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James Webb Space Telescope

From Wikipedia, the free encyclopedia

James Webb Space Telescope
JWST spacecraft model 3.png
Rendering of the James Webb Space Telescope fully deployed.
NamesNext Generation Space Telescope (NGST; 1996–2002)
Mission typeAstronomy
OperatorSTScI (NASA)[1]
COSPAR ID2021-130A Edit this at Wikidata
SATCAT no.50463[2]
WebsiteOfficial website
Mission duration
  • 20 years (expected)[3]
  • 10 years (planned)
  • 5½ years (primary mission)[4]
  • 6 months, 2 days (elapsed)
Spacecraft properties
Manufacturer
Launch mass6,161.4 kg (13,584 lb)[5]
Dimensions20.197 m × 14.162 m (66.26 ft × 46.46 ft), sunshield
Power2 kW
Start of mission
Launch date25 December 2021 (2021-12-25), 12:20 UTC
RocketAriane 5 ECA (VA256)
Launch siteCentre Spatial Guyanais, ELA-3
ContractorArianespace
Entered serviceEnd of June 2022 (planned)
Orbital parameters
Reference systemSun–Earth L2 orbit
RegimeHalo orbit
Periapsis altitude250,000 km (160,000 mi)[6]
Apoapsis altitude832,000 km (517,000 mi)[6]
Period6 months
Main telescope
TypeKorsch telescope
Diameter6.5 m (21 ft)
Focal length131.4 m (431 ft)
Focal ratiof/20.2
Collecting area25.4 m2 (273 sq ft)[7]
Wavelengths0.6–28.3 μm (orange to mid-infrared)
Transponders
Band
  • S-band, telemetry, tracking, and control
  • Ka-band, data acquisition
Bandwidth
  • S-band up: 16 kbit/s
  • S-band down: 40 kbit/s
  • Ka-band down: up to 28 Mbit/s
Instruments
Elements
JWST Launch Logo.png

James Webb Space Telescope mission logo  

The James Webb Space Telescope (JWST) is a space telescope designed primarily to conduct infrared astronomy. The most powerful telescope ever launched into space, its greatly improved infrared resolution and sensitivity will allow it to view objects too old, distant, or faint for the Hubble Space Telescope. This is expected to enable a broad range of investigations across the fields of astronomy and cosmology, such as observations of first stars and the formation of the first galaxies, and detailed atmospheric characterization of potentially habitable exoplanets. JWST was launched in December 2021 on a European Space Agency (ESA) Ariane 5 rocket from Kourou, French Guiana, and entered orbit in January 2022; as of June 2022, the telescope is undergoing testing and alignment. Once operational, expected about the end of June 2022, JWST is intended to succeed the Hubble as NASA's flagship mission in astrophysics. NASA has scheduled the first official science images release event for Tuesday, 12 July 2022 at 10:30 am EDT/usa.[8]

NASA led JWST's development in collaboration with ESA and the Canadian Space Agency (CSA). The NASA Goddard Space Flight Center (GSFC) in Maryland managed telescope development, the Space Telescope Science Institute in Baltimore operates JWST, and the prime contractor was Northrop Grumman. The telescope is named after James E. Webb, who was the administrator of NASA from 1961 to 1968 during the Mercury, Gemini, and Apollo programs.

JWST's primary mirror consists of 18 hexagonal mirror segments made of gold-plated beryllium which combined create a 6.5-meter (21 ft) diameter mirror, compared with Hubble's 2.4 m (7.9 ft). This gives the Webb telescope a light-collecting area of about 25 square meters, about 6 times that of Hubble. Unlike Hubble, which observes in the near ultraviolet, visible, and near infrared (0.1–1.7 μm) spectra, JWST will observe in a lower frequency range, from long-wavelength visible light (red) through mid-infrared (0.6–28.3 μm). The telescope must be kept extremely cold, below 50 K (−223 °C; −370 °F), to observe faint signals in the infrared without interference from other sources of warmth. It is deployed in a solar orbit near the Sun–Earth L2 Lagrange point, about 1.5 million kilometers (930,000 mi) from Earth, where its five-layer, kite-shaped sunshield protects it from warming by the Sun, Earth, and Moon.

Development began in 1996 for a launch initially planned for 2007 with a US$500 million budget. There were many delays and cost overruns, including a major redesign in 2005, a ripped sunshield during a practice deployment, recommendations from an independent review board, a threat by the U.S. Congress to cancel the project, the COVID-19 pandemic, and problems with the telescope itself. The high-stakes nature of the launch and the telescope's complexity were remarked upon by the media, scientists and engineers. Construction was completed in late 2016, followed by years of extensive testing before launch. The total project cost is now expected to be about US$9.7 billion.

Features

Rough plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation, including visible light
Rough plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation, including visible light

The James Webb Space Telescope has a mass that is about half of Hubble Space Telescope's mass. The JWST has a 6.5-meter (21 ft)-diameter gold-coated beryllium primary mirror made up of 18 separate hexagonal mirrors. The mirror has a polished area of 26.3 m2 (283 sq ft), of which 0.9 m2 (9.7 sq ft) is obscured by the secondary support struts,[9] giving a total collecting area of 25.4 m2 (273 sq ft). This is over six times larger than the collecting area of Hubble's 2.4-meter (7.9 ft) diameter mirror, which has a collecting area of 4.0 m2 (43 sq ft). The mirror has a gold coating to provide infrared reflectivity and durability.

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. It can detect objects up to 100 times fainter than Hubble can, and objects much earlier in the history of the universe, back to redshift z≈20 (about 180 million years cosmic time after the Big Bang).[10] For comparison, the earliest stars are thought to have formed between z≈30 and z≈20 (100-180 million years cosmic time),[11] and the first galaxies may have formed around redshift z≈15 (about 270 million years cosmic time). Hubble is unable to see further back than very early reionization[12][13] at about z≈11.1 (galaxy GN-z11, 400 million years cosmic time).[14][15][10]

The design emphasizes the near to mid-infrared for three main reasons:

  • high-redshift (very early and distant) objects have their visible emissions shifted into the infrared, and therefore their light can only be observed today via infrared astronomy;
  • colder objects such as debris disks and planets emit most strongly in the infrared;
  • these infrared bands are 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.[16]

JWST can also observe nearby objects, including objects in the Solar System, having an apparent angular rate of motion of 0.030 arc seconds per second or less. This includes all planets and satellites, comets, and asteroids beyond Earth's orbit, and "virtually all" known Kuiper Belt Objects.[11] In addition, it can observe opportunistic and unplanned targets within 48 hours of a decision to do so, such as supernovae and gamma ray bursts.[11]

Location and orbit

JWST operates in a halo orbit, circling around a point in space known as the Sun-Earth L2 Lagrange point, approximately 1,500,000 km (930,000 mi) beyond Earth's orbit around the Sun. Its actual position varies between about 250,000 km (160,000 mi) and 832,000 km (517,000 mi) from L2 as it orbits, keeping it out of both Earth and Moon's shadow. 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. Objects near this Sun-Earth L2 point can orbit the Sun in synchrony with the Earth, allowing the telescope to remain at a roughly constant distance[17] with continuous orientation of its unique sunshield and equipment bus toward the Sun, Earth and Moon. Combined with its wide shadow-avoiding orbit, the telescope can simultaneously block incoming heat and light from all three of these bodies and avoid even the smallest changes of temperature from Earth and Moon shadows that would affect the structure, yet still maintain uninterrupted solar power and Earth communications on its sun-facing side. This arrangement keeps the temperature of the spacecraft constant and below the 50 K (−223 °C; −370 °F) necessary for faint infrared observations.[18][19]

Sunshield protection

Test unit of the sunshield stacked and expanded at the Northrop Grumman facility in California, 2014
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 Sun-Earth L2 keeps all three bodies on the same side of the spacecraft at all times.[20] 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.[17] 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 in space.[18]

The five-layer sunshield, each layer as thin as a human hair,[21] is constructed from Kapton E, a commercially available polyimide film from DuPont, with membranes specially coated with aluminum on both sides and a layer of doped silicon on the Sun-facing side of the two hottest layers to reflect the Sun's heat back into space.[18] Accidental tears of the delicate film structure during testing in 2018 were among the factors delaying the project.[22]

The sunshield was designed to be folded twelve times so that it 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. The shield's fully deployed dimensions were planned as 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.[23]

Because of the sunshield, JWST does not have an unlimited field of regard at any given time. The telescope can see 40 percent of the sky from one position and can see all of the sky over a period of six months,[24] the amount of time it takes to complete half its orbit around the Sun.

