The classical rocket equation—M/m = e^(delta-v/exhaust velocity)—is a harsh mistress. If you want increased velocity, you have to increase fuel. For every increase in delta-v, you increase the ratio between the dry mass of a spacecraft and the fully fuelled mass. The ship gets heavier, needs more fuel, yadda yadda.
This is a pain for the sort of SF author who aims at a patina of verisimilitude: chemical rockets, for example, are limited to comparatively small delta-vs (which is why, for example, so few probes have been sent to Mercury). There are a number of ways to sidestep the limitations imposed by the rocket equation, the most straight forward of which is to somehow obtain the necessary thrust from some external source…which brings us to light sails.
Light bouncing off a mirrored surface does not exert much force. A light sail one square kilometre in area, located at 1 AU, would experience about 8 newtons of force from the sunlight bouncing off it. 8 newtons is about the force two blocks of butter would exert on your hand as you held them up against gravity. Still, small forces for very long times can provide surprisingly large delta-vs. This eight newtons/kilometre squared is free1 and available for as long as the sun shines. Wikipedia is kind enough to provide some idea of the potential this offers:
You might think that taking almost two years to park nine tons in Mercury orbit is slow…but remember that it took conventional rockets (and flybys) seven years to get MESSENGER’s single ton to Mercury.
Rather surprisingly, while it should have been intuitively obvious after the mid-19th century that light exerts pressure, it took about another century for science fiction authors to pick up on this fact. Indeed, even though the first popular science piece on light sails of which I am aware2, Russell Saunders’ “Clipper Ships of Space,” appeared in 1951, the first SF short stories and novellas featuring light sails of which I am aware (Cordwainer Smith’s “The Lady Who Sailed the Soul,” Jack Vance’s “Gateway to Strangeness” aka “Sail 25,” Poul Anderson’s “Sunjammer,” and Arthur C. Clarke’s “Sunjammer,” later renamed “The Wind From the Sun”) did not appear until 1960, 1962, 1964, and 1965, respectively.
Possibly the reason that light sails took a while to become popular tropes is that the scientifically-clued-in authors who would have been aware of the light sail possibility would also have known just how minuscule light sail accelerations would be. They might also have realized that it would be computationally challenging to predict light sail trajectories and arrival times. One-g-forever rockets may be implausible, but at least working how long it takes them to get from Planet A to Planet B is straightforward. Doing the same for a vehicle dependent on small variable forces over a long, long time would be challenging.
Still, sailing ships in space are fun, so it’s not surprising that some authors have featured them in their fiction. Here are some of my favourites:
Light sails feature prominently in the late Vonda N. McIntyre’s Starfarers quartet. However, since I already mentioned those books in an earlier essay (and don’t like featuring the same books over and over) consider 1974’s “The Mountains of Sunset, the Mountains of Dawn.” One of two stories about a race of birdlike aliens, this explores a period in their history when their species has set out in light sail-propelled sublight ships. Their voyage is by its nature slow and by the time they reach their first destination, the newest generation has grown accustomed to living in their artificial world. Nevertheless, one elder is determined to descend to a world in which its fellows have little interest.
Joan D. Vinge’s 1978 Hugo-nominated “View from a Height” was inspired by a Robert Forward proposal regarding one-way interstellar missions. In the real world we sidestep the issues involved by dispatching expendable robots on one-way journeys. Vinge wondered what would motivate someone to volunteer for a one-way crewed mission into deep space, and this novella resulted.
The spacecraft in Yūichi Sasamoto’s Bodacious Space Pirates light-novel series are, for the most part, fairly conventional rockets (with a side order of faster than light travel to circumvent the whole “dying of old age while waiting to reach one’s destination” thing). Plucky schoolgirl-turned-privateer Marika Kato and her crew of equally plucky schoolgirl space-yacht-club members set out on the Odette II, a light sail craft with a dubious history. They thought that history was safely buried, never to return; this being an adventure series, they are proved wrong.
The Congregation of Alastair Reynold’s 2016 Revenger is merely the latest of a long line of civilizations that have occupied the myriad worlds orbiting the Old Sun. Eager to plunder the treasures left by their predecessors, travelers ply the space lanes in ships propelled by high-tech light sails. Of course, some very bad people have figured out the obvious: that it is easier to let other people do the searching, then simply commandeer their hard-earned treasure. The Ness sisters hoped to find freedom and fortune in space. What they find instead is considerably darker.
If, however, fictional light sails are not enough for you, I have good news. The same strengths that appeal to writers also appeal to people eager to explore our solar system. Although space exploration bureaucracies are by their nature technologically conservative (preferring conventional but reliable methods to the cool but untested) there have been some trials of light sail craft. There have, of course, been setbacks—Sunjammer was cancelled in 2014—but there have also been successes: JAXA’S light sail IKAROS successfully made its way past Venus in 2010. In June, The Planetary Society’s LightSail 2’s launch window opens. I hope it is as successful as IKAROS.
