Solar Sail

This image depicts the Japanese IKAROS spacecraft, which used its solar sail to control altitude and rotation speed. Yes, this trope is Truth in Television.

In frigid darkness they slept, and as they slept, the cold winds of light between the stars swept them onward, slowly...

— At the Mountains of Madness (Atlanta Radio Theater Company version)

So you've got your nice big spaceship, but you have one big problem — fuel. It's heavy and runs out real quick.

The solution? Build a paper-thin mirrored sail a hundred kilometers across, and hang it in front of your ship — the pressure of light from stars will slowly propel your ship outwards. As you get closer to a star, you get more thrust, and as you get further away, the thrust drops off according to the inverse-square law. Using the thrust to move directly away from the sun is rarely the goal, since the acceleration is low and continuous and doesn't usually equal the sun's gravity.

Like a sailing ship, which uses the contrary force vectors of the water and the wind to choose its direction, a solar sail ship uses the contrary vectors of the sun's gravity (always in) and the sun's light (straight out). A sail ship angles relative to the sun to either increase or decrease its tangential orbital velocity, and therefore the radius of its orbit. In some cases, a solar sail might be used to "hang" a ship or satellite over a star in a place orbital mechanics don't allow (see: Dyson Sphere), but in this case it's more of a solar parachute. Unlike a sailing ship at the waters, however, a solar sail cannot "tack" into the solar wind. Tacking requires friction, which is famously lacking in space.

A close cousin is the electromagnetic sail, any of several similar systems that use a huge conductor or set of conductors (like kilometers of superconducting wire or a cloud of ionized gas) and an electromagnetic field to steal kinetic energy from passing solar wind, the corona of charged particles constantly streaming from a star. While the reflected sunlight used by a reflective solar sail provides orders of magnitude more push per surface area in the inner parts of a solar system, the working area of an electromagnetic sail is a "virtual sail" of electromagnetic force generated by a comparatively minuscule amount of material, and the average energy density of solar wind remains fairly potent far into the outreaches of a solar system. The net result is that these systems make up for the lower inner-system energy density by sheer size and operating range, while still being far lighter than a reflective solar sail, at the cost of requiring input energy, and the invisible "sail" being less visually interesting. An additional advantage for these systems, or hybrid systems mixing electromagnetic and solar sails, is that tacking and breaking are possible through a rare case of reversing the polarity actually being meaningful- because manipulating the orientation and strength of the field alters the direction and intensity of force between interacting electromagnetic fields, a keel's opposing drag can be replicated via secondary sails or "kite-tail"-type structures tuned to work at angles or opposition to the primary sail(s).

For interstellar voyages, the ship is usually propelled by a giant laser cannon in space shooting at the sail.

Real-life solar sailing is limited to attitude and rotational control, but it has still advanced by quite a lot despite the technology's infancy — Japan has already sent a solar sailing spacecraft on a Venus flyby trajectory in 2010, and other agencies and groups are planning to launch solar sails for purposes other than to test the sail itself.

Common in hard science fiction, or as Technobabble for Space Sailing starships.

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Examples

Anime

• Martian Successor Nadesico uses this brilliantly. To lure out a hidden Jovian weapon that's been attacking them, Yurika shuts off the Nadesico and has the Aestivalises hooked up to solar sails to keep them powered up, making it akin to fishing. Once the weapon is finally revealed, everything's reactivated and the crew forces it to escape.
• The Mobile Suit Gundam SEED CE.73: Stargazer ONA has the "Stargazer" Gundam. It uses a Voiture Lumiere system as a magnetic sail. It is meant to be an unmanned probe controlled by a rudimentary AI for extrasolar exploration, though it can carry a couple of passengers/pilots if necessary. It also takes the Gundam about a month to make it from Venus to Earth using just the Sun's output for thrust (it rode a giant particle beam to get out to Venus in the space of a couple of minutes).

Comic Books

• Guardians of the Galaxy character Starhawk has these built into his spandex super-suit, one set one his back and a matching set on his head — which is impractical and unlikely, but...
• Dyltah from Star Crossed was genetically engineered into a "living lightsail" so she can glide on solar currents.