Optics

Main mirror assembly from the front with primary mirrors attached, November 2016
Main mirror assembly from the front with primary mirrors attached, November 2016

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 (a technique pioneered by Guido Horn d'Arturo), which unfolded after the telescope was launched. Image plane wavefront sensing through phase retrieval is used to position the mirror segments in the correct location using very precise micro-motors. Subsequent to this initial configuration, they only need occasional updates every few days to retain optimal focus.[25] 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 132 small motors (called actuators) to position and occasionally adjust the optics as there are few environmental disturbances of a telescope in space.[26] Each of the 18 primary mirror segments is controlled by 6 positional actuators with a further ROC (radius of curvature) actuator at the center to adjust curvature (7 actuators per segment), for a total of 126 primary mirror actuators, and another 6 actuators for the secondary mirror, giving a total of 132.[27][28] The actuators can position the mirror with 10 nanometer (10 millionths of a millimeter) accuracy.[29]

The actuators are critical in maintaining the alignment of the telescope's mirrors, and are designed and manufactured by Ball Aerospace & Technologies. Each of the 132 actuators are driven by a single stepper motor, providing both fine and coarse adjustments.[30] The actuators provide a coarse step size of 58 nanometers for larger adjustments, and a fine adjustment step size of 7 nanometers.[31]

JWST's optical design is a three-mirror anastigmat,[32] which makes use of curved secondary and tertiary mirrors to deliver images that are free from optical aberrations over a wide field. The secondary mirror is 0.74 m (2.4 ft) in diameter. In addition, there is a fine steering mirror which can adjust its position many times per second to provide image stabilization. The primary mirror segments are hollowed at the rear in a honeycomb pattern, to reduce weight.

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.[33][34] The mirrors, plus flight spares, were fabricated and polished by Ball Aerospace & Technologies based on beryllium segment blanks manufactured by several companies including Axsys, Brush Wellman, and Tinsley Laboratories.[35]

Scientific instruments

NIRCam wrapped up in 2013
NIRCam wrapped up in 2013
The Calibration Assembly, one component of the NIRSpec instrument
The Calibration Assembly, one component of the NIRSpec instrument
MIRI
MIRI

The Integrated Science Instrument Module (ISIM) is a framework that provides electrical power, computing resources, cooling capability as well as structural stability to the Webb telescope. It is made with bonded graphite-epoxy composite attached to the underside of Webb's telescope structure. The ISIM holds the four science instruments and a guide camera.[36]

  • NIRCam (Near InfraRed Camera) is an infrared imager which will have a spectral coverage ranging from the edge of the visible (0.6 μm) through to the near infrared (5 μm).[37][38] There are 10 sensors each of 4 megapixels. NIRCam will also serve as the observatory's wavefront sensor, which is required for wavefront sensing and control activities, used to align and focus the main mirror segments. NIRCam was built by a team led by the University of Arizona, with principal investigator Marcia J. Rieke. The industrial partner is Lockheed-Martin's Advanced Technology Center in Palo Alto, California.[39]
  • NIRSpec (Near InfraRed Spectrograph) will also perform spectroscopy over the same wavelength range. It was built by the European Space Agency at ESTEC in Noordwijk, Netherlands. The leading development team includes members from Airbus Defence and Space, Ottobrunn and Friedrichshafen, Germany, and the Goddard Space Flight Center; with Pierre Ferruit (École normale supérieure de Lyon) as NIRSpec project scientist. The NIRSpec design provides three observing modes: a low-resolution mode using a prism, an R~1000 multi-object mode, and an R~2700 integral field unit or long-slit spectroscopy mode.[40] Switching of the modes is done by operating a wavelength preselection mechanism called the Filter Wheel Assembly, and selecting a corresponding dispersive element (prism or grating) using the Grating Wheel Assembly mechanism.[40] Both mechanisms are based on the successful ISOPHOT wheel mechanisms of the Infrared Space Observatory. The multi-object mode relies on a complex micro-shutter mechanism to allow for simultaneous observations of hundreds of individual objects anywhere in NIRSpec's field of view. There are two sensors each of 4 megapixels. The mechanisms and their optical elements were designed, integrated and tested by Carl Zeiss Optronics GmbH (today Hensoldt) of Oberkochen, Germany, under contract from Astrium.[40]
  • MIRI (Mid-InfraRed Instrument) will measure the mid-to-long-infrared wavelength range from 5 to 27 μm.[41][42] It contains both a mid-infrared camera and an imaging spectrometer.[33] MIRI was developed as a collaboration between NASA and a consortium of European countries, and is led by George Rieke (University of Arizona) and Gillian Wright (UK Astronomy Technology Centre, Edinburgh, Scotland, part of the Science and Technology Facilities Council (STFC)).[39] MIRI features similar wheel mechanisms to NIRSpec which are also developed and built by Carl Zeiss Optronics GmbH (today Hensoldt) under contract from the Max Planck Institute for Astronomy, Heidelberg, Germany. The completed Optical Bench Assembly of MIRI was delivered to Goddard Space Flight Center in mid-2012 for eventual integration into the ISIM. The temperature of the MIRI must not exceed 6 K (−267 °C; −449 °F): a helium gas mechanical cooler sited on the warm side of the environmental shield provides this cooling.[43]
  • FGS/NIRISS (Fine Guidance Sensor and Near Infrared Imager and Slitless Spectrograph), led by the Canadian Space Agency under project scientist John Hutchings (Herzberg Astronomy and Astrophysics Research Centre, National Research Council), is used to stabilize the line-of-sight of the observatory during science observations. Measurements by the FGS are used both to control the overall orientation of the spacecraft and to drive the fine steering mirror for image stabilization. The Canadian Space Agency is also providing a Near Infrared Imager and Slitless Spectrograph (NIRISS) module for astronomical imaging and spectroscopy in the 0.8 to 5 μm wavelength range, led by principal investigator René Doyon at the Université de Montréal.[39] Because the NIRISS is physically mounted together with the FGS, they are often referred to as a single unit; however, they serve entirely different purposes, with one being a scientific instrument and the other being a part of the observatory's support infrastructure.

NIRCam and MIRI feature starlight-blocking coronagraphs for observation of faint targets such as extrasolar planets and circumstellar disks very close to bright stars.[42]

The infrared detectors for the NIRCam, NIRSpec, FGS, and NIRISS modules are being provided by Teledyne Imaging Sensors (formerly Rockwell Scientific Company). The James Webb Space Telescope (JWST) Integrated Science Instrument Module (ISIM) and Command and Data Handling (ICDH) engineering team uses SpaceWire to send data between the science instruments and the data-handling equipment.[44]

Spacecraft bus

Diagram of the spacecraft bus. The solar panel is in green and the light purple panels are radiators.
Diagram of the spacecraft bus. The solar panel is in green and the light purple panels are radiators.

