A solar sail is a device that uses the radiation pressure of sunlight to propel a spacecraft in the same way that a sailing ship uses wind. It consists of a large but extremely thin sheet of strong, flexible material, such as Mylar film, and a rigid framework to keep it extended and to transmit its pressure to the spacecraft. The sheet is coated on one side with a highly reflective surface so that it acts as a very efficient mirror. Sending most of the light back nearly doubles the pressure when the sail squarely faces the Sun. The sail can also be tilted at an angle to the Sun's rays so as to reflect them in some chosen direction, e.g. in the direction opposite from the one in which the spacecraft is moving, which gradually increases the vehicle's velocity. When the sail faces the Sun squarely, the pressure of sunlight counteracts the Sun's pull and only enables the satellite to circle the Sun in a slightly larger orbit. On the other hand, placing it at an angle can put the spacecraft on an outward or inward spiral which makes it useful for interplanetary missions.

 

A solar sail is only able to function where sunlight is sufficiently intense. At Earth's distance from the Sun, the pressure of sunlight on a square km of sail is so feeble that if sail and payload had a mass of 5,000 kg the spacecraft would accelerate at a maximum of 0.0001g (1/10,000th the acceleration to gravity on Earth). Although this seems tiny, it would lead over a six-month period to a velocity of about 12 kilometers per second.

 

History of the idea

An early hint that radiant energy from the Sun could be harnessed for propulsion came almost four centuries ago when Johannes Kepler observed comet tails pointing away from the Sun, apparently being blown by a solar breeze. Taking this as evidence that winds blew objects about in the heavens, he suggested that eventually ships might be able to navigate beyond the Earth by using sails to catch the air currents of space. Of course, we now know that space contains a pretty good vacuum. Moreover, the "wind" that future solar sails will catch is not the so-called solar wind of charged particles that escapes from the Sun's surface. The propulsive force will be supplied by the pressure of sunlight itself, which typically provides about 1,000 times more pressure than that available from the solar wind.

 

The concept of the solar sail was first mooted in the 1920s by Konstantin Tsiolkovsky and Fridrikh Tsander. However, the first in-depth treatment of the idea wasn't published until 1951 when Carl Wiley's article "Clipper Ships of Space" appeared in the May issue of Astounding Science Fiction. Wiley, an aeronautical engineer, was conscious of the risk to his career of expressing far-out ideas in a science-fiction magazine and hence wrote under the pseudonym "Russell Saunders."

 

In 1958, Richard Gamin, a Defense Department consultant with IBM, authored the first article in a professional publication. Gamin's paper in Jet Propulsion, which included preliminary calculations of sail-vehicle performance, encouraged others to produce technical papers; these subsequently appeared in the engineering literature and in NASA and university publications. Also in 1958, Ted Cotter, at the Los Alamos Scientific Laboratory, discussed the notion of a spinning solar sail. Time magazine picked up on his ideas in a 1958 editorial, "Trade Winds in Space."

 

In 1960, Philippe Villers at the Massachusetts Institute of Technology wrote his master's thesis on the subject. A meeting on solar sail design was held that same year at NASA's Langley Research Center, and a short course on solar sailing was offered at the University of California at Los Angeles the following year. NASA carried out various technology studies on solar sails in the mid-1960s but as funding for the space program declined in the wake of Apollo this work came to an end. Then, in the mid-1970s, NASA interest was revived and NASA administrator James Fletcher authorized a new study after communicating with Gamin about the concept. This new work was assigned to Jerome Wright at Battelle Memorial Institute in Ohio. While considering the possibilities of solar sailing for interplanetary missions, Wright hit upon the idea of using a solar sail-powered craft to encounter Halley's Comet upon its return in 1986. Wright came up with a scheme whereby a sail-driven probe would be able to reach and rendezvous with the comet in a mere four years after launch. His ideas were taken up by a JPL (Jet Propulsion Laboratory) study group but were eventually rejected by NASA as being overly ambitious. The European Space Agency (ESA) also looked at the possibility of a Halley mission using solar sails but finally settled for a more conventional approach with its Giotto craft.

 

Although, in the 1970s, both NASA and ESA decided against sailing missions at the time because the technology available was insufficiently mature, interest in solar sails caught on elsewhere. Several groups around the world began considering practical missions to solar sail from Earth to the Moon or even to Mars. Efforts to build solar sails were made by amateur groups, including World Space Foundation, U3P Union for the Promotion of Photonic Propulsion, and Solar Sail Union of Japan, and semi-professional groups such the VSE European Solar Sail and the Russian Space Regatta Consortium. The idea, first envisioned by Arthur Clarke in the 1960s, of racing solar sails between Earth and Moon, was developed by U3P since 1981 and resulted in an official set of rules with the International Astronautical Federation.

 

Recent developments

On 4 February 1993, solar sail design finally left the drawing board and was tested for the first in the vacuum of space. A 2-m thin film structure onboard Progress M-15 was deployed in orbit, receiving its first illumination before sunrise over Western Europe. This was followed in 2001 by the first launch of the Cosmos 1 solar sail test spacecraft, sponsored by the Planetary Society and the Cosmos studio. Unfortunately, the command for the spacecraft to deploy from its launcher malfunctioned; however, a second attempt to deploy Cosmos 1, following its launch from a Vona missile, is expected to be made in late 2004.

 

Another giant step in the progress of solar sails was taken on 9 August 2004, when the Japanese space agency ISAS, now part of JAXA, successfully deployed a large film in space for the first time. An S-310 rocket, launched from Uchinoura Space Center at Kagoshima, carried two kinds of deploying schemes for films of 7.5-micrometer thickness. A cloverleaf-type deployment (see illustration) began 100 sec after liftoff at an altitude of 122 km, followed by a fan-type deployment 230 sec after launch at an altitude of 169 km altitude. Both experiments were successful.

 

The solar sail is a just one type of space sail. More ambitious space sail projects have been proposed that would work using magnetic fields (magsails) or directed laser beams, possibly enabling interstellar flight.