James Webb Space Telescope

James Webb Space Telescope

James Webb Space Telescope model

Artist's Concept of the James Webb Space Telescope
Names Next Generation Space Telescope
James Webb Space Telescope
Mission type Astronomy
Operator NASA / ESA / CSA / STScI[1]
Website jwst.nasa.gov
sci.esa.int/jwst
asc-csa.gc.ca
jwst.stsci.edu
Mission duration 5 years (design)
10 years (goal)
Spacecraft properties
Manufacturer Northrop Grumman
Ball Aerospace
Launch mass 6,500 kg (14,300 lb)[2]
Dimensions 20.197 m × 14.162 m (66.26 ft × 46.46 ft) (sunshield)
Power 2,000 watts
Start of mission
Launch date October 2018[3]
Rocket Ariane 5 ECA
Launch site Kourou ELA-3
Contractor Arianespace
Orbital parameters
Reference system Sun–Earth L2
Regime Halo orbit
Periapsis 374,000 km (232,000 mi)[4]
Apoapsis 1,500,000 km (930,000 mi)[4]
Period 6 months
Epoch planned
Main
Type Korsch telescope
Diameter 6.5 m (21 ft)
Focal length 131.4 m (431 ft)
Collecting area 25 m2 (270 sq ft)
Wavelengths from 0.6 µm (orange)
to 28.5 µm (mid-infrared)
Transponders
Band S-band (TT&C support)
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
JWST logo

James Webb Space Telescope insignia

The James Webb Space Telescope (JWST), previously known as Next Generation Space Telescope (NGST), is a part of NASA's ongoing Flagship Program which has developed in cooperation with the European Space Agency and Canadian Space Agency. The space telescope is under construction and scheduled to launch in October 2018.

JWST will offer unprecedented resolution and sensitivity from long-wavelength (orange-red) visible light, through near-infrared to the mid-infrared (0.6 to 27 micrometers). While the Hubble Space Telescope has a 2.4-meter (7.9 ft) mirror, the JWST features a larger and segmented 6.5-meter-diameter (21 ft 4 in) primary mirror and will be located near the Earth–Sun L2 point. A large sunshield will keep its mirror and four science instruments below 50 K (−220 °C; −370 °F).

JWST's capabilities will enable a broad range of investigations across the fields of astronomy and cosmology.[5] One particular goal involves observing some of the most distant events and objects in the Universe, such as the formation of the first galaxies. These types of targets are beyond the reach of current ground and space-based instruments. Another goal is understanding the formation of stars and planets. This will include direct imaging of exoplanets, and novas.

In gestation since 1996,[6] the project represents an international collaboration of the European Space Agency, Canadian Space Agency and team members of other countries led by NASA.[7] It is named after James E. Webb, the second administrator of NASA, who played an integral role in the Apollo program.[8]

NASA has described JWST as the scientific successor of the Hubble Space Telescope, but not a replacement, because the capabilities are not identical.[9] JWST will have the ability to see high-redshift objects, typically both older and farther away than previous instruments could assess.[10] The result was to extend the life of Hubble until JWST as the next generation telescope could go online.[10] This led to a radically altered design for JWST to obtain images deeper into the infrared than Hubble, and beyond the capabilities of the Infrared Space Observatory and Spitzer Space Telescope.

In December 2016, NASA announced that the JWST has passed major milestones, including completion of its primary mirror and integration of science instruments with the payload module, and is undergoing acoustic and extreme vibration testing to simulate launch conditions.[11][12]

Overview

JWST in Ariane 5 launch configuration

The JWST originated in 1996 as the Next Generation Space Telescope (NGST). In 2002 it was renamed after NASA's second administrator (1961–1968) James E. Webb (1906–1992), noted for playing a key role in the Apollo program and establishing scientific research as a core NASA activity.[13] The JWST is a project of the National Aeronautics and Space Administration, the United States space agency, with international collaboration from the European Space Agency and the Canadian Space Agency.

