by our planet’s gravity and formed stable satellites was under active investigation, and under debate, in the academic literature of the time. Frederic Petit, who was indeed the Director of Toulouse Observatory, had claimed the discovery of an asteroid in an elliptical orbit in 1846 - although this claim was quickly disputed, not least because the orbit’s perigee lay well within the atmosphere, rendering it extremely unstable.

Other astronomers in the late 1800s and early 1900s made similar claims - largely based on chance observations of moving objects which were pieced together into estimated orbits. None stood the test of repeatable and independent observations, and all were small (asteroidal) bodies Perhaps the most interesting and plausible claim came in the wake of a “Great Meteor Procession” observed over the continental United States and Canada in 1913. Analysis of the recorded observations suggested that they may have originated from an asteroid drawn into a decaying Earth orbit, and then fragmenting as it entered the atmosphere. Such a body would likely have been a natural satellite of Earth for long enough for the orbit to become nearly circular and decay, but was observed only in its demise.

Moving forward into the era of the pulp science fiction magazines, the possibility that a second moon might still have escaped observation began to recapture the imagination of science fiction writers and readers. Early examples were located squarely in the planetary romance genre, in which strapping young men endured desperate adventures on Earth-like worlds whose resemblance to any true astronomical body was already known to be remote.

Perhaps typifying these is Edmund Hamilton’s The Second Satellite. The contents page of Astounding Stories of Super Science for August 1930 summarises the work with the rather remarkable sentence: “Earth-men War on Frog-vampires for the Emancipation of the Human Cows of Earth's Second Satellite. (A Novelet.)”. In fairness, this is something of an oversimplification. The story starts with a fairly firm grounding: some pilots trying for altitude records (a popular trend of the time) go missing, and two friends of one of the missing men calculate that the disappearances correspond to the locus in time and space of a low-Earth orbit. Hypothesising the existence of some body - orbiting too fast to be seen from the ground in the upper reaches of the atmosphere - they launch on an intercept flight, and find themselves crashing on an Earth-like world. Thence they do indeed find frog-people who feed on blood tapped from green men, and lead the latter in rebellion.

Nothing in the science of the story is particularly good - there’s no reason, for example, that low gravity would enable Earthmen to bend metal bars (as they do), and being large enough to hold a small atmosphere of its own wouldn’t (as claimed) prevent friction from Earth’s atmosphere heating and fragmenting the world. Nor could any such body pass without leaving a broadly visible wake, miss observation from astronomers, or avoid exerting a gravitational influence on Earth itself. It’s nonetheless interesting that Hamilton considers these issues when positioning his planetary romance.

Such romances weren’t abandoned overnight, and remained popular escapism even as the readership became more scientifically educated and the constraints became tighter. A master of the such classical pulp science fiction romances was Ray Cummings, and an example of his work is Onslaught of the Druid Girls (published in Fantastic Adventures, June 1941). The story begins with the assertion that

“For days the nation laughed at the poor old man in Wisconsin who claimed to have discovered a new moon, a second satellite of Earth, out beyond the orbit of Luna, some 440,000 miles away. No other astronomers would support the theory. None had observed it. Not even Grandfather had seen it. But he could prove its existence to his own satisfaction. He had called it Zonara.”

Taking a one-man rocket, the Wisconsonite’s grandson crash lands on the new satellite, finding it fully habitable, rescuing a beautiful druid girl, and fighting evil foes in true planetary romance style. Despite the rather unscientific premise of the rest of the story, it’s worth noting that Zonara is unusual in lying beyond the Moon (as seen from Earth), rather than closer to hand, and also in noting that the inference of a planet or satellite might remain robust even if its physical presence has never been observed [1].

By the start of the 1940s though, science fiction was already looking to place stories of the second moon on a more robustly scientific basis. A detailed popular science article on the question, written by well-known science communicator Willy Ley, appeared in Astounding magazine in October 1939, and similar factual articles were printed in other magazines into the 60s and 70s. Stories from this point onwards generally had to be more creative in explaining how their second moons had escaped previous observation.

The Second Moon by R R Winterbotham appeared in Marvel magazine in February 1939. Rather than launching rapidly into romance, the story starts by describing an American scientist with a mathematical theory which explains all the Solar System bodies - except for predicting an Earth-sized second satellite for our planet. As becomes clear, this second moon, known to its inhabitants as Irtl, had been displaced into hyperspace. When the people of that world try to invade, they need the human physicist who originated the theory to solve a key equation for them - he does so, defeating the invasion with a rather bizarre mathematical trick. The story itself is indifferently written and the mathematics (and astrophysics) here is problematic, but the narrative is interesting in that it pitches astronomical discovery in (the then-distant future of) 1976 as a competition based on arrogance and national pride between Europe and America, name checking the (real-world) observatories at Harvard and Mount Palomar in the process. 

