NASA’s James Webb Space Telescope is the largest and most powerful telescope ever launched to space. Its mirror is composed of 18 individual segments that have been aligned so accurately, that they effectively work as a single giant (21.6-foot, or 6.5-meter) reflector. NASA’s James Webb Space Telescope is the largest and most powerful telescope ever launched to space. Its mirror is composed of 18 individual segments that have been aligned so accurately, that they effectively work as a single giant (21.6-foot, or 6.5-meter) reflector. Astronomy Phys.org – latest science and technology news stories