Setting Sail --- Navigating the Ocean of the Universe

The Emergence of a Wonderful Concept

Since the launch of the first artificial satellite by the Soviet Union on October 4, 1957, marking the beginning of the space age, every spacecraft launched into space has carried its own chemical fuel, electrical energy, or nuclear fuel. However, in recent years, some aerospace scientists have been studying how to utilize solar light particles—which are more reliable and “freer” than wind—to serve as the power for probes.

In fact, the concept of the solar sail emerged very early. The first modern concept of a solar sail appeared in the novel “From the Earth to the Moon” (1865) by the famous 19th-century French science fiction writer Jules Verne. Verne’s ideas undoubtedly laid the foundation for the work of Scottish physicist James Clerk Maxwell. In the 1870s, Maxwell discovered that light is composed of energy packets made of very small particles (which we now call “photons”). Photons produced by the sun diffuse into space in massive quantities at a speed of approximately 3 \times 10^5 km/s. At this speed, a photon can circle the Earth’s equator seven times in one second. Photons exert a force on any object they strike and bounce off. Although the energy produced by a single collision is extremely small, the total number of photons emitted by the sun is so vast that they can eventually push an object like a solar sail forward.

In the 1920s, Russian scientists first began research on using solar sails for interplanetary navigation and wrote books on the subject. In the 1950s and 60s, through the efforts of a new generation of scientists, engineers, and science fiction writers, this idea was revived. In 1964, science fiction author Arthur C. Clarke published the famous short story “The Wind from the Sun,” telling the story of an international manned solar sail race to the moon. Around this time, NASA began to seriously consider the solar sail concept and initiated several studies.

The “Perpetual Motion Machine” of the Universe

A solar sail is a spacecraft that does not carry an engine and is powered by sunlight. Traditional spacecraft are propelled by the impulse generated by the combustion of internal energy, whereas a solar sail is pushed solely by photons reflected by its massive, mirror-like sail. Solar sails do not fly by the “solar wind.” The solar wind consists of ions ejected by the sun, which move much slower than the speed of light. While the solar wind might transfer some force to the sail, the power obtained from it is less than 1/100 of the radiation pressure (the force transferred to the sail by sunlight).

For an interplanetary solar sail, its speed depends on the duration for which it is pushed by radiation pressure. In the initial stages of flight, a solar sail is quite slow. At such speeds, it might take a year to reach the moon, whereas the “Apollo” manned lunar spacecraft took only three days using conventional thrust. However, the true advantage of a solar sail is that, unlike chemical rockets whose thrust lasts only for a short time, the push of sunlight on a solar sail is continuous. Theoretically, after 100 days of flight, an interplanetary solar sail could reach a speed of approximately 1.6 \times 10^4 km/h, and 5.8 \times 10^4 km/h after one year. In just three years, the speed could reach 1.6 \times 10^5 km/h. At this rate, a solar sail could reach Pluto in less than five years. In comparison, the U.S. “New Horizons” Pluto probe, using chemical thrusters and gravity assists from Jupiter, is planned to take nine years to reach its destination.

To provide sufficient radiation pressure, the solar sail must capture as much sunlight as possible, meaning the surface area of the sail must be very large. In the 1970s U.S. plan to probe Halley’s Comet (later cancelled), the sail area was designed to be 600,000 square meters, roughly the size of 10 city blocks in New York. Even with such a large surface area, a solar sail accelerates very slowly compared to traditional spacecraft. However, because a rocket only burns for a few minutes before releasing its payload to fly at a constant speed, while a solar sail accelerates continuously, it eventually becomes faster than other probes.

Given enough time, through small but continuous acceleration, a solar sail can reach any desired speed. If the acceleration decreases due to increasing distance from the sun, some scientists suggest using lasers to push it forward, though this idea is not yet feasible with current technology and resources. However, if one day solar sails are to carry humans to other star systems, lasers will be necessary because solar radiation pressure becomes very weak once past the orbit of Jupiter. Humanity’s first interstellar probe might be a craft with a giant sail propelled by lasers; perhaps the first extraterrestrial visitors will arrive at Earth in a similar fashion.

