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sō’lar
sāil, n. - A gossamer material that, when unfurled
in the vacuum of space, feels the pressure of sunlight and
propelled by said pressure may carry a ship among the stars.
July
31, 2008: Long ago, someone stood alone on a sandy
shore and gazed longingly out at the seemingly endless expanse
of ocean, over a horizon suffused softly with ocean mist,
musing "I wonder, what's out there?" Then, they
fashioned a boat, rigged it with a large cloth to catch the
wind, and set sail.
Not
quite so long ago, someone stood alone on a sandy shore and
gazed longingly up at the seemingly endless expanse of space,
suffused softly with sparkling stars, musing "I wonder,
what's out there?" Then, they fashioned a spacecraft,
rigged it with a large cloth to catch the sun, and set sail.
The
first paragraph: Already happened. The second: Any day now….
Above:The
Milky Way beckons to a sky watcher in the south of France.
Photo credit: Laurent Laveder, July 28, 2008.
Two
very special missions are planned for the near future; both
aim to deploy a solar sail to harness the power of sunlight.
NASA's NanoSail-D is a small solar sail slated for launch
as early as August 2008. The Planetary Society's Cosmos 2
does not yet have a specific launch target date. Its goal
is to make "a controlled flight under sunlight pressure."1
To
fully appreciate these two missions, let's travel back in
time for a brief history of solar sailing:
Almost
400 years ago, German astronomer Johannes Kepler observed comet
tails being blown by what he thought to be a solar "breeze."2
This observation inspired him to suggest that "ships and
sails proper for heavenly air should be fashioned" to glide
through space.
Little
did Kepler know, the best way to propel a solar sail is not
by means of solar wind, but rather by the force of sunlight
itself. In 1873, James Clerk Maxwell first demonstrated that
sunlight exerts a small amount of pressure as photons bounce
off a reflective surface. This kind of pressure is the basis
of all modern solar sail designs.
In
1960, Echo-1 felt these solar pressure effects loudly and
clearly. "Photon pressure played orbital soccer with
the Echo-1 thin-film balloon in orbit.... The shards were
flung far and wide by sunlight."3
NASA
had a more positive experience with solar sailing in 1974
when the Mariner 10 spacecraft ran low on attitude control
gas. Because Mariner 10 was on a mission to Mercury, there
was plenty of sunlight around and this gave mission controllers
an idea: They angled Mariner's solar arrays into the sun and
used solar radiation pressure for attitude control. It worked.
Though Mariner 10 was not a solar sail mission, and though
the radiation pressure it used was incredibly small, this
ingenious use of Mariner's solar arrays did demonstrate the
principle of solar sailing.
Right:
Mariner 10, circa 1974, was not designed for solar sailing,
but the spacecraft's solar arrays worked surprisingly well
as impromptu sails for attitude control.
Also
in the 1970s, Dr. Louis Friedman, then at NASA's Jet Propulsion
Laboratory, led a project to try the first solar sail flight.
Halley's Comet was to make its closest approach to Earth in
1986, and NASA conceived the exciting idea of propelling a
probe via solar sail to rendezvous with the comet. Eventually,
the project was scrapped. Still "the year-long work on
preliminary design demonstrated that, indeed, solar sailing
was a feasible spacecraft-propulsion technique."4
In
1993, the Russian Space Agency launched a 20-meter diameter,
spinning mirror called Znamya 2, hoping to beam solar power
back to the ground.
"Some
call Znamya 2 a sail because it was made of a large, lightweight
reflector and unfurled like a solar sail might be unfurled,"
says Les Johnson of the NASA Marshall Space Flight Center,
co-author of the soon-to-be-published book Solar Sails:
A Novel Approach to Interplanetary Travel. "In fact,
if I were asked to demonstrate solar sail technology and was
constrained to deploy it from a large spacecraft, I might
design a 'sail' like Znamya."
The
foil reflector unfurled and when illuminated produced a spot
of light which crossed Europe from France to Russia. Unable
to control its own flight, however, the mirror burned up in
the atmosphere over Canada. Russia's proto-sail program was
abandoned in 1999 after a larger, follow-up mission (Znamya
2.5) failed to deploy properly.
Solar
sails were an accessory on India's INSAT 2A and 3A communications
satellites, circa 1992 and 2003. The satellites were powered
by a 4-panel solar array on one side. A solar sail was mounted
on the north side of each satellite to offset the torque resulting
from solar pressure on the array.
Right:
A clover-shaped Japanese solar sail unfurling in 2004. [more]
In
2004, the Japanese deployed solar sail materials sub-orbitally
from a sounding rocket. Although it was not a demonstration
of a free-flying solar sail that could be used for deep-space
exploration, the deployment was nevertheless "a valuable
milestone" remarks Friedman, who appreciates the challenges
of deploying gossamer sheets from fast-moving spacecraft.
