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December 5-7 was a red-letter weekend for the MESSENGER team - the solar panels were mounted on the spacecraft and successfully tested. While MESSENGER travels to and then orbits Mercury, these custom-developed panels will be the spacecraft's sole source of electric power and will need to operate at distances ranging from 150 million to 50 million kilometers (93 million to 31 million miles) from the Sun.
The panels will face intense solar energy when Mercury's orbit reaches its perihelion - or closest approach to the Sun - but the team devised a two-part strategy to handle the strong sunlight. Small mirrors placed between the power-generating solar cells will reflect nearly 70 percent of the Sun's energy and keep the panel cooler. While there are more than twice as many mirrors than solar cells, however, the mirrors alone will only keep the panels at "survival" temperatures. So, at peak heating times, MESSENGER's onboard computer will command the Solar Array Drive Actuators (SADAs) to tilt the panels back from the Sun. This points the panels close enough to the Sun to get power but far enough away to maintain a normal operating temperature of about 150� Celsius, or 302� Fahrenheit.
The dark stripes between the mirrors are strings of highly efficient triple-junction solar cells - the business portion of the panel. To run MESSENGER's systems and charge its onboard nickel-hydrogen battery, the two panels, each about 1.5 meters (5 feet) by 1.65 meters (5.5 feet), will supply 465 watts of power near Earth and more than 850 watts when close to the Sun.
Developing these specialized solar panels was one of the biggest engineering challenges the MESSENGER team faced, but with installation and testing behind them the panels are now ready for the trip to Mercury. Click here for large and small time-lapse movies of the team installing one of the panels on Dec. 6.
Last updated: December 12, 2003
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