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How Solar Sails Work
by Craig C. Freudenrich, Ph.D.

Special thanks to Dr. Louis Friedman, Executive Director of The Planetary Society, for his technical assistance.
In the 1970s, NASA scientists proposed sending a probe to Halley's comet that would be propelled by the pressure of sunlight against a giant solar sail. Although the proposal was rejected as being too risky and unproven, the idea of solar-sail-propelled spacecraft has endured. Numerous developments and tests of solar-sail materials have been conducted over the years, but no one had designed, successfully launched and sailed such a spacecraft.

In mid-April 2001, The Planetary Society, in collaboration with several Russian space organizations, will test launch the deployment mechanism for the first solar-sail mission, Cosmos-1, in a sub-orbital flight. If the test is successful, they will launch the Cosmos-1 spacecraft into Earth orbit in October 2001.

Photo courtesy The Planetary Society
Solar sails will use the sun's energy to propel spacecraft across the cosmos.

What is a solar sail? How can you use sunlight to move a spacecraft in outer space? In this edition of HowStuffWorks, we'll show you how solar sails work, take an in-depth look at the Cosmos-1 mission and find out what solar-sails mean for future space travel.

Solar Sails
Solar sails may evoke images of large sailing vessels of old, such as clipper ships, or modern America's Cup racing yachts. However, the principles, construction and operation of solar sails are quite different from sailboats.

What is a Solar Sail?
A solar sail is a very large mirror that reflects sunlight. As the photons of sunlight strike the sail and bounce off, they gently push the sail along by transferring momentum to the sail. Because there are so many photons from sunlight, and because they are constantly hitting the sail, there is a constant pressure (force per unit area) exerted on the sail that produces a constant acceleration of the spacecraft. Although the force on a solar-sail spacecraft is less than a conventional chemical rocket, such as the space shuttle, the solar-sail spacecraft constantly accelerates over time and achieves a greater velocity. It's like comparing the effects of a gust of wind versus a steady, gentle breeze on a dandelion seed floating in the air. Although the gust of wind (rocket engine) initially pushes the seed with greater force, it dies quickly and the seed coasts only so far. In contrast, the breeze weakly pushes the seed during a longer period of time, and the seed travels farther. Solar sails enable spacecraft to move within the solar system and between stars without bulky rocket engines and enormous amounts of fuel.

What is a Solar Sail Made of?
For a solar sail to be practical way of propelling a spacecraft, it must have the following characteristics:

  • Large area - It must collect as much sunlight as possible, because the larger the area, the greater the force of sunlight.
  • Light weight - The sail must be thin and have a minimal mass, because the more mass, the less acceleration that sunlight imparts to the sail.
  • Durable - It must withstand the temperature changes, charged particles and micrometeoroid hazards of outer space.
To meet these characteristics, most solar sails are made of thin, metal-coated, durable plastics such as Mylar or Kapton. For example, the solar sail of Cosmos-1 is made of aluminum-coated Mylar, has a thickness of 0.0002 inches or 5 microns (ordinary Saran Wrap is about 0.001 inches or 25 microns thick) and an area of 6,415 square feet (600 square meters).

Solar sails come in three major designs:

    Photo courtesy NASA
    Square solar sail
  • Square sail - requires booms to support the sail material
  • Heliogyro sail - bladed like a helicopter, the sail must be rotated for stability
  • Disc sail - circular sail that must be controlled by moving the center of mass relative to the center of pressure
Cosmos-1 has a solar sail that is a cross between a square sail and a heliogyro sail. It is a rounded solar sail that is divided into eight triangular blades with inflatable booms for support. The sail does not have to be rotated for stability.

Cruising by Sunlight
Maneuvering a solar-sail spacecraft requires balancing two factors: the direction of the solar sail relative to the sun and the orbital speed of the spacecraft. By changing the angle of the sail with respect to the sun, you change the direction of the force exerted by sunlight.

Maneuvering a solar sail to change orbits (For purposes of illustration, the change in orbit shown here occurs faster than in reality.)

Pressure from Sunlight
Using the following equations and values, you can calculate the force of sunlight on and acceleration of the spacecraft:
  • Force (F) = 2(P x A)/c
  • Acceleration (a) = F/M
At 1 astronomical unit (AU), the power of sunlight is about 131 watts/foot2 (1,400 watts/meter2). Our spacecraft weighs 2.2-lb (1-kg) and has a sail area of 0.38 mi2 (1 km2 or 1-million m2), so:
  • P (power) = 1,400 watts/m2
  • A (area) = 1-million m2
  • c (speed of light) = 3x108 m/s
  • M (mass) = 1 kg
This works out to a force (F) of about 2 lb or 9 newtons (N). This force leads to an acceleration (a) of about 29 ft/s 2 (9 m/s2), slightly less than the acceleration due to Earth's gravity. In comparison, a space-shuttle main engine can produce 367,000 lb (1.67-million N) of force during liftoff and 462,000 lb (2.1-million N) of thrust in a vacuum.

