Solar Sail

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 Earth's gravitational well, it seems for now that the extra punch from exothermic processes is needed.

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

What is a Solar Sail?

A solar sail, simply put, is a spacecraft propelled by sunlight. Whereas a conventional rocket is propelled by the thrust produced by its internal engine burn, a solar sail is pushed forward simply by light from the Sun. This is possible because light is made up of packets of energy known as “photons,” that act like atomic particles, but with more energy. When a beam of light is pointed at a bright mirror-like surface, its photons reflect right back, just like a ball bouncing off a wall. In the process the photons transmit their momentum to the surface twice – once by the initial impact, and again by reflecting back from it. Ever so slightly, propelled by a steady stream of reflecting photons, the bright surface is pushed forward.

A solar sail is made up of just such a reflective surface, or several surfaces, depending on the sail’s design. When the bright sails face the Sun directly, they are subjected to a steady barrage of photons that reflect off the shiny surfaces and impel the spacecraft forward, away from the Sun. By changing the angle of the sail relative the Sun it is possible to affect the direction in which the sail is propelled – just as a sailboat changes the angle of its sails to affect its course. It is even possible to direct the spacecraft towards the Sun, rather than away from it, by using the photon’s pressure on the sails to slow down the spacecraft’s speed and bring its orbit closer to the Sun.

In order for sunlight to provide sufficient pressure to propel a spacecraft forward, a solar sail must capture as much Sunlight as possible. This means that the surface of the sail must be big – very big. Cosmos 1 is a small solar sail intended only for a short mission. Nevertheless, once it spreads its sails even this small spacecraft will be 10 stories tall, as high as the rocket that will launch it. Its eight triangular blades are 15 meters (49 feet) in length, and have a total surface area of 600 square meters (6500 square feet). This is about one and a half times the size of a basketball court.

For a true exploration mission the requirements are far greater: when a NASA team in the 1970s, headed by Louis Friedman, suggested using a solar sail spacecraft for a rendezvous with Halley’s comet, they proposed a sail with a surface area of 600,000 square meters (6.5 million square feet). This is equivalent to a square of 800 meters (half-mile) by 800 meter – the size of 10 square blocks in New York City!

Even with such a gigantic surface, a solar sail spacecraft will accelerate very slowly when compared to a conventional rocket. Under optimal conditions, a solar sail on an interplanetary mission would gain only 1 millimeter per second in speed every second it is pushed along by Solar radiation. The Mars Exploration Rovers, by comparison, accelerated by as much as 59 meters (192 feet) per second every second during their launch by conventional Delta II rockets. This acceleration is 59,000 times greater than that of a solar sail!

But the incomparable advantage of a solar sail is that it accelerates CONSTANTLY. A rocket only burns for a few minutes, before releasing its payload and letting it cruise at a constant speed the rest of the way. A solar sail, in contrast, keeps on accelerating, and can ultimately reach speeds much greater than those of a rocket-launched craft. At an acceleration rate of 1 millimeter per second per second (20 times greater than the expected acceleration for Cosmos 1), a solar sail would increase its speed by approximately 310 kilometers per hour (195 mph) after one day, moving 7500 kilometers (4700 miles) in the process. After 12 days it will have increased its speed 3700 kilometers per hour (2300 mph).

While these speeds and distances are already substantial for interplanetary travel, they are insignificant when compared to the requirements of a journey to the stars. Given time, however, with small but constant acceleration, a solar sail spacecraft can reach any desired speed. If the acceleration diminishes due to an increasing distance from the Sun, some scientists have proposed pointing powerful laser beams at the spacecraft to propel it forward. Although such a strategy is not practicable with current technology and resources, solar sailing is nevertheless the only known technology that could someday be used for interstellar travel.

 How do they work?

Solar Sails propel a spacecraft by utilizing the pressure created by the stream of photons (tiny units of light energy) from the sun. Once a spacecraft is in orbit, a lightweight sail would unfurl. Changing the position of the sail would increase or decrease speed. The thrust created by the photon stream is very low and interplanetary journeys would take years. For long missions, an on-board laser or microwave transmitter would be fitted to provide power when the Sun is distant.

Hundreds of space missions have been launched since the last lunar mission, including several deep space probes that have been sent to the edges of our solar system. However, our journeys to space have been limited by the power of chemical rocket engines and the amount of rocket fuel that a spacecraft can carry. Today, the weight of a space shuttle at launch is approximately 95 percent fuel. What could we accomplish if we could reduce our need for so much fuel and the tanks that hold it?

International space agencies and some private corporations have proposed many methods of transportation that would allow us to go farther, but a manned space mission has yet to go beyond the moon. The most realistic of these space transportation options calls for the elimination of both rocket fuel and rocket engines -- replacing them with sails. Yes, that's right, sails.

