JPL Accomplishes Laser Sail First
Source: Space.com
March 1, 2000
A project run by NASA's Jet Propulsion Laboratory (JPL)
is claiming a new first: Engineers have used a laser beam to move a
sample of extremely lightweight material using nothing more than the
pressure of light.
If true, the experiment would be an important step in
the development of solar, laser and microwave sails to propel
spacecraft at speeds that can -- for now -- only be imagined. It could
mark the germination of technologies that one day may yield spacecraft
that ride at the front of high-powered laser beams, traversing the
space between nearby stars at 10 percent the speed of light.
Such visionary ideas have been kicking around for years.
But if the recent experiments prove correct, they mark the first time
that scientists have demonstrated that what works in laser-sail theory
can actually be put to practical use.
The milestone occurred during the last week of December
at the high energy laser facility at Wright-Patterson Air Force Base
in Ohio. The experiment was conducted under the direction of Leik
Myrabo, as part of JPL's Interstellar Technology development program.
Myrabo's team aimed a laser and a pendulum that had a swatch of
super-lightweight carbon mesh attached to the end. The team was able
to use the laser beam to deflect the sail and pendulum upward, much
the way that a breeze would push a windsock hanging at the bottom of a
the same swinging arm.
"By throttling the laser we were able to get it to a
steady-state position," said Henry Harris, who manages the
interstellar technology development at JPL. "I'm very proud of the
team."
Harris compares the experiment's significance to Robert
Goddard's firing of the first liquid-fueled rocket in 1926. In March
of that year Goddard used a mixture of gasoline and liquid oxygen to
shoot a small rocket about 40 feet (12 meters) into the air above a
cabbage plot in Auburn, Massachusetts. The feat kicked off the age of
rocketry.
"Before Goddard, they knew that you could produce steam
in a chemical reaction and it was hypothesized that you could build a
rocket," Harris said. "Goddard started using real fuels, and getting
motion in a rocket using practical materials. That's what we're doing.
We're using practical sail materials that are extremely light weight
and resistant to very high temperature to point the way to future
transportations."
Miracle Mesh
Scientists have long recognized that carbon would be the
perfect material for solar sails, but had no way of making a carbon
sail. Now a remarkable new mesh made of an almost chaotic arrangement
of carbon fibers could be what solar sail supporters have longed for.
Although others in the scientific community have not yet
assessed the experiment, Harris said there is no question that the
sail was really being moved by the pressure of the laser light. The
team took extensive precautions to eliminate the possibility that any
other forces were acting on the on the sail, he said.
"We performed the experiments many times under different
conditions. We weighed the sails. We examined the sails carefully
before and after the experiments. We took the same sails and
repeatedly drove them at very high temperatures to see if the
characteristics were changing.
"We're convinced -- and I think the paper we're writing
will be convincing -- that we actually showed photon-momentum
propulsion," Harris said.
The research group has been analyzing data from the
laser-sail experiment since it was completed. The results will be
submitted for publication in a major scientific journal in the coming
months, Harris said.
Those transportation methods include all sorts of sails
that would use light as a direct energy source.
Solar sails are essentially large mirrors in space that
harness the momentum of photons of light by reflecting them. Photons
are the particles that make up light that have no mass, but do have
momentum. A photon perfectly reflected will transfer twice its
momentum to whatever it bounces off of, but since this pressure is so
slight, it can only be harnessed by extremely light, extremely
reflective materials.
Breakthrough material
The experiment was conducted with a remarkable new
material developed by Energy Science Laboratories, Inc. (ELSI) in San
Diego, California. It is a mesh of randomly-organized carbon fibers
linked into a sort of criss-cross matrix that is mostly empty space.
Thus it is extremely light, but relatively thick, so it has stiffness.
"It's one of those things that comes along every so
often that's kind of a technology breakthrough and at the same time,
an honest-to-goodness paradigm shift," said Robert Frisbee, who
analyses advanced propulsion mission concepts at JPL.
The material is particularly promising because it can
withstand much hotter temperatures than traditional sails. This
property contributed a great deal to the apparent success of the laser
sail experiment, Harris indicated, allowing the team to illuminate the
sail at much higher temperatures than it anticipated.
"Last year we were talking about hundreds of degrees
centigrade. This year we realized, with this new material we can go to
as much as 2,500 degrees centigrade," Harris said.
Although others in the scientific community have not yet
assessed the experiment, Harris said there is no question that the
sail was really moved by the pressure of the laser light. The team
took extensive precautions to eliminate the possibility that any other
forces were acting on the on the sail, he said.
"We performed the experiments many times under different
conditions. We weighed the sails. We examined the sails carefully
before and after the experiments. We took the same sails and
repeatedly drove them at very high temperatures to see if the
characteristics were changing.
"We're convinced - and I think the paper we're writing
will be convincing - that we actually showed photon-momentum
propulsion," Harris said.
The research group has been analyzing data from the
laser-sail experiment since it was completed. The results will be
submitted for publication in a major scientific journal in the coming
months, he said.
Beam-riding cone
The next step for the laser sail project is to test the
sail materials under a wider regime of laser intensities, and to
continue to develop the carbon materials, Harris said. He is already
working on various concepts for developing a sail that could ride a
laser beam through space. One of the challenges of laser sailing is
keeping track of the
sail as it travels through the vast void of interstellar space.
A sail set in the glare of a laser beam would has
tendency to flip out of the beam, just as a paper plate would be
tossed out of a high-speed jet of water. Engineers need to develop a
"beam-riding" sail, Harris explained. Ideally, this beam-rider would
not only stay centered on a beam, but could be guided simply by moving
the column of light.
"You don't want to be half way to Alpha Centauri and
loose this thing," he said.
One proposed design would build a sail that is
essentially two cones joined along their rims. One cone would be
composed of carbon fiber material with a highly reflective coating on
the inside. The other cone would be clear, so that laser light could
pass through it unimpeded to strike the reflective coating on the
inside of the first cone. The reflected photons would impart their
momentum to the structure.
The center of pressure would be focused at the nose of
the carbon cone, pushing it forward. Because the center of mass would
be at the open end of this cone, in the middle of the whole structure,
the apparatus would be stable, and would keep itself centered on the
beam.
As it turns out, the carbon mesh material seems
perfectly suited for building such a beam-riding cone, Harris said.
Unlike the extremely thin plastic films that have been
used until now, the carbon-fiber material is somewhat rigid, and can
be fabricated in various shapes. In addition, the material can be
folded and compressed, but will pop back into its original shape when
deployed.
Of course, this laser-powered sail-ship would require a
mammoth and expensive laser-generating station orbiting around the sun
and powered by huge solar concentrators. Such a power station is
technologically several decades away, but some scientists think laser
sailing poses the best chances for completing interstellar missions
within a reasonable time frame, say, within a human lifetime.
by Greg Clark
Staff Writer
http://www.space.com/news/lasersail_000301.html