When the spacecraft is in orbit around the Earth or sun, it is traveling in a circular or elliptical path at a given speed and distance. To go to a higher orbit (travel farther away from the object), you angle the solar sail with respect to the sun so that the pressure generated by sunlight is in the direction of your orbital motion. The force accelerates the spacecraft, increases the speed of its orbit and the spacecraft moves into a higher orbit. In contrast, if you want to go to a lower orbit (closer to the object), you angle the sail with respect to the sun so that the pressure generated by the sunlight is opposite the direction of your orbital motion. The force then decelerates the spacecraft, decreases the speed of its orbit and the spacecraft drops into a lower orbit.

The pressure of sunlight decreases with the square of the distance from the sun. Therefore, sunlight exerts greater pressure closer to the sun than farther away. Future solar-sail spacecraft may take advantage of this fact by first dropping to an orbit close to the sun -- a solar fly-by -- and using the greater sunlight pressure to get a bigger boost of acceleration at the start of the mission. This is called a powered perihelion maneuver.

Cosmos-1 Spacecraft Design
The first solar-sail spacecraft, called Cosmos-1, is being developed, built and flown by The Planetary Society, a private, non-profit organization whose goal is to encourage the exploration of our solar system. The Planetary Society contracted a Russian space organization, the Babakin Space Center, to build, launch and operate the spacecraft. The cost of the project is about $4-million and is funded by Cosmos Studios, a new science-based media company.

Cosmos-1 spacecraft

The spacecraft itself weighs 88 lb (40 kg) and can sit on a tabletop. After a first-phase test launch, the spacecraft will be launched into Earth orbit -- 522 mi (840 km) perigee and 528 mi (850 km) apogee. The spacecraft systems include:

    Photo courtesy The Planetary Society
    One solar-sail blade
  • Solar sail
    • made of aluminized Mylar
    • thickness of 0.0002 inches (5 microns)
    • area of 6,415 square feet (600 square meters)
    • arranged in eight triangular blades:
      • each about 49 ft (15 m) long
      • consist of inflatable plastic tubes that support the sail (a foam may be used inside the tubes to hold them rigid once inflated)
      • can be pivoted (like a helicopter blade) by electric motors to change its angle relative to the sun
  • Solar-sail deployment - A pressurized gas-filling system inflates the plastic tubes.

Solar-sail deployment

    Folding Solar Sails
    The original design of the sail blade had it folded into rolls. However, recent tests have indicated that it may be more reliable to fold the sail blades into accordion-like structures, which would still be deployed by inflating the tubes. The first suborbital launch phase of the Cosmos-1 mission will test the deployment of two solar-sail blades in flight.

  • Power - A small array of solar cells supplies all of the electrical power.
  • Navigation - It is essential for the spacecraft to know where it is and where the sun is at all times.
    1. A sensor detects the position of the sun.
    2. A global positioning system (GPS) receiver detects the spacecraft's position. (From the ground, the spacecraft orbit will be determined from Doppler tracking data with the aid of on-board accelerometers, which we'll discuss later.)
    3. The information from the sun sensor and the GPS receiver are continuously relayed to the spacecraft's on-board computer.
    4. The on-board computer operate the motors that turn the sail blades to maintain the proper orientation of the sail blades with respect to the sun.
    5. The on-board computer can accept corrections or override commands from the ground.
  • Communications - Redundant radio systems are used to communicate with flight controllers on the ground.
    • one UHF band, 400 megahertz
    • one S-band, 2210 MHz
  • On-board computer
    • Two 386EX series microprocessors
      • old, but reliable in the harsh environment of outer space
      • can be run in low-power modes, similar to laptop computers
      • programmed to operate the on-board systems, relay information to the ground and receive commands from the ground
    • A software program assigns tasks to each microprocessor based on workload and performance (speed, delay).
    • Each processor has its own small amount of read-only memory (ROM) -- enough to boot the computer and load the operating system into random-access memory (RAM).
    • Three re-writable ROMs contain the operating systems and programs. The copies of ROM are checked before use for errors caused by radiation in outer space.
    • Three RAMs are present to receive the operating system. Again, the integrity of each RAM is checked for errors before loading.
    • The ROM architecture allows programmers on the ground to update and re-boot the spacecraft's software at any time. It also allows the spacecraft to function in the case of severe radiation damage.
    • Data are stored in two separate databases connected by serial and parallel systems.
  • Instruments
    • Two on-board imaging cameras (Russian and American) to document the mission
    • On-board accelerometers to measure the acceleration of the spacecraft due to sunlight pressure (non-gravitational acceleration)

In the next section, we'll discuss the details of the Cosmos-1 mission.

