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INTRODUCTION Riding
on the light from the sun, Solar Sails are large, elegant structures that can
deliver payloads to unique locations in our solar system. Just as the sailing
ships of past centuries opened up new frontiers for affordable transport and exploration,
solar sails offer revolutionary capabilities for in-space propulsion, transport,
and exploration of the earth, the sun, the planets, and even interstellar travel. A
BRIEF HISTORY Nearly
400 years ago, as much of Europe was still involved in naval exploration of the
world, Johannes Kepler proposed the idea of exploring the galaxy using sails.
Through his observation that comet tails were blown around by some kind of solar
breeze, he believed sails could capture that wind to propel spacecraft the way
winds moved ships on the oceans. What Kepler observed was the pressure of solar
photons on dust particles that are released by the comet as it is orbiting. While
Kepler's idea of a solar wind has been disproving, scientists have since discovered
that sunlight does exert enough force to move objects. Photonic pressure is a
very gentle force which is not observable on earth because the frictional forces
in the atmosphere are so much larger. To take advantage of this force, NASA has
been experimenting with giant solar sails that could be pushed through the cosmos
by light. Solar sails were seriously studied by NASA in the 1960s as possible
manned transportation around the solar system. In those days of optimism serious
plans were formed for lunar bases by 1975 nuclear launchers and interplanetary
engines, and unmanned interstellar probes. None of these ever received serious
funding, and they all died on the drawing boards and test beds by the early 1970s.
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 conventional chemical
rockets, 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 travel
farther. Solar sails enable spacecraft to move within the solar system and between
stars without bulky rocket engines and enormous amounts of fuel.
COMPONENTS
OF SOLAR SAIL
There
are three components to a solar sail-powered spacecraft:
" Continuous
force exerted by sunlight
" A large, ultra thin mirror
" A separate
launch vehicle
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