The Sunshield on NASA's James Webb Space Telescope is the largest part of the observatory-five layers of thin membrane that must unfurl reliably in space to precise tolerances. Last week, for the first time, engineers stacked and unfurled a full-sized test unit of the Sunshield and it worked perfectly.

The Sunshield is about the length of a tennis court, and will be folded up like an umbrella around the Webb telescope's mirrors and instruments during launch.

Once it reaches its orbit, the Webb telescope will receive a command from Earth to unfold, and separate the Sunshield's five layers into their precisely stacked arrangement with its kite-like shape.

The Sunshield test unit was stacked and expanded at a cleanroom in the Northrop Grumman facility in Redondo Beach, California.

The Sunshield separates the observatory into a warm sun-facing side and a cold side where the sunshine is blocked from interfering with the sensitive infrared instruments.

The infrared instruments need to be kept very cold (under 50 K or -370 degrees F) to operate. The Sunshield protects these sensitive instruments with an effective sun protection factor or SPF of 1,000,000 (suntan lotion generally has an SPF of 8-50).

In addition to providing a cold environment, the Sunshield provides a thermally stable environment. This stability is essential to maintaining proper alignment of the primary mirror segments as the telescope changes its orientation to the sun.

The James Webb Space Telescope is the successor to NASA's Hubble Space Telescope. It will be the most powerful space telescope ever built. Webb is an international project led by NASA with its partners, the European Space Agency and the Canadian Space Agency.