Cloth Would Provide Solar Power to Space Vehicles
By Tim Furniss  

LONDON Japanese scientists are using a material normally made into handkerchiefs to produce a huge "solar sail" that will provide electric power for craft in deep space.

Japan's National Space Development Agency (Nasda), is creating an ultra-light 1,500 sq ft sail that can generate 2.5 megawatts of electrical power for spacecraft so they can travel out of the solar system without the need for heavy nuclear-powered engines.

The silk-like furoshiki material under investigation is normally used for handkerchiefs and wrapping. Its ability to be tightly packed and then unfolded without wrinkles has attracted the attention of the scientists, who will line it with tiny solar cells to gather power from the sun.

Working alongside the University of Tokyo, Nasda claims it could have a small $5 million demonstration mission in orbit within two years.

The ultra-thin material would be folded and stored in a case the size of a parachute pack.

Once in space, it would be deployed by four small space vehicles attached to each corner of the cloth. As the craft moves further apart in space, they would pull out the sail completely, keeping it taut. Deployment could take less than 10 minutes.

The sail would be covered with thin solar cells, which are under development. Once unfurled, it would act as a giant solar array, converting the sun's energy into electricity.

On a craft traveling into deep space beyond Jupiter, a furoshiki sail could provide enough electricity even though the energy from the sun has diminished sharply — the solar energy that reaches Jupiter is only 4 percent of that which reaches Earth.

Conventional solar arrays would have to be extremely large to power vehicles in deep space. It has been impossible to produce a conventional solar array large enough to provide the required power so far away from the sun. Such an array would also too heavy to be launched easily from Earth.

Today's deep-space craft use nuclear generators instead of solar arrays. They create electric power from the energy given off by plutonium 238 as it decays. The furoshiki sail would avoid the need for such generators.

Nasda's first use of the sail would be for planetary exploration — of Jupiter, for example.

Furoshiki sails could be used for many other purposes besides power generation. They could, for example, be turned into huge communications antennas capable of large volume, ultra-high-speed data transmissions from geostationary orbit.

The deployment of large dish-like antennas on communications satellites is difficult and risky. These arrays are like fine-mesh frames that have to unfurl perfectly into a dish shape when the satellite reaches its orbit.

The sail could also be used as a shield. The material could be toughened and placed around a spacecraft to stop it being hit by space debris. It could also be used as a net to recover spacecraft in orbit.

The sail may even work as a propulsion system, with the sun's radiation acting on it to push a spacecraft along much as the wind pushes a yacht along.

Some remarkable schemes have been dreamt up by the researchers. They believe an aluminum-coated sail could be used as a mirror in space to focus sunlight on dark areas of the Earth, such as the polar regions during the winter. Another idea is to use the sail as a huge dish to focus sunlight into a unit on the craft to convert the solar energy into microwave power, which would be beamed to receivers on the Earth to provide electricity for cities.

Nasda is studying how the material behaves in microgravity. It is also looking at how to control the four small craft that deploy the sail in space and estimating how much fuel they would need. This work is being done with computer simulations.

The scientists believe that a furoshiki sail could eventually be several miles wide.

Posted: 4/14/99 9:43:50 AM