What is JWST's sunshield SPF compared with suntan lotion?

JWST’s sunshield provides protection comparable to an SPF of 1,000,000. This is an engineering analogy for how strongly its five layers reduce the sunlight and heat reaching Webb’s cold telescope side, not a bottle of cosmetic sunscreen.

The sunshield is made from five large, kite-shaped Kapton membranes coated with reflective metals. The layers are separated so that heat radiated by one layer does not simply pass straight through to the next. Sun-facing layers also use treated silicon coatings to reflect additional solar energy back into space.

The shield divides the observatory into a hot, Sun-facing side and a cold side containing the primary mirror and infrared instruments. Webb’s detectors need very low temperatures to measure faint infrared signals without being overwhelmed by heat from the Sun, Earth, the spacecraft, or the telescope itself.

The SPF comparison is often misunderstood as a literal dermatological rating. It is a memorable way to express the sunshield’s attenuation of solar energy; NASA has also described the five layers as reducing exposure by a factor of about one million. The tennis-court-sized shield had to fold for launch and deploy precisely in space.