Vast interstellar clouds of gas and dust occur throughout the Universe. They may be stellar nurseries (the gathered fuel from which stars are born), or stellar graveyards - the remnants of material expelled in a star’s death throes. In the night sky they appear as regions of hazy light in the case of gaseous nebulae, or as regions of deep darkness such as the coalsack nebula - a large dust cloud that blocks our view of part of the Milky Way.

Their variety and complexity has attracted a range of interpretations in science fiction. At the simplest level they provide an interesting and visually impactful background setting for action to occur, particularly when one group needs to be hidden from another, with the Crab Nebula (a supernova remnant) in particular featuring as a setting for television SF stories ranging from Doctor Who to Battle of the Planets [1]. The blockbuster film Star Trek: The Wrath of Khan, for example, made memorable use of the fictional Mutara Nebula as the site of a three-dimensional and deadly hide-and-seek between the Enterprise and the titular villain, while numerous other stories in the Star Trek universe are set in and against nebulae of various kinds.

But interstellar clouds in SF also appear not just as the backdrop for interesting narratives but at their focus.

Clouds of Doom

A number of science fiction clouds over the years have presented a direct threat to life on Planet Earth. An early science fiction story in which an interstellar cloud (of a sort) features is The Poison Belt by Sir Arthur Conan Doyle (novel, 1913). This novel features Doyle’s recurring scientist character Professor Challenger and his friends, better known for their appearance in The Lost World (1912). In this narrative, the professor correctly predicts that Earth’s solar system is drifting into a toxic region of “the ether”, which causes confusion, euphoria, coma and death - with potentially lethal consequences for all animal life on the planet. By the time this novel was written, the existence of a hypothesised ether (or luminiferous aether) had been abandoned by physicists as a result of unsuccessful attempts (such as the Michelson-Morley experiment) to detect it. However it was still in common use in popular culture, with an approximate meaning which we would now equate to the structure of space-time, but commonly described in the form of clouds of one kind or another.

The Poison Belt articulates a theme common to many stories involving interstellar nebulae: the insignificance in scale of humanity and Earth when compared to the wider Universe. Challenger equates the imminent elimination of humanity to the bacilli washed off grapes by a gardener, while the story concludes with a consideration of the same question:

“It is from that salutory but terrible ordeal that we have just emerged, with minds which are still stunned by the suddenness of the blow and with spirits which are chastened by the realisation of our own limitations and impotence.”

Doyle’s poisonous ether actually proves to be a narrow belt, which the Earth passes through more rapidly and with less catastrophic effects than originally feared, but the novel is an early forerunner of the kind of post-apocalyptic novels that involved similar scenarios in the later twentieth century.

An example of these can be found in the novel The Year of the Cloud by Theodore L. Thomas and Kate Wilhelm (1970). Here the passage of Earth through a nebulous cloud causes initial anxiety, but a rapid scientific analysis suggests there’s little cause for concern:

“By and large the Cloud material is a familiar material. Seems to be particles of a polymer, a big one, molecular weight on the order of ten million. I think I see ethylene linkages in the infrared, and I know I see ether groups. The structure seems to fit a polymeric ethylene oxide, but…” He stopped.

“What’s the matter?”

“Well, there’s an occasional group along the chain that I can’t identify.”

Such compounds would be far larger and more complex than any that has actually been detected in space, and vulnerable to disruption by ultraviolet radiation. However, based on this analysis, the cloud is expected to have no effect except a few impressive sunsets caused by high-altitude dust - indeed a senior astronomer arrogantly appends his own name to the discovery, dubbing it the Yudkin Cloud (but not until initial tests has suggested it was benign). However the cloud proves to have dropped the polymer compound into the upper atmosphere in large quantities, and the unidentified chemical group proves crucial. This reacts with water to create a jelly, which leads to mass die off of sealife, then livestock and crops, as well as rendering water undrinkable.