The spacecraft bus is the primary support component of the James Webb Space Telescope which hosts a multitude of computing, communication, electric power, propulsion, and structural parts.[45] Along with the sunshield, it forms the spacecraft element of the space telescope.[46] The other two major elements of JWST are the Integrated Science Instrument Module (ISIM) and the Optical Telescope Element (OTE).[47] Region 3 of ISIM is also inside the spacecraft bus; region 3 includes ISIM Command and Data Handling subsystem and the MIRI cryocooler.[47] The spacecraft bus is connected to Optical Telescope Element via the Deployable Tower Assembly, which also connects to the sunshield.[45] The spacecraft bus is on the Sun-facing "warm" side of the sunshield and operates at a temperature of about 300 K (27 °C; 80 °F).[46]

The structure of the spacecraft bus has a mass of 350 kg (770 lb), and must support the 6,200 kg (13,700 lb) space telescope.[48] It is made primarily of graphite composite material.[48] It was assembled in California, assembly was completed in 2015, and then it had to be integrated with the rest of the space telescope leading up to its 2021 launch. The spacecraft bus can rotate the telescope with a pointing precision of one arcsecond, and isolates vibration down to two milliarcseconds.[49]

In the central computing, memory storage, and communications equipment,[45] the processor and software direct data to and from the instruments, to the solid-state memory core, and to the radio system which can send data back to Earth and receive commands.[45] The computer also controls the pointing of the spacecraft, taking in sensor data from the gyroscopes and star tracker, and sending commands to the reaction wheels or thrusters.[45]

Webb has two pairs of rocket engines (one pair for redundancy) to make course corrections on the way to L2 and for station keeping—maintaining the correct position in the halo orbit. Eight smaller thrusters are used for attitude control—the correct pointing of the spacecraft.[50] The engines use hydrazine fuel (159 liters or 42 U.S. gallons at launch) and dinitrogen tetroxide as oxidizer (79.5 liters or 21.0 U.S. gallons at launch).[51]

Servicing

JWST is not intended to be serviced in space. A crewed mission to repair or upgrade the observatory, as was done for Hubble, would not currently be possible,[52] and according to NASA Associate Administrator Thomas Zurbuchen, despite best efforts, an uncrewed remote mission was found to be beyond current technology at the time JWST was designed.[53] During the long JWST testing period, NASA officials referred to the idea of a servicing mission, but no plans were announced.[54][55] Since the successful launch, NASA have stated that limited accommodation was made to facilitate future servicing missions, if any. This included: precise guidance markers in the form of crosses on the surface of JWST, for use by remote servicing missions, as well as refillable fuel tanks, removable heat protectors, and accessible attachment points.[56][53]

Comparison with other telescopes

Comparison with Hubble primary mirror
Comparison with Hubble primary mirror
Primary mirror size comparison between JWST and Hubble

The desire for a large infrared space telescope traces back decades. In the United States, the Space Infrared Telescope Facility (SIRTF, later called the Spitzer Space Telescope) was planned while the Space Shuttle was in development, and the potential for infrared astronomy was acknowledged at that time.[57] Unlike ground telescopes, space observatories were free from atmospheric absorption of infrared light. Space observatories opened up a whole "new sky" for astronomers.[57]

The tenuous atmosphere above the 400 km nominal flight altitude has no measurable absorption so that detectors operating at all wavelengths from 5 μm to 1000 μm can achieve high radiometric sensitivity.

— S. G. McCarthy and G. W. Autio, 1978.[57]

However, infrared telescopes have a disadvantage: they need to stay extremely cold, and the longer the wavelength of infrared, the colder they need to be.[16] If not, the background heat of the device itself overwhelms the detectors, making it effectively blind.[16] This can be overcome by careful spacecraft design, in particular by placing the telescope in a dewar with an extremely cold substance, such as liquid helium.[16] This means that most infrared telescopes have a lifespan limited by their coolant, as short as a few months, maybe a few years at most.[16]

In some cases, it is possible to maintain a temperature low enough through the design of the spacecraft to enable near-infrared observations without a supply of coolant, such as the extended missions of Spitzer Space Telescope and Wide-field Infrared Survey Explorer. Another example is Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) instrument, which started out using a block of nitrogen ice that depleted after a couple of years, but was then converted to a cryocooler that worked continuously. The James Webb Space Telescope is designed to cool itself without a dewar, using a combination of sunshields and radiators, with the mid-infrared instrument using an additional cryocooler.[58]

Selected space telescopes and instruments[59]
Name Launch Year Wavelength
(μm)
Aperture
(m)
Cooling
Spacelab Infrared Telescope (IRT) 1985 1.7–118 0.15 Helium
Infrared Space Observatory (ISO)[60] 1995 2.5–240 0.60 Helium
Hubble Space Telescope Imaging Spectrograph (STIS) 1997 0.115–1.03 2.4 Passive
Hubble Near Infrared Camera and Multi-Object Spectrometer (NICMOS) 1997 0.8–2.4 2.4 Nitrogen, later cryocooler
Spitzer Space Telescope 2003 3–180 0.85 Helium
Hubble Wide Field Camera 3 (WFC3) 2009 0.2–1.7 2.4 Passive, and thermo-electric[61]
Herschel Space Observatory 2009 55–672 3.5 Helium
James Webb Space Telescope 2021 0.6–28.5 6.5 Passive, and cryocooler (MIRI)

JWST's delays and cost increases can be compared to those of the Hubble Space Telescope.[62] When Hubble formally started in 1972, it had an estimated development cost of US$300 million (or about US$1 billion in 2006 constant dollars),[62] but by the time it was sent into orbit in 1990, the cost was about four times that.[62] In addition, new instruments and servicing missions increased the cost to at least US$9 billion by 2006.[62]

Of the other NASA observatories that were proposed around the same time, most have already been canceled or put on hold, including Terrestrial Planet Finder (2011), Space Interferometry Mission (2010), International X-ray Observatory (2011), MAXIM (Microarcsecond X-ray Imaging Mission), SAFIR (Single Aperture Far-Infrared Observatory), SUVO (Space Ultraviolet-Visible Observatory), and SPECS (Submillimeter Probe of the Evolution of Cosmic Structure).[citation needed]

History

Background (development to 2003)

Selected events
Year Events
1996 Next Generation Space Telescope project started (8 m)
2002 Named James Webb Space Telescope, Chg to 6 m
2003 TRW contract awarded to build
2004 NEXUS cancelled[63]
2007 ESA/NASA MOU
2010 MCDR passed
2011 Proposed cancel
2016 Final assembly completed
2021 Launch

Discussions of a Hubble follow-on started in the 1980s, but serious planning began in the early 1990s.[64] The Hi-Z telescope concept was developed between 1989 and 1994:[65] a fully baffled[a] 4 m (13 ft) aperture infrared telescope that would recede to an orbit at 3 Astronomical unit (AU).[66] This distant orbit would have benefited from reduced light noise from zodiacal dust.[66] Other early plans called for a NEXUS precursor telescope mission.[67][68]

Correcting the flawed optics of the Hubble Space Telescope in its first years played a significant role in the birth of the JWST. In 1993, NASA readied STS-61, the Space Shuttle mission that would carry a replacement for HST's camera and a retrofit for its imaging spectrograph to compensate for the spherical aberration in its primary mirror. While the astronomical community eagerly awaited this mission, NASA cautioned that this extraordinary advance in working in space carried significant risk and that its successful completion was in no way guaranteed.[citation needed]

Consequently, the "HST & Beyond Committee", was formed in 1995 to evaluate the effectiveness of the HST repair mission and to explore ideas for future space telescopes that would be needed if the repair mission fell short.[69] It had the good fortune to see the success of the Space Shuttle Servicing Mission 1 in December 1993 and the unprecedented public response to the stunning images that the HST delivered.[citation needed]

Emboldened by HST's success, and recognizing innovative work in Europe for future missions[70][71] its 1996 report explored the concept of a larger and much colder, infrared-sensitive telescope that could reach back in cosmic time to the birth of the first galaxies. This high-priority science goal was beyond the HST's capability because, as a warm telescope, it is blinded by infrared emission from its own optical system. In addition to recommendations to extend the HST mission to 2005 and to develop technologies for finding planets around other stars, NASA embraced the chief recommendation of HST & Beyond[72] for a large, cold space telescope (radiatively cooled far below 0 °C), and began the planning process for the future JWST.

Beginning in the 1960s, and at the beginning of each decade since, the National Academies had organized the community of U.S. astronomers to think creatively about astronomical instruments and research for the subsequent decade, and to reach consensus on goals and priorities. A faithful supporter of these 'Decadal Surveys of Astronomy and Astrophysics', NASA has also been extraordinarily successful in developing programs and tools to accomplish Survey recommendations. So, even with the substantial support and excitement in the mid-1990s for NASA's beginning to work on a successor to the HST, the astronomical community regarded as essential a high prioritization by the 2000 Decadal Survey.

