Sailing to the Stars

Stories of space travel have always reflected their kinship with tales of adventures on the high seas. Perhaps inevitably, the vacuum oceans of deep space have their own sailing ships - a technology which is perhaps more plausible and exciting than many realise. Here we look at the science and science fiction of solar sails.

Under Pressure

The idea that light can not only heat or illuminate but actually exert a force is perhaps counterintuitive. Forces in nature result from changes of momentum. A ball bounces off you, its momentum is reversed due to the force your body exerts on it, and so you feel a force at the impact. Mathematically, momentum is calculated by multiplying mass and velocity (i.e. p=mv): if the ball is heavier, or travelling faster, you feel a proportionally larger force. If it is more massive than you, then you are the one bounced away instead!

However light, whether considered as a wave or particle, has no mass, so how can it have momentum? Well, in both Maxwellian electromagnetism and Einsteinian relativity [1], energy and mass is equivalent. Photons (particles of light) have energy, therefore they have a mass-equivalence (m=E/c²) and a momentum (p=mc=E/c). While the mathematics are unimportant, the result is that when light bounces from a reflector, it exerts a force - just as if a series of balls were being bounced off it. Of course, the amount of momentum carried by each photon is very small. In normal conditions - as in air on the surface of Earth - the reflector doesn’t feel it any more than we would feel a speck of dust (or the molecules of the air around us) bouncing away. Only the most sensitive of instruments could show this pressure when it was first demonstrated in 1899.

Where the potential of this radiation pressure becomes exciting is in situations where a great many photons can be collected, and where there is little else to resist the tiny force. These conditions are met outside the deep sea of atmosphere, in the vast emptiness of interplanetary space. This was recognised early in the history of both spaceflight and science fiction. Rocket pioneers such as Tsiolkovsky (the Russian aerospace engineer after whom a lunar crater is named) proposed their use in the 1920s, and science fiction sees mentions of the possibility in the work of Jules Verne, and in J.B.S. Haldane’s The Last Judgement (short story, 1925, [2]):

“on leaving the atmosphere, wings of metallic foil of a square kilometre or more in area are spread out to catch the sun’s radiation pressure, and voyages are thus made on principles analogous to those employed in the ancient sailing-ships.”

Surfing the Solar System

The potential for solar sails to be used as a slow but low-energy method of transport around the solar system was widely recognised in the 1960s, when rocket technology capable of reaching Earth orbit was first being developed and further exploration looked likely to follow. The first serious and rigorously calculated scientific proposal for plausible solar-sail craft in vacuum was laid out in a factual article which appeared in Astounding SF magazine in May 1951. Published by aeronautics engineer Carl Wiley under the pen-name Russell Saunders, and titled The Clipper Ships of Space, it went through calculations of the radiation pressure that might be obtainable and discussed the material requirements for a sail robust enough yet light enough to be extended over areas of square miles. This, together with subsequent articles such as Cosmic Carousel by Frank Tinsley (Amazing Stories, May 1962, which reported work by Dr. Theodore P. Cotter on a possible solar-sail vessel and imagined its exploration of Saturn and its moons), raised the potential of solar sails in the consciousness of both scientific research and SF.

Jack Vance was amongst the early authors to adopt this idea, with his story Gateway to Strangeness (Amazing Stories, August 1962). This described the experiences of a group of cadet spacemen who are sent on an extended, multi-month training cruise with a tyrannical, alcoholic, irascible old instructor who placed them in a variety of difficult situations. His initial briefing lays out the rationale for why they are being sent in a solar sail vessel (albeit one with a weak plasma engine to provide back-up):

“First, why bother with the sail, when nuclear jet-ships are faster, more dependable, more direct, safer and easier to navigate? The answer is three-fold. First, a sail is not a bad way to move heavy cargo slowly but cheaply through space. Secondly, the range of the sail is unlimited, since we employ the mechanical pressure of light for thrust, and therefore need carry neither propulsive machinery, material to be ejected, nor energy source. The solar sail is much lighter than its nuclear-powered counterpart, and may carry a larger complement of men in a larger hull. Thirdly, to train a man for spасе there is no better instrument than the handling of a sail.”