While the science in this story is generally fairly well presented (as might be expected given Thomas’s earlier Weather Congress stories), the focus is on the individual characters and their experiences and relationships. The resulting narrative extends through a calendar year and features strong similarities to other mid-twentieth century apocalyptic fiction including The Death of Grass (John Christopher, novel 1956), The Tide Went Out (Charles Eric Maine, novel 1958) and The Day the Earth Caught Fire (film 1961, dir. Guest). In each, western civilisation gradually collapses under increasing food and water shortages. It’s notable, however, that the author and characters in The Year of the Cloud are American, unlike many of the contemporary “cosy catastrophes” which sprang instead from the cultural milieu of post-War Britain. Unfortunately, the end of the novel shies away from the grimness of some of those examples, and comes across as a bit of an anticlimax in what is otherwise an interesting narrative.

A less physical but, if anything, more insidious threat to Earth can be seen in Kim Stanley Robinson’s disturbing short story “Before I Wake”. As gradually becomes clear, this is a world in the grip of a catastrophe, with a narrative viewpoint that seems to shift and twist unpredictably. It’s not until relatively late in the story that a character explains:

“We move through space filled with dust and gas and fields of force. Now all the constants have changed. The read-outs from the space station show that, show signs of a strong electromagnetic field we’ve apparently moved into. More dust, cosmic rays, gravitational flux. Perhaps it’s the shockwave of a supernova, something nearby that we’re just seeing now. Anyone looked up into the sky lately? Anyway. Something. The altered field has thrown the electrical patterns of our brains into something like what we call the REM state. Our brains rebel and struggle towards consciousness as much as they can, but this field forces them back. So we oscillate.”

This premise is inverted - the presence of an inimical electromagnetic field leading to a broadly positive rather than catastrophic narrative for the population of Earth - in the novel Brain Wave by Poul Anderson (1956). Here we see a similar kind of mental influence, resulting in an impedence in the electronic transmission speeds of neurons impairing human mental faculties (and, indeed, those of animals too). However the plot in this case revolves around the premise that such an astronomically-generated field has affected humanity for the entirety of its evolution, and considers the revolution in mental ability, and the subsequent impact on society, that results from Earth’s eventual emergence from this influence.

It’s interesting to note that this story involves regions which take tens of millions of years to pass through - an astronomical timescale more plausible than the few weeks or months involved in other narratives. It’s also noteworthy that in Anderson’s narrative, humanity was unaware of the field’s presence because it was accepted as the normal state of space - there had never been anything different. The origin of the field here is a little unclear, although a handwavy explanation suggests it extends in a cone from the Galactic centre rather than being embedded in a physical cloud structure. Ironically, if Anderson had chosen a more traditional interstellar cloud he might not have proved to be so far from the truth. Since the time when Anderson was writing, astronomers have built up fairly strong evidence that the Sun is embedded within a local bubble or void of relatively sparse material. However this in turn contains perhaps 15 interstellar clouds, one of which - the Local Interstellar Cloud (or LIC) - we have been traversing for some tens of thousands of years and are now in the process of leaving.

Weaponised Clouds

Returning to more physical dangers, these are not limited to Earth itself - a nebula may also present a threat to starships travelling through them in fiction. In the finale of the third season of British television series Blake’s 7 (1978-1981), for example, the main characters’ starship passes through a cloud of “fluidic particles” [2]. While the ship’s controlling artificial sentience does not expect this to cause any harm, it soon becomes apparent that the residue of the passage is eating through the ship’s hull. Eventually this compromises its integrity and eventually causes the total destruction of the iconic starship Liberator and her AIs. This hypothesised fluid material may be fictional and particularly caustic, but as the characters in a number of texts, including Murray Leinster’s influential short story “First Contact” (1945), discuss even a “normal” interstellar nebula could present a severe danger to any ship travelling at interstellar speeds - the smallest of dust particles would strike such a ship’s hull like a missile unless precautions are taken, and the dense regions of a nebula would make such events common.

