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| A rendering of the James Webb Space Telescope with its components fully deployed | |
| Names | Next Generation Space Telescope (NGST) |
|---|---|
| Mission type | Astronomy |
| Operator | NASA / ESA / Canadian Space Agency / STScI[1] |
| Website | webbtelescope.org |
| Mission duration | 10 years (planned) |
| Spacecraft properties | |
| Manufacturer | Northrop Grumman Ball Aerospace & Technologies |
| Launch mass | 6,500 kg (14,300 lb)[2] |
| Dimensions | 20.197 m × 14.162 m (66.26 ft × 46.46 ft), sunshield |
| Power | 2 kW |
| Start of mission | |
| Launch date | 24 December 2021, 12:20 UTC[3][4] |
| Rocket | Ariane 5 ECA (Ariane flight VA256) |
| Launch site | Centre Spatial Guyanais, ELA-3 |
| Contractor | Arianespace |
| Orbital parameters | |
| Reference system | Sun–Earth L2 orbit |
| Regime | Halo orbit |
| Perigee altitude | 374,000 km (232,000 mi)[5] |
| Apogee altitude | 1,500,000 km (930,000 mi) |
| Period | 6 months |
| Main telescope | |
| Type | Korsch telescope |
| Diameter | 6.5 m (21 ft) |
| Focal length | 131.4 m (431 ft) |
| Focal ratio | f/20.2 |
| Collecting area | 25.4 m2 (273 sq ft)[6] |
| Wavelengths | 0.6–28.3 μm (orange to mid-infrared) |
| Transponders | |
| Band | S-band, telemetry, tracking, and controlKa-band, data acquisition |
| Bandwidth | S-band up: 16 kbit/sS-band down: 40 kbit/sKa-band down: up to 28 Mbit/s |
| showInstruments | |
James Webb Space Telescope mission patch |
The James Webb Space Telescope (JWST) is a space telescope being jointly developed by NASA, the European Space Agency (ESA), and the Canadian Space Agency (CSA). It is planned to succeed the Hubble Space Telescope as NASA’s flagship astrophysics mission.[7][8] JWST is scheduled to be launched no earlier than Friday 24 December 2021 during Ariane flight VA256. It 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, and detailed atmospheric characterization of potentially habitable exoplanets.
The primary mirror of JWST, the Optical Telescope Element, consists 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, 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.[9][10] 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 (which is 0.010 au – or 3.9 times the Lunar distance – away from Earth)[11] and a large sunshield made of silicon– and aluminum-coated Kapton will keep its mirror and instruments below 50 K (−223 °C; −370 °F).[12]
The NASA Goddard Space Flight Center (GSFC) is managing the development effort, and the Space Telescope Science Institute will operate Webb after launch.[13] The prime contractor is Northrop Grumman.[14] It is named after James E. Webb,[15] who was the administrator of NASA from 1961 to 1968 and played an integral role in the Apollo program.[16][17]
Development began in 1996 for a launch that was initially planned for 2007 and a US$500 million budget,[18] but the project had numerous delays and cost overruns and underwent a major redesign in 2005.[19] Construction was completed in late 2016, after which its extensive testing phase began.[20][21] In March 2018, NASA further delayed the launch after the telescope’s sunshield ripped during a practice deployment.[22] Launch was delayed again in June 2018 following recommendations from an independent review board.[23][24][25] Work on integration and testing of the telescope was suspended in March 2020 due to the COVID-19 pandemic,[26] adding further delays. Following work resumption, the launch date was delayed to 31 October 2021.[27][28] Problems with the Ariane 5 launch vehicle and the telescope itself subsequently pushed the launch date to 22 December 2021.[29][30][31]
On being mounted on the launch vehicle, a communication problem between the telescope and the launch vehicle led to a further delay to “no earlier than 24 December 2021”.[4] Concerns among the involved scientists and engineers about the launch and deployment of the telescope have been well described.[32][33] On 18 December 2021, NASA announced that JWST is scheduled to be launched at 6:20 am Central(UTC−05:00) on Friday, 24 December 2021 by an Ariane 5 rocket from Kourou, French Guiana, on the northeastern coast of South America.[3][34]
Features
Rough plot of Earth’s atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation, including visible light
The James Webb Space Telescope has an expected mass about half of Hubble Space Telescope‘s, but its primary mirror, a 6.5 m (21 ft) diameter gold-coated beryllium reflector will have a collecting area over six times as large, 25.4 m2 (273 sq ft), using 18 hexagonal mirrors with 0.9 m2 (9.7 sq ft) obscuration for the secondary support struts.[35]
JWST is designed primarily for 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
- this band is difficult to study from the ground or by existing space telescopes such as Hubble