In the words of Wikipedia editor TexasAndroid, prolific book reviewer and perennial Darwin Award nominee James Davis Nicoll is of “questionable notability.” His work has appeared in Publishers Weekly and Romantic Times as well as on his own websites, James Nicoll Reviews and Young People Read Old SFF (where he is assisted by editor Karen Lofstrom and web person Adrienne L. Travis). He is a finalist for the 2019 Best Fan Writer Hugo Award, and is surprisingly flammable.
[1]Obviously the sail is not free.
[2]Assuming we don’t count Methuselah’s Children, which managed to use light pressure to get its starship up to relativistic speeds by...well, let me just quote Slipstick Libby: “I saw a discontinuity in the mathematical model of the aspect of mass-energy called inertia.” When put that way, it seems so easy.
See also Star Trek: Deep Space Nine episode “Explorers” (Season 3, Episode 22). Sisko and Jake build a replica of an ancient Bajoran solar sailcraft and use it to demonstrate whether Bajorans could have reached Cardassian space in a sublight space vessel. It occurs to me that this also sounds a lot like “Kon-Tiki in Space.”
JAXA’S light sail IKAROS successfully made its way past Venus in 2010.
This could be interpreted as suggesting that IKAROS traveled to Venus just by using solar-sail propulsion, which isn’t really true; it was launched on a trajectory to pass by Venus by a conventional rocket (the same one that sent the Venus orbiter Akutaksi on its way to Venus). IKAROS did demonstrate the ability to modify its trajectory along the way, of course, accumulating a delta-V of about 100 m/s in six months. Not quite enough to get you to from low earth orbit to Venus (about 3.5 km/s, as I understand it), but still impressive.
“The Mote In God’s Eye” has an interstellar light sail launched by lasers and the sail itself turns out to be a dandy weapon (at close range against unshielded ships.)
Kissing cousin to light sails are laser assisted light sails. From the Motie ship from The Mote in God’s Eye to the Venture Star from Avatar, they are making more appearances in fiction.
If I recall correctly, the two travellers who find the manuscript for the original novel of Planet of the Apes are travelling in a solar-wind powered craft.
The belters in C.J. Cherryh’s Heavy Time use laser-assisted sails to move from home base to their assigned mining/survey areas & back.
Count Dooku has a solar sail that briefly appears in Attack of the Clones. It’s a pretty nifty design that only gets a few moments of screen time.
GURPS addressed this almost 27 years ago in the Roelplayer. That one inspired a _lot_ of thought on my part for a campaign set in the solar system.
http://www.sjgames.com/gurps/Roleplayer/Roleplayer29/MagSails.html
As You Know, James, there’s also the less-solid versions of solar sails, the magnetosail and the one you personally pointed me to for Threshold, the Dusty Plasma sail (called the Nebula Drive in both the Boundary and the Castaway books).
I was just thinking of the bounce-a-photon-off-a-reflective-surface kind.
@@.-@ The most extreme assisted light sail is in Niven’s “The Fourth Profession”, where aliens threaten to blow up the Sun to propel their sail.
You may want to google the Breakthrough Starshot initiative. It is surprising how close the current technology is to the goals of the project.
Light sales in Edgar Rice Burroughs “John Carter of Mars” series, 1917.
Whoops. Sails, not sales. Sale’s were actually heavy.
Michael F. Flynn’s The Wreck of the River of Stars.
Schlock Mercenary had some dumb sapient robots on light sails, and Dyson star-ennclosures that were basically Mylar balloons around a star, supported by light pressure.
The Pioneer anomaly has been explained as an inadvertent thermal photon drive at work. Not the same as a light sail but similar in the “light has momentum” front.
@15: I was thinking of mentioning Wreck – but that ship used a magnetic sail, not a light sail.
There’s a light sail (laser assisted) in Forward’s Rocheworld.
@4/Kedamono
The wiki entry (https://james-camerons-avatar.fandom.com/wiki/Interstellar_Vehicle_Venture_Star) claims the ISV Venture Star uses laser boost from Sol, but in its few shots in the 2009 movie during Pandora approach, we don’t see the sail — or, for that matter, the deployed stack of debris shields, or dialog confirmation that antimatter is used. What we see is a Charles Pellegrino-style tractor configuration in which the engines and their red-glowing radiators vastly outmass the payload (which itself is rare in visual SF).
(Love or hate the movie’s plot, there was a lot of worldbuilding behind it, and I am vaguely annoyed that James Cameron didn’t execute on the novel that he teased.)
@11: note that turning Sol into a nova is an extreme measure, used only when cultures can’t or won’t build a laser (as in the later Mote) to send the trader on its way. Laser boosts are a cute way to get around the very low acceleration of natural-light sails while still shutting out the mass-ratio equation, but I haven’t seen calculations of how many power plants the lasers would need.
Sadly, iirc the relevant equation is something like P = FC/2, where P is power, F force and C the speed of light, so matching the ten Newton/km^2 a light sail gets for free from the sun takes 3 GW, which I think will run you sixty or seventy bucks a second. The important lesson here is sunlight is inherently socialist and we should encase the sun to better monetize its output.
