Voyager 1 is now an interstellar spacecraft, according to the latest reports (and I’ll have thoughts on Voyager, its progress and its implications, on Monday). For today, though, Keith Cooper is envisioning other ways of going interstellar, methods that take advantage of natural objects like comets. Can we harness their resources and change the paradigm of deep space flight? Keith has written often for Centauri Dreams despite a busy schedule as editor of the British monthly Astronomy Now and equivalent duties at Principium, the newsletter of the Institute for Interstellar Studies. This look at how we might expand into the Oort Cloud and beyond takes us into a future in which our species may well differentiate as we explore different ways of reaching the stars.

by Keith Cooper


Amidst the clamor for giant metal-hulled ships, fusion engines and warp drive, our interstellar pioneers may be missing a trick. Why go to all the trouble of building a starship when there are trillions of natural ones right on our doorstep?

After I reviewed W. Patrick McCray’s book The Visioneers in my last column I picked up some old copies of Omni magazine from eBay on the strength of the magazine’s portrayal in the book. Flicking through the June 1984 issue I stumbled upon an article about using comets ejected from our Solar System as vessels to transport us through interstellar space. This instantly took me back to a few months earlier and a conversation with space artist Jon Lomberg, who mentioned artwork he did for Carl Sagan of ‘treeships’ – living, hollow trees grown out of comets, riffing on an original premise by physicist Freeman Dyson.

Comet ships are an old idea that does not seem to have received much airtime recently, but with all eyes on Comet ISON this winter it is the perfect opportunity to revisit this novel notion. ISON itself is a five-kilometre wide chunk of ice, rock and dirt that is racing towards the Sun. It hit headlines last year with claims that it could, for a short time, outshine the full Moon, but expectations have been significantly downgraded since then. Nevertheless, there is still hope that it will grow as bright as Venus near perihelion on 28 November – assuming it isn’t vaporised in the heat of the Sun as it flies within 1.8 million kilometres of our star.

If it survives its solar encounter, the comet will be flung back into the Oort Cloud – the distant realm of the comets – and it may even be ejected from the Solar System entirely on a parabolic trajectory. That’s a remarkable thought: the comet would enter interstellar space, to wander lonely between the stars and to maybe, one day, be picked up by another star. The comet highway could work both ways – the short period comet 96P/Maccholz is suspected by some scientists to have originated from another star system, based on what appears to be a carbon depletion seen in the abundance of molecules like cyanogen compared to other comets in our Solar System (although it should be stated that there are also more mundane explanations for 96P’s weird chemistry). Computer simulations of the formation and evolution of the Oort Cloud indicate that up to a hundred times more comets are ejected into interstellar space than remain in the Sun’s grasp. The Solar System is literally shedding comets in their billions.


Image: Hubble’s view of Comet ISON (C/2012 S1) on April 10, 2013. This image was taken in visible light. The blue false color was added to bring out details in the comet structure. Credit:NASA, ESA, J.-Y. Li (Planetary Science Institute), and the Hubble Comet ISON Imaging Science Team.

Cometary Trajectories Outward

No astronomer has ever observed the Oort Cloud directly – it is too distant, its inhabitants too small and too faint – but we know the cloud must exist there as the orbits of long-period comets indicate they originate in all directions from a region that exists between as close as 2,000 and as far away as 50,000 astronomical units. Models suggest that the Oort Cloud comes in two parts – an inner doughnut shaped region and an outer spherical halo, together containing more than a trillion icy bodies. It is from the loosely bound outer cloud that the long-period comets like ISON hail – and which interstellar comets must also originate.

We could take advantage of these escaping comets, eschewing the need to scratch-build complete starships and loading millennia’s worth of supplies onto them when we can just hitch a ride on a comet. After all, comets come ready-made, endowed with resources and volatiles: plenty of water, metal ores, even large stocks of deuterium to help fire a fusion reactor. One downside is that we might not have too much say about where the comet is heading, but given that the Solar System will eject comets randomly in all directions, hopping aboard them may pave the way for the full blown diaspora of humanity into the Milky Way at large.

Catching a comet when it is near the Sun might not be the best option. As a comet receives greater solar energy it warms, developing the familiar fuzz of the coma and a trailing tail as ice on its surface sublimates into vapour. More violently, gases trapped within pockets inside the comet’s porous structure begin to expand, often bursting out explosively. The comet would be unstable, even out beyond the orbit of Mars, as 2008’s outburst from Comet 17P/Holmes proved. Instead the best time to latch onto a comet could be when it is receding back into the outer Solar System and things have settled back down on its surface.

There’s an added problem. Human activity on a comet could kick the activity off again, so actually building habitats on or inside comets is not going to work. Consequently, the best tactic might be to live in habitats that are tethered to and which siphon materials from a comet. It is not yet clear how we could mine a comet safely, but the comet’s deuterium, locked in its water-ice, will be essential to the interstellar travellers if they want to survive their journey.


Image: Comet Halley, as seen in 1986 by the European spacecraft Giotto. Data from Giotto’s camera were used to generate this enhanced image of the potato shaped nucleus that measures roughly 15 kilometers across. Some surface features on the dark nucleus are on the right, while gas and dust flowing into Halley’s coma are on the left. Credit & Copyright: Halley Multicolor Camera Team, Giotto Project, ESA.

