The sun is both Webb's friend and enemy. Sunlight is the spacecraft's sole electric power source, so Webb's single solar array will be the first component to unfurl after launch. But sunlight and heat would overwhelm Webb's infrared detections, so an impenetrable tennis court-sized sunshield must protect its topside primary mirror, secondary mirror and science instruments, known collectively as OTIS, Optical Telescope and Integrated Science Instrument Module. Another problem: Momentum imparted by photons striking the shield could destabilize Webb as it lines up its cosmic targets. To compensate without wasting station-keeping thruster fuel, Webb will unfurl a momentum trim tab, a flap aft of the sunshield.
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