Another creative approach to this theme can be found in our second narrative named The Second Satellite, this time by Martin Pearson (aka Donald A. Wollheim) and appearing in Future magazine in 1943. In this story, an eccentric millionaire’s will leaves money to the first man to set foot on another world. A Michigan garage mechanic and amateur astronomer, Sanders Mikkelsen, suggests that an asteroid, captured into temporary inspiral orbit around Earth, would qualify. He locates a 10 mile wide body in a temporary orbit that’s about to enter the atmosphere, breaking up and become a meteor shower (reminiscent of the 1913 Procession). Persuading a businessman to back him in his bid for the fortune, Mikkelsen launches and - in a slightly less plausible turn of events - he finds traces of an ancient civilisation disintegrating around him as the asteroid plunges into the atmosphere and turns into a fireball. There is another slightly odd twist in the (undoubtedly World War II-inspired) ending when the falling moonlet wipes out Japan during its war with China, leading to general rejoicing.

Another first-man-in-space adventure, very much in the tradition of both Second Satellite tales, is Shipwreck in the Sky by Eando Binder (Fantastic Universe, 1954). Here military-trained pilot Dan Barstow is launched as the first man to travel into space. His mission takes a turn for the worse:

"Hello, Rough Rock.... Listen ... nobody expected this ... hold your hat, sir, and sit down. I've discovered a second moon of Earth!... Uh huh, you heard me right! a second moon! Tie that, will you?... Sure, it's tiny, less than a mile in diameter I'd say. Dead black in color. Guess that's why telescopes never spotted it. Tiny and black, blends into the black backdrop of space. It has terrific speed. And that little maverick's gravitational field caught my rocket.... Of course it can't yank me away from Earth gravity, but the trouble is—yipe! my rocket and that moonlet may be in for a mutual collision course...."

After crashing and being given up for lost by his military commander, Barstow gets a quick reminder of Newton’s Second Law and figures out a way to escape the tiny moon before parachuting to Earth.

Carrying the same optimistic air into the realm of children’s science fiction is the novel Tom Swift on the Phantom Satellite (Victor Appleton II, 1956). This series frequently describes artificial human-built satellites as little moons. However in this story, a new moonlet thirty miles in diameter comes zooming through space, apparently on a crash course with Earth, before settling into an orbit 50,000 miles from Earth (under the influence of benign aliens) and becoming tidally locked. Already equipped with the world’s best telescope and an anti-gravity drive from earlier adventures, the eponymous teenage inventor promptly comes up with an atmosphere-making machine and sets off to render Earth’s new moonlet habitable (and, perhaps more importantly, stop it being claimed by a hostile nation). It's interesting to note that this is one of few such stories to actually seriously consider the consequences of such a moon's gravitational forces:

“Certainly it could be dangerous!” exclaimed the professor. “Tremendous forces may be unleashed - these high tides are only a sample! Even worse, a slight miscalculation could lead to a collision between the object and earth!”

The same world, renamed from Little Luna to Nestria after Swift’s mother, reappears in Tom Swift and the Asteroid Pirates (1969), where it has become a habitable base for both the Swifts and the aforementioned menace.

In all of the stories mentioned above, explanations are provided for why the second moon has never been seen: it may be tiny, may not exist in our space time, may be newly brought into orbit, or may exist only for a few brief weeks or months before orbital decay.

Another children’s novel takes verisimilitude a step further. Second Satellite (yes, a third one!) by Robert S Richardson was published in 1956. It follows young Cyril Westcott, known as Cy, who is the son of a prominent astronomer. Initially unenthused by the prospect of spending a summer holiday at a mountain observatory, he is gradually drawn into the thrill of discovery - especially when he discovers that his father is hunting for an extremely faint second moonlet, and learns to help out with the search himself:

“Cy could hardly wait while the film was developing. But when the satellite turned up in the very center of the field it seemed like magic. It was magic. For millions of years this little body had been whirling around the Earth out there in space. Nobody could see it. Nobody had the slightest idea it was there. Then you went through a certain ritual with a telescope. You took a photograph of a particular region of the sky. And when the photograph was developed there was the body exposed to view.” (pg 169)

The novel is science fiction only in the sense that it describes scientific discoveries that hadn’t been made at the time (or indeed since [2]). Really though, it’s a careful description of life at an observatory site in the 1950s [3], and the hard work, observational procedures and steady series of photographic observations needed to first find a faint object and then establish its orbit. Richardson himself was a professional astronomer working at Mount Wilson (on which the Mount Hawthorne observatory of the story is closely modelled), and his knowledge of professional astronomical practice shines through. Although he also wrote science fiction under the pseudonym Philip Latham, this novel was published under his professional name - as were a number of factual science articles written for both the popular press and pulp science fiction magazines.

