Upon Reflection . . .
MESSENGER will rely on solar panels for power during its long voyage
to and orbit around Mercury. The custom-developed panels, installed
on the spacecraft June 24-25, were the next-to-last major components
to be attached before the spacecraft is moved to the Delta II launch
vehicle this month. This week's annotated Webcam image captures the
installation in progress.
To run MESSENGER's systems and charge its 23-ampere-hour nickel-
hydrogen battery, the panels, each about 1.5 meters (5 feet) by 1.65
meters (5.5 feet), will support between 385-485 watts of spacecraft
load power during the cruise to Mercury and 640 watts during the
science orbit. The panels could produce more than two kilowatts of
power near Mercury, but to prevent stress on MESSENGER's
electronics, onboard power processors take in only what the
spacecraft actually needs.
The panels are 67 percent mirrors (called optical solar reflectors)
and 33 percent triple-junction solar cells, which convert 28 percent
of the sunlight hitting them into electricity. Each panel has two
rows of mirrors for every row of cells; a total of 648 cells and
1,296 mirrors per panel. The small mirrors reflect the Sun's energy
and keep the panel cooler. The panels also rotate, so MESSENGER's
flight computer will tilt the panels away from the Sun, positioning
them to get the required power while maintaining a normal surface
operating temperature of about 150 degrees Celsius, or 302 degrees
Fahrenheit.
Major events before moving the spacecraft include fueling - the team
is loading the hydrazine fuel and nitrogen tetroxide oxidizer this
week - and final installation of the sunshade.
