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The James Webb Space Telescope (JWST or "Webb") is a joint NASA–ESA–CSA space telescope that is planned to succeed the Hubble Space Telescope as NASA's flagship astrophysics mission. The JWST will provide improved infrared resolution and sensitivity over Hubble, and will enable a broad range of investigations across the fields of astronomy and cosmology, including observing some of the most distant events and objects in the universe, such as the formation of the first galaxies.
The primary mirror of the JWST, the Optical Telescope Element, is composed of 18 hexagonal mirror segments made of gold-plated beryllium which combine to create a 6.5 m (21 ft) diameter mirror—considerably larger than Hubble's 2.4 m (7 ft 10 in) mirror. Unlike the Hubble telescope, which observes in the near ultraviolet, visible, and near infrared (0.1 to 1 μm) spectra, the JWST will observe in a lower frequency range, from long-wavelength visible light through mid-infrared (0.6 to 28.3 μm), which will allow it to observe high redshift objects that are too old and too distant for Hubble to observe. The telescope must be kept very cold in order to observe in the infrared without interference, so it will be deployed in space near the Sun–Earth L2 Lagrange point, and a large sunshield made of silicon-coated and aluminium-coated Kapton will keep its mirror and instruments below 50 K (−223.2 °C; −369.7 °F).The JWST is being developed by NASA, the European Space Agency and the Canadian Space Agency. The NASA Goddard Space Flight Center is managing the development effort, and the Space Telescope Science Institute will operate Webb after launch. It is named for James E. Webb, who was the administrator of NASA from 1961 to 1968 and played an integral role in the Apollo program. The prime contractor is Northrop Grumman.Development began in 1996 for a launch that was initially planned for 2007 and a 500-million-dollar budget, but the project has had numerous delays and cost overruns, and underwent a major redesign in 2005. The JWST's construction was completed in late 2016, after which its extensive testing phase began. In March 2018, NASA further delayed the launch after the telescope's sunshield ripped during a practice deployment. Launch was delayed again in June 2018 following recommendations from an independent review board. Work on integration and testing of the telescope was suspended in March 2020 due to the COVID-19 pandemic, adding further delays. Following work resumption, NASA announced that the launch date had been delayed to 31 October 2021. Problems with the Ariane 5 launch vehicle have threatened to push the launch date back even further to November or early December. The total cost of developing the telescope has increased to over US$10 billion.

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