NASA is one of the organizations that has been studying this amazing technology called solar sails that will use the sun's power to send us into deep space.

History

On February 4, 1993, Znamya 2, a 20-meter wide aluminized-mylar reflector, was successfully tested from the Russian Mir space station. Although the deployment test was successful, the experiment only demonstrated the deployment, not propulsion. A second test, Znamaya 2.5, failed to deploy properly.

On August 9, 2004, the Japanese ISAS successfully deployed two prototype solar sails from a sounding rocket. A clover type sail was deployed at 122 km altitude and a fan type sail was deployed at 169 km altitude. Both sails used 7.5 micrometer thick film. The experiment was purely a test of the deployment mechanisms, not of propulsion.

In June 21, 2005 a Volna rocket launched from a Russian submarine in the Barents Sea launched the privately built Cosmos-1 spacecraft. However, a rocket failure prevented it from reaching its intended orbit. The Cosmos-1 spacecraft was designed to use solar sails to move through space. Had the mission been successful, it would have been the first ever orbital use of a solar sail to speed up a spacecraft, as well as the first space mission by a space advocacy group (Planetary Society).

The IKAROS probe is the world's first spacecraft to use solar sailing as the main propulsion.

LightSail-1, a second orbital spacecraft by the Planetary Socity is under construction and is expected to be ready by the end of 2010.

How It Works: The sun’s radiation exerts force on the ultrathin fabric of solar sails, much like wind propels a sailboat. These sails can clean up orbit by slowing debris enough that it deorbits. For example, an expandable 97-square-foot sail can be launched as a secondary payload on even a small 110-pound satellite, and an onboard system or ground control triggers its deployment. Conductive coils embedded in the sail control its angle, so it can maneuver a satellite out of orbit; sail and satellite disintegrate together in the atmosphere.

Pros: The material is cheap and portable.

Cons: The sail’s expansion and the spacecraft’s altitude need to be carefully calculated beforehand.

Plausibility: The technology has existed for decades, from a sail-equipped craft in the mid-1970s intended to ride along with Halley’s Comet (the craft never flew, and the project was scrapped) to large solar arrays currently affixed to the Messenger spacecraft, which are helping steer it to Mercury. Most recently, the Japan Aerospace Exploration Agency launched a solar-sail-propelled “space yacht” called Ikaros in May. But the solar sails precisely controllable enough to remove debris are still years away.

Cloud Cover: Tanks of liquid gas would emit mist that slows down objects passing through enough that the debris would fall from orbit.  Kevin Hand

Space Mist

How It Works: One of the more novel solutions, first proposed by researchers at NASA’s Ames Research Center in 1990 and recently resurrected, is to use frozen mist to drag an object out of orbit. A rocket launches a tank filled with liquid gas, such as carbon dioxide, and thrusters position the tank near the path of the target object.Thousands of miles from the debris, the tank sprays a cloud of frozen mist. The droplets slow down and deorbit anything they encounter. According to the idea’s originator, aerospace engineer George Sarver of Ames, a spaceship emitting a 220-pound, 62-foot-diameter cloud of frozen mist could deorbit dense objects such as steel nuts. Larger clouds could stop less-dense items like insulation.

Pros: Once the mist dissipates, nothing remains in orbit. The tank falls back into the atmosphere, which means less clutter that could otherwise create more debris.

Cons: It requires precise aim, because each tank sprays its shot just once.

Plausibility: “We could probably test it in orbit within 18 months if NASA funded it,” says Creon Levit, the chief scientist for programs at Ames.

Astro-Velcro: Large balls with adhesive shells could stick themselves to debris and carry it out of orbit. Kevin Hand

Robots and Adhesives

How It Works: One idea to capture tumbling debris employs orbiting spacecraft that would use robotic arms to grab and release debris, essentially tossing it out of orbit so it burns up in the atmosphere. Sean Shepherd, a curriculum coordinator at Eastern New Mexico University, proposes a more novel idea, called Adhesive Synthetic Trash Recovery Orbital Spheres, or Astros—essentially a collection of sticky balls. The balls consist of a layer of metallic foam (such as silicon carbide) and an outer shell of adhesive (such as aerogels or resins). They attach themselves to debris and then dive into the atmosphere, where both are incinerated.

Pros: Spacecraft that carry and position the robotics are easier to control than tethers or sails and can remove multiple pieces of debris. Adhesives are cheap.

Cons: The adhesive balls can’t be compressed, so packing them into a rocket for launch will be difficult.

Plausibility: Hanspeter Schaub, the associate chair of graduate studies in the Aerospace Engineering Sciences department at the University of Colorado, says the orbiting robot concept could be tested in space within five years.