Energy can be derived from nuclear fusion using deuterium. It is particularly abundant in comets – Comet Hale-Bopp, which graced our skies in 1997, was shown to contain an abundance of deuterium twice that of Earth’s oceans, at a ratio of 1:6,410 compared to regular hydrogen. This is three times as much as astrophysicist Eric Jones predicted when writing on the subject in 1985’s Interstellar Migration and the Human Experience, where he speculated that one deuterium atom for every 20,000 hydrogen atoms in a comet should be sufficient to power fusion to sustain a significant population of interstellar voyagers for many centuries. We see this scenario being played out in [be warned, spoilers] Greg Bear’s novel Hull Zero Three, where three cylinders are affixed on stanchions to a central comet from which resources are being drawn. Alas, interstellar travel times for comets are measured in millennia, not centuries, so an additional power source will also be required.

Deep into interstellar space the singular power of the Sun fades into insignificance. However, despite being at great distance the collective starlight of all the stars could, says Jones, produce several hundred megawatts of power. Huge parabolic mirrors built out of aluminium mined from a comet could be constructed to capture and focus diffuse starlight. Inspired by Robert Forward’s book The Flight of the Dragonfly, in which the main spacecraft used similar mirrors to capture and focus the laser light that is beamed towards it from the Solar System and which is driving it forward to Barnard’s Star, Jones suggests that, at around 1.6 light years from the Sun, the mirrors would require a diameter of 3,000 kilometres. Cluster a bunch of comets together, says Jones, and the inhabitants could tend to a farm of mirrors 30,000 kilometres across. Combined with fusion reactors, these mirrors should be able to supply power to the comet cluster’s inhabitants for as long as the community exists.

Biotechnology and the Interstellar Diaspora

Nevertheless, it all sounds like a terribly hard way to live, a subsistence existence. Small communities in tiny tin cans eking out their lives surviving off a trickle of energy derived from raw starlight and a few centuries worth of fusion power, light years from any planetary system. It is difficult to think of a reason why anybody would want to travel to the stars this way. Jones suggests that bands of nomads, clans and tribes residing in the Oort Cloud in the future would be most likely to embark on such a lonely journey, mirroring the voyage into the unknown Pacific made by the Polynesian explorers a thousand years ago. It is a thought that Freeman Dyson concurs with, a thought that he voiced at his talk at the recent Starship Century symposium in May this year. For Dyson, in a few centuries there could well be more people living in the Oort Cloud than are living on Earth. If this is to be the shape of the future diaspora it couldn’t be further from the organised, planned launch of gun-metal grey starships with hand-picked crews that we envisage today.

Before dismissing cometary travel as a venture only for a rabble of humans, Dyson perhaps has a better idea, one that raises itself above the mundane toil of living off a comet in the conventional way, to arrive at a level of magnificent eloquence and beauty.

During his Starship Century speech, Dyson revisited an old idea of his dating back to 1972. He postulated with a degree of confidence that biotechnology will come to dominate the future, inevitably instigating a number sociological changes in humanity, some that we might predict, others that we cannot. One possibility that Dyson does predict is the existence of ‘big trees’ – genetically modified plants designed to survive in space that would grow to enormous sizes, many kilometres across, with their roots ensconced within a nutrient-rich comet. The tree would be so huge that it would contain hollow, airtight spaces where people could live. Furthermore, the plants could grow their own greenhouses, “just as turtles grow shells and polar bears grow fur and polyps build coral reefs in tropical seas,” according to Dyson. They would produce their own oxygen supply from photosynthesis, like any other plant, while an array of lenses and mirrors could focus light from the distant Sun and stars into the greenhouse. It is an idea that has had a little play in science fiction, such as in Dan Simmons’ novels like Hyperion:

“The Consul remembered his first glimpse of the kilometre-long treeship as he closed for rendezvous… it’s leafy bulk clearly ablaze with thousands of lights which shone softly through leaves and thin-walled environment pods, or along countless platforms, bridges, command decks stairways and bowers. Around the base of the treeship, engineering and cargo spheres clustered like oversized galls while blue and violet drive streamers trailed behind like ten-kilometre roots.”


Image: This painting by Jon Lomberg was commissioned for the book Comet by Carl Sagan and Ann Druyan, to illustrate a concept proposed by physicist Freeman Dyson. Dyson suggested that in the future, comets could be used as the source of minerals and water to support the growth of gigantic, genetically engineered trees. These trees would use sunlight to grow to large size, and provide a habitat for space colonists. Credit: Jon Lomberg.

Biotechnology could play a significant role in adapting life for the harsh conditions on the rim of the Solar System. Rather than toiling away on comets, Oort Cloud denizens could use biotechnology to adapt to be more efficient with limited resources and create a balanced ecosphere in their habitats. While we are currently busy tinkering with our paper designs for starship drives and metal spaceships, are we blind to the oncoming biotechnological revolution and could it be this that leads us to the stars rather than fusion engines, antimatter or warp drive?

Perhaps, but there would be a downside. Sticking engines on a comet or a ‘treeship’ as in Simmons’ novel is probably impractical if we are limiting ourselves to the resources available only on the comet or via captured in starlight. Instead, riding on a comet will mean taking the slow road to the stars. We shouldn’t expect to get anywhere on them for tens or even hundreds of thousands of years. By necessity they would become generation ships, at their greatest a fleet of mobile O’Neill colonies, or a literal floating forest spurring the “greening of the Galaxy” where starships are grown, not built.

If we are to pursue the faster route, constructing engines of enormous power settled into vast metal hulks, then perhaps the the comet travellers play a different role. Maybe they will never reach the stars, but instead settle into the dark spaces between, way-stations for lost or weary travellers who have followed later in their faster metal starships. Two different populations moving out into the Galaxy and occasionally meeting, each pursuing distinctly different technologies, each growing and changing in their own different ways, one driven by the green efficiencies of biotech, the other by the gleam of a metal hull and the hum of a starship engine. If you could choose, which would you be?