 Vance also describes the necessary sail material and recognises the necessity of having a tough but light-weight material:

“We use a fluoro-siliconic film a tenth of a mil in gauge, fogged with lithium to the state of opacity. I believe the layer of lithium is about a thousand two hundred molecules thick. Such a foil weighs about four tons to the square mile. It is fitted to a hoop of thin-walled tubing, from which mono-crystalline iron cords lead to the hull.”

The story focuses on the crew’s struggle to survive the trip, which takes them to the orbit of Jupiter and beyond, but this is intimately tied to the cumbersome navigation and limited capabilities of their vessel.

A solar-sailed pleasure yacht was described by Pierre Boule in the framing story of his 1963 novel Planet of the Apes. However the degree to which solar sail technology was in the scientific and science-fictional spotlight is demonstrated by the near-simultaneous publication of two science fiction stories from leading authors in early 1964, both of which were entitled Sunjammer.

Poul Anderson (writing as Winston P Sanders) published his story, Sunjammer in Analog magazine in April 1964. This is set in Anderson’s sequence of stories in which an rebelliously independent asteroid belt tries to find a balance of power with Earth, establishing a trading system known as the Beltline. In this story, a solar-sailed cargo vessel carrying volatile materials to Earth is about to be caught in a solar flare. Although this is a future which has mastered predicting space weather, the crew of a Beltline maintenance vessel receives just a few hours warning to get the cargo to safety in order to prevent debris from contaminating Earth’s environs and causing vast financial damages.

As well as the danger to the crew, the scale of the problem in capturing and handling the sailship is described in the text:

“The sail now nearly bisected the sky. Four and a half miles across, the foam-filled members that stiffened it marching across the field of view like Brobdingnagian spokes with its slow rotation, that disk massed close to a hundred tons. And yet it was ghostly thin, a micron's breadth of tough aluminized plastic, the spin as necessary as the ribs to keep it from collapsing backward under the torque at its edge. For while the pressure of sunlight in Earth's neighborhood is only some eighty microdynes per square centimeter, this adds up unbelievably when dimensions stretch out into miles. The sunjammers were slow, their shortest passages measured in months, but that vast steady wind never ended for them.”

The other Sunjammer was published by Arthur C Clarke in Boy’s Life, March 1964, and was renamed The Wind from the Sun in its many subsequent publications to avoid confusion with the Anderson title. This focussed less on routine cargo transport and more on a solar-sail yacht race around the Solar System for a technology still in the demonstration stage. The vessels here are much more compact and light-weight versions of the solar sail concept, designed to carry crews of one or two and no cargo, but gaining speed and manoeuvrability as a result. The story’s drama builds as the various one-off ships in the race encounter technical difficulties and drop out. 

An important technical point, and one farsighted for the early sixties, is Clarke’s recognition that adjusting the sails second by second would require both miniaturised winches and an extremely small, extremely powerful computer to calculate the adjustments and suppress any oscillations which might otherwise give the sail “the wiggles” which could tear them apart. Despite his protagonist being equipped with such a computer, the race ultimately comes down to a straightforward competition between two crews, but is threatened by a solar flare, which would be unsurvivable to crews in the unprotected, lightweight craft.

Since the 1960s solar-sailed vessels are relatively common in solar-system based vessels in SF, although still spectacular. A recent example can be found in the animated television series Thunderbirds Are Go (2015-2020), in which space rocket Thunderbird 3 deploys solar sails for a relaxed return from a mission which takes it to Mercury (episode “Night and Day”, 2018). A solar sail also features in television series For All Mankind (2019-present), which imagines an alternate history in which the Cold War space race did not end at the Moon, where it is used for the first crewed mission to Mars (season 3, 2022).

Sailing to the Stars

The idea that solar sails might be used for travel not just within the Solar System but also for travel between stars, and particularly for early interstellar colonisation, developed in parallel with the stories above.