Rather stranger and marking the shift to the less optimistic adult science fiction of the 1950s is The Green Earth Forever by Christopher Monig. Here a second moon appears suddenly in the sky on exactly the same day that global nuclear war breaks out.

“The new satellite was approximately 263,000 miles from Earth, only a little farther away than the old moon. It seemed nearer because its diameter was 7,700 miles instead of the 2,160 miles of the moon. It had an orbital velocity of 18.5 miles per second; its mass was 0.996, as compared to Earth; its volume was 0.995 as compared to Earth; its density was 5.51; its albedo per cent was 50; and its mean daily motion was 0.985 per cent.”

Curiously, despite these measurements the newly-appeared satellite exerts no gravity, and soon becomes forgotten amidst the conflict. When Earth is broken into fragments by the nuclear war, it slips into the gap, and becomes gravitating, apparently under the influence of alien spacecraft and their technology. The remnants of old Earth settle into a planetary ring, and the rest of the solar system settles down after an initial wobble. The reason for all this remains unclear, with the reader left to draw their own inferences. Presumably a moon-stabilised orbit, in the habitable zone of a stable star and shielded by an outer gas giant, is real estate that is too precious for some alien race to let go to waste.

When Sputnik, the world’s first artificial satellite, was launched in 1957, it was widely hailed as a new, Russian moon, and this term - moon or moonlet for orbiting human spacecraft - was common for some years. However after the success of the American and Russian space programmes, as well as the continued development of telescope and imaging technology, it became very obvious that no asteroid of any significant size could be orbiting between the Earth and the Moon, let alone in the Earth’s upper atmosphere, without being noticed, and the frequency of second satellite stories rather died down.

A rare example from the 1970s is the novel Dhalgren by Samuel R Delaney [4]. This novel focuses on a mid-West American town that has been cut off from the rest of the world by some unspecified disaster and the experience of an unreliable and amnesiac narrator. It becomes clear that it has also been cut-off from reality as we know it. While the skies are usually shrouded in cloud, a giant red sun is sometimes seen… and so is a second moon which is promptly and irreverently dubbed George. Here the second satellite isn’t the focus of the story, but instead a way to challenge the reader’s assumptions and question the presumed realities that underlie the world we perceive. Indeed, two of the characters use it to discuss how their perceptions changed both after the 1969 Apollo 11 Moon landing and after seeing George:

“Do you remember the next time you were outside and you looked up and saw the moon in the sky instead of on television?”

He frowned.

“It was different, remember. I realised that for the last fifty thousand science-fiction novels it had still been just a light hanging up there. And now it was… a place.”

“I just figured somebody had taken a shit up there, and why weren’t they telling.” He stopped laughing. “But it was different; yeah.”

“Then tonight.” she looked a the featureless smoke. “Because there was another one, that you don’t know if anybody’s walked on, suddenly both of them were…”

“Just lights again.”

“Or…” she nodded. “Something else.” [Gollancz masterworks edition, pg 98]

Delaney himself has discussed how this reflects his own experience of the Apollo landings, and another of the characters in the story actually proves to have been an Apollo astronaut. In Dhalgren, then, both moons are symbols both of ways humans look out from their own everyday reality and how they probe inside it.

As Richardson’s novel (one of our Second Satellites) pointed out, photographic surveys for moving objects on the sky were already starting to appear in the late 1950s. With the advent of modern, large format detectors on survey telescopes, systematic mapping of the faint and moving sky has become possible. The realisation that the dinosaurs became extinct due to an asteroid impact, together with a growing understanding of the threat presented by such impacts more generally, led to investment in all sky surveys for asteroids funded by organisations such as the US military.

