Artist's impression of a solar sail leaving Earth orbit. Is this the future of space travel? Yes and no.
Artist's impression of a solar sail leaving Earth orbit. Is this the future of space travel? Yes and no.
updated 9/6/2012 3:23:26 PM ET 2012-09-06T19:23:26

It's interesting when you look back at the history of space exploration and realize that propulsion technology hasn't really changed very much.

The earliest rocket prototypes were nothing more than elaborate versions of weapons used during World War 2 and fireworks used during civil celebrations. Even the Space Shuttle made use of solid rocket fuel technology in its pair of solid rocket boosters. But, with the liquid rocket fuel propulsion in the external tank, this combination has proved to be highly effective and launched hundreds of astronauts into space.

The approach works -- albeit not very efficiently -- and to get out of the gravitational well of the Earth, it seems for now that the extra punch from exothermic processes is needed.

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In deep space, however, there are alternatives receiving very serious consideration -- such as the "eco-friendly" solar sail.

The solar sail concept is simple: any surface exposed to electromagnetic radiation 'feels' a pressure known as radiation pressure and it's this pressure that exerts a tiny pushing force against the surface. If the surface happens to be a spacecraft or part of a spacecraft, it could act against it to provide propulsion through space.

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The concept of radiation pressure isn't particularly new. The idea was first alluded to by Johannes Kepler in 1610 when he suggested the reason why the tail of a comet points away from the sun was in some way caused by the sun. Kepler even made reference to using this unknown force for exploration when he wrote in a letter: 'Provide ships or sails adapted to the heavenly breezes, and there will be some who will brave even that void.'

NEWS: NASA's Nanosail-D Released into the Winds of Space

By 1864, it was accepted that the light carried momentum and would exert a pressure on anything it meets. A great demonstration of this can be seen in the Nichols radiometer, which is a sealed bulb with tiny silvered glass mirrors attached by a very thin wire inside the glass. On being exposed to light the mirrors start to rotate, driven by radiation pressure excerted by photons from the bulb filament.

The technique is already being used in space exploration to for course corrections and fuel savings. For example, NASA's Mercury MESSENGER probe successfully used solar radiation pressure to make small course corrections during its journey to the innermost planet.


To make the most out of radiation pressure for space exploration every bit of solar energy needs to be eked out. For any useful form of propulsion, giant solar sails need to be used and exposed to as much light as possible.

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The first interplanetary test of a solar sail was conducted by the Japanese Aerospace Exploration Agency in May 2010 with the launch of Ikaros. This was the first time a solar sail was deployed and tested in space and used as its primary propulsion. NASA also launched the orbital solar sail prototype Nanosail-D in November 2010, successfully completing its mission after 240 days in Earth orbit. Ikaros, on the other hand, continues its journey around the sun after passing Venus in December 2010.

With Ikaros' 27 square meter sail deployed, the full effect of radiation pressure from the sun on the sail produces about 0.0002 pounds of force, thats equal to about 0.1 grams -- less than the average goose feather! The acceleration offered by this method of propulsion is small but over a long period of time, incredible speeds could be reached.

The downside to this mode of transportation is that heavier craft will take longer to accelerate, so larger sails would need to be manufacturered. Ikaros' sail was impregnated with solar cells to power the electronic equipment and a matrix of liquid crystals around the outside whose reflectivity could be altered to change the attitude of the spacecraft.

Future missions will take these tests further from the sun. The challenge here is that the further away from the sun you go, the weaker the radiation pressure, so acceleration through interstellar space will be limited. Innovations in laser technology may extend the range of solar sails.

The technology is no doubt in its infancy, but new ideas of rotating solar sails in various configurations shows great promise.

© 2012 Discovery Channel

Photos: Month in Space: January 2014

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  1. Southern stargazing

    Stars, galaxies and nebulas dot the skies over the European Southern Observatory's La Silla Paranal Observatory in Chile, in a picture released on Jan. 7. This image also shows three of the four movable units that feed light into the Very Large Telescope Interferometer, the world's most advanced optical instrument. Combining to form one larger telescope, they are greater than the sum of their parts: They reveal details that would otherwise be visible only through a telescope as large as the distance between them. (Y. Beletsky / ESO) Back to slideshow navigation
  2. A balloon's view

    Cameras captured the Grandville High School RoboDawgs' balloon floating through Earth's upper atmosphere during its ascent on Dec. 28, 2013. The Grandville RoboDawgs’ first winter balloon launch reached an estimated altitude of 130,000 feet, or about 25 miles, according to coaches Mike Evele and Doug Hepfer. It skyrocketed past the team’s previous 100,000-feet record set in June. The RoboDawgs started with just one robotics team in 1998, but they've grown to support more than 30 teams at public schools in Grandville, Mich. (Kyle Moroney / AP) Back to slideshow navigation
  3. Spacemen at work