The telescope 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 vs. 4.5 m2). 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.

The JWST will operate near the Earth-Sun L2 (Lagrange) point, approximately 930,000 mi (1,500,000 km) beyond the Earth. By way of comparison, Hubble orbits 340 miles (550 km) above Earth's surface, and the Moon is roughly 250,000 miles (400,000 km) 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[14] 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.[15][16] The prime contractor is Northrop Grumman.[17]

Outline

Main mirror assembled at Goddard Spaceflight Center, May 2016
Components of the JWST being tested, 2012

Major JWST components:

Three major sections of the JWST include:[22][23]

Sunshield protection

Test unit of the sunshield stacked and expanded at the Northrop Grumman facility in California, 2014

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.[24] Its halo orbit around L2 avoids the shadow of the Earth and Moon, maintaining a constant environment for the sunshield and solar arrays.[14] The sunshield is made of polyimide film, has membranes coated with aluminum on one side and silicon on the other.

The sunshield is designed to be folded twelve times so it will fit within the Ariane 5 rocket's 4.57 m × 16.19 m shroud. Once deployed at the L2 point, it will unfold to 21.197 m × 14.162 m. The sunshield was hand-assembled at Man Tech (NeXolve) in Huntsville, Alabama before it was delivered to Northrop Grumman in Redondo Beach, California for testing.[25]

Optics

The first six primary mirror segments being prepared for final cryogenic acceptance testing, 2011

JWST's primary mirror is a 6.5-meter-diameter gold-coated beryllium reflector with a collecting area of 25 m2. This is too large for existing launch vehicles, so the mirror is 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.[26] This is unlike terrestrial telescopes like the Keck which continually adjust their mirror segments using active optics to overcome the effects of gravitational and wind loading, and is made possible because of the lack of environmental disturbances of a telescope in space.

JWST's optical design is a three-mirror anastigmat,[27] which makes use of curved secondary and tertiary mirrors to deliver images that are free of optical aberrations over a wide field. In addition, there is a fast steering mirror, which can adjust its position many times per second to provide image stabilization.

Ball Aerospace & Technologies Corp. is the principal optical subcontractor for the JWST program, led by prime contractor Northrop Grumman Aerospace Systems, under a contract from the NASA Goddard Space Flight Center, in Greenbelt, Maryland.[2][28] Eighteen primary mirror segments, secondary, tertiary and fine steering mirrors, plus flight spares have been fabricated and polished by Ball Aerospace based on beryllium segment blanks manufactured by several companies including Axsys, Brush Wellman, and Tinsley Laboratories.

The final segment of the primary mirror was installed on February 3, 2016,[29] and the secondary mirror was installed on March 3, 2016.[30]

Scientific instruments

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.[31]

NIRCam model
NIRSpec model
MIRI model

NIRCam and MIRI feature starlight-blocking coronagraphs for observation of faint targets such as extrasolar planets and circumstellar disks very close to bright stars.[37]

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.[39]

Spacecraft Bus

Diagram of the Spacecraft Bus. The solar panel is in green and the light purple flats are radiators shades

The Spacecraft Bus is the primary support component of the James Webb Space Telescope, that hosts a multitude of computing, communication, propulsion, and structural parts, bringing the different parts of the telescope together.[20] Along with the Sunshield, it forms the Spacecraft Element of the space telescope.[23] The other two major elements of the JWST are the Integrated Science Instrument Module (ISIM) and the Optical Telescope Element (OTE).[22] Region 3 of ISIM is also inside the Spacecraft Bus; region 3 includes ISIM Command and Data Handling subsystem and the MIRI cryocooler.[22]

The Spacecraft Bus is connected to Optical Telescope Element via the Deployable Tower Assembly, which also connects to the sunshield.[20]

