Projects: Solar Sailing
Basic Facts on Cosmos 1 and Solar Sailing
Solar Sailing
How does a solar sail work?
When the light from the Sun reflects off the surface of the solar
sail, the energy and momentum of light particles known as "photons" is
transferred to the sail. This gives the sail a "push" that accelerates
it through space. Although the acceleration is very slight, it is also continuous,
enabling the sail to reach very high speeds in a relatively short time.
The direction of the push is controlled by the angle of the sail with respect
to the Sun, adding to or subtracting from the orbital velocity.
How does light push a solar sail?
Photons, which are "particles" of light, bounce off the reflective
material of the sail. (Newton's Third Law of Motion states that for every
action there is an equal and opposite reaction.) The reaction here causes
a change in momentum, pushing the sail and accelerating the spacecraft.
A photon reflecting off the mirror-like surface of a solar sail gives the
sail a double kick -- a push equal to twice the photon's momentum (one push
from the sail stopping the photon and one from it reflecting the photon and
accelerating it away).
Does a solar sail fly on the solar wind?
No! The solar wind is made up of ionized particles ejected by the Sun. These
particles move much slower than light. A solar sail does not stop or reflect
them, although they also may impart some of their momentum to the solar sail.
However, the force from the solar wind is less than one percent of that from
light pressure.
How fast does a solar sail go?
The speed of an interplanetary solar sail spacecraft will depend on how long
it has been propelled by the pressure of sunlight. The acceleration from
sunlight is very small -- approximately five ten-thousandths of a meter
per second per second, depending on the size and weight of the sail and the
spacecraft. Over one day, that is a velocity increase of 45 meters per second
or about 100 miles per hour.
But the real advantage of solar sailing is that,
unlike a chemical rocket that applies a lot of thrust but only for an instant,
sunlight hitting the sail applies thrust continuously. In 100 days a sail
could reach 16,000 kilometers per hour (10,000 miles per hour); in one year
it could reach 58,000 kilometers per hour (36,000 miles per hour). In just
three years, a solar sail could reach a speed of over 160,000 kilometers
per hour (100,000 miles per hour). At that speed you could reach Pluto in
less than five years. In comparison, the New Horizons misson to Pluto, using
chemical propulsion and a gravity-assist from Jupiter, is planned to take
nine years to reach its target.
Still, 160,000 kilometers per hour (100,000
miles per hour) is still only 0.00015 the speed of light. It would take about
1,000 years for a solar sail to reach one-tenth the speed of light, even with
light shining on it continuously. This emphasizes just how hard interstellar
flight is. It will take advanced sails much thinner than today’s technology,
plus a laser power source in space that can operate over interstellar distances
to reach one-tenth the speed of light in less than 100 years. Some researchers
of beamed-power sailing think that use of high-temperature materials may make
such speeds possible in a few decades.
What can a solar sail be used for?
Solar sails can be used to boost or decrease the orbits of spacecraft, travel
between the planets within our solar system, and someday may take us to worlds
around other stars.
However, once you get much beyond the orbit of Jupiter, energy from sunlight
is too weak. When far from the Sun, lasers can be directed at the sails. Lasers
stay in a tight beam so that most of their energy can be imparted to the sail
and not diffused into space. Very large lasers in Earth orbit or in the inner
solar system could be used to help us travel to other stars. In the future,
people may travel to distant stars using laser powered solar sails.
What is the advantage of using a solar sail?
The great advantage of a solar sail is that it requires no fuel. Today, we
use chemical rockets to give our spacecraft a quick boost into Earth orbit,
and then out of Earth orbit. The spacecraft then coasts most of the way to
its destination, with some small blasts from thrusters to adjust its trajectory.
This requires a lot of fuel. Solar sails give a very low thrust, but they
can work continuously, pushing spacecraft faster and faster. A solar sail
can, in time, move the spacecraft even faster than a chemical rocket. For
a round trip solar sails have great advantage since no fuel is needed for
the return.
Can a solar sail only provide thrust away from the Sun?
No, thrust can be generated inward or outward with respect to the sun. By
turning the sail at different angles, we can add or subtract velocity to the
spacecraft. When we add velocity, the sail flies away from the Sun. When we
subtract velocity, its orbit spirals inward.