To
date, no solar sail has been successfully deployed in space
as a primary means of propulsion.
The
Planetary Society hoped to demonstrate the technology with
its Cosmos 1 mission in 2005. "Cosmos 1 was a fully developed
solar sail spacecraft intended to fly only under the influence
of solar pressure for control of the spacecraft's orbit,"
says Friedman, now the director of the Planetary Society.
"If
all had gone as planned, the US-based Planetary Society, working
with Russia, would have been the first to fly a fully functional,
though performance-limited, solar sail in space," says
Johnson. "It would have been the first spin-stabilized,
free-flying solar sail to fly in space."5
Cosmos
1, however, was lost when the launch vehicle failed.
Meanwhile,
NASA has also continued to dabble in solar sailing. Between
2001 and 2005, the Agency developed two different 20-meter
solar sails (fabricated by ATK Space Systems and L'Garde,
Inc., respectively) and tested them on the ground in vacuum
conditions. "These sail designs are robust enough for
deployment in a one atmosphere, one gravity environment and
are scalable to much larger solar sails -- perhaps as much
as 150 meters on one side." "A NASA flight test
is possible by the year 2010."6
Meanwhile,
NanoSail-D is shooting for this summer, and space.
Above:
NanoSail-D poses after a successful laboratory deployment
test: movie.
Edward
E. Montgomery's team from the Marshall Space Flight Center
is working in cooperation with Elwood Agasid's Ames team toward
deploying the NanoSail-D solar sail--in fact, "any day
now," says Montgomery. It will travel to orbit onboard
a SpaceX Falcon 1 rocket, to be launched from Omelek Island
in the Pacific Ocean.
"Our
primary objective is to demonstrate successful deployment
of a lightweight solar sail structure in low Earth orbit,"
says Montgomery. NanoSail-D will feel two kinds of pressure:
(1) aerodynamic drag from the wispy top of Earth’s atmosphere
and (2) the pressure of sunlight. Montgomery's team hopes
to measure both types of pressure as the sail circles Earth.
Johnson
cautions, "If--and it's a big if—they can measure the
solar pressure, they will have demonstrated solar pressure
as a primary means of orbital maneuvering. They'll have to
show conclusively the effects of solar pressure, with a convincingly
high signal-to-noise ratio (above the forces resulting from
the residual atmospheric pressure)."
Montgomery
acknowledges the challenge: "The orbit available to us
in this launch opportunity is so low, it may not allow us
to stay in orbit long enough for solar pressure effects to
accumulate to a measurable degree. We are going to have to
look closely at the flight data to see if we can make that
determination."
And
what of Cosmos 2? The mission is a privately funded project,
a partnership of The Planetary Society and Cosmos Studios.
Work has begun at the Russian Space Research Institute on
some Cosmos 2 spacecraft hardware. They are also studying
possible launch configurations on a reliable launch vehicle.7
Right:
Next stop, the stars? An artist's rendering of an interstellar
solar sail. [more]
If
successful, NanoSail-D and Cosmos 2 could profoundly affect
the future of science and exploration missions.
"Success
would be huge for the future of space exploration," says
Montgomery.
"Solar
sailing is the only means known to achieve practical interstellar
flight," says Friedman. "It is our hope that the
first solar sail flight will spur the development of solar
sail technology so that this dream can be made real."
Each
effort is a stepping stone, in the great visionary Carl Sagan's
words, along "the shore of the cosmic ocean,"8
leading us closer to sailing among the stars. Future attempts
will surely take us the rest of the way.
"'Twas
all so pretty a sail it seemed
As if it could not be,
And some folks thought 'twas a dream they'd dreamed
Of sailing that beautiful sea."9
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Author: Dauna Coulter
| Editor:
Dr. Tony Phillips | Credit: Science@NASA
footnotes
and more information |
1,7New
developments on the road to Cosmos 2 by Dr. L. Friedman
2Measuring
Up to a Solar Sail -- NASA feature story
3Solar-sail
mission reflects past and future -- MSNBC
4The
History of Solar Sailing by Dr. L. Friedman
5,6From
Solar Sails: A Novel Approach to Interplanetary
Travel, by Giovanni Vulpetti, Les Johnson (Author),
Gregory L. Matloff, to be published in 2008.
8From
Cosmos, by Carl Sagan, page 5.
9Excerpt
from Winken, Blinken, and Nod, by Eugene Field,
19th century poet.
SpaceX
-- home page
More
information on Cosmos
I
The
basics of solar sailing
NASA's
Future: US
Space Exploration Policy
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