Cosmos-1 Mission

Photo courtesy The Planetary Society
Cosmos-1 will be launched from a submarine.
The mission of Cosmos-1 will occur in two phases. Phase 1 will test the deployment of two solar-sail blades, and Phase 2 will launch the Cosmos-1 spacecraft into Earth orbit.

Launch Vehicle
To get Cosmos-1 into Earth orbit, the spacecraft will be loaded into a modified intercontinental ballistic missile (ICBM) of Russian design, called the Volna. The ICBM will be launched from a Russian submarine in the Barents Sea. Typically, the Volna ICBM does not have enough thrust to reach orbit, but the missile used for Cosmos-1 will have an added rocket engine (kick stage) that is used to de-orbit satellites. The kick-stage engine will provide the additional thrust required to get Cosmos-1 into orbit.

Phase 1
Phase 1 of the Cosmos-1 Solar Sail Project will take place in mid-April. The goal of Phase 1 is to test the deployment of the solar sails. To do this, a payload consisting of two inflatable solar-sail blades and a solar-sail platform with an imaging camera will be packaged inside a Volna ICBM and launched from a Russian submarine in the Barents Sea. The flight will be a suborbital flight that will last about 15 minutes. At about 248 mi (400 km) high, the two solar-sail blades will be deployed. The camera in the platform will image the sail deployment. This test spacecraft will use an aerobrake to slow down in the upper atmosphere and an additional inflatable braking device as it approaches the ground. It will return to Earth in the Kamchatka Peninsula, where it hopefully will be recovered.

Photo courtesy The Planetary Society
Phase 1 (larger version of the image)

If all goes well with Phase 1, the second phase will be launched in October 2001.

Phase 2
Phase 2 will be an orbital flight of the actual Cosmos-1 spacecraft. Again, it will be launched from a Russian submarine on a modified Volna ICBM. The kick motor will put the spacecraft in orbit, and the solar sails will be deployed. The mission could last anywhere from a few days to a few months. The mission will be deemed a success if the spacecraft can move to a higher orbit using the solar sails. If the goal of the mission is achieved, and if the mission lasts longer than a few days, there may be an additional test to determine if Earth-based lasers can supply sufficient light to push the spacecraft in orbit.

Photo courtesy The Planetary Society
Phase 2 (larger version of the image)

To see videos of these two mission profiles, check The Planetary Society's Solar Sail Mission Animation Page (you will need either Quicktime or RealPlayer Plus to view the videos).

Other Solar Sail Missions
Groups other than The Planetary Society have proposed and are developing solar-sail missions. NASA is developing a solar-sail spacecraft for launch in 2010. The German Space Agency (DLR) and European Space Agency (ESA) also have a solar-sail spacecraft in development, and Carnegie Mellon University is working on a heliogyro solar sail. Team Encounter, a private organization, is developing a solar-sail spacecraft to send archives from individual contributors to the stars -- the Encounter 2001 Deep Space Probe has passed the design review stage and is scheduled for launch sometime in 2003.

Future of Solar Sails
Specific Impulse
In rocket science, the fuel efficiency of a rocket engine is measured by its specific impulse. Specific impulse refers to the units of thrust per units of propellant consumed over time. Because a solar-sail spacecraft carries no fuel, it has infinite specific impulse.
The major advantage of a solar-sail spacecraft is its ability to travel between the planets and to the stars without carrying fuel. Solar-sail spacecraft need only a conventional launch vehicle to get into Earth orbit, where the solar sails can be deployed and the spacecraft sent on its way. These spacecraft accelerate gradually, unlike conventional chemical rockets, which offer extremely quick acceleration. So for a fast trip to Mars, a solar-sail spacecraft offers no advantage over a conventional chemical rocket. However, if you need to carry a large payload to Mars and you're not in a hurry, a solar-sail spacecraft is ideal. As for traveling the greater distances necessary to reach the stars, solar-sail spacecraft, which have gradual but constant acceleration, can achieve greater velocities than conventional chemical rockets and so can span the distance in less time. Ultimately, solar-sail technology will make interstellar flights and shuttling between planets less expensive and therefore more practical than conventional chemical rockets.

Lots More Information!

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