Why hasn't anyone flown a solar sail before?
No one had been able to organize a simple flight using the very low cost launch
vehicles in Russia. The Planetary Society has unique international team building
capability, a willingness to take risk, and accepts the very limited objective
of a first solar sail flight.
Cosmos 1
What is Cosmos 1?
Cosmos 1 was the first solar sail spacecraft ever built and launched. To make
it happen, The Planetary Society and Cosmos Studios teamed up with an international
group of scientists and engineers in the United States and Russia. NPO Lavochkin
in Russia developed and constructed the spacecraft, and the Makeev Rocket
Design Bureau built the Volna launch vehicle. Cosmos 1 was the first space
mission conducted by a public interest organization and was done without
government funding.
What happened to Cosmos 1?
Cosmos 1 was launched into space on June 21, 2005 on board a Volna rocket
from the Russian submarine Borisoglebsk. Because of a technical malfunction,
the engine of the rocket’s first stage shut down prematurely, and Cosmos
1 never reached orbit. Sadly, Cosmos 1 never had a chance to spread its wings
and fly as a true solar sail in space.
Who funded Cosmos 1?
The project was funded primarily by Cosmos
Studios, a science-based media and entertainment venture by Ann Druyan,
the wife of our co-founder, the late Carl Sagan. The Planetary Society also
gratefully acknowledges the donation of Mr. Peter Lewis of Cleveland, Ohio,
and the many contributions of Society members around the world.
Who developed
and launched the spacecraft?
The Planetary Society coordinated the development of Cosmos 1, with Cosmos
Studios as the project sponsor. The lead spacecraft contractor was NPO Lavochkin
in Russia. The Space Research Institute of the Russian Academy of Sciences
was also a principal contractor, developing all electronics for the mission.
The launch was conducted in Russia from a nuclear submarine in the Barents
Sea. The launch vehicle was the Volna,
a converted SS-N-18 Soviet ICBM. The Makeev Rocket Design Bureau developed
the rocket.
How would the sail have been controlled?
The Cosmos 1 spacecraft had eight rotating solar sail blades. Each
one was triangular in shape, about 15 meters (50 feet) in length, with the
total area equalling 600 square meters. When combined, their total
structure is equivalent to a disk. Each blade can be pitched, similar to how
a helicopter blade changes angle so the pilot can control the motion of the
aircraft. The pitch angle varies the direction of the reflected sunlight pressure
so that force may be applied in any direction. The control algorithms for
the spacecraft are programmed onto the on-board computer and can be operated
by commands from the ground or, if necessary, autonomously.
How thin were Cosmos 1's reflective sails, and what material are they
made of?
The sails were made of 5-micron-thin aluminized reinforced Mylar. That is
about ¼ the
thickness of a trash bag. The sail must be as light as possible to maximize
the acceleration.
How much did Cosmos 1 weigh?
The spacecraft mass was approximately 100 kilograms (220 pounds). This is extremely
light for a spacecraft.
Where was the flight to take place?
The flight was to take place in Earth orbit, above the atmosphere. Solar
sails can't be used in Earth's atmosphere because the air pressure and wind
greatly overwhelm any pressure from sunlight. The initial orbit was to be
approximately circular with an altitude of about 850 kilometers. The orbit
was inclined to Earth's equator at approximately 78 degrees - nearly polar.
How long was the flight meant to last?
We hoped the sail would fly for weeks, perhaps even more than one month.
What is the mission destination?
Our goal was to fly in Earth's orbit to demonstrate control under
sunlight pressure.
Who was the Project Director?
Dr. Louis Friedman, Executive Director of The Planetary Society was
the Cosmos 1 Project Director. He was the program leader of the NASA Solar Sail
program in the 1970s and is author of the book, Starsailing: Solar Sails
and Interstellar Travel.
With Special Thanks to Cosmos Studios!
As the major sponsor and partner of Cosmos 1, we thank Cosmos
Studios and its founder and CEO, Ann Druyan, for years of financial
and inspirational support for this cutting edge project that will pave
the
way to future travel to the stars.
We gratefully acknowledge your unwavering commitment and that of Planetary
Society Members around the world for their many donations to this project.
Together, we will make our way to the stars. |
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