Largest space telescope

The largest space telescope is the James Webb Space Telescope (JWST), which has a primary mirror diameter of 6.5 metres (21 ft 3 in). The telescope launched on 25 December 2021, and achieved first light in early February 2022. The first images from the un-calibrated primary mirror array, were published on 11 February 2022.

The James Webb Space Telescope is a novel design that incorporates many design elements that had never been flown on a spacecraft before. The most important, and technically challenging, of these os the telescope's multi-layer fabric sunshield. This is a 21.19 m (69 ft 6 in) long and 14.16 m (46 ft 5 in) wide expanse of lightweight kapton which was unfolded and tensioned as the spacecraft travelled away from Earth.

The primary mirror, which also had to be unfolded in space, is made from 18 hexagonal segments, each made from berylium coated with a microscopic layer of gold. Every 1.32-m (4-ft 3-in) segment is adjusted using motors capable of nanometre-level precision, allowing them – once calibrated – to function as a single 6.5-m diameter mirror. The mirror is designed for optimal reflectivity in the infra-red wavelength, and channels light to a series of instruments optimized for particular aspects of its science mission.

To prevent the faint signals it is being designed to detect being drowned out by relefected light from Earth, JWST is located some 1.6 million km (1 million mi) away in an orbit around a position called the Earth-Sun Langrange Point 2. This is a location where the gravitational forces of the Sun and Earth are perfectly balanced, allowing the JWST to orbit the Sun in unison with Earth, but far enough away for its sunshield to be able to block out our planet.

Development of the JWST started in the early 1990s, around the time that the Hubble Space Telescope launched. NASA awarded the first construction contracts in 2003, with an expected budget of $825 million and a planned launch date of 2010. The design and construction of such an ambitious project proved to be vastly more challenging than anyone had anticipated, however. Many of the problems the team had to overcome required wholly new technological solutions that used techniques and designs that had never been tried before – these technologies also had to be manufactured to impossibly high standards as they would have to work flawlessly first time.

As result, over the next two decades, the JWST's budget ballooned and its scheduled slipped over and over again. By the time it finally launched, on Christmas Day 2021, JWST was more than a decade late and its development cost had risen to $8.8 billion, making it the most expensive single spacecraft ever constructed.