Propulsion

Moving Extremely Lightweight Materials Using Nothing More than the Pressure of Light

 

JPL Accomplishes Laser Sail First
by Greg Clark
Staff Writer

Source: Space.com
http://www.space.com/news/lasersail_000301.html

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.

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