The structure of the Spacecraft Bus must support the 6.5 ton space telescope, while it itself weighs 350 kg (about 772 lb).[5] It is made primarily of graphite composite material.[5] It was assembled in California by 2015, and after that it had to be integrated with the rest of the space telescope leading up to its planned 2018 launch.[40] The bus can provide pointing of one-arcsecond and isolates vibration down to two (2) milliarcseconds.[41]

The Spacecraft Bus is on the sun-facing "warm" side and operates at a temperature of about 300 K.[23] Everything on the Sun facing side must be able to handle the thermal conditions of JWST's halo orbit, which has one side in continuous sunlight and the other in shade by the spacecraft sunshield.[23]

Another important aspect of the Spacecraft Bus is the central computing, memory storage, and communications equipment.[20] 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.[20] The computer also controls the pointing and moment of the spacecraft, taking in sensor data from the gyroscopes and star tracker, and sending the necessary commands to the reaction wheels or thrusters depending.[20]

Launch and mission length

Launch is scheduled for October 2018 on an Ariane 5 rocket. The observatory attaches to the Ariane 5 rocket 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.[2] Its nominal mission time is five years, with a goal of ten years.[21] JWST needs to use propellant to maintain its halo orbit around L2, which provides an upper limit to its designed lifetime, and it is being designed to carry enough for ten years.[42] The planned five year science mission begins after a 6-month commissioning phase.[42] An L2 orbit is meta-stable so it requires orbital station-keeping or an object will drift away from this orbital configuration.[43]

Comparisons

Comparison with Hubble primary mirror
Calisto architecture for SAFIR would be a successor to Spitzer, requiring even cooler passive cooling than JWST (5 Kelvin).[44]

The desire for a large infrared space telescope traces back decades; in the United States the Shuttle Infrared Telescope Facility was planned while the Space Shuttle was in development and the potential for infrared astronomy was acknowledged at that time.[45] Compared to ground telescopes, space observatories were free from atmospheric absorption of infrared light; this would be a whole "new sky" for astronomers.[45]

The tenuous atmosphere above the 400 km nominal flight altitude has no measurable absorption so that detectors operating at all wavelengths from 5µm to 1000µm can achieve high radiometric sensitivity.
— S. G. McCarthy & G. W. Autio, 1978[45]

However, infrared telescopes have an Achilles heel—they need to stay extremely cold, and the longer the wavelength of infrared, the colder they need to be.[46] If not, the background heat of the device itself overwhelms the detectors, making it effectively blind.[46] This can be overcome by careful spacecraft design, in particular by placing the telescope in dewar with an extremely cold substance, such as liquid helium.[46] This has meant most infrared telescopes have a lifespan limited by their coolant, as short as a few months, maybe a few years at most.[46] It has been 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 or NEOWISE. Another example is Hubble's NICMOS instrument, which started out using a block of nitrogen ice that depleted after a couple 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 sun-shield and radiators with the mid-infrared instrument using an additional cryocooler.[47]

The telescope's delays and cost increases can be compared to the Hubble Space Telescope.[48] When it formally started in 1972, what came to be known as Hubble then had an estimated development cost of $300 million (or about $1 billion in 2006 constant dollars),[48] but by the time it was sent into orbit in 1990, the cost was about four times that.[48] In addition new instruments and servicing missions increased the cost to at least $9 billion by 2006.[48]

In contrast to other proposed observatories, most of which have already been canceled or put on hold, including Terrestrial Planet Finder (2011), Space Interferometry Mission (2010), Laser Interferometer Space Antenna (2011), and the International X-ray Observatory (2011), MAXIM (Microarcsecond X-ray Imaging Mission), SAFIR (Single Aperture Far-Infrared Observatory), SUVO (Space Ultraviolet-Visible Observatory), SPECS (Submillimeter Probe of the Evolution of Cosmic Structure), the JWST is the last big NASA astrophysics mission of its generation to be built.