Film — Animation

• Jim Hawkins' sailboard and the RLS Legacy from Treasure Planet are referred to as solar sails, but they have little in common with actual solar sails — here, they're seen as being pushed by "solar winds" much like sailing ships on oceans use planetary winds.

Film — Live-Action

• The titular ship deploys one to recharge near the start of Alien: Covenant, working more like solar panels than this trope.
• In Attack of the Clones, Count Dooku uses a ship equipped with a (unrealistically small) solar sail to escape from a battle.
• The Venture Star in Avatar uses a solar sail to accelerate to its destination several lightyears away. A laser in the Sol System provides thrust for the sail to get up to 70% lightspeed. However, the ship uses an antimatter rocket to slow down when it reaches Alpha Centauri. The reverse is true on return trip: accelerating away from Alpha Centauri with antimatter rocket and slow down to Sol with laser.
• The pod people from Invasion of the Body Snatchers are Starfish Aliens that are propelled interstellar distances by solar winds.
• In TRON, a simulated vehicle called a Solar Sailer is stolen by two of the movie's protagonists to make a hasty exit from the facility where it just happens to have been under construction. To avert the need for steering, a Solar Sailer's path follows the path of whatever data beam it rides on. A much larger version is used in TRON: Legacy for bulk transport of abducted programs.

Literature

Examples by author:

• Cordwainer Smith has interstellar solar sails in two stories, "The Lady Who Sailed The Soul" and "Think Blue, Count Two." They're the basis of humanity's first expansion to other solar systems, in decades-long voyages with passengers in suspended animation.

Examples by title:

• The original Starfish Aliens, the Elder Things introduced in At the Mountains of Madness, have fan-like wings that act as biomechanical ones allowing them to fly through space in the nude. They're apparently very good at holding their breath (although holding your breath in outer space would be a really bad idea as the air inside your lungs will expand and rupture your insides).
• The Odette II, the yacht club's training vessel (and a former pirate ship), in Bodacious Space Pirates.
• In Allen Steele's Captain Future novel Avengers of the Moon, Leigh Brackett makes use of an enormous lens located at a Lagrange point near the Moon to concentrate photonic energy against its sail. Smaller vessels use it as The Mothership to carry them to Mars. This becomes a plot point when a saboteur tries to detach Future's spaceship from its mooring cradle so they'll be stranded in space, as a vessel with this form of propulsion can't just stop and pick them up again without becoming stranded themselves.
• A derelict alien-built solar sail vessel appears in Decency by Robert Reed. The sail is wider than the Moon, and is punctured from impacts with stray atoms of dust in interstellar space. When it impacts with the Earth, the impact itself is harmless due to the sail being so fragile.
• Nathan Lowell's Golden Age of the Solar Clipper series has the titular ships using the magnetic sail variant to cruise out of star systems on the solar wind, then use a jump drive to reach the edge of the next system and a "gravity keel" to tack against the solar wind there.
• The Martians in Last and First Men, being creatures whose bodies are composed of trillions of nanoscopic (not microscopic) viruses that communicate on a central nervous system of radiation, are able to conform their bodies into thin solid sheets that can traverse interplanetary distances. This is how they reach an Earth inhabited by the Second Men to begin their crusade to 'liberate' terrestrial diamonds which they believe are being handled sacrilegiously by the planet's dominate species, which the Martians also mistakenly think are the humans' radio devices.
• The arrival of an alien solar sail vessel in a human colonized system in The Mote in God's Eye starts off the plot — although this one was the variation that is initially accelerated by a planet-based laser emitter.
• The Nano Flower: Nearing the end, rather than destroy or be destroyed by humanity, the alien organism leaves to find an uninhabited solar system, by shapeshifting into a solar sail 500 kilometers in diameter.
• Solar sails are used as weapons in the Revelation Space Series—a monolayer solar sail does wonders at killing the starships flying at 90% the speed of light. In an interstellar chase, a sail is released behind the ship being pursued, and a powerful laser pushes the sail back towards the pursuer; with the speed difference between the two objects, contact would result in instantaneous destruction.
• In Road to Mars, the multinational crew of the Ares receives orders from Mission Control to beat the Chinese Millennium Boat in a race to be the first humans on the red planet. The Captain of the Ares has the crew adjust the course to get dangerously close to the Sun in order to shave off a few hours and orders the crew into the living pod in order to shield everyone from increased heat and radiation. Just then, he gets a message from Mission Control, letting him know that there is a secret container aboard the craft, containing an experimental solar sail (the knowledge was kept secret in order to allow the company to file a patent on the new lightweight material). He is also told that a solar flare is about to occur just as they're passing the Sun. This will not only allow the sail to unfurl and give them the needed boost to beat the Chinese, but the magnetic field will also shield the craft from radiation, allowing the crew to go EVA to set up the sail (unfortunately, a tool failure results in one of the crew members receiving a lethal dose of radiation).
• The Rocheworld series features solar sails heavily. They are used for cargo vessels in the solar system, and the interstellar ship sent to the Barnard system uses a solar sail propelled by a laser on Mercury.
• The Wind from the Sun by Arthur C. Clarke is about an international race by solar sailing ships from the Earth to the Moon, patterned on America's Cup style yacht racing.

