leads the NanoSail-D mission. “We are
ready now with the technology to make
Riding the wind
In principle, solar sailing could not be
easier. Scottish physicist James Clerk
Maxwell described in 1873 how light can
exert pressure: A particle of light transfers up to nearly twice its momentum to
an object it bounces off of.
Each individual transfer amounts to
no more than a mosquito’s breath, but
over time that breath accumulates to a
steady wind that a spacecraft can ride
just as a sailboat rides the wind on Earth.
After 100 days, a solar sail could reach
14,000 kilometers per hour; after three
years it could be zipping along at 240,000
kilometers per hour. At that rate it could
get to Pluto in less than five years, rather
than the nine years the plutonium-powered New Horizons spacecraft, now
on its way, is taking. Solar sails are the
tortoise to the hare of chemical rocketry.
Scientists have long wanted such
a tortoise. In the 1920s Konstantin
Tsiolkovsky, the founder of Soviet astronautics, and colleague Fridrikh Tsander
separately wrote of the idea of using solar
radiation pressure to accelerate sails.
After a few decades on the back burner,
the idea took off in the ’50s and ’60s, with
engineers drafting up grandiose designs
and Arthur C. Clarke plotting a solar sail
race in his short story “The Wind from
the Sun.” By 1976 engineers at NASA’s
Jet Propulsion Laboratory in Pasadena,
Calif., were dreaming of sending a massive
solar sail to fly alongside Comet Halley as
it passed close to Earth the next decade.
Without the need to carry fuel, solar
sails promised to be a cheaper way to
SOURCE: JEROME L. WRIGHT/SPACE SAILING, CALTECH
of the Planetary Society, headquartered
in Pasadena. “That was kind of unimaginable. Now that we’re talking about
things 5 meters or 10 meters on a side,
you realize that a lot of people might be
able to build them and use them a little
A CubeSat like this one has carried
solar sails into space. Once there, the
sails unfurl and begin sunjamming.
explore Earth and its environs. They
could also make visits, such as hovering
above the North Pole, that traditional
spacecraft can’t because of the dictates
of gravity. But solar sails lost the funding battle to other alternative propulsion
systems — at least in the United States.
By the early 1990s a few other sporadic
attempts, including a plan for a solar sail
race to Mars, also fell apart.
Now, tiny satellites may be saving
the big dreams of some would-be solar
sailors. One of the hottest things in satellite technology today is the CubeSat,
a box just 10 centimeters on a side that
weighs about 1 kilogram. Such boxes can
be mixed and matched in “nanosatellite”
combinations of up to three cubes yet still
be launched using a shared deployment
system. CubeSats are thus relatively
cheap and easy to work with, so researchers have used them to carry a variety of
science experiments. A small solar sail,
thinner than a trash bag and weighing
just grams, turns out to be nearly the perfect payload to fly on a CubeSat.
“When we first thought of solar sails
back in the ’70s and ’80s, it was these
huge structures, a mile or half a mile on
a side,” says Louis Friedman, cofounder
Two flights and failure
Existing solar sail designs fall into
two main categories: ones that deploy
rigid booms to hold the sail taut, like a
sailboat’s mast, and ones that spin to
blossom sail blades outward using centrifugal force. The main challenges are
to unfold the whisker-thin sail in space
without ripping, and to direct the sail to
move in the right direction.
Thanks to hefty government funding,
Japan’s space agency, JAXA, was the first
to conquer both challenges. It built a large
square sail, too big to fit in a CubeSat, and
launched it on board a probe headed to
Venus. In June of last year, the probe
released the still-folded solar sail, named
IKAROS after the mythological boy who
flew too close to the sun and melted the
wax anchoring his wings. In the great
space acronym tradition, the name also
stands for Interplanetary Kite-craft
Accelerated by Radiation Of the Sun.
On cue, IKAROS unfolded its sail,
20 meters across diagonally, and made
its way toward Venus, flying past that
planet in December. By turning on and
off an innovative set of liquid crystals,
project engineers showed they could
change the sail’s reflectivity and thus
direct its motion.
JAXA has extended the mission to
March 2012 so engineers can test some
more risky flight maneuvers. “I’m just so
in admiration of them,” says Friedman.
Driving a solar sail A solar sail gets its momentum from particles of light hitting it and bouncing off. By tilting the sail so the photons hit at
different angles, the sail can be made to fall in toward the sun (a shrinking orbit) or travel outward (an expanding orbit).
September 10, 2011 | SCIENCE NEWS | 19