Cordwainer Smith explored this form of slow travel in his story The Lady who Sailed the Soul (Galaxy SF, April 1960). The story is framed by a parent in the very distant future looking back at a nearer future:

“There used to be sailors, darling, a long time ago. They were brave men who took the ships out to the stars, the very first ships that took people away from our sun. And they had big sails. I don’t know how it worked, but somehow the light pushed them, and it took them a quarter of a life to make a single one-way trip.”

In fact, in this story, the solar sail ships carry cargos of settlers in suspended animation, accelerated away from the solar system by the Sun's radiation pressure, then coasting for decades before being decelerated by the star of the target planetary system. However the ships require pilots, a single individual in each case who is drugged until they only perceive one month of subjective time passing, while their bodies (and the rest of the Universe) experience forty years of aging. The story the parent tells is actually a romance between a male pilot who arrives back on Earth still feeling themself to be a young man, but in an aged body, and a young woman.

While the story explores the difference between subjective (experienced) and objective (elapsed) time, it neglects the time dilation effect that would occur as a result of even the relatively low relativistic speeds reached by the colony ships.

By contrast Survey Ship by Marion Zimmer Bradley (novel, 1980) explores an earlier stage of interstellar colonisation. This focuses on a group of adolescents who are despatched on the titular survey ship not to a known destination (as was the case for Cordwainer Smith's Lady) but instead in the hope of finding a habitable world. Since in such a circumstance, the ship's crew must be self-sustaining and potentially able to establish a viable colony itself before others join them, the crew consists of a mixture of male and female members. The solar sails are controlled by a series of controls in the central cabin and deployed by a crew member, Moira, when the ship is clear of lunar orbit.

“She felt like a spider, spinning out her silken web into every direction, surrounded by the feathery streaming of filmy sails, responding to the light... feeding the endless energies of light into the drives. The awareness shimmered inside her nerves with the violence of orgasm, and she closed her eyes in momentary ecstasy.”

However, as Moira tells another member of the crew, and like Clarke’s Sunjammer, the process is largely automated:

“the sails are programmed to trim themselves; strictly speaking, we could leave the Bridge now and spend the next four years or so playing string quartets and making love in our cabins.”

Launched in their slow-moving solar sail ship, and travelling for an extended period through the solar system to build up speed (targeting better than 0.9c) before escaping the Sun's gravity, other than dealing with the occasional shipboard emergency and asteroid impact, the young crew have little to entertain them except for rather frequent sexual encounters in a range of iterations.

A somber note touched upon at the end of the story is the realisation that the vast majority of these survey crews are not expected to survive or find habitable worlds. Instead many are being cast adrift in the hope that a lucky few might find a solution to Earth's chronic overpopulation problem.

The idea that long range colony ships might adopt solar sail acceleration and deceleration (and so avoid the need to accelerate a substantial fuel burden) continues to occur in science fiction. Ken Liu's 2012 novella The Waves (Asimov's, Dec 2012) uses this premise to explore a possible posthuman future. The much larger group of colonists about Sea Foam must orbit the Sun many times in order to pick up enough velocity even to start their journey. as we're told:

"The great solar sail strained silently in the vacuum of space as the Sea Foam spiraled away from the sun, accelerating year after year until the sun had shifted into a dull red, a perpetual, diminishing sunset.” 

However even then their journey will not be rapid and, without suspended animation, this is a generation ship:

“it would take close to four hundred years (by the ship's frame of reference) to reach 61 Virginis, even taking into account the modest time-dilation effects. That required planning for children and grandchildren so that, one day, the colonists' descendants might carry the memory of the three hundred original explorers onto the surface of an alien world.”

However the story goes on to explore the consequences of immortality treatments, lightspeed leapfrog (i.e. being overtaken by faster craft that leave Earth later), digitization of humanity and ultimately their transition into beings of pure energy, each wave of advancement seeming shocking but inexorable and ultimately retaining the core of what it means to be human, despite the remaking of humanity.