Selected space telescopes and instruments[49]
Name Year Wavelength Aperture Cooling
Human eye — 0.39–0.75 μm 0.007 m N/A
IRT 1985 1.7-118 μm 0.15 m Helium
ISO[50] 1995 2.5-240 μm 0.60 m Helium
Hubble STIS 1997 0.115–1.03 μm 2.4 m Passive
Hubble NICMOS 1997 0.8-2.4 μm 2.4 m Nitrogen, later cryo-cooler
Spitzer 2003 3–180 μm 0.85 m Helium
Hubble WFC3 2009 0.2–1.7 μm 2.4 m Passive + Thermo-electric [51]
Herschel 2009 55–672 μm 3.5 m Helium
JWST Planned 0.6–28.5 μm 6.5 m Passive + Cryo-cooler (MIRI)

History

Development and construction

Selected Events
Year Events
1996 NGST started
2002 named JWST, 8 to 6 m
2004 NEXUS cancelled[52]
2007 esa/nasa MOU
2010 MCDR passed
2011 Proposed cancel
2018 Planned launch
Primary mirror segments made of beryllium
Mirror segments undergoing cryogenic tests at the X-ray & Cryogenic Facility at Marshall Space Flight Center
Mirror segment after being coated with gold

Early development work for a Hubble successor between 1989 and 1994 led to the Hi-Z[53] telescope concept, a fully baffled[Note 1] 4-meter aperture infrared telescope that would recede to an orbit at 3 AU.[54] This distant orbit would have benefited from reduced light noise from zodiacal dust.[54] Other early plans called for a NEXUS precursor telescope mission.[55][56]

In the "faster, better, cheaper" era in the mid-1990s, NASA leaders pushed for a low-cost space telescope.[57] The result was the NGST concept, with an 8-meter aperture and located at L2, estimated to cost $500 million.[57] In 1997, NASA worked with the Goddard Space Flight Center,[58] Ball Aerospace,[59] and TRW[60] to conduct technical requirement and cost studies, and in 1999 selected Lockheed Martin[61] and TRW for preliminary design concepts.[62]

A JWST mirror segment, 2010

In 2002, NASA awarded the $824.8 million prime contract for the NGST, now renamed the James Webb Space Telescope, to TRW. The design called for a descoped 6.1-meter (20 ft) primary mirror and a launch date of 2010.[63] Later that year, TRW was acquired by Northrop Grumman in a hostile bid and became Northrop Grumman Space Technology.[62]

NASA's Goddard Space Flight Center in Greenbelt, Maryland, is leading the management of the observatory project. The project scientist for the James Webb Space Telescope is John C. Mather. Northrop Grumman Aerospace Systems serves as the primary contractor for the development and integration of the observatory. They are responsible for developing and building the spacecraft element, which includes both the spacecraft bus and sunshield. Ball Aerospace has been subcontracted to develop and build the Optical Telescope Element (OTE). Northrop Grumman's Astro Aerospace business unit has been contracted to build the Deployable Tower Assembly (DTA) which connects the OTE to the spacecraft bus[64] and the Mid Boom Assembly (MBA) which helps to deploy the large sunshields on orbit.[65] Goddard Space Flight Center is also responsible for providing the Integrated Science Instrument Module (ISIM).[31] A solar panel converts sunlight into electrical power that recharges batteries needed to operate the other subsystems, as well as the science instruments, but heat from these operations must be dissipated for optimal instrument performance at 50 K (−220 °C; −370 °F).[2][66]

Cost growth revealed in spring 2005 led to an August 2005 re-planning.[67] 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 micrometers. Other major features of the observatory were unchanged. Following the re-planning, the program was independently reviewed in April 2006. The review concluded the program was technically sound, but that funding phasing at NASA needed to be changed. NASA re-phased its JWST budgets accordingly.

In the 2005 re-plan, the life-cycle cost of the project was estimated at about 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.[67] ESA is contributing about €300 million, including the launch,[68] and the Canadian Space Agency about $39M Canadian.[69]

In January 2007, nine of the ten technology development items in the program successfully passed a non-advocate review.[70] These technologies were deemed sufficiently mature to retire significant risks in the program. 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 program into its detailed design phase (Phase C). By May 2007, costs were still on target.[71] 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.