As a result, we now know of a number of asteroidal bodies in orbits which keep them close to Earth as it travels around the Sun - known as 1:1 mean motion resonance orbits. None are true natural satellites. However we now know of quasi-satellites (bodies which orbit the sun in one year and remain close to Earth for extended periods before migrating elsewhere), asteroids in horseshoe orbits (sun-orbiting orbits which travel part way around their resonant planet before apparently returning in their path due to ellipticity of their orbits and the relative motion of the two objects), Earth trojan asteroids (locked in gravitationally stable 1-year orbits around the Sun trailing or leading Earth), and even a few temporary satellites (observed to be orbiting Earth for short periods but in rapidly unbound or rapidly decaying orbits, similar to that described by Pearson above).

Amongst these bodies is (3753) Cruithne, named after a mythic Pictish character. Classified as an asteroid on its discovery in 1986, and its peculiar orbit was published in 1997. The Nature paper announcing this commented that: “although it is not a satellite of our planet per se, it is, apart from the Moon, the only known natural companion of the Earth.” The few-kilometre wide asteroid actually follows a horseshoe orbit, which takes Cruithne from well outside Earth’s orbital distance from the Sun to inside it, apparently returning to a near-Earth location about forty times more distant than the Moon’s orbit every 364.02 days. This object rose to some prominence when it was widely described as Earth’s “second moon”, and featured in that context on the popular trivia-based BBC television panel show QI in 2003. The series corrected itself, discussing other near-Earth objects, in a later episode.

However Cruithne also appears as a key location in Stephen Baxter’s novel, Time. In this first part of the Manifold series, the asteroid is identified in an encoded message from the future, and an innovative commercial mining expedition is diverted there. Crewed by a genetically-enhanced intelligent and self-aware squid, it is intended to send back valuable resources to Earth orbit. However the squid proves to be expecting young, an unsettling theory about the future of the Universe appears and a new breed of superhuman children emerges. As a result, the first human crew to reach “Earth’s second moon”, led by maverick entrepeneur Reid Malenfant, is both oddly composed and about to find a breed of sentient squid waiting for them - together with an artefact shaped in the far future from space and time itself.

Baxter does use the “second moon” term, but also explains Cruithne’s orbit and qualifies the statement. Indeed, his characters’ adventures give quite an accurate view (glowing portal across time and space excluded) of what might be expected on a carbonaceous chondrite in a near-Earth orbit of this kind:

“Where her blue-booted feet hit the regolith, with dreamy slowness, she kicked up a little coal-black asteroid dirt. It sailed into the air - no, just upward - for a few feet, before settling back, following perfect parabolas….

“The ground was coal-black, layered with dust, and very uneven, extensively folded. She could see maybe a hundred yards in any direction before the ground fell away, but the horizon was crumpled, as if she was standing on a hill-top.” (pg 276, Voyager paperback edition, 1999)

In the years since, the existence of small co-orbital asteroids, and even of occasional temporarily captured moons, has been well established. It has been equally well established that the Earth certainly does not have a larger companion, let alone one which may be habitable. Indeed the orbital dynamics of our system are now so well constrained by both observations and satellites that only very small debris can remain to be found. Perhaps naturally then, science fiction has moved onwards and outwards, focussing on other possible solar system environments, or back onto the more obviously fundamental targets of Luna and Mars.

Nonetheless, the decades of the second moons demonstrate the potential for discovery that was opened up by the dawn of rocket science, and the interest and excitement that potential engendered. An interesting aspect of this is that the discovery or investigation of the new worlds is often undertaken accidentally or unofficially by maverick lone operators - even Richardson’s Second Satellite, which focusses on a painstaking and legitimately authorised scientific investigation, is undertaken largely by two people in a small shed. The science - and particularly the space science - of the time was seen as accessible to all (at least in the developed west). The new frontier was a new opportunity for discovery which did not necessarily have to be associated with governments, major industries or huge projects. In many ways Baxter’s Time picked up on this idea, with his inventor protagonist Malenfant riding the New Space paradigm of interplanetary commercialisation towards the stars [5]. Harsh economic and technical realities, as well as ever-more-sensitivity to the safety of astronauts, have somewhat slowed the advancement expected by science fiction authors from Verne to Baxter. Although industrial concerns still cite space exploration as an objective, the majority of development is still government funded, much of it directed or contracted by national or military space organisations.

These stories of exploration and discovery captured a brief phase in the history of our understanding of our solar system, and one glimpse of a possible future. It was a fun dream while it lasted. We will most likely never find another true moon in Earth orbit. However humanity continues to explore - albeit via robot - moons throughout the solar system. And we are now searching for moons around planets orbiting entirely different stars. Other dreams will continue.