Failure is the Mother of Success

In recent years, Japan and the United States have conducted several solar sail tests in space. Although Japan’s small-scale tests were successful, the most influential large-scale test, the U.S. “Cosmos 1,” failed due to a launch vehicle malfunction. Nevertheless, the implementation plan for Cosmos 1 remains highly valuable for reference.

On August 9, 2004, the Japan Aerospace Exploration Agency (JAXA) launched an S-310 small rocket from Kagoshima. While spinning in space, the rocket successfully deployed two solar sails of different shapes with diameters of 10 meters (using heat-resistant and radiation-resistant polyimide film, only 0.0075 mm thick). The purpose was to test deployment, not propulsion. When the rocket reached an altitude of over 150 km, it first opened a clover-shaped sail; at its peak altitude of 170 km, the sail separated from the rocket. Subsequently, the rocket opened a sail composed of six fan-shaped polyimide films. About 6 minutes and 40 seconds after launch, both sails fell into the sea east of Uchinoura. This was the first time in the world that a frameless solar sail was successfully opened. In February and September 2006, Japan launched the solar sail secondary payloads “SSSAT-1” and “SSSAT-2,” with film diameters of 15 meters. In August 2006, Japan also used a high-altitude balloon to successfully deploy a 20-meter diameter rectangular solar sail. Clearly, Japan has built a strong technical foundation in the field of solar sails.

On June 21, 2005, the world’s first large-scale solar sail, “Cosmos 1,” was launched from a submerged submarine using a “Volna” rocket (a modified Soviet SS-N-18 ICBM). Funded by the U.S. Planetary Society and built by Russia, the design was ambitious: after entering orbit, Cosmos 1 would deploy eight triangular sails like windmill blades and orbit for at least a month. The mass of Cosmos 1 was about 100 kg, with sail lengths of 15 meters made of aluminized reinforced polyester film only 5 microns thick (about 1/4 the thickness of a plastic trash bag). These sails were arranged around the main body and deployed via inflatable tubes, forming a flower-like shape with a total area of 600 square meters. While in orbit, the sails could adjust their angles like helicopter rotors to reflect sunlight from different directions. Control algorithms were pre-programmed into the onboard computer, allowing operation via ground commands. If controllers wanted to stop the push, they could rotate the blades edge-on to the sun. The payload included imagers and accelerometers to measure acceleration.

Actually, as early as July 20, 2001, a suborbital flight test of Cosmos 1 was attempted, but the rocket’s computer failed to command the separation of the third stage, preventing the sail from deploying. This second attempt in 2005 also failed due to a first-stage malfunction of the Volna rocket. Later, on August 3, 2008, the U.S. attempted to launch the NanoSail-D1 using a “Falcon 1” rocket, which also failed due to rocket malfunction. NanoSail-D1 weighed 4.5 kg and had a total area of 9.3 square meters.

Beautiful Interstellar Kites

The aforementioned failures provided humanity with a wealth of experience and lessons, which finally bore fruit with the preliminary success of Japan’s “IKAROS” solar sail launched on May 21, 2010.

IKAROS is the world’s first deep-space probe to use solar sail technology, as previous tests were conducted only in Earth orbit. IKAROS has four main missions: first, to deploy the sail and verify the deployment mechanism and procedures; second, to study the reflection coefficient of the sail and verify photon propulsion technology; third, to generate power using thin-film solar cells embedded in the sail; and fourth, to verify the guidance, navigation, and control technologies of solar sail propulsion. If this mission achieves final success, it could rewrite the history of aerospace propulsion, making deep-space exploration possible and marking the true beginning of the “Great Age of Space Navigation.”

