How big is Webb going to be?
The most important size of a telescope is the diameter of the primary mirror, which will be about 6.5 meters (21 feet) for Webb. This is about 2.7 times larger than the diameter of Hubble, or about 6 times larger in area. The Webb will have a mass of approximately 6,500 kg, with a weight of 14,300 lbs on Earth (in orbit, everything is weightless), a little more than half the mass of Hubble. The largest structure of Webb will be its sunshade, which must be able to shield the deployed primary mirror and the tower that holds the secondary mirror. The sunshade is approximately the size of a tennis court. • Why does the sunshield have five layers rather than one thick one? Each successive layer of the sunshield is cooler than the one below. The heat radiates out from between the layers, and the vacuum between the layers is a very good insulator. One big thick sunshield would conduct the heat from the bottom to the top more than 5 layers separated by vacuum. • How will Webb communicate with
