James Webb Space Telescope

The James Webb Space Telescope (JWST or 'Webb') is a space telescope that is planned to be the successor to the Hubble Space Telescope. The JWST will provide greatly improved resolution and sensitivity over the 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. Other goals include understanding the formation of stars and planets, and direct imaging of exoplanets and novas. The primary mirror of the JWST, the Optical Telescope Element, is composed of 18 hexagonal mirror segments which combine to create a 6.5-meter (21 ft; 260 in) diameter mirror that is much larger than the Hubble's 2.4-meter (7.9 ft; 94 in) mirror. Unlike the Hubble, 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 27 μm), which will allow it to observe high redshift objects that are too old and too distant for the 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 Earth–Sun L2 Lagrangian point, and a large sunshield will keep its mirror and instruments below 50 K (−220 °C; −370 °F). The JWST is being developed by NASA—with significant contributions from the European Space Agency and the Canadian Space Agency—and 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. Development began in 1996 for a launch that was initially planned for 2007, 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 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, and is currently scheduled for March 2021. The JWST has an expected mass about half of Hubble Space Telescope's, but its primary mirror (a 6.5 meter diameter gold-coated beryllium reflector) will have a collecting area about five times as large (25 m2 or 270 sq ft vs. 4.5 m2 or 48 sq ft). The JWST is oriented toward near-infrared astronomy, but can also see orange and red visible light, as well as the mid-infrared region, depending on the instrument. The design emphasizes the near to mid-infrared for three main reasons: High-redshift objects have their visible emissions shifted into the infrared, cold objects such as debris disks and planets emit most strongly in the infrared, and this band is difficult to study from the ground or by existing space telescopes such as Hubble. Ground-based telescopes must look through the atmosphere, which is opaque in many infrared bands (see figure of atmospheric transmission). Even where the atmosphere is transparent, many of the target chemical compounds, such as water, carbon dioxide, and methane, also exist in the Earth's atmosphere, vastly complicating analysis. Existing space telescopes such as Hubble cannot study these bands since their mirrors are not cool enough (the Hubble mirror is maintained at about 15 °C (288 K)) and hence the telescope itself radiates strongly in the infrared bands. The JWST will operate near the Earth–Sun L2 (Lagrange) point, approximately 1,500,000 km (930,000 mi) beyond Earth's orbit. By way of comparison, Hubble orbits 550 kilometres (340 mi) above Earth's surface, and the Moon is roughly 400,000 kilometres (250,000 mi) from Earth. This distance makes post-launch repair or upgrade of the JWST hardware virtually impossible. Objects near this point can orbit the Sun in synchrony with the Earth, allowing the telescope to remain at a roughly constant distance and use a single sunshield to block heat and light from the Sun and Earth. This will keep the temperature of the spacecraft below 50 K (−220 °C; −370 °F), necessary for infrared observations. The prime contractor is Northrop Grumman. To make observations in the infrared spectrum, the JWST must be kept very cold (under 50 K (−220 °C; −370 °F)), otherwise infrared radiation from the telescope itself would overwhelm its instruments. Therefore, it uses a large sunshield to block light and heat from the Sun, Earth, and Moon, and its position near the Earth–Sun L2 point keeps all three bodies on the same side of the spacecraft at all times. Its halo orbit around L2 avoids the shadow of the Earth and Moon, maintaining a constant environment for the sunshield and solar arrays. The shielding maintains a stable temperature throughout the structures on the dark side, which is critical to maintaining precise alignment of the primary mirror segments. The five-layer sunshield is constructed from polyimide film, with membranes coated with aluminum on one side and silicon on the other. Accidental tears of the delicate film structure during testing are one factor delaying the project. The sunshield is designed to be folded twelve times so it will fit within the Ariane 5 rocket's 4.57 m (5 yards) × 16.19 m (17.7 yards) payload fairing. Once deployed at the L2 point, it will unfold to 21.197 m (23.18 yards) × 14.162 m (15.55 yards). The sunshield was hand-assembled at ManTech (NeXolve) in Huntsville, Alabama, before it was delivered to Northrop Grumman in Redondo Beach, California, USA for testing.