IKAROS is a small cylindrical spacecraft 1 meter high and 1.6 meters in diameter, using a helicopter-like configuration. After being released in space, the ultra-thin solar sail, which is wrapped around the spacecraft, is slowly deployed by the centrifugal force generated by rotating the cylindrical body (at 25 rpm). It forms a square sail with a side length of 14 meters, a diagonal distance of 20 meters, and a thickness of only 0.0075 mm. The sail surface is embedded with silicon thin-film solar cells (25 microns thick), covering about 5% of the total area and expected to generate 500 watts of power. This power is used to verify the functionality of thin-film solar cells and to power the onboard equipment. The total mass of IKAROS is 315 kg, of which the sail accounts for 15 kg. It is currently flying toward Venus, propelled by solar photons. During the mission, IKAROS uses spin stabilization at a speed of about 20 rpm.

The deployment of the ultra-thin solar sail was carried out in two stages: the first stage was completed on June 8, forming a “cross” shape with a diagonal span of 15 meters. The second stage was completed on June 10, achieving full deployment with a diagonal span of 20 meters. JAXA announced on June 10 that at a distance of approximately 7.7 million kilometers from Earth, IKAROS successfully deployed its sail using centrifugal force. On June 15, a camera separated from the central body captured images of the deployed sail and transmitted them back to Earth. This cylindrical camera, about 6 cm in diameter and height, took photos while moving away from the spacecraft and transmitted them via radio; the camera was not recovered. The successful deployment of the IKAROS sail marks the completion of half the mission. The remaining half involves flying to Venus, located about 8 million kilometers from Earth, within six months using solar radiation pressure.

The IKAROS sail is made of polyimide, only 1/10 the thickness of a human hair and weighing only 1/5 of a coin. It can adjust light reflection to accelerate, decelerate, and change direction. During the six-month journey to Venus, repeated tests will be conducted to accumulate experience for future deep-space probes. Furthermore, because IKAROS carries solar cells to improve efficiency in distant regions of the solar system, it is a hybrid spacecraft utilizing both solar radiation pressure and electrical power.

If this mission is successful, Japan plans a second mission in the coming years using a medium-sized solar sail (50 meters in diameter) combined with an ion engine. This would be the world’s first photon/ion hybrid propulsion system, with the destination being Jupiter and the Trojan asteroids. To this end, Japan is researching large thin-film structures, hybrid propulsion, and high-performance thin-film solar cells.

Despite the failure of Cosmos 1, the U.S. Planetary Society remains confident. Society President Louis Friedman stated that even if the sail is controlled for only a short segment of the orbit, demonstrating solar-powered propulsion would constitute a success. On November 9, 2009, the Planetary Society announced that 2010 could be the “Year of the Solar Sail,” with at least two launches planned. The Society will launch “LightSail-1” at the end of 2010, based on NASA’s “NanoSail” concept (NASA itself is scheduled to launch NanoSail-D2 in September 2010). LightSail-1 is built on a “CubeSat” platform provided by Cal Poly, weighing about 5 kg and shaped like a loaf of bread before deployment. It consists of three modules: two for electronics and control, and one for the folded sail. The sail is made of polyester film about 4.6 microns thick (1/4 the thickness of a trash bag). Once deployed, it forms a square of 32 square meters, capable of producing an acceleration of 5.7 \times 10^{-5} m/s^2. The Planetary Society also plans to develop LightSail-2 and LightSail-3 for higher orbits and solar climate monitoring.

Although humanity has yet to successfully deploy a spacecraft primarily propelled by a solar sail in deep space, research continues globally. 2010 is a pivotal year for solar sail technology. For a long time, humans have yearned to move beyond a sole reliance on rockets and find new ways to roam the stars. Solar sail propulsion represents the future of spaceflight and a significant step toward developing new types of cosmic engines. It is believable that in the future, humans will use solar sails for deep-space exploration, bringing a revolution to space travel.

Link: http://www.jspec.jaxa.jp/e/activity/ikaros.html