NASA MESSENGER Mission News December 12, 2003

Meeting a Powerful Challenge

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.