Sailing by Sunlight

Background Information:

Voyagers closes with a 3-dimensional computer animation produced by the University of Pennsylvania. This video animation depicts 3 ocean vessels sailing across the sky, symbolic of Columbus' fleet. The last ship transforms into a spacecraft. The exotic sails mentioned in the program are solar sails, an alternative propulsion system to the familiar chemical engines used on today's rockets.

Figure 11 is an artist's rendition of a British entry into a proposed race to Mars, called the Columbus Cup. This solar spacecraft consists of a circular sail over 750 feet in diameter. Called the Manta, this ship will also bear the name Niña. This dual name has historical precedence. The Niña, which was the only vessel from Columbus' original fleet to return to the New World, was re-christened the Santa Clara on its second voyage.

The following section is condensed from an article on solar sailing written by Kevin Polk (The Interplanetary News, Vol. I, No. 2, September 1991).

In conjunction with International Space Year '92 and the 500th anniversary of Columbus' voyage across the Atlantic, the Christopher Columbus Quincentenary Jubilee Commission is sponsoring the launches of 3 flagship entries from Europe, Asia and the Americas. The catch is that these ships must sail to the moon and Mars, propelled by sunlight alone.

British solar sailcraft Manta

This technique, called "solar sailing," is not a new idea. In 1924, the Russian rocket pioneer Konstantin Tsiolkovsky suggested that the pressure of sunlight on a spaceship could propel it. By then it was long known that light can be thought of as tiny particles. When these photons bounce off a surface, they give it a minuscule kick. The resultant force is small, however. The sunlight falling on Aloha Stadium in Honolulu, for example, exerts less than an ounce of force on the artificial turf. This is millions of times weaker than the thrust from chemical rockets. Yet over time, the constant pressure of sunlight can accelerate a large, thin-sheet, called a solar sail, to a hefty speed.

The romantic image of vehicles sailing majestically through space has inspired many science fiction authors. Modern interest in solar sailing in the United States traces back to Russell Saunders' story, "Clipper Ships of Space," published in Astounding Magazine in 1951. Perhaps the most famous of all is Arthur C. Clarke's story "Sunjammer" (later renamed "The Wind from the Sun"), which appeared in Boy's Life in 1963. The story, which details a clipper cup-style races between piloted solar sailcraft, may have helped to inspire the Columbus Cup Race.

solar sailcraft tracking

Solar sails are actually powered by sunlight, NOT the solar wind, which is a varying stream of ions (i.e., atoms that have lost electrons) that escape the outer atmosphere of the sun. During a solar storm, the solar wind whips into a frenzy and enough ions spiral down the earth's magnetic fields to create the auroras, i.e., the Northern and Southern Lights. Even the solar wind is thousands of times weaker than the virtually constant stream of sunlight that propels solar sailcraft.

Now, sunlight only flows away from the sun. Does this mean that solar sailcraft can only move outward, away from it? Surprisingly, no. An orbit is essentially a balance between an object's speed and the force of gravity. If you are orbiting the sun (as shown in figure 12) and want to get closer to it, reduce your orbital speed, and the sun's gravity will bring you closer. To do this with a solar sail, you just adjust the sail to reflect light roughly in the direction you are traveling (position A). This is like firing rocket thrusters forward to reduce your speed. To move outward, you position the sail to reflect light opposite your direction of travel (position B), which boosts your velocity.

Activity:

1.) Use a mirror to illustrate that light reflects at the same angle it strikes a reflective surface.

2.) Assume a solar sailcraft is orbiting the earth in a circular orbit (as shown in figure 13) and that its solar sail has a reflective coating on both sides. How would you orient the sails at points A, B, C and D of its orbit so that the sailcraft would lift higher above the earth?

Solution:

Turn the sail perpendicular to the rays of sunlight at position A to get the full impact of the photons from the sun. At position D (where the sailcraft travels head-on toward the sun), aim the sail parallel to the incoming sunlight to prevent the ship from slowing down. At locations C and D, the sail should be turned at the angle shown to allow sunlight to reflect off its surface. This will impart a forward push that increases the sailcraft's speed. These combined maneuvers will boost the sailcraft to a higher orbit, as illustrated by the dashed path.

Maneuvering a solar sailcraft

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