Ground-based telescopes must look through Earth’s atmosphere, which is opaque in many infrared bands (see figure of atmospheric absorption). 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 insufficiently cool (the Hubble mirror is maintained at about 15 °C (288 K; 59 °F)) thus the telescope itself radiates strongly in the infrared bands.[36]
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 km (340 mi) above Earth’s surface, and the Moon is roughly 400,000 km (250,000 mi) from Earth. This distance made post-launch repair or upgrade of JWST hardware virtually impossible with the spaceships available during the telescope design and fabrication stage. Objects near this Lagrange point can orbit the Sun in synchrony with the Earth, allowing the telescope to remain at a roughly constant distance[37] and with constant orientation of the single heatshield and the Bus toward the earth and the sun to block heat and light from the Sun and Earth and maintain communications. This arrangement will keep the temperature of the spacecraft below 50 K (−223 °C; −370 °F), necessary for infrared observations.[12][38]
Sunshield protection
Main article: Sunshield (JWST)Test unit of the sunshield stacked and expanded at the Northrop Grumman facility in California, 2014
To make observations in the infrared spectrum, JWST must be kept under 50 K (−223.2 °C; −369.7 °F); otherwise, infrared radiation from the telescope itself would overwhelm its instruments. It therefore 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.[39] Its halo orbit around the L2 point avoids the shadow of the Earth and Moon, maintaining a constant environment for the sunshield and solar arrays.[37] The shielding maintains a stable temperature for the structures on the dark side, which is critical to maintaining precise alignment of the primary mirror segments.[citation needed]
The five-layer sunshield, each layer as thin as a human hair,[40] is constructed from Kapton E, a commercially available polyimide film from DuPont, with membranes specially coated with aluminum on both sides and doped silicon on the Sun-facing side of the two hottest layers to reflect the Sun’s heat back into space.[41] Accidental tears of the delicate film structure during testing in 2018 were among the factors delaying the project.[42]
The sunshield is designed to be folded twelve times so that it will fit within the Ariane 5 rocket’s payload fairing, which is 4.57 m (15.0 ft) in diameter, and 16.19 m (53.1 ft) long. Once deployed at the L2 point, it will unfold to 14.162 m × 21.197 m (46.46 ft × 69.54 ft). The sunshield was hand-assembled at ManTech (NeXolve) in Huntsville, Alabama, before it was delivered to Northrop Grumman in Redondo Beach, California, for testing.[43]
Optics
Main article: Optical Telescope ElementEngineers cleaning a test mirror with carbon dioxide snow, 2015Main mirror assembled at Goddard Space Flight Center, May 2016
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JWST’s primary mirror is a 6.5 m (21 ft)-diameter gold-coated beryllium reflector with a collecting area of 25.4 m2 (273 sq ft). If it were built as a single large mirror, this would have been too large for existing launch vehicles. The mirror is therefore composed of 18 hexagonal segments which will unfold after the telescope is launched. Image plane wavefront sensing through phase retrieval will be used to position the mirror segments in the correct location using very precise micro-motors. Subsequent to this initial configuration, they will only need occasional updates every few days to retain optimal focus.[44] This is unlike terrestrial telescopes, for example the Keck telescopes, which continually adjust their mirror segments using active optics to overcome the effects of gravitational and wind loading. The Webb telescope will use 126 small motors to occasionally adjust the optics as there are few environmental disturbances of a telescope in space.[45]
JWST’s optical design is a three-mirror anastigmat,[46] which makes use of curved secondary and tertiary mirrors to deliver images that are free from optical aberrations over a wide field. In addition, there is a fine steering mirror which can adjust its position many times per second to provide image stabilization.
Ball Aerospace & Technologies is the principal optical subcontractor for the JWST project, led by prime contractor Northrop Grumman Aerospace Systems, under a contract from the NASA Goddard Space Flight Center, in Greenbelt, Maryland.[2][47] Eighteen primary mirror segments, secondary, tertiary and fine steering mirrors, plus flight spares have been fabricated and polished by Ball Aerospace & Technologies based on beryllium segment blanks manufactured by several companies including Axsys, Brush Wellman, and Tinsley Laboratories.[citation needed]
The final segment of the primary mirror was installed on 3 February 2016,[48] and the secondary mirror was installed on 3 March 2016.[49]
More at: https://en.wikipedia.org/wiki/James_Webb_Space_Telescope
(Contributed by Sarah Flynn)