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Solar Sail
This image depicts the Japanese IKAROS spacecraft, which used its solar sail to accelerate itself on its way to Venus. 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, however, a solar sail cannot "tack" into the solar wind. Tacking requires a keel submerged in a medium with much greater drag (e.g. water), which a solar sailing vessel lacks.

A close cousin is the magnetic sail, a system that uses a huge conductor (kilometers of superconducting wire or a cloud of ionized gas) and a magnetic field to steal kinetic energy from passing solar wind. Since the charged particles streaming from the sun are more massive than photons, this system delivers more thrust, but it requires input energy, and isn't as visually interesting since the "sail" is invisible.

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

Real-life solar sailing has advanced by quite a lot despite the technology's infancy - Japan has already sent a solar sailing spacecraft to Venus 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 Techno Babble for Space Sailing starships.


Examples

    open/close all folders 

    Anime 
  • The Odette II, the yacht club's training vessel (and a former pirate ship), in Bodacious Space Pirates.
  • The Mobile Suit Gundam Seed Destiny 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).
  • Martian Successor Nadesico used 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.

    Comics 
  • Highly obscure Marvel Comics character Starhawk had 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...

    Film 
  • 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.
  • Count Dooku in Star Wars: Episode II uses a ship equipped with a (tiny) solar sail to escape from a battle.
  • Jim Hawkings' sailboard and the RLS Legacy from Treasure Planet.
  • 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 
  • 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.
  • 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 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.
  • Robert L. Forward's Roche World 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.
  • Solar sails are used as weapons in Alastair Reynolds' Revelation Space trilogy - a paper thin 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 the ship's exhaust pushes the sail back towards the pursuer; with the speed difference between the two objects, contact would result in instantaneous destruction.
  • The original Starfish Aliens, the Elder Things introduced in H.P. Lovecraft's 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.

    Live Action Television 
  • Star Trek: Deep Space Nine has Captain Sisko repair a derelict Bajoran spacecraft with solar sails. He and his son decide to go on a voyage and accidentally end up in Cardassian space. Earlier in the episode shows a Cardassian and Bajoran arguing whether ancient Bajorans did make it to Cardassia.
  • The Reveal in The Outer Limits episode "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.

    Real Life 
  • The Japanese IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun) space probe was the first spacecraft to use a solar sail as its main method of propulsion. The craft was launched in 2010. It used the radiation pressure from the Sun to propel itself toward Venus, making history as the first solar sail-powered interplanetary spacecraft. The craft's journey to Venus was 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. Currently JAXA, the Japanese space agency, is using the craft to further refine control methods for solar sail-powered craft.
  • NASA's first attempt at launching a solar sail-powered craft was much less ambitious - the Nano Sail-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 Nano Sail-D2, which was launched successfully in 2010, after IKAROS was launched. Unlike IKAROS, Nano Sail-D2 does not exist anymore - it reentered the atmosphere and burned up as planned.
    • Although Nano Sail-D2 was NASA's replacement for Nano Sail-D, the original Nano Sail-D mission was handed to the non-profit Planetary Society. Nano Sail-D was renamed Light Sail-1, and although Nano Sail-D's successor had been launched successfully in 2010, Light Sail-1 still awaits launch. The Planetary Society had earlier tried to launch in 2005 the first ever solar sail-powered spacecraft, Cosmos 1, but the launch vehicle failed to reach orbit.
  • 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.

    Role Playing Games 
  • 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.
  • The Jovian Chronicles have 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 to slow to be worth it except for sport.

    Tabletop Games 
  • Eldar ships in Battlefleet Gothic use solar sails for propulsion.
  • 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.

    Video Games 
  • 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.
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