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Image Credits: NASA/JPL
Satellites, space stations, probes, manned vehicles, and other spacecraft all need power to operate, and because it’s never cloudy in our Solar System many space agencies have turned to solar power.
Photoelectric cells made their debut in 1954, and at that time they were able to convert about 6 percent of the solar energy that hit them into electricity. Today’s silicon-based cells operate at efficiencies ranging from 12% to 17%, and state-of-the-art gallium arsenide cells can hit rates of around 29%.
Deploying Solar Panels in Space
With an output of around 220 watts per square meter, solar panels must be significantly larger than the craft they are designed to power. Interplanetary craft require between 300 W and 2.5 kW of electrical power, according to NASA.
In order to get into space intact, solar panels must be tucked away inside of a launch draft as it blasts against Earth’s gravity. The panels must then deploy properly or the craft would not be able to power up its systems.
After the panels are deployed, the craft must keep them constantly pointed toward the Sun in order to keep electricity flowing. On satellites that are stabilized by spinning around a single axis, the entire hull is covered with panels and only about one third are effective at once. On satellites that use three gyroscopes for stabilization, a motorized mechanism keeps the solar panels pointed toward the Sun.
Satellites that pass through Earth’s shadow and crafts that need to draw extra power are equipped with battery systems for such occasions. According to spacecraft maker Airbus, its battery system can be fully charged in about 12 hours. Like other types of batteries, these systems have a limited life that grows shorter and shorter with each charge-discharge cycle.
One other major problem that affects solar panels is degradation by constant exposure to solar radiation. Performance can degrade 1 or 2 percent each year from normal radiation and even faster when bombarded by solar flares.
Long-Distance Space Travel
Beyond the orbit of Mars, light from the Sun becomes very weak. Because of this, a craft needs inappropriately large conventional solar panels to continue harvesting enough power to operate.
Thanks to massively large solar arrays, NASA’s Juno probe recently set the record for furthest solar-powered craft on its way to Jupiter. The record was set back in January at 493 million miles from the Sun. The previous record was held by the European Space Agency’s Rosetta craft, which hit 492 million miles on its way to comet 67P/Churyumov-Gerasimenko.
Launched in 2011, Juno is the first solar-powered craft intended to function such a long way from the Sun. The four-ton spacecraft carries three 30-foot solar arrays, and owes its record-breaking ability to high-performance solar cells, energy-sipping instruments, a flight path that averts Jupiter's shadow, and a polar orbit that reduces radiation exposure. During its 16-month mission, Juno is expected to reach approximately 517 million miles from the Sun.
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Image Credits: NASA/JPL-Caltech
NASA isn’t writing off solar power for deep space missions just yet. The space agency is currently developing a system that concentrates sunlight onto a compact solar cell in order to generate electric power.
Developed through a partnership between NASA and Entech Solar, the system is based on one that successfully powered NASA’s Deep Space 1 craft in the late 90s called SCARLET (Solar Concentrator Array with Refractive Linear Element Technology). Cheaper to produce, lighter and more robust, the newer system uses a specially-designed lens focus dim sunlight onto multi-junction solar cells.
NASA officials said although this system was developed to power spaceflight beyond Mars; it could also see terrestrial applications.
Sources and Further Reading
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