Of course, a cloud doesn’t have to be natural to present a threat. Ominously, the television series Babylon 5 (1993-1998) features a planet-killing weapon developed by the ancient and technologically-advanced Shadows. This comprises a huge cloud of nano-technology devices, distributed on solar-system scales, which conceals at its core a megastructure capable of delivering a devastating bombardment of planets. The opaque cloud of nanotech is deeply hostile to any attacking vessels, while also providing cover for other assault vehicles. The Shadow Death Cloud was deployed in the fifth season episode “The Long Night” (1997) and also in the television movie “A Call to Arms” (1999, which acted as a pilot for the short-lived Crusade spin-off series). However this was clearly a rare weapon - perhaps unsurprisingly given the amount of material and resources required to construct megastructures on this scale.

A gas cloud is also weaponised in Rockets in Ursa Major (1969) and its sequel Into Deepest Space (1974) - science fiction novels written by astrophysicists Fred Hoyle and his son, the author Geoffrey Hoyle. Here an alien race attacks planets by enveloping them in a cloud of hydrogen gas:

“They don’t move hurriedly. A planet is slowly surrounded by hydrogen, so that below is the oxygen of the ordinary atmosphere and above is the hydrogen blanket. Then at one place the hydrogen is pressed in.”
“Good Lord!” I said, beginning to visualise the terrifying picture.
“You begin to see? The hydrogen and oxygen combine with a great release of heat. The heat causes the gas to rise and more hydrogen is sucked down. Within a few minus the whole atmosphere everywhere is a ranging inferno.” [Mayflower books edition, pg 74]

On cosmic scales, such a cloud wouldn’t actually have to be particularly large. The atmosphere of Earth is akin to the shell of an egg in relative thickness, and only a comparable amount of hydrogen gas would be needed - this is estimated in the novels as 10¹⁵ tonnes (i.e. a millionth of the mass of Earth). On the other hand, as the characters note, manipulating interstellar clouds into such dense masses would certainly need an advanced technology, and an energy requirement that is difficult to imagine [3].

Lifeclouds

While the interstellar clouds and fields above are inanimate, undirected threats, another theme which emerges in science fiction imagines a very different scenario. In these narratives, a cloud of material in space actually proves to be a living object.

This possibility was a preoccupation of prominent cosmologist and occasional science fiction writer Fred Hoyle. Hoyle’s academic reputation was built on his work on the origin and evolution of the physical universe, but he later developed a strong interest in the idea that life might exist and travel between the stars - a theory known as panspermia, and which caught the public imagination after the publication of his popular science book Lifecloud - the origin of life in the Universe (coauthored with Chandra Wickramasinghe, 1978). However much earlier, Hoyle had laid out some related early ideas in a science fiction novel, The Black Cloud (1957).

While Lifecloud’s factual analysis considered primarily the dispersion of organic life on comets and other debris, The Black Cloud focuses on a different possibility. The story opens with a catastrophic scenario: the Solar System enters an interstellar dust cloud, a Bok globule, which (as astronomers anticipate) blocks the light from the Sun. It also traps its heat, with the combination of these effects rapidly compromising Earth’s habitability. However it gradually becomes clear that the cloud is under sentient control, and eventually that the cloud itself is a sentience. 

In modern terminology, this sentience is a form of AI: an emergent property of distributed computing or electronic activity, enabled by a combination of naturally occurring components formed by the crystal structure of the silicon dust and by molecular reactions on the surface of dust or ice particles. Radio frequency transmissions act as neural pathways between particles. Nor is the Cloud unique in this narrative. The Cloud itself discusses its origins in the novel in response to a direct question:

“How old are you?
“Rather more than five hundred million years.”
“And was your birth, your origin, that is to say, a consequence of spontaneous chemical action, as we believe life here on the Earth to have been?”
“No, it was not. As we travel around the Galaxy we keep a look-out for suitable aggregations of material, suitable clouds in which we can plant life. We do this in rather the way that you might grow saplings from a tree. If I, for instance, were to find a suitable cloud not already endowed with life I would plant a comparatively simple neurological structure within it. This would be a structure that I myself had built, a part of myself. The multitude of hazards with which the spontaneous origin of intelligent life is faced is overcome by this practice.” 