The use of sub-light speed solar-sailed ships in interstellar space is also seen in a range of Star Trek episodes, where it is usually shown as an archaic or traditional technology used in the early stages of space exploration. In the Deep Space 9 episode “Explorers” (1995), for example, ancient Bajoran light sails are shown to be capable of reaching nearby star systems (albeit with space-phenomenon assistance. In the Strange New Worlds episode “Spock Amok” (2022), an alien civilisation the R'ongovians, signifies its new allegiance to the Federation by decorating the solar sails on the antique vessel it uses as a flagship for ceremonial purposes, while in Lower Decks episode “Shades of Green” (2024) two Orion characters are ordered to settle their differences through a traditional solar sail race through the Excellon nebula.

Sailing on Laser Light

Like many proposed interstellar technologies, solar sail vessels would obtain their highest accelerations at the start of the journey, when the solar radiation intercepted by their sails is strongest, before coasting through interstellar space. However a number of science fiction stories have considered a way of boosting or extending this period of acceleration. 

Writing in Nature in July 1966, Prof Gyorgy Marx proposed that a craft could be propelled to relativistic speeds by the pressure exerted by an Earth-based laser [3]. Physicist Robert L Forward had proposed laser propulsion of light sails in articles in 1962 and expanded on the technical requirements in his 1984 scientific paper “Roundtrip interstellar travel using laser-pushed lightsails”.

In Rocheworld (novel, 1984, first serialised as Flight of the Dragonfly in 1982), Forward used his expertise in solar sail technologies in his own science fiction writing. The early chapters of his story describes a training visit to a “sunhook” on Mercury - a floating space station which is not in orbit but rather levitated above the surface by the sun’s intense radiation pressure. On the same visit, the astronauts also visit the thousand solar-powered laser stations orbiting Mercury (to avoid being pushed away themselves by radiation pressure!). Located at the Mercury-Sun L-2 Lagrange point, a beam combiner will feed this energy into a single, thirteen-hundred terawatt infrared laser.

In order to reach the Rocheworld (binary planet system in orbit of Barnard’s Star) which forms the story’s focus, Forward describes a solar-sail equipped interstellar vessel which is accelerated out of the Solar System by this system, which can provide propulsion long after the more diffuse direct light from the Sun becomes ineffective. Indeed, one of the greatest threats to the mission is political pressure to stop the laser construction programme after the ship leaves the system. Since one of Forward’s innovations is the use of an Earth-based laser to decelerate as well as accelerate the ship, this presents a problem. When the laser is finally switched on as the ship approaches Barnard’s Star, it is reflected from the outer ring of the ship’s solar sail onto the now-reversed central section and then reflected again, providing a net deceleration.

Since these initial proposals, the idea of laser boosting of solar sail ships has become mainstream. It appears, for example in The Mote in God’s Eye (novel, 1974) by Larry Niven and Jerry Pournelle. This is a story of first contact between humanity and an alien race. In it, the alien’s lightship is launched by a powerful laser beam. The authors explained some of their thinking and worldbuilding in a January 1976 article in Pournelle’s science column A Step Further Out (which featured regularly in Galaxy SF magazine). This article, Building the Mote in God’s Eye included the story’s original prologue (omitted in the final novel) in which astronomers, working against a backdrop of interstellar war, initially believe a star has gone nova, only to realise they are detecting coherent laser light. It’s more than a century before the laser goes out, and longer before the lightship that was launched is detected.

Laser propulsion was used by John Barnes and Buzz Aldrin in their 1996 novel, Encounter with Tiber. This novel reflected astronaut Aldrin’s vision for the future of space technology, although it is notable that the authors gave solar sail technology to the aliens in their story rather than to humans, recognising that the technology is perhaps beyond the near-future setting of the human spaceflight elements. In the story, finding their own planet’s habitability threatened, the natives of a planet orbiting Alpha Centauri launch a sailship called Wahkopem Zomos towards Earth in the hope of establishing a settlement. The material of the sail is described in fair (if not technical) detail:

 “The sail for Wahkopem Zomos was wider across than the Ring Island, and all the billions of people on Nisu lying down on it wouldn't have covered a twentieth of its surface area. But I had held scraps of the sail material in my hand, and they were so light that they couldn't be felt. In essence the sail was one big beryllium-boron molecule, the individual atoms woven in an intricate matrix that was stronger than any other material (except for the spun-diamond shroud lines that held it to our ship) and yet so thin that if you put a sheet of it next to the ceiling in an ordinary room and let it drop, its air resistance was so much greater than its weight that it might spend an eightday spiraling down to the floor.”