Preparation for the Survey included further development of the scientific program for what became known as the "Next Generation Space Telescope",[73] and advancements in relevant technologies by NASA. As it matured, studying the birth of galaxies in the young universe, and searching for planets around other stars – the prime goals coalesced as "Origins" by HST & Beyond became prominent.

Late in the 1990s NASA created the 'Origins Subcommittee' to guide this effort and the 'Beyond Einstein Subcommittee' to oversee missions where the universe is a laboratory for fundamental astrophysics, for example, black holes and supernovae. As hoped, the NGST received the highest ranking in the 2000 Decadal Survey of Astronomy & Astrophysics,[74] which allowed the project to proceed with the full endorsement of a community consensus.

An administrator of NASA, Dan Goldin, coined the phrase "faster, better, cheaper", and opted for the next big paradigm shift for astronomy, namely, breaking the barrier of a single mirror. That meant going from "eliminate moving parts" to "learn to live with moving parts" (i.e. segmented optics). With the goal to reduce mass density a tenfold, silicon carbide with a very thin layer of glass on top was first looked at, but beryllium has been selected at the end.[64]

"Faster, better, cheaper" mid-1990s era produced the NGST concept, with an 8 m (26 ft) aperture to be flown to L2, roughly estimated to cost US$500 million.[75] In 1997, NASA worked with the Goddard Space Flight Center,[76] Ball Aerospace & Technologies,[77] and TRW[78] to conduct technical requirement and cost studies of the three different concepts, and in 1999 selected Lockheed Martin[79] and TRW for preliminary concept studies.[80] Launch was at that time planned for 2007, but the launch date was pushed back many times (see table further down).

In 2002, the project was renamed after NASA's second administrator (1961–1968), James E. Webb (1906–1992).[81] Webb led the agency during the Apollo program and established scientific research as a core NASA activity.[82]

In 2003, NASA awarded TRW the US$824.8 million prime contract for JWST. The design called for a de-scoped 6.1 m (20 ft) primary mirror and a launch date of 2010.[83] Later that year, TRW was acquired by Northrop Grumman in a hostile bid and became Northrop Grumman Space Technology.[80]

JWST is a project of NASA, with international collaboration from the European Space Agency (ESA) and the Canadian Space Agency (CSA) who formally joined in 2004 and 2007 respectively.

Development - (re) planning - 2005

Development was managed by NASA's Goddard Space Flight Center in Greenbelt, Maryland, with John C. Mather as its project scientist. The primary contractor was Northrop Grumman Aerospace Systems, responsible for developing and building the spacecraft element, which included the satellite bus, sunshield, Deployable Tower Assembly (DTA) which connects the Optical Telescope Element to the spacecraft bus, and the Mid Boom Assembly (MBA) which helps to deploy the large sunshields on orbit,[84] while Ball Aerospace & Technologies has been subcontracted to develop and build the OTE itself, and the Integrated Science Instrument Module (ISIM).[36]

Cost growth revealed in spring 2005 led to an August 2005 re-planning.[85] The primary technical outcomes of the re-planning were significant changes in the integration and test plans, a 22-month launch delay (from 2011 to 2013), and elimination of system-level testing for observatory modes at wavelength shorter than 1.7 μm. Other major features of the observatory were unchanged. Following the re-planning, the project was independently reviewed in April 2006.

In the 2005 re-plan, the life-cycle cost of the project was estimated at US$4.5 billion. This comprised approximately US$3.5 billion for design, development, launch and commissioning, and approximately US$1.0 billion for ten years of operations.[85] The ESA agreed in 2004 to contributing about €300 million, including the launch.[86] The Canadian Space Agency pledged $39 million Canadian in 2007[87] and in 2012 delivered its contributions in equipment to point the telescope and detect atmospheric conditions on distant planets.[88]

Construction (detailed design - from 2007)

A JWST mirror segment, 2010
Mirror segments undergoing cryogenic tests at the X-ray & Cryogenic Facility at Marshall Space Flight Center
The assembled telescope following environmental testing
The assembled telescope following environmental testing

In January 2007, nine of the ten technology development items in the project successfully passed a Non-Advocate Review.[89] These technologies were deemed sufficiently mature to retire significant risks in the project. The remaining technology development item (the MIRI cryocooler) completed its technology maturation milestone in April 2007. This technology review represented the beginning step in the process that ultimately moved the project into its detailed design phase (Phase C). By May 2007, costs were still on target.[90] In March 2008, the project successfully completed its Preliminary Design Review (PDR). In April 2008, the project passed the Non-Advocate Review. Other passed reviews include the Integrated Science Instrument Module review in March 2009, the Optical Telescope Element review completed in October 2009, and the Sunshield review completed in January 2010.[91]

In April 2010, the telescope passed the technical portion of its Mission Critical Design Review (MCDR). Passing the MCDR signified the integrated observatory can meet all science and engineering requirements for its mission.[92] The MCDR encompassed all previous design reviews. The project schedule underwent review during the months following the MCDR, in a process called the Independent Comprehensive Review Panel, which led to a re-plan of the mission aiming for a 2015 launch, but as late as 2018. By 2010, cost over-runs were impacting other projects, though JWST itself remained on schedule.[93]

By 2011, the JWST project was in the final design and fabrication phase (Phase C). As is typical for a complex design that cannot be changed once launched, there are detailed reviews of every portion of design, construction, and proposed operation. New technological frontiers were pioneered by the project, and it passed its design reviews. In the 1990s it was unknown if a telescope so large and of such low mass was possible.[94]

Assembly of the hexagonal segments of the primary mirror, which was done via robotic arm, began in November 2015 and was completed on 3 February 2016. The secondary mirror was installed on 3 March 2016.[95][96] Final construction of the Webb telescope was completed in November 2016, after which extensive testing procedures began.[97]

In March 2018, NASA delayed JWST's launch an additional 2 years to May 2020 after the telescope's sunshield ripped during a practice deployment and the sunshield's cables did not sufficiently tighten. In June 2018, NASA delayed the launch by an additional 10 months to March 2021, based on the assessment of the independent review board convened after the failed March 2018 test deployment.[98] The review identified that JWST launch and deployment had 344 potential single-point failures - tasks that had no alternative or means of recovery if unsuccessful, and therefore had to succeed for the telescope to work.[99] In August 2019, the mechanical integration of the telescope was completed, something that was scheduled to be done 12 years before in 2007.[100]

After construction was completed, JWST underwent final tests at a Northrop Grumman factory in Redondo Beach, California.[101] A ship carrying the telescope left California on 26 September 2021, passed through the Panama Canal, and arrived in French Guiana on 12 October 2021.[102]

Cost and schedule issues

NASA's lifetime cost for the project is expected to be US$9.7 billion, of which US$8.8 billion was spent on spacecraft design and development and US$861 million is planned to support five years of mission operations.[103] Representatives from ESA and CSA stated their project contributions amount to approximately €700 million and CA$200 million, respectively.[104]

JWST has a history of major cost overruns and delays which have resulted in part from outside factors such as delays in deciding on a launch vehicle and adding extra funding for contingencies. By 2006, US$1 billion had been spent on developing JWST, with the budget at about US$4.5 billion at that time. A 2006 article in the journal Nature noted a study in 1984 by the Space Science Board, which estimated that a next generation infrared observatory would cost US$4 billion (about US$7 billion in 2006 dollars).[62]

Then-planned launch and total budget
Year Planned
launch
Budget plan
(billion USD)
1997 2007[94] 0.5[94]
1998 2007[105] 1[62]
1999 2007 to 2008[106] 1[62]
2000 2009[41] 1.8[62]
2002 2010[107] 2.5[62]
2003 2011[108] 2.5[62]
2005 2013 3[109]
2006 2014 4.5[110]
2008: Preliminary Design Review
2008 2014 5.1[111]
2010: Critical Design Review
2010 2015 to 2016 6.5[112]
2011 2018 8.7[113]
2013 2018 8.8[114]
2017 2019[115] 8.8
2018 2020[116] ≥8.8
2019 March 2021[117] 9.66
2021 Dec 2021[118] 9.70