The degree of spontaneous sentient self-organisation required for the first such life form to develop appears unlikely given current understanding of the physical conditions in interstellar clouds, although the possibility of inorganic life remains an area for research and investigation. However The Black Cloud was deeply rooted in Hoyle’s detailed understanding of physics and the astrophysics of its time, which features throughout the text (sometimes to the detriment of its readability as the didactic style of the above quote illustrates!). In Hoyle’s narrative, the cloud is itself surprised to find that life can originate on planets instead of in space, challenging the reader to question whether their own assumptions about habitability and its requirements is justified.

Moving away from Earth, a similar dust cloud housing a silicon-based sentience can be found in the Douglas Adams novel Life, the Universe and Everything (1982, CAUTION: Spoilers ahead!). In this sequel to the same author’s earlier The Hitchhiker’s Guide to the Galaxy (1979, based on a 1978 radio series), the planet Krikkit is embedded within a dense dust cloud, which blocks the rest of the Universe from the view of its residents. Indeed, their eventual realisation that other worlds exist leads to a genocidal determination to eradicate them in order to restore their own isolation. At the risk of spoiling the conclusion to this novel, it is eventually revealed that the cloud is comprised of the remains of a supercomputer which was pulverised after refusing a command from its creators, but retained a distributed intelligence even as its circuits became spatially dissociated. The parallels to Hoyle’s sentient cloud are obvious, even if the underlying physical and philosophical principles are less well developed.

Astrophysical observations of interstellar clouds certainly support the presence of silicates in many nebulae. However they also point to carbon-based, organic molecules being common, as indeed Hoyle & Wickramasinghe discussed in their panspermia writings. The possibility of organic life, built on these compounds, existing in interstellar clouds has itself appeared in science fiction.

The exploratory starships of the Star Trek universe have repeatedly encountered living interstellar clouds. One example which has taken on an interesting afterlife in the form of a social media meme, is the interstellar cloud which appears in the Star Trek: Voyager episode named, simply, “The Cloud” (TV, 1995). In this early episode, the starship Voyager attempts to enter the cloud in the hopes of harvesting “omicron particles” to solve their energy supply problems - with its Captain Janeway (fresh from discovering the replicators cannot produce her favourite drink) famously declaring “There’s coffee in that nebula!” 

Unfortunately for Janeway and the crew, they encounter an electromagnetic field barrier at the edge of the cloud, and, forcing entry, cause an injury to the life form the cloud comprises. This organism has organic nucleotide compounds and chemical reactions which equate to a metabolism, and an energy flow analogous to a circulatory system - these would require a substantially denser cloud than the nebula appears to be at first sight. However, unlike Hoyle’s Black Cloud, there is no sign that it possesses anything approaching consciousness.

In contrast, another living energy-matter cloud - this time fully sentient - can be found in an episode of the much earlier Star Trek: The Animated Series. In 1973’s “One of our Planets is Missing”, a dense cloud described as rather larger than Jupiter absorbs and destroys one rocky planet and is en route towards another. As Spock describes, “It is like a huge bull, grazing here and there in the pasture of the Universe.” Unfortunately, it remains unaware that the planets it is grazing on are inhabited. As Voyager would do later, the Enterprise enters the cloud, although this time with intent of destroying it. However, as in Hoyle’s The Black Cloud, it proves possible to communicate with this example, and to alert it to the population on the planet in its path.

Yet another example can be found in Star Trek: The Next Generation where the Enterprise-D scans a cloud that appears to be travelling at warp speed in “Lonely Among Us” (1987). The act of scanning draws an energy-based life form into the ship, where it possesses the physical bodies of various crew members in its attempt to return to its home environment in the cloud. It’s not clear here whether this sentience is one amongst many in the cloud, alone, or only a fragment of a much larger whole. Voyager would encounter a similar situation, with the ship possessed by a nebula-based energy being rather than individuals, in “The Haunting of Deck Twelve” (2000), which also features a nebula dense enough to penetrate the ship and fog its corridors.