Interestingly, this story describes a combination of both solar acceleration (from both the system’s binary suns) and a laser boost. Even so the four-light-year journey will not be particularly fast:

“We would use this wisp of a sail first to catch the fury of the sun, kicking us up to fourteen times our current speed, and then to pick up another kick in our close pass by Zoiroy, so that as we left the system we would be moving at over twenty times our present speed. Then we would move onto the beam of the giant laser that had been built in solar orbit, and that would speed us up until at peak we were moving at two-fifths the speed of light, just about eighteen years from now.”

The use of laser acceleration with lightships is now widely assumed in mainstream science fiction, including the novels of Alastair Reynolds and others. Another, still higher profile, example can be found in the 2009 film Avatar. 

The extensive backstory and reference work of this franchise prepared by the creators, including director James Cameron, describes extensive technical details never seen on screen. Writing in his accompanying 2011 non-fiction book The Science of Avatar, Stephen Baxter mentions a ten page ‘bible’ that describes the Venture Star - the vessel that travels between Earth and Pandora. This is a 1500m long solar sail vessel, which uses a laser for acceleration and deceleration in Earth’s solar system, intercepted by a sixteen-kilometre-wide bowl mirror. The result is a six-month-long acceleration at 1.5g, before coasting at 70% of the speed of light through intervening space. Antimatter-annihilation engines are used for braking (and then reacceleration for the return trip) in the Alpha Centauri system where Pandora is located (and where the laser infrastructure is unavailable).

Symbiotic Sails

One of the outstanding problems of solar sail construction, as mentioned in several of the works above, is to produce a reflective material (and cables) light enough to benefit from radiation pressure acceleration, but robust enough to be deployed over vast areas and haul a payload. Materials suggested have ranged from ultra-thin aluminium films to plastics and synthetic nanomaterials such as molecular structures. However several authors have considered an alternative, wondering if biological materials might be found, modified or engineered to survive in vacuum and act as solar sails.

John Varley’s Eight Worlds series (novels and short stories, beginning with The Ophiuchi Hotline, 1977) introduced an interesting example. Varley’s symbs are bio-engineered organisms which are symbiotic with human beings. Completely enclosing their telepathically-connected human partners, and extending through their digestive tract, these creatures act as biological spacesuits or even spaceships. Several short stories in the series focus on these symbiotic partnerships, who populate the rings of Saturn and lean towards artistic and creative endeavours. The symbs are described as photosynthetic beings dependent on sunlight, often extending themselves into thin films across square kilometres of asteroid surface in order to catch the relatively weak light flux reaching Saturn’s orbit from the Sun.

The stories also describe the space manoeuvring of symb partnerships. In Equinoctal (short story, 1976), which describes a human, Parameter, adapting to a new symbiotic partner after a separation, we’re told that:

“Equinox was in her extended configuration. The pair looked like a gauzy parabolic dish, two hundred meters across. The dish was transparent, with veins that made it look like a spider web. The illusion was heightened by the small figure spreadeagled in the center of it, like a fly. The fly was Parameter.”

and

“In her extended configuration, Equinox was a moderately efficient solar sail. By controlling the angle she presented to the incoming sunlight she could slowly alter velocity.”

More generally, symbs use a combination of this light pressure and pushing off ice rocks to navigate through the Saturnian system, although they can also carry supplies of gas for propulsion or spare a small amount of bio-manufactured gas from their otherwise-closed biosystems.