Live-Action Television

• For All Mankind: In Season 3, the Sojourner-1 Mars mission has a solar sail.
• The Reveal in The Outer Limits (1995) episode "The Message" is that the 1s and 0s the woman keeps furiously scribbling at 3:10 AM every night are a cry of help from an alien ship that is propelled by a solar sail. The ship is moving on a trajectory towards the Sun, much too fast for the solar sail to stop. They instruct the woman how to build a high-energy laser to push the craft onto a safer course. It works.
• The Star Trek: Deep Space Nine episode "Explorers" has Captain Sisko recreate an ancient Bajoran spacecraft with solar sails. It supposedly made it all the way to a neighboring star system, though some characters doubt the voyage could be achieved at sublight speeds. It turns out that the sails can also catch tachyons in order to achieve Faster-Than-Light Travel.

Tabletop Games

• In Battlefleet Gothic, Eldar Craftworld and Corsair starships are propelled by large solar sails. In game this is represented by Eldar ships having three movement speeds representing whether they are moving towards, away from or parallel to the sun, meaning that they have to tack like a sailing ship to get the best speed across the battlefield.
• In BattleTech, Jumpships and Warships use jump sails to recharge their jump drives. The sails take a long time to set up and it takes a week to fully recharge the drives. It's worth noting that in this case the sails only act as basically giant flexible solar panels; any actual sublight propulsion worth mentioning is done via fusion rocket, it's just that the average Jumpship is essentially just a single FTL drive core with some station-keeping engines and quarters for the crew as an afterthought.
• The Big Eyes, Small Mouth sourcebook Centauri Knights uses beam-assisted magsail ships as the only means of travel between Alpha Centauri and the Sol system. They travel at a significant fraction of light speed, but the trip still takes seven years.
• Jovian Chronicles has solar sail and mag-sail craft as the primary bulk cargo carriers. The trade fleets operate from Mercury, because its central location next to the sun makes it a convenient stopping point. Solar sails are used in the inner systems, but only magsails are practical to reach the Jovian colonies and the asteroid belt. Past that, the solar wind is so diffuse that the Jupiter to Saturn trade relies on fusion-powered ships — fortunately, there are big sources of hydrogen at both ends of that run.
• An optional technology in Traveller, but usually too slow to be worth it except for sport.