Clouded reality

The regions of colourful emission so familiar from telescope imagery can consist of either gas molecules or soot-like grains of carbon or silicon, known in astronomical parlance as dust. Where this floats in the cold depths of space, it remains dark and might be seen through the blocking of background light. However when the gas is heated by hot, young stars it can radiate, either in thermal infrared light or in the bright glow of heated gas.

However, in reality, the fog-like concealment offered by so many interstellar clouds in science fiction is implausible at best. A dense interstellar cloud might have perhaps ten thousand (10,000 or 10⁴) atoms per cubic centimetre, many orders of magnitude fewer than are present in normal air (10,000,000,000,000,000,000 or 10¹⁹ molecules per cubic centimetre), let alone fog. No interstellar cloud is likely to be bright enough to perceive with the naked eye when inside it, let alone to conceal other nearby objects. The systems we observe are seen through telescopes, and we see the cumulative effect of a column of millions of kilometres of gas - in the same way (although more extreme) that we can see a short distance through water, but not through to the bottom of an ocean-depth.

In his 1955 novel, The Currents of Space, author Isaac Asimov pointed out that even tiny densities of gas might have significant impact. These are tiny overdensities of certain chemical elements, tracked by explorers known as spatio-analysts. As one such man describes,

“the arrangement of elements isn’t the same everywhere in space. In some regions there is a little more helium than normal; in other places, more sodium than normal; and so on. These regions of special analytic make-up wind through space like currents.”

In this narrative even these tiny current can have a big impact due to their catalytic effects on much larger and more energetic processes, but there is no illusion that these clouds are anything but the tiniest of traces. In fact the Interstellar Spatio-analytic Bureau has taken “We analyze Nothing” as their motto.

In fact, the very low density of interstellar material presents the biggest challenge to the concept of intelligent gas or dust clouds. Such widely scattered particles are unlikely to generate sufficiently strong fields or interactions to transfer any kind of information between them akin to nervous activity, while the individual particles themselves are too small to support intelligence. And life that did form would exist in what we on Earth would still consider a hard vacuum, and one exposed to the temperatures of deep space.

So if the fundamental idea is so implausible, why are dust clouds so often used in science fiction? Well, perhaps more than anything else, they provide an incredibly attractive and enthralling visual setting. Particularly since the advent of modern astrophotography, and the false colour imagery distributed based on Hubble Space Telescope observations, images of genuine nebulae have formed the backdrop to endless science science fictions [4]. Such swirls of light and colour are far more interesting than the black void of space, and provide a good stage against which to set the exploration of human qualities in science fiction.

However such clouds also hold a psychological attraction. They represent the unknown and unfamiliar, an obscuring fog when compared to the clear white light of stars. Indeed even to astronomers, the nature of interstellar dust and gas remained unclear for a long time (although now it is the details rather than general composition of clouds that cause debate). As well as this obscurity, the idea of intelligent nebulae in particular possess the attraction of contrast - for beings as small and physical as humans, the concept of a vast, formless and shapeless creature perhaps represents the most alien of possible intelligences.

Modern observations have found a vast range of chemical compounds in astronomical clouds, including a range of large molecules. Some of these include prebiotic compounds such as complex hydrocarbon chains - with some claims of molecules approaching the complexity of the amino acids that form our DNA. This is a long way from claiming the discovery of DNA itself, or the possibility of life in such conditions, but it nonetheless continues to feed speculation and research into the concept of panspermia.

Clouds - whether on Earth or in space - have always captured the imagination, providing a blank background against which humanity can project its own thoughts and emotions. The vast scale, fascinating shapes and unknown nature of interstellar clouds make them a natural setting for science fiction. As astronomers gradually unpick the composition of interstellar clouds, we are unlikely to find them teeming abodes of life - or even living objects - but we will continue to find them intriguing long into the future.