The telescope was originally estimated to cost US$1.6 billion,[119] but the cost estimate grew throughout the early development and had reached about US$5 billion by the time the mission was formally confirmed for construction start in 2008. In summer 2010, the mission passed its Critical Design Review (CDR) with excellent grades on all technical matters, but schedule and cost slips at that time prompted Maryland U.S. Senator Barbara Mikulski to call for an independent review of the project. The Independent Comprehensive Review Panel (ICRP) chaired by J. Casani (JPL) found that the earliest possible launch date was in late 2015 at an extra cost of US$1.5 billion (for a total of US$6.5 billion). They also pointed out that this would have required extra funding in FY2011 and FY2012 and that any later launch date would lead to a higher total cost.[112]

On 6 July 2011, the United States House of Representatives' appropriations committee on Commerce, Justice, and Science moved to cancel the James Webb project by proposing an FY2012 budget that removed US$1.9 billion from NASA's overall budget, of which roughly one quarter was for JWST.[120][121][122][123] US$3 billion had been spent and 75% of its hardware was in production.[124] This budget proposal was approved by subcommittee vote the following day. The committee charged that the project was "billions of dollars over budget and plagued by poor management."[120] In response, the American Astronomical Society issued a statement in support of JWST,[125] as did Maryland US Senator Barbara Mikulski.[126] A number of editorials supporting JWST appeared in the international press during 2011 as well.[120][127][128] In November 2011, Congress reversed plans to cancel JWST and instead capped additional funding to complete the project at US$8 billion.[129]

Some scientists expressed concerns about growing costs and schedule delays for the Webb telescope, which competed for scant astronomy budgets and thus threatened funding for other space science programs.[130][114] Because the runaway budget diverted funding from other research, a 2010 Nature article described JWST as "the telescope that ate astronomy".[131]

A review of NASA budget records and status reports noted that JWST was plagued by many of the same problems that have affected other major NASA projects. Repairs and additional testing included underestimates of the telescope's cost that failed to budget for expected technical glitches, and missed budget projections, thus extending the schedule and increasing costs further.[114][119][132]

On 27 March 2018, NASA announced that JWST's launch would be pushed back to May 2020 or later, admitting that the project's costs might exceed US$8.8 billion.[116] NASA committed to releasing a revised cost estimate after a new launch window was determined with the European Space Agency (ESA).[133][134][135] The project was paused in 2020 due to the COVID-19 pandemic.[136]

In February 2019, despite expressing criticism over cost growth, Congress increased the mission's cost cap by US$800 million.[137]

Partnership

NASA, ESA and CSA have collaborated on the telescope since 1996. ESA's participation in construction and launch was approved by its members in 2003 and an agreement was signed between ESA and NASA in 2007. In exchange for full partnership, representation and access to the observatory for its astronomers, ESA is providing the NIRSpec instrument, the Optical Bench Assembly of the MIRI instrument, an Ariane 5 ECA launcher, and manpower to support operations.[86][138] The CSA will provide the Fine Guidance Sensor and the Near-Infrared Imager Slitless Spectrograph plus manpower to support operations.[139]

Several thousand scientists, engineers, and technicians spanning 15 countries have contributed to the build, test and integration of the JWST.[140] A total of 258 companies, government agencies, and academic institutions are participating in the pre-launch project; 142 from the United States, 104 from 12 European countries (including 21 from the U.K., 16 from France, 12 from Germany and 7 international[141]), and 12 from Canada.[140] Other countries as NASA partners, such as Australia, have or will be involved in post-launch operation.[142]

Participating countries

Public displays and outreach

Early full-scale model on display at NASA Goddard Space Flight Center (2005)
Early full-scale model on display at NASA Goddard Space Flight Center (2005)

A large telescope model has been on display at various places since 2005: in the United States at Seattle, Washington; Colorado Springs, Colorado; Greenbelt, Maryland; Rochester, New York; New York City; and Orlando, Florida; and elsewhere at Paris, France; Dublin, Ireland; Montreal, Canada; Hatfield, United Kingdom; and Munich, Germany. The model was built by the main contractor, Northrop Grumman Aerospace Systems.[143]

In May 2007, a full-scale model of the telescope was assembled for display at the Smithsonian Institution's National Air and Space Museum on the National Mall, Washington, D.C. The model was intended to give the viewing public a better understanding of the size, scale and complexity of the satellite, as well as pique the interest of viewers in science and astronomy in general. The model is significantly different from the telescope, as the model must withstand gravity and weather, so is constructed mainly of aluminum and steel measuring approximately 24 m × 12 m × 12 m (79 ft × 39 ft × 39 ft) and weighs 5,500 kg (12,100 lb).[144]

The model was on display in New York City's Battery Park during the 2010 World Science Festival, where it served as the backdrop for a panel discussion featuring Nobel Prize laureate John C. Mather, astronaut John M. Grunsfeld and astronomer Heidi Hammel. In March 2013, the model was on display in Austin for SXSW 2013.[145][146] Amber Straughn, the deputy project scientist for science communications, has been a spokesperson for the project at many SXSW events from 2013 onwards in addition to Comic Con, TEDx, and other public venues.[147]

The public was able to follow the progress of the launch and deployment in real-time on NASA's Where is Webb? web site.

Controversy over name

In 2002, NASA administrator (2001–2004) Sean O'Keefe made the decision to name the telescope after James E. Webb, the administrator of NASA from 1961 to 1968 during the Mercury, Gemini, and much of the Apollo programs.[81][82]

In 2015, allegations surfaced around Webb's role in the lavender scare, the mid-20th-century persecution by the U.S. government against accused homosexuals in federal employment.[148][149] The scare led to the dismissal of nearly 300 U.S. State Department employees between 1950 and 1952; Webb served as undersecretary of state from early 1949 to early 1952.[150] Astrophysicist Hakeem Oluseyi argued that accusations against Webb were based on a quote falsely attributed to him on Wikipedia and could find little to no evidence that he took part in anti-gay discrimination.[151][152] In March 2021, four scientists published an opinion piece in Scientific American urging NASA to reconsider the name of the telescope, based on Webb's alleged complicity.[151] The controversy was widely reported by the press.[153][154][155] In September 2021, NASA announced its decision not to rename the telescope.[156] O'Keefe, who made the decision to name the telescope after Webb, stated that to suggest Webb should "be held accountable for that activity when there's no evidence to even hint [that he participated in it] is an injustice."[81][152][157] The American Astronomical Society sent NASA administrator Bill Nelson two letters requesting NASA release a public report detailing their investigation.[158][159] The inquiry is ongoing; documents from a 1969 appeals ruling (regarding the 1963 firing of an employee) suggests that firing gay people was considered customary within the agency.[160][161]

Mission goals

The James Webb Space Telescope has four key goals:

These goals can be accomplished more effectively by observation in near-infrared light rather than light in the visible part of the spectrum. For this reason, JWST's instruments will not measure visible or ultraviolet light like the Hubble Telescope, but will have a much greater capacity to perform infrared astronomy. JWST will be sensitive to a range of wavelengths from 0.6 to 28 μm (corresponding respectively to orange light and deep infrared radiation at about 100 K or −173 °C).

JWST may be used to gather information on the dimming light of star KIC 8462852, which was discovered in 2015, and has some abnormal light-curve properties.[163]

Additionally, it will be able to tell if an exoplanet has methane in its atmosphere, allowing astronomers to determine whether or not the methane is a biosignature.[164]

Orbit design

JWST is not exactly at the L2 point, but circles around it in a halo orbit.
JWST is not exactly at the L2 point, but circles around it in a halo orbit.
Two alternate Hubble Space Telescope views of the Carina Nebula, comparing ultraviolet and visible (top) and infrared (bottom) astronomy. Far more stars are visible in the latter.
Two alternate Hubble Space Telescope views of the Carina Nebula, comparing ultraviolet and visible (top) and infrared (bottom) astronomy. Far more stars are visible in the latter.