Video Games

• The Interstellar Game Mod for Kerbal Space Program is a roughly 2500m^2 square sail. It has a truly pathetic amount of thrust even when close to the sun, but it's fairly lightweight and has infinite delta-V, useful for trimming interplanetary orbits. Navigation with the solar sail is Difficult, but Awesome, as its flight mechanics are completely different from traditional rockets and only receives thrust with the sun in direct line-of-sight.
• In Kingdom Hearts II, the second visit to the Space Paranoids world has the party take the Solar Sailer from TRON to the MCP's location for the final battle with it.
• The Outer Wilds: Echoes of the Eye DLC is set on "the Stranger," a Ringworld hiding behind an Invisibility Cloak. It originated from outside your solar system, but was flown there on solar sails hundreds of thousands of years ago. Said sails deploy shortly after you board the station (in response to detecting that your sun is about to go supernova), and unfortunately, because the Stranger is so old, the shock of this is The Last Straw for the dam inside its habitat, resulting in a Giant Wall of Watery Doom sweeping along the inside of the ring.
• The titular ship in Seedship uses one.
• Starting in Space Empires IV, you can place solar sails on your ships for bonus movement. They still work even if there isn't an actual star in the system, though.
• A random event in Stellaris has you discovering a derelict solar sail ship. Sadly, you can't reverse-engineer the tech to use it yourself, but you do get some bonus engineering research off of it.
• Treasure Planet: Battle at Procyon: Most of the ships are powered by solar sails, with the exception being the Ironclads, which are powered by dark matter. The sails on ships that have them can be targetted to cripple the ship's mobility. Ships with sails will also lose power if the ship moves into a Nebula, as it obstructs light from nearby stars.

Webcomics

• Schlock Mercenary:
  • The Gatekeepers use "solar sail material" to make Dyson Spheres, their word for which translates to "this was expensive to buil.".
  • In one arc, the crew encounter a small STL starship with a solar sail that "you could giftwrap planets with".

Western Animation

• The Back to the Future episode "Solar Sailors" is mostly set on an interplanetary cruise ship powered by a solar sail.

Real Life

• The billion-dollar interstellar travel company Breakthrough Starshot aims to send solar sails to our nearest star, Alpha Centauri, by sending unmanned probes with light propelled sails to the star system using lasers at 20% the speed of light. Because of the distance between Sol (our sun) and Alpha Centauri, it will take about 20 years to reach it. The probes will take pictures of planets in Alpha Centauri while never landing on any of them. It is planned to send manned crafts to Alpha Centauri after enough probe observations of atmospheric conditions. Since this manned craft will probably not launch until a decently far future, there is a possibility cryogenic technology could have been invented to allow astronauts or potential colonists to remain in stasis on the way there.
• The Japanese IKAROS (Interplanetary Kite-craft Accelerated by Radiation of the Sun) space probe was launched in 2010. It was the first spacecraft to use a solar sail as its main method of control (a traditional chemical rocket placed the probe on its Venus flyby trajectory) and was the first interplanetary solar sail spacecraft. The craft's journey to Venus was intended to test the sail's control system — LCD panels on the sail itself can adjust their reflectivity, giving the craft a method of attitude control. Cycling the panels on and off also allows the spacecraft to control its rotational speed. Since its launch in 2010, IKAROS has completed its original mission successfully and now orbits the Sun once every ten months, periodically waking up and transmitting data back to Earth.
• NASA's first attempt at launching a solar sail-powered craft was much less ambitious- the NanoSail-D mission was designed to test a solar sail in low Earth orbit. The first launch in 2008 failed, destroying the probe; however, a launch spare was modified and improved to become NanoSail-D2, which was launched successfully in 2010, after IKAROS was launched. Unlike IKAROS, NanoSail-D2 does not exist anymore — it reentered the atmosphere and burned up as planned.
  • Although NanoSail-D2 was NASA's replacement for NanoSail-D, the original NanoSail-D mission was handed to the non-profit Planetary Society. NanoSail-D was renamed LightSail-1, and although NanoSail-D's successor had been launched successfully in 2010, LightSail-1 launched in May 2015 and although battling problems and gaps in communication, it completed the mission successfully and entered the Earth's atmosphere in June 2015 (presumably burning in it). LightSail-2 launched in June 2019, and entered the athmosphere in November 2022.
• Proposed as a far less dangerous method of dealing with a meteor heading towards Earth than launching a giant nuke at it. A robot spacecraft would attach a solar sail which would gradually pull it off course — if the meteor is caught early, even the minuscule acceleration from a solar sail is enough to change its trajectory substantially by the time it comes close to Earth.

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