Another organic solar sail is described in If I fell, would I fall? (short story, 1988, aka Rescue at L-5) by Kevin J Anderson with Doug Beason. Again this is a bio-engineered creature, but while Varley’s symbs are conscious, through sharing their human’s nervous system, Anderson’s sail-creatures are experimental and lack sentience:

“The creatures had both mitochondria and chloroplasts within their cell walls: They were both plant and animal, a one-in-a-million accidental success from the bioengineering labs. Somehow everything had worked exactly right; everything fit together, everything functioned as it should.”

The creature’s reaction to exposure to vacuum is to prioritise the plant elements, extending into a thin film in order to maximise their solar radiation intake.

“The bioengineers oriented the sail-creature’s proto-sails edge-on to the Sun to prevent the solar photons from accelerating the sail before the process was complete. Day by day the creature’s fins spread out, becoming vast cell-thin sails, immense and opaque, hundreds of kilometers to a side, as the creature desperately tried to soak up light. The main bodily core became rigid and exceedingly tough, like an organic “hull.””

This metamorphosis becomes a vital skill in Anderson and Beason’s scenario in which off-world human habitats are left in a desperate state, and lacking usable space craft, after a war on Earth cuts off supplies. The story follows a character encased in a cyst in the organic hull, trying to manoeuvre the prototype creature from its origin at the Filipino colony Aguinaldo at L-4 to another station at L-5 [4]. In the cyst with him, he carries both embryos of more sail creatures and organisms that can provide food to the starving Orbitech facility. However, as the creature ails, he is forced to ever more desperate efforts to navigate, including prodding the creature with a knife to trigger flinches that modify the sail angle.

While this treatment of the prototype creature appears harsh, the story ends with the possibility of survival for the space colonies, and more sail-creatures as enabling routine transport between them [5].

Sailing into Space

Throughout the history of solar sails, the conversation between science fiction and science fact has gone in both directions. From the first essay by aeronautical engineer Wiley, science fiction and its writers have formed an integral part of solar sail development. Many of the writers of hard science fiction, notably including Robert L Forward, have had professional careers in physics or astronautical design.

Launched the early 1960s, at the very dawn of the space age, NASA’s Echo 1 mission was a technology demonstrator intended to prove the effectiveness of microwave reflectors in orbit as communication satellites. However its orbit clearly showed the impact of solar radiation pressure on the large, reflective balloon. Since then, dealing with radiation pressure impulses on their large surfaces (particularly solar panels) has become routine in satellite and spacecraft operations. In the sphere of astronomy, for example, the Hubble Space Telescope balances radiation pressure with forces from the Earth’s magnetic field to stabilise its orbit. By contrast, the James Webb Space Telescope, outside the Earth’s magnetic field at the Earth-Sun L-2 point, is forced to compensate with fuel instead, contributing to its more limited operational lifetime. Laser pressure de-orbiting is also a demonstrated and in-active-development technology for clearing space-junk from low Earth orbit.

This proof of the concept of solar radiation pressure having a measurable effect in orbit inspired a rush of more ambitious plans for use of solar sails in either interplanetary or interstellar missions. Advanced concept designs for solar-sail craft were delivered both by national organisations such as NASA and by private organisations, particularly in the 1970s and again in the 1990s. NASA concepts included the “Yankee Clipper” project, and a proposal floated by the Jet Propulsion Laboratory in 1976 that a solar sail probe should be launched to rendezvous with Halley’s comet in 1987. Unfortunately budget and technological constraints cancelled both projects.

Privately-funded bodies such as the British Interplanetary Society, The Planetary Society and The World Space Foundation considered a range of more ambitious projects, often including science-fiction-engaged scientists engineers (including Carl Sagan, Robert Forward and Arthur C Clarke) amongst their designers. Notable projects (none of which came to fruition, but all of which involved extensive effort and technical project design) include Starwisp and Project Solar Sail. 