JWST orbits the Sun near the second Lagrange point (L2) of the Sun-Earth system, which is 1,500,000 km (930,000 mi) farther from the Sun than the Earth's orbit, and about four times farther than the Moon's orbit. Normally an object circling the Sun farther out than Earth would take longer than one year to complete its orbit. But near the L2 point, the combined gravitational pull of the Earth and the Sun allow a spacecraft to orbit the Sun in the same time that it takes the Earth. Staying close to Earth allows data rates to be much faster for a given size of antenna.

The telescope circles about the Sun-Earth L2 point in a halo orbit, which is inclined with respect to the ecliptic, has a radius varying between about 250,000 km (160,000 mi) and 832,000 km (517,000 mi), and takes about half a year to complete.[17] Since L2 is just an equilibrium point with no gravitational pull, a halo orbit is not an orbit in the usual sense: the spacecraft is actually in orbit around the Sun, and the halo orbit can be thought of as controlled drifting to remain in the vicinity of the L2 point.[165] This requires some station-keeping: around 2.5 m/s per year[166] from the total v budget of 93 m/s.[167] Two sets of thrusters constitute the observatory's propulsion system.[168] Because the thrusters are located solely on the Sun-facing side of the observatory, all station-keeping operations are designed to slightly undershoot the required amount of thrust in order to avoid pushing the JWST beyond the semi-stable L2 point, a situation which would be unrecoverable. Randy Kimble, the Integration and Test Project Scientist for the James Webb Space Telescope, compared the precise station-keeping of the JWST to "Sisyphus [...] rolling this rock up the gentle slope near the top of the hill – we never want it to roll over the crest and get away from him".[169]

Animation of James Webb Space Telescope trajectory
Top view
Side view
Side view from the Sun

Infrared astronomy

Infrared observations can see objects hidden in visible light, such as the HUDF-JD2 shown here.
Infrared observations can see objects hidden in visible light, such as the HUDF-JD2 shown here.
Atmospheric windows in the infrared: Much of this type of light is blocked when viewed from the Earth's surface. It would be like looking at a rainbow but only seeing one color.
Atmospheric windows in the infrared: Much of this type of light is blocked when viewed from the Earth's surface. It would be like looking at a rainbow but only seeing one color.

JWST is the formal successor to the Hubble Space Telescope (HST), and since its primary emphasis is on infrared astronomy, it is also a successor to the Spitzer Space Telescope. JWST will far surpass both those telescopes, being able to see many more and much older stars and galaxies.[170] Observing in the infrared spectrum is a key technique for achieving this, because of cosmological redshift, and because it better penetrates obscuring dust and gas. This allows observation of dimmer, cooler objects. Since water vapor and carbon dioxide in the Earth's atmosphere strongly absorbs most infrared, ground-based infrared astronomy is limited to narrow wavelength ranges where the atmosphere absorbs less strongly. Additionally, the atmosphere itself radiates in the infrared spectrum, often overwhelming light from the object being observed. This makes a space telescope preferable for infrared observation.[171]

The more distant an object is, the younger it appears; its light has taken longer to reach human observers. Because the universe is expanding, as the light travels it becomes red-shifted, and objects at extreme distances are therefore easier to see if viewed in the infrared.[172] JWST's infrared capabilities are expected to let it see back in time to the first galaxies forming just a few hundred million years after the Big Bang.[173]

Infrared radiation can pass more freely through regions of cosmic dust that scatter visible light. Observations in infrared allow the study of objects and regions of space which would be obscured by gas and dust in the visible spectrum,[172] such as the molecular clouds where stars are born, the circumstellar disks that give rise to planets, and the cores of active galaxies.[172]

Relatively cool objects (temperatures less than several thousand degrees) emit their radiation primarily in the infrared, as described by Planck's law. As a result, most objects that are cooler than stars are better studied in the infrared.[172] This includes the clouds of the interstellar medium, brown dwarfs, planets both in our own and other solar systems, comets, and Kuiper belt objects that will be observed with the Mid-Infrared Instrument (MIRI).[41][173]

Some of the missions in infrared astronomy that impacted JWST development were Spitzer and the Wilkinson Microwave Anisotropy Probe (WMAP).[174] Spitzer showed the importance of mid-infrared, which is helpful for tasks such as observing dust disks around stars.[174] Also, the WMAP probe showed the universe was "lit up" at redshift 17, further underscoring the importance of the mid-infrared.[174] Both these missions were launched in the early 2000s, in time to influence JWST development.[174]

Ground support and operations

The Space Telescope Science Institute (STScI), in Baltimore, Maryland, on the Homewood Campus of Johns Hopkins University, was selected as the Science and Operations Center (S&OC) for JWST with an initial budget of US$162.2 million intended to support operations through the first year after launch.[175] In this capacity, STScI will be responsible for the scientific operation of the telescope and delivery of data products to the astronomical community. Data will be transmitted from JWST to the ground via the NASA Deep Space Network, processed and calibrated at STScI, and then distributed online to astronomers worldwide. Similar to how Hubble is operated, anyone, anywhere in the world, will be allowed to submit proposals for observations. Each year several committees of astronomers will peer review the submitted proposals to select the projects to observe in the coming year. The authors of the chosen proposals will typically have one year of private access to the new observations, after which the data will become publicly available for download by anyone from the online archive at STScI.

The bandwidth and digital throughput of the satellite is designed to operate at 458 gigabits of data per day for the length of the mission (equivalent to a sustained rate of 5.42 megabits per second (Mbps)).[26] Most of the data processing on the telescope is done by conventional single-board computers.[176] The conversion of the analog science data to digital form is performed by the custom-built SIDECAR ASIC (System for Image Digitization, Enhancement, Control And Retrieval Application Specific Integrated Circuit). NASA stated that the SIDECAR ASIC will include all the functions of a 9.1 kg (20 lb) instrument box in a 3 cm (1.2 in) package and consume only 11 milliwatts of power.[177] Since this conversion must be done close to the detectors, on the cool side of the telescope, the low power use of this IC will be crucial for maintaining the low temperature required for optimal operation of JWST.[177]

Micrometeoroid strike

The C3[b] mirror segment suffered a micrometeoroid strike from a dust mote-sized particle between 23 and 25 May, the fifth such since launch, reported 8 June 2022, which required engineers to compensate for the strike using a mirror actuator.[179]

Mission progress

Launch and mission length

Scientists and engineers who worked on the project described their feelings of anticipation and anxiety about the launch of the exhaustively tested[180][181] nearly $10 billion instrument, commenting that it would be "an exciting moment" and they would feel "terrified the entire time".[182][183] The launch (designated Ariane flight VA256) took place as scheduled at 12:20 UTC on 25 December 2021 on an Ariane 5 rocket that lifted off from the Guiana Space Centre in French Guiana.[184][185] Upon successful launch, NASA administrator Bill Nelson called it "a great day for planet Earth".[186] The telescope was confirmed to be receiving power, starting a two-week deployment phase of its parts[187] and traveling to its target destination.[188][189][190] The observatory was attached to the Ariane 5 via a launch vehicle adapter ring which could be used by a future spacecraft to grapple the observatory to attempt to fix gross deployment problems. However, the telescope itself is not serviceable, and astronauts would not be able to perform tasks such as swapping instruments, as with the Hubble Telescope.[33] The telescope was released from the upper stage 27 minutes 7 seconds after launch, beginning a 30-day adjustment to place the telescope in a Lissajous orbit[191] around the L2 Lagrange point.

