Solar energy is a rich source of power for spaceships floating in the inner region of the solar system. But how far from Earth can photoelectric activity work?
By: Gil Knier and Dr Tony Phillips, FIRST SCIENCE website
Above: The Sun, as seen from distant Pluto, is just another star in the night sky, albeit the brightest of them all. Below: Right: The concept presented by the artist shows a ground station radiating power to a distant spacecraft; Left: Concept of a solar sail
Translation: Eli Ben David, for the "Hidan" website
Try this: close your eyes and try to imagine the International Space Station (ISS), bright and shining as it orbits Earth.
what did she look like The image in your mind is surely dominated by beautiful and wide wings - the impressive solar array of the space station.
It is no coincidence that the solar panels (solar collectors) dominate the profile of the station. On the ISS (as well as on the ISS) solar energy ultimately drives everything that happens. Our sun radiates an enormous power of energy output measured in 4 X10 to the power of 23 kilowatts, which is 4 with 23 zeros! of photoelectric cells, which convert sunlight into electricity, which only a tiny fraction of it needs to be intercepted to power the station.
But not all spacecraft hover near Earth where sunlight is abundant. Many of NASA's research spacecraft travel far beyond Earth's orbit, and as they do so, the size of the distance between them and the Sun and its light dims. Where it is up there, the sun's power ceases to be an effective source of energy for the spacecraft - but where?
That's what NASA's spacecraft designers want to know: Where is the final edge of the effectiveness of relying on energy from the sun's rays? What is their final range?
The space station's solar cells, which were developed decades ago, convert 14% of the sun's energy hitting it into electricity, and the broad-spectrum photovoltaic cells, which convert light in many parts of the spectrum into electricity, reach an efficiency of approximately 30%. Devices of this type work well in the brightly-lit inner circumference of the solar system, but for longer journeys and in places where solar photons are rare, more efficient cells and a much larger solar array will surely be needed. Far from the reach of the sun, for example, the ability to convert even single photons into electricity would be of great importance.
"Sunlight is reduced in intensity as a result of distance, by a factor of one part of R squared, where R is the distance from the Sun," explains Geoff Landis, from the Glenn Research Center, NASA, "This means that a square meter of a solar array producing 400 watts at a distance of 1 AU , would have to be 25 square meters in size near Jupiter - and almost 2,000 square meters near Pluto to produce the same power." (Note: an astronomical unit or "AU" is the average distance between Earth and the Sun. 1 AU equals 150 million kilometers).
Landis and his colleagues at the Glenn Center for Photovoltaics and Space Environment are researching new ways to harness the sun's power—including more efficient solar cells, laser energy for distant spacecraft, and methods of solar power for the moon and Mars. "The use of solar power is a complex study," says Landis. "Finding a solution requires that we consider the factors of distance, weight, the energy of different bands of light and the actual material we have."
"Using today's technology," he says, "the range of the sun's rays that we can use is four astronomical units from the sun, where the sun's rays shine at 1/16th the intensity of the Earth's environment." It is beyond the orbit of Mars (1.5 AU), but closer to the Sun than Jupiter (5.2 AU).
"We hope that with the technologies of tomorrow we will be able to push the range far inside the solar system", he says. "Future solar collectors, for example, may use advanced films - almost like the Saran wraps - and very lightweight solar cells, which can stretch over an area of an acre (an area equal to 4 kmXNUMX-AB) or more in size. Instead of a spacecraft carrying a solar array, there will be a solar array carrying a spacecraft."
Such broadsheet sails would be a target for fast-moving space-dust, so they would be designed to be puncture-proof and of self-sealing material – another challenge for spacecraft builders.
To date, the farthest distance a solar-powered spacecraft has reached was 2.35 AU - a feat accomplished last October by NASA's Stardust research satellite. Stardust will extend its feat every day until April 2002, when it will reach a maximum distance from the Sun of 2.72 AU on its way to comet WILD 2. Stardust's solar array actually produces more energy than expected. Perhaps because its photoelectric cells work more efficiently in the cold of the depths of space than the laboratories at DHA. No one is sure, this is uncharted territory.
Not so far from the sun as the Stardust, the experimental spacecraft 'Deep Space 1' recently tested a "solar concentrator" - 720 lenses that concentrated sun rays on 3,600 solar cells. Deep Space 1 was the first solar-powered research spacecraft to rely exclusively on TRIPLE-JUNCTION MULTIBANDGAP cells. The small but innovative system generates 2,500 watts: enough to run three microwave devices and more than enough to power a spacecraft with an ion engine.
Such progress will eventually push solar energy deep into space - perhaps, even, outside the solar system altogether.
"In the long term, solar arrays will not have to rely on the sun," says Landis, "We are exploring the idea of using laser beams to beam photons into the solar array. If you make a powerful enough laser device and if you can direct the beam, then there really will be no end to the range of the sun's rays - with sufficiently large lenses we can beam light to research spacecraft halfway to Alpha Centauri."
The projection of light energy for targets on the earth, in space, on the moon or on Mars and other planets - or distant spacecraft - is the stuff of science fiction. This is the path Geoff Landis is treading. He is also a Hugo and Nebiola award winner for science fiction writers! As a scientist, he and his team from NASA are in the daily business of reaching out to reach the edge of the sun's rays, and see how fiction quickly turns into fact.