Starwisp was a proposal led by Robert L Forward in 1985 (and later developed by another SF-author-engineer Geoffrey A Landis). It would comprise an ultra-small uncrewed probe, propelled by a 100m^2 solar sail fed by a microwave-frequency laser. It would reach Alpha Centauri in a matter of decades, and send back signals from a 40 hour fly-through of the system (it had no deceleration mechanism).

Project Solar Sail was an umbrella phrase used by the World Space Foundation, a private organisation based in Pasadena. While their initial plans were for a modest “engineering demonstration mission” or EDM, the ultimate goal was interplanetary and interstellar travel. As part of their fundraising efforts, they recruited Arthur C Clarke and David Brin to edit an anthology of science fiction stories and science fact essays with the title Project Solar Sail. Published in 1990, it included several of the stories mentioned here, along with others and essays from engineers including one from Robert Forward on Astrowisp. The World Space Foundation still exists, but no longer lists solar sailing as amongst its aims.

More recently, the Breakthrough Starshot initiative was a philanthropic programme founded in 2016 by entrepreneurs, scientists and science fiction enthusiasts Yuri Milner, Stephen Hawking, and Mark Zuckerberg. It aimed to develop a laser-launched sailship mission to Proxima Centauri, which would reach our neighbour system in a matter of a few decades. This would in turn deploy a fleet of tiny centimetre-sized probes, known as Starchips, each with a meter-wide sail of their own. The programme contributed millions of dollars to technology development and concept design, before being placed on hold in late 2025.

By comparison the actual deployment of solar-sail propelled craft has been problematic, due to both launch problems and problems with deploying the thin films of the sails. It’s notable that success was delayed until improvements in both materials technology and micro-scale computing (as predicted by Clarke and others). Following several failed attempts, the IKAROS probe, launched by the Japanese space agency JAXA in 2010 was the first successful demonstration of solar sail use for deliberate navigation, using a 200m² sail for a mission to Venus. NASA followed suit later in 2010 with the successful deployment of NanoSail-D, a 10m² solar sail-powered craft in low Earth orbit. The main goal of this mission was to explore the potential for solar sails to deorbit defunct spacecraft and space junk. Controlled solar sail propulsion was also demonstrated by the privately-funded Planetary Society’s LightSail 2 project in 2019, fulfilling an aim of its founder, physicist and science fiction writer Carl Sagan. 

Even with modern technologies though, and despite investment from governments and foundations, problems remain. All the demonstrators to date have been far smaller than those proposed for practical spaceships. Whether they could be scaled to much larger areas, remain intact in the face of abrasion by the solar wind, and be actively controlled, remains to be seen. In addition to deploying, controlling and protecting the sensitive sails, sail ships also face dilemmas about how best to manoeuvre (while various ‘tacking’ procedures have been proposed, these differ from ocean sail-ship tacking because the latter has a rudder that can press against the water pressure), and whether either charged ions collected on the sails or some balance against interstellar magnetic fields might be used. Given the overriding requirement for a small mass relative to their area, sail ships would also not easily be able to carry the dense radiation shielding needed to transport humans safely through space.

Despite that, solar sail vessels, and particularly those launched on laser arrays, remain one of the most plausible technologies for human space travel, and interstellar travel in particular. And perhaps for space travel by others as well, as noted in this rewriting of The Night Before Christmas which appeared in the anthology Project Solar Sail in 1990, and explored a problem faced by a seasonal visitor when human colonies were first established off-world:

“He was done in a flash (as he had been for years),
And then upwards he flew into thin atmospheres,
With his reindeer cut free, a new helmet he donned,
And his upward trajectory took him beyond, 

Poor old Terra below, and out of the blue,
And in blackness of space that old sleigh became new,
The bells were all silenced by the vacuum of space,
So bright red and green lasers were put in their place!

From the back of the sleigh, robot arms did deploy,
And they shook out a sail of metal alloy,
It was bright, shining Mylar that caught the Sun’s light,
And it gave him a push for a Luna-bound flight!”

- from Goodnight Children - Joe Clifford Faust (1990)

As materials technology, computer technology and continued private investment in near-Earth space continue to evolve, perhaps Santa won’t be alone in sailing to the stars.