The telescope was launched with slightly less speed than needed to reach its final orbit, and slowed down as it travelled away from Earth, in order to reach L2 with only the velocity needed to enter its orbit there. The telescope reached L2 on 24 January 2022. The flight included three planned course corrections to adjust its speed and direction. This is because the observatory could recover from underthrust (going too slowly), but could not recover from overthrust (going too fast) - to protect highly temperature-sensitive instruments, the sunshield must remain between telescope and Sun, so the spacecraft could not turn around or use its thrusters to slow down.[192]

The telescope's nominal mission time is five years, with a goal of ten years.[193] The planned five-year science mission begins after a six-month commissioning phase.[194] An L2 orbit is unstable, so JWST needs to use propellant to maintain its halo orbit around L2 (known as station-keeping) to prevent the telescope from drifting away from its orbital position.[195] It was designed to carry enough propellant for 10 years,[194] but the precision of the Ariane 5 launch and the first midcourse correction were credited with saving enough onboard fuel that JWST may be able to maintain its orbit for around 20 years instead.[196][197][198]

Transit and structural deployment

Planned structural deployment timeline[33]
Planned structural deployment timeline[33]

JWST was released from the rocket upper stage 27 minutes after a flawless launch.[184][200] Starting 31 minutes after launch, and continuing for about 13 days, JWST began the process of deploying its solar array, antenna, sunshield, and mirrors.[201] Nearly all deployment actions are commanded by the Space Telescope Science Institute in Baltimore, except for two early automatic steps, solar panel unfolding and communication antenna deployment.[202][203] The mission was designed to give ground controllers flexibility to change or modify the deployment sequence in case of problems.[204]

Planned structural deployment sequence

The electricity-generating solar panel deployed on the day of launch, one and a half minutes after the telescope separated from the Ariane rocket second stage;[196][204] this took place slightly sooner than expected because launch rotation was much closer to ideal than deployment plans had envisaged.[205] The separation and solar panel extension were both visible in a live feed from a camera on the rocket.[206]

After deployment of the solar arrays, power output was reduced due to a factory pre-set duty cycle in the array regulator module which was set prior to launch. Power usage was greater than that supplied by the solar arrays and this resulted in increased drawdown of the telescope's batteries and higher than expected voltage. To ensure power delivery would be sufficient for spacecraft and science operations, the solar panels were reset and duty cycles were optimized to account for the real world conditions observed including array temperatures.[207] Higher than desired temperatures were observed in some of the shade deployment motors. While the motors remained well within their operational tolerances, to ensure greater margins the spacecraft's attitude was adjusted to aid the motors in reaching their desired temperatures and the motors were rebalanced. This was done based on results from simulator testing.[207] The majority of forecast models of vehicle behavior and conditions matched the operational evolution[jargon] in space.[207]

At 7:50 p.m. EST on 25 December 2021, about 12 hours after launch, the telescope's pair of primary rockets began firing for 65 minutes to make the first of three planned mid-course corrections.[208] On day two, the high gain communication antenna deployed automatically.[204]

On 27 December 2021, at 60 hours after launch, Webb's rockets fired for nine minutes and 27 seconds to make the second of three mid-course corrections for the telescope to arrive at its L2 destination.[209] On 28 December 2021, three days after launch, mission controllers began the multi-day deployment of Webb's all-important sunshield. Controllers sent commands that successfully lowered the forward and aft pallet structures, which contain the sunshield. This deployment precedes the actual unfolding and extension of the delicate shield membranes, which are pulled out of the pallets by telescoping beams in a subsequent step.[210][211]

On 29 December 2021, controllers successfully extended the Deployable Tower Assembly, a pipe-like column, which moved apart the two main segments of the observatory, the telescope with its mirrors and scientific instruments, and the "bus" holding electronics and propulsion. The assembly lengthened 48 in (1,200 mm) in a process that lasted six and a half hours, including many preparatory commands. Deployment created the needed distance between the JWST segments to allow extreme cooling of the telescope and room for the sunshield to unfold.[212][213] On 30 December 2021, controllers successfully completed two more steps in unpacking the observatory. First, commands deployed the aft "momentum flap", a device that provides balance against solar pressure on the sunshield, saving fuel by reducing the need for thruster firing to maintain Webb's orientation.[214] Next, mission control released and rolled up covers that protect the sunshield, exposing it to space for the first time.[215][24]

On 31 December 2021, the ground team extended the two telescoping "mid booms" from the left and right sides of the observatory, pulling the five sunshield membranes out of their folded stowage in the fore and aft pallets, which were lowered three days earlier.[216] Deployment of the left side boom (in relation to pointing direction of the main mirror) was delayed when mission control did not initially receive confirmation that the sunshield cover had fully rolled up. After looking at extra data for confirmation, the team proceeded to extend the booms.[217] The left side deployed in 3 hours and 19 minutes; the right side took 3 hours and 42 minutes.[217][216] With that step, Webb's sunshield resembled its complete, kite-shaped form and extended to its full 14-meter (47 ft) width. Commands to separate and tension the membranes were to follow[216] and were expected to take several days.[207]

After resting on New Year's Day, the ground team delayed sunshield tensioning one day to allow time to optimize the observatory's array of solar panels and to adjust the orientation of the observatory slightly to cool the slightly hotter-than-expected sunshield deployment motors.[218] Tensioning of layer one, closest to the Sun and largest of the five in the sunshield, began on 3 January 2022, and was completed at 3:48 p.m. EST.[219] Tensioning of the second and third layers began at 4:09 p.m. EST and took two hours and 25 minutes.[220] On 4 January, controllers successfully tensioned the last two layers, four and five, completing the task at 11:59 a.m. EST.[221]

On 5 January 2022, mission control successfully deployed the telescope's secondary mirror, which locked itself into place to a tolerance of about one and a half millimeters.[222]

The last step of structural deployment was to unfold the wings of the primary mirror. Each panel consists of three primary mirror segments and had to be folded to allow the space telescope to be installed in the fairing of the Ariane rocket for the launch of the telescope. On 7 January 2022, NASA deployed and locked in place the port-side wing,[223] and on 8 January, the starboard-side mirror wing. This successfully completed the structural deployment of the observatory.[224][225][226]

On 24 January 2022, at 2pm EST,[227] nearly a month after launch, a third and final course correction took place, inserting JWST into its planned halo orbit around the Sun-Earth L2 point.[228][229]

Animation of JWST's halo orbit

Commissioning and testing

On 12 January 2022, while still in transit, mirror alignment began. The primary mirror segments and secondary mirror were moved away from their protective launch positions. This took about 10 days, because the 132[27] actuator motors are designed to fine-tune the mirror positions at microscopic accuracy (10 nanometer increments) and must each move over 1.2 million increments (12.5 mm) during initial alignment.[230][29] Additionally, to reduce risk and complexity, and to minimize heat production near the cooling mirrors, only one actuator was moved at a time and the actuators only operated for short periods at a time, limiting total speed to about 1mm per day.[230][29] The 18 radius of curvature (ROC) actuators, which adjust curvature of the primary mirror segments, were also moved from launch position at the same time.[27]

After being freed from launch protection, the 18 mirror segments are being fine tuned and aligned to work as a single mirror, a process expected to take around three of the five months allowed for commissioning and testing.[29][27] Commissioning is complicated by the fact that the telescope's performance and precise shapes of some components will also change microscopically as it continues to cool.[231] Furthermore, its remaining scientific instruments have all been powered up for testing (31 January 2022),[232] and heaters used to protect against water and ice condensation will no longer be needed and will gradually be switched off.[233]

Mirror alignment requires each of the 18 mirror segments, and the secondary mirror, to be positioned to within 50 nanometers. NASA compares the required accuracy by analogy: "If the Webb primary mirror were the size of the United States, each [mirror] segment would be the size of Texas, and the team would need to line the height of those Texas-sized segments up with each other to an accuracy of about 1.5 inches".[234]

Mirror alignment is a complex operation split into seven phases, that has been repeatedly rehearsed using a 1/6 scale model of the telescope.[234] Once the mirrors reach 120 K (−153 °C; −244 °F),[233] NIRCam targets a bright star, the 6th magnitude star HD 84406 in Ursa Major.[c][236][237] (HD 84406 is bright and easily identified, will stay in view for the entire 3 months of commissioning, and is in part of the sky with fewer other stars.) [238] To do this, NIRCam takes 1560 images of the sky (156 images with each of its 10 sensors) and uses these wide-ranging images to determine where in the sky each segment of the main mirror is initially pointing. [238] Initially, the individual primary mirror segments will be greatly misaligned, so the image will contain 18 separate, blurry, images of the star field, each containing an image of the target star. The 18 images of HD 84406 are matched to their respective mirror segments, and the 18 segments are brought into approximate alignment centered on the star ("Segment Image Identification").[234] Each segment is then individually corrected of its major focusing errors, using a technique called phase retrieval, resulting in 18 separate, but individually good quality, images from the 18 mirror segments ("Segment Alignment").[234] The 18 images from each segment, are then moved so they precisely overlap to create a single image ("Image Stacking").[234]