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.[72] 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 programs, though JWST itself remained on schedule.[73]

By 2011, the JWST program 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 have been pioneered by the program, and it has passed its design reviews. In the 1990s it was unknown if a telescope so large and low mass was possible.[74]

Assembly of the hexagonal segments of the primary mirror, which was done via robotic arm, began in November 2015 and was completed in February 2016.[75] Final construction of the Webb telescope was completed in November 2016, after which extensive testing procedures began.[76]

Cost and schedule issues

Then-planned launch and costs
Year Launch Budget Plan
1997 2007[74] 0.5 Billion USD[74]
1998 2007[77] 1[48]
1999 2007 to 2008[78] 1[48]
2000 2009[36] 1.8[48]
2002 2010[79] 2.5[48]
2003 2011[80] 2.5[48]
2005 2013 3[81]
2006 2014 4.5[82]
2008 2014 5.1[83]
2010 2015 to 2016 6.5
2011 2018 8.7[84]
2013 2018 8.8[85]

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 $4 billion (about $7 billion in 2006 dollars).[48] In June 2011, it was reported that the Webb telescope would cost at least four times more than originally proposed, and launch at least seven years late. Initial budget estimates were that the observatory would cost $1.6 billion and launch in 2011. NASA has now scheduled the telescope for a 2018 launch.

The JWST has a history of major cost overruns and delays which have resulted from outside factors such as delays in deciding on a launch vehicle and adding extra funding for contingencies. The first budget estimates were that the observatory would cost $1.6 billion and launch in 2011, but NASA has now scheduled the telescope for a 2018 launch on a budget that has grown to $8.8 billion.[86][85] In 2011, the United States House of Representatives voted to terminate funding, after about $3 billion had been spent and 75% of its hardware was in production.[87] Funding was restored and capped at $8 billion.[88] As of winter 2015–2016, the telescope remained on schedule for an October 2018 launch and within the 2011 revised budget.[89] One of the reasons why the cost grew so much is that it is difficult to forecast the cost of development, and in general budget predictability improved when initial development milestones were achieved.[85]

Some scientists have expressed concerns about growing costs and schedule delays for the Webb telescope, which competes for scant astronomy budgets and thus threatens funding for other space science programs.[85] A review of NASA budget records and status reports noted that the JWST is 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, missed budget projections, and evaluation of components to estimate extreme launch conditions, thus extending the schedule and increasing costs further.[85][90][91]

By 2006, $1 billion had been spent on developing JWST, with the budget at about $4.5 billion at that time. By the mid-2010s, the U.S. contribution was still expected to cost $8.8 billion,[85] but this does not include the international contributions.[92] With the combined U.S. and international funding, the overall cost not including extended operations is projected to be over $10 billion when completed.[92]

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 $1.9bn from NASA's overall budget, of which roughly one quarter was for JWST.[93][94][95][96] This budget proposal was approved by subcommittee vote the following day; however, in November 2011, Congress reversed plans to cancel the JWST and instead capped additional funding to complete the project at $8 billion.

The committee charged that the project was "billions of dollars over budget and plagued by poor management". The telescope was originally estimated to cost $1.6bn but the cost estimate grew throughout the early development reaching about $5bn by the time the mission was formally confirmed for construction start in 2008. In summer 2010, the mission passed its Critical Design Review with excellent grades on all technical matters, but schedule and cost slips at that time prompted Maryland US 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 $1.5bn (for a total of $6.5bn). 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.[97] Because the runaway budget diverted funding from other research, the science journal Nature described the James Webb as "the telescope that ate astronomy".[98] However, termination of the JWST project as proposed by the House appropriation committee also would have imperiled funding to other missions, such as the Wide-Field Infrared Survey Telescope.[99]

The American Astronomical Society issued a statement in support of JWST in 2011,[100] as did Maryland US Senator Barbara Mikulski.[101] A number of editorials supporting JWST appeared in the international press during 2011 as well.[93][102][103]