Since noone else has mentioned it, I feel the need to link to XKCD’s Laser Pointers and More Power, which quite well demonstrates the sorts of energies involved on the broadcast side.
I see others have already pointed out one of the best-known light-sail stories, The Mote In God’s Eye (Niven & Pournelle, 1974).
Much more recently, I’ve been enjoying Nathan Lowell’s Solar Clipper Trader series (and related spin-off series), which starts with Quarter Share. He cheats, using “force fields” instead of a physical sail, allowing them to be unreasonably large and hence unreasonably powerful (plus there’s a space-folding drive for the big distances, once they get far enough away from the local sun).
@9, @10: FWIW, the “solar sail” in the academy ship in Bodacious Space Pirates seems to be a magnetosail. I also suspect its size to be on the order of at least a couple hundred miles in diameter, given its performance.
@9, @10, @23, re: Bodacious Space Pirates with a so-called “solar sail” that acts like a magnetosail: this isn’t the only case of SF anime misusing established English-language terminology. The not-Gundam mecha series Valvrave has space colonies called “Dyson spheres” that are merely sub-planetary-scale volvox-shaped clusters of domes (with, evidently, non-centrifugal artificial gravity). To be fair, in Star Trek IV the Whaleprobe-becalmed USS Yorktown rigged a “makeshift solar sail” for power, which is equally a misnomer.
In the anime series Martian Successor Nadesico (1996), the Aestivalis mecha (specifically, its cockpit/escape pod) can deploy a solar sail as an emergency measure. From the context it’s intended to be propulsive, as the pilot had previously jettisoned the limbs for thrust. (In general, this series feels like the writer has been reading pop-physics and is showing off, as with the hero-ship’s “vacuum transition engine”, or an enemy artillery-piece that fires micro black holes of a size where the Hawking radiation is dangerously explosive.)
The microgravity mecha (“gardes”) of Knights of Sidonia can deploy (non-propulsive) sails to collect “Heigus particles” (possibly intended as “Higgs particles”, but definitely the show’s answer to Gundam’s “Minovski particles”).
Many years ago, I think as part of the Science Book Club, I picked up a copy of Space Sailing by Jerome Wright, which seems like a good introductory text for anyone who’s interested in the math and the practicalities of solar sails.
https://www.amazon.com/Space-Sailing-Jerome-Wright/dp/288124842X/ref=sr_1_8?keywords=solar+sail&qid=1559745771&rnid=2941120011&s=books&sr=1-8
If I’m not mistaken, that Arthur C. Clarke story first appeared in Boy’s Life. And the Poul Anderson story appeared in Analog under one of his pen names, Winston Sanders, at almost the exact same time. Both under the name “Sunjammer.”
I think Gene Wolfe’s Urth of the New Sun has some action on a interstellar yacht-like ship with multiple light sails and masts and complicated topography, which was fascinating to try and visualise.
Other commenters have mentioned two other instances of Larry Niven using light sails. The third instance I can think of are the Star Seed Pods in his Known Space series. Living, non-sapient (?), beings who sail between the stars on light sails and are followed, for mysterious reasons, by the Outsiders.
@20 And that’s for light actually hitting the sail… If your laser is not perfectly focused on the sail, factor in losses. Not to mention the efficiency of the laser at converting source power into laser light…
@11 If I remember correctly, and I may well be wrong, but I think that’s in his Draco’s Tavern series.
@28 Not the Draco Tavern series, but another bartender as a protagonist.
I’m confused by that chart. So for that last one, the “70 tons to Mars with aerobrake” includes the mass of the sail (12 metric tons)? So it would be 58 metric tons, both ship and payload (although it says “w/o cargo”). Is that right?
That’s pretty good for cargo hauling, especially if you can re-use the sail because you’re just cycling it between Earth and Mars. Not sure if it would be an improvement over solar-electric propulsion.
Oh, and my favorite use of light sails in fiction is in the highly scientifically implausible Star Winds by Barrington J. Bayley, a textbook example of a book I judged by its cover. (See, those wacky physicists with all of their “atoms” and “electrons” were trying to make things too complicated, and everything really is composed of just the four elements; and you can make sails that catch the ethereal currents that flow between planets and stars.)
At first I thought no one had mentioned that Forward used his light sail in The Flight of the Dragonfly, but then I saw it mentioned under Rocheworld. I always thought it was a really cool idea.
‘The Mote in God’s Eye’ rides again!
https://www.abc.net.au/news/2021-06-08/canberra-anu-scientists-laser-sail-interstellar-space-discovery/100198228
Back in the early 1980s I remember reading a short story about a family where the mom/dad were scientists and they were going to be testing a solar sail. I remember that they unfurled the sail and slowed down but then started speeding up and started approaching light speed. I went searching for that story and found this thread so now I gotta go through all these stories to see if I can find it. If anyone knows which one it is offhand, I’m all ears.
Might as well plug my lightsail story, “The Caress of a Butterfly’s Wing,” published in the online BuzzyMag in 2014 and still available for free. Let’s just say I took the concept of a lightweight, minimal sailcraft to the extreme.