With the mirrors now positioned for almost correct images, they must be fine tuned to their operational accuracy of 50 nanometers, less than one wavelength of the light that will be detected. A technique called dispersed fringe sensing compares images from 20 pairings of mirrors, allowing most of the errors to be corrected ("Coarse Phasing"), and then the same technique is used with special optical elements to introduce ±4 and ±8 waves of defocus to each segment's image, allowing detection and correction of almost all remaining errors ("Fine Phasing").[234] These two processes are repeated three times, and Fine Phasing will be routinely checked throughout the telescope's operation.[234]

After three rounds of Coarse and Fine Phasing, the telescope will be well aligned at one place in the NIRCam field of view.[234] Measurements will be made at various points in the captured image, across all instruments, and corrections calculated from the detected variations in intensity, giving a well-aligned outcome across all instruments ("Telescope Alignment Over Instrument Fields of View").[234]

Finally, a last round of Fine Phasing and checks of image quality on all instruments is performed, to ensure that any small residual errors remaining from the previous steps, are corrected ("Iterate Alignment for Final Correction").[234] The telescope's mirror segments are then aligned and able to capture precise focused images.[234] If needed, the process allows for earlier steps to be retested again, to ensure accuracy.[234]

In preparation for alignment, NASA announced at 19.28 UTC on 3 February 2022, that NIRCam had detected the telescope's first photons (although not yet complete images).[234][239] On 11 February 2022, NASA announced the telescope had almost completed phase 1 of alignment, with every segment of its primary mirror having located and imaged the target star HD 84406, and all segments brought into approximate alignment.[238] Phase 1 alignment was completed on 18 February 2022,[240] and a week later, phases 2 and 3 were also completed on 25 February 2022.[241] This means the 18 segments are working in unison, however until all 7 phases are complete, the segments still act as 18 smaller telescopes rather than one larger one.[241] At the same time as the primary mirror is being commissioned, hundreds of other instrument commissioning and calibration tasks are also ongoing.[242]

Allocation of observation time

JWST observing time is allocated through a General Observers (GO) program, a Guaranteed Time Observations (GTO) program, and a Director's Discretionary Early Release Science (DD-ERS) program.[248] The GTO program provides guaranteed observing time for scientists who developed hardware and software components for the observatory. The GO program provides all astronomers the opportunity to apply for observing time and will represent the bulk of the observing time. GO programs are selected through peer review by a Time Allocation Committee (TAC), similar to the proposal review process used for the Hubble Space Telescope.

Early Release Science program

In November 2017, the Space Telescope Science Institute announced the selection of 13 Director's Discretionary Early Release Science (DD-ERS) programs, chosen through a competitive proposal process.[249][250] The observations for these programs will be obtained during the first five months of JWST science operations after the end of the commissioning period. A total of 460 hours of observing time was awarded to these 13 programs, which span science topics including the Solar System, exoplanets, stars and star formation, nearby and distant galaxies, gravitational lenses, and quasars. These 13 ERS programs will use a total of 242.8 hours of observing time on the telescope (not including JWST observing overheads and slew time).

Early Release Science programs
Name PI Category Science Observation Time (hours)
Radiative Feedback from Massive Stars as Traced by Multiband Imaging and Spectroscopic Mosaics Olivier Berne Stellar Physics 8.3[251]
IceAge: Chemical Evolution of Ices during Star Formation Melissa McClure Stellar Physics 13.4[252]
Through the Looking GLASS: A JWST Exploration of Galaxy Formation and Evolution from Cosmic Dawn to Present Day Tommaso Treu Galaxies and the IGM 24.3[253]
A JWST Study of the Starburst-AGN Connection in Merging LIRGs Lee Armus Galaxies and the IGM 8.7[254]
The Resolved Stellar Populations Early Release Science Program Daniel Weisz Stellar Populations 20.3[255]
Q-3D: Imaging Spectroscopy of Quasar Hosts with JWST Analyzed with a Powerful New PSF Decomposition and Spectral Analysis Package Dominika Wylezalek Massive Black Holes and their Galaxies 17.4[256]
The Cosmic Evolution Early Release Science (CEERS) Survey Steven Finkelstein Galaxies and the IGM 36.6[257]
Establishing Extreme Dynamic Range with JWST: Decoding Smoke Signals in the Glare of a Wolf-Rayet Binary Ryan Lau Stellar Physics 6.5[258]
TEMPLATES: Targeting Extremely Magnified Panchromatic Lensed Arcs and Their Extended Star Formation Jane Rigby Galaxies and the IGM 26.0[259]
Nuclear Dynamics of a Nearby Seyfert with NIRSpec Integral Field Spectroscopy Misty Bentz Massive Black Holes and their Galaxies 1.5[260]
The Transiting Exoplanet Community Early Release Science Program Natalie Batalha Planets and Planet Formation 52.1[261]
ERS observations of the Jovian System as a Demonstration of JWST's Capabilities for Solar System Science Imke de Pater Solar System 9.3[262]
High Contrast Imaging of Exoplanets and Exoplanetary Systems with JWST Sasha Hinkley Planets and Planet Formation 18.4[263]

General Observer Program

For GO Cycle 1, 6,000 hours of observation time were available to allocate, and 1,173 proposals were submitted requesting a total of 24,500 hours of observation time.[264] Selection of Cycle 1 GO programs was announced on 30 March 2021, with 266 programs approved. These include 13 large programs and treasury programs producing data for public access.[265]

See also

Notes

  1. ^ "Baffled", in this context, means enclosed in a tube in a similar manner to a conventional optical telescope, which helps to stop stray light entering the telescope from the side. For an actual example, see the following link: Freniere, E.R. (1981). "First-order design of optical baffles". Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, First-order design of optical baffles. Radiation Scattering in Optical Systems. Vol. 257. pp. 19–28. Bibcode:1981SPIE..257...19F. doi:10.1117/12.959598.
  2. ^ The C3 mirror segment is positioned in the outer ring of segments, located at the '5 o'clock' number of a clock face, when viewing the primary mirror face-on.[178]
  3. ^ HD 84406 is a star approximately 258.5 light-years away in the constellation of Ursa Major. The star is a spectral type G star and has a high proper motion.[235]
  4. ^ 2MASS J17554042+6551277, also known as UNSW-V 084 and TYC 4212-1079-1,[243] is a star in the constellation Draco, in the Milky Way. It is located almost 2,000 light years away from Earth, within a degree of the north ecliptic pole. Its visual apparent magnitude mv is 10.95, which makes it much too faint to be observed with the naked eye. It is cooler than the Sun, but some 13 to 16 times brighter in visible light,[244] and is consequently not a sun-like star. Its motion vector in the direction of the Sun is 51 km/s.[243]
  5. ^ For this test, Webb pointed at part of the Large Magellanic Cloud, a small satellite galaxy of the Milky Way, providing a dense field of hundreds of thousands of stars across all the observatory's sensors. Webb's three imaging instruments are NIRCam (images shown here at a wavelength of 2 microns), NIRISS (image shown here at 1.5 microns), and MIRI (shown at 7.7 microns, a longer wavelength revealing emission from interstellar clouds as well as starlight). NIRSpec is a spectrograph rather than imager but can take images, such as the 1.1 micron image shown here, for calibrations and target acquisition. The dark regions visible in parts of the NIRSpec data are due to structures of its microshutter array, which has several hundred thousand controllable shutters that can be opened or shut to select which light is sent into the spectrograph. Lastly, Webb’s Fine Guidance Sensor tracks guide stars to point the observatory accurately and precisely; its two sensors are not generally used for scientific imaging but can take calibration images such as those shown here. This image data is used not just to assess image sharpness but also to precisely measure and calibrate subtle image distortions and alignments between sensors as part of Webb’s overall instrument calibration process.

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Further reading

External links

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