Try this: Close your eyes and imagine the International
Space Station (ISS), sunlit and gleaming as it circles our planet.

What did the ISS look like? The lingering image in your mind is probably
dominated by broad, beautiful wings — the station’s awesome solar arrays.

It’s no accident that solar panels dominate the station’s profile. On the
ISS (as on the earth below) solar energy ultimately powers everything that
happens. Our Sun, a star named Sol, radiates enormous power: a constant
output of 4 x 1023 kilowatts (kW), which is a 4 followed by 23 zeros!
Photovoltaic cells, which convert sunlight to electricity, need only
intercept a tiny fraction of that total to energize the station.

But not all spacecraft linger near Earth where sunlight is plentiful. Many
NASA probes travel far beyond our planet’s orbit. And as they do, the Sun
grows more distant and dim. Somewhere out there, solar power ceases to be a
useful source of energy for spacecraft. But where?

That’s what NASA spacecraft builders want to know: Where is the edge of

The Space Station’s solar cells, developed decades ago, convert 14% of the
Sun’s energy that hits them into electricity, and modern multibandgap cells,
which convert light in multiple parts of the spectrum into electric power,
reach efficiencies of 30% or so. Such devices work well enough in the
brightly-lit inner solar system, but more efficient cells and larger arrays
will be needed as spacecraft travel to places where solar photons are
scarce. In the outer reaches of the solar system, for instance, the ability
to convert even single photons into electricity would be important.

"Sunlight decreases in intensity over distance by a factor of 1/r^2, where r
is the distance from the Sun," explains Geoff Landis, a scientist at NASA’s
Glenn Research Center. "This means a 1-meter-square solar array producing
400 watts at a distance of 1 AU would have to be 25 square meters in size
out at Jupiter — and almost 2,000 square meters at Pluto to yield the same
power." (Note: An astronomical unit or "AU" is the mean distance between
Earth and the Sun. 1 AU equals 150 million kilometers.)

Landis and his colleagues at Glenn’s Photovoltaics and Space Environment
Branch are exploring new ways to harness the Sun’s power — including more
efficient solar cells, laser-beaming energy to distant spacecraft, and solar
power systems for the Moon and Mars. "The use of solar power is a complex
field of study," says Landis. "Finding solutions requires that we balance
such factors as distance, weight, the energy of different light bands, and
the actual materials available to us."

"Using today’s technologies," he says, "the ‘edge’ of sunshine we can use is
about four astronomical units away from the Sun, where the sunlight is about
one-sixteenth as bright as it is near the Earth." That’s beyond the orbit of
Mars (1.5 AU), but closer to the Sun than Jupiter (5.2 AU).

"With tomorrow’s technologies we hope to push that edge further out into the
solar system," he says. "Future solar collectors, for example, might use
advanced thin films — almost like Saran Wrap — and very lightweight solar
cells, which can roll out to an acre or more in size. Instead of a
spacecraft that carries a solar array with it, you would have a solar array
that carries a spacecraft."

Such expansive sails would also be targets for fast-moving space-dust, so
they would need to be crafted from puncture-resistant or self-sealing
materials. Yet another challenge for spacecraft builders!

Sail for the Stars."

To date, the farthest any solar-powered spacecraft has ventured from the Sun
is 2.35 AU — a record set last October by NASA’s Stardust probe. Stardust
will extend its own record every day until April, 2002, when it will reach a
maximum distance from the Sun of 2.72 AU en route to Comet Wild 2.
Stardust’s solar arrays are actually producing more energy than expected,
perhaps because its photovoltaic cells operate more efficiently in the cold
of deep space than in Earth labs. No one is certain; this is unexplored

Not quite as far from the Sun as Stardust, NASA’s experimental spacecraft
Deep Space 1 recently tested a "solar concentrator" — 720 lenses that
focused sunlight onto 3600 solar cells. Deep Space 1 was the first
solar-powered probe to rely entirely on triple-junction multibandgap cells.
The small but innovative system generated 2500 watts: enough to energize
three microwave ovens and more than enough to power the craft’s ion engine.

Such advances will eventually propel solar power into deep space — perhaps
out of the solar system altogether.

"In the long term, solar arrays won’t have to rely on the Sun," Landis said.
"We’re investigating the concept of using lasers to beam photons to solar
arrays. If you make a powerful-enough laser and can aim the beam, there
really isn’t any edge of sunshine– with a big enough lens, we could beam
light to a space-probe halfway to alpha-Centauri!"

Beaming light power to targets on Earth, in orbit, on the Moon or on Mars
and other planets — or to distant spacecraft — is the stuff of science
fiction. That’s right up Geoff Landis’ alley. He’s also a Hugo and Nebula
award-winning science fiction writer! As a scientist he and his NASA cohorts
are in the business of reaching out to the edge of sunshine every day,
seeing fiction very rapidly and certainly turning into fact.