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.[68][104] The CSA will provide the Fine Guidance Sensor and the Near-Infrared Imager Slitless Spectrograph plus manpower to support operations.[105]

Participating countries

Public displays and outreach

Early full-scale model on display at NASA Goddard (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; Manhattan, New York; and Orlando, Florida; and elsewhere at Paris, France; Dublin, Ireland; Montreal, Quebec, Canada; Hatfield, United Kingdom; and Munich, Germany. The model was built by the main contractor, Northrop Grumman Aerospace Systems.[106]

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 24×12×12 m (79×39×39 ft) and weighs 5.5 tonnes (12,000 lb).

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, Texas for SXSW 2013.[107][108]

Mission

The JWST's primary scientific mission has four key goals: to search for light from the first stars and galaxies that formed in the Universe after the Big Bang, to study the formation and evolution of galaxies, to understand the formation of stars and planetary systems and to study planetary systems and the origins of life.[109] 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 the 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. The JWST will be sensitive to a range of wavelengths from 0.6 (orange light) to 28 micrometers (deep infrared radiation at about 100 K (−170 °C; −280 °F)).

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.[110]

Orbit

JWST will not be exactly at the L2 point, but circle 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.

The JWST will be located near the second Lagrange point (L2) of the Earth-Sun system, which is 1,500,000 kilometers (930,000 mi) from Earth, directly opposite to the Sun. 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 it takes the Earth. The telescope will circle about the L2 point in a halo orbit, which will be inclined with respect to the ecliptic, have a radius of approximately 800,000 kilometers (500,000 mi), and take about half a year to complete.[14] 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.[111] This requires some station-keeping: around 2–4 m/s per year[112] from the total budget of 150 m/s.[113] Two sets of thrusters constitute the observatory's propulsion system.[114]

Infrared astronomy

Infrared observations can see objects hidden in visible light, such as HUDF-JD2 shown.

JWST is the formal successor to the Hubble Space Telescope (HST), and since its primary emphasis is on infrared observation, 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.[115] Observing in the infrared is a key technique for achieving this, because it better penetrates obscuring dust and gas, allows observation of dim cooler objects, and because of cosmological redshift. 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, often overwhelming light from the object being observed. This makes space the ideal place for infrared observation.[116]

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 these objects are therefore easier to see if viewed in the infrared.[117] 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.[118]

Infrared radiation can pass more freely through regions of cosmic dust that scatter radiation in the visible spectrum. Observations in infrared allow the study of objects and regions of space which would be obscured by gas and dust in the visible spectrum,[117] such as the molecular clouds where stars are born, the circumstellar disks that give rise to planets, and the cores of active galaxies.[117]

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.[117] 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) requiring an additional cry-cooler.[36][118]

Some of the missions in infrared astronomy that impacted JWST development were Spitzer and also the WMAP probe.[119] Spitzer showed the importance of mid-infrared, such as in its observing dust disks around stars.[119] Also, the WMAP probe showed the universe was "lit up" at redshift 17, further underscoring the importance of the mid-infrared.[119] Both these missions launched in the early 2000s, in time to influence JWST development.[119] On JWST the mid-infrared science instrument is MIRI, and it required an additional cry-cooler.

Ground support and operations

The Space Telescope Science Institute (STScI), located 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 $162.2 million intended to support operations through the first year after launch.[120] 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 NASA's 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 programs 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.

Most of the data processing on the telescope is done by conventional single-board computers.[121] 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 kg (20 lb) instrument box in a 3 cm package and consume only 11 milliwatts of power.[122] 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 the JWST.[122]

Images

After-launch deployment

JWST after-launch deployment planned timeline, FAQ 16: What happens after Webb is launched?[2]

Nearly a month after launch, a trajectory correction will be initiated to place the JWST into a halo orbit at L2.[123]

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. 257. pp. 19–28.

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Further reading

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