    Russian cosmonauts Oleg Kotov, right, and Sergey Ryazanskiy perform maintenance on the International Space Station on Jan. 27. During the six-hour, eight-minute spacewalk, Kotov and Ryazanskiy completed the installation of a pair of high-fidelity cameras that experienced connectivity issues during a Dec. 27 spacewalk. The cosmonauts also retrieved scientific gear outside the station's Russian segment. (NASA) Back to slideshow navigation
  4. Special delivery

    The International Space Station's Canadian-built robotic arm moves toward Orbital Sciences Corp.'s Cygnus autonomous cargo craft as it approaches the station for a Jan. 12 delivery. The mountains below are the southwestern Alps. (NASA) Back to slideshow navigation
  5. Accidental art

    A piece of art? A time-lapse photo? A flickering light show? At first glance, this image looks nothing like the images we're used to seeing from the Hubble Space Telescope. But it's a genuine Hubble frame that was released on Jan. 27. Hubble's team suspects that the telescope's Fine Guidance System locked onto a bad guide star, potentially a double star or binary. This caused an error in the tracking system, resulting in a remarkable picture of brightly colored stellar streaks. The prominent red streaks are from stars in the globular cluster NGC 288. (NASA / ESA) Back to slideshow navigation
  6. Supersonic test flight

    A camera looking back over Virgin Galactic's SpaceShipTwo's fuselage shows the rocket burn with a Mojave Desert vista in the background during a test flight of the rocket plane on Jan. 10. Cameras were mounted on the exterior of SpaceShipTwo as well as its carrier airplane, WhiteKnightTwo, to monitor the rocket engine's performance. The test was aimed at setting the stage for honest-to-goodness flights into outer space later this year, and eventual commercial space tours.

    More about SpaceShipTwo on PhotoBlog (Virgin Galactic) Back to slideshow navigation
  7. Red lagoon

    The VLT Survey Telescope at the European Southern Observatory's Paranal Observatory in Chile captured this richly detailed new image of the Lagoon Nebula, released on Jan. 22. This giant cloud of gas and dust is creating intensely bright young stars, and is home to young stellar clusters. This image is a tiny part of just one of 11 public surveys of the sky now in progress using ESO telescopes. (ESO/VPHAS team) Back to slideshow navigation
  8. Fire on the mountain

    This image provided by NASA shows a satellite view of smoke from the Colby Fire, taken by the Multi-angle Imaging SpectroRadiometer aboard NASA's Terra spacecraft as it passed over Southern California on Jan. 16. The fire burned more than 1,863 acres and forced the evacuation of 3,700 people. (NASA via AP) Back to slideshow navigation
  9. Where stars are born

    An image captured by NASA's Spitzer Space Telescope shows the Orion Nebula, an immense stellar nursery some 1,500 light-years away. This false-color infrared view, released on Jan. 15, spans about 40 light-years across the region. The brightest portion of the nebula is centered on Orion's young, massive, hot stars, known as the Trapezium Cluster. But Spitzer also can detect stars still in the process of formation, seen here in red hues. (NASA / JPL-Caltech) Back to slideshow navigation
  10. Cygnus takes flight

    Orbital Sciences Corp.'s Antares rocket rises from NASA's Wallops Flight Facility on Wallops Island, Va, on Jan. 9. The rocket sent Orbital's Cygnus cargo capsule on its first official resupply mission to the International Space Station. (Chris Perry / NASA) Back to slideshow navigation
  11. A long, long time ago...

    This long-exposure picture from the Hubble Space Telescope, released Jan. 8, is the deepest image ever made of any cluster of galaxies. The cluster known as Abell 2744 appears in the foreground. It contains several hundred galaxies as they looked 3.5 billion years ago. Abell 2744 acts as a gravitational lens to warp space, brightening and magnifying images of nearly 3,000 distant background galaxies. The more distant galaxies appear as they did more than 12 billion years ago, not long after the Big Bang. (NASA / NASA via AFP - Getty Images) Back to slideshow navigation
  12. Frosty halo

    Sun dogs are bright spots that appear in the sky around the sun when light is refracted through ice crystals in the atmosphere. These sun dogs appeared on Jan. 5 amid brutally cold temperatures along Highway 83, north of Bismarck, N.D. The temperature was about 22 degrees below zero Fahrenheit, with a 50-below-zero wind chill.

    Slideshow: The Year in Space (Brian Peterson / The Bismarck Tribune via AP) Back to slideshow navigation
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