John Hutchings

James Webb Space Telescope

The James Webb Space Telescope (JWST) is a planned space infrared observatory, the successor to the aging Hubble Space Telescope. The main scientific goal is to observe the most distant objects in the universe, those beyond the reach of either ground based instruments or the Hubble. JWST is a NASA led international collaboration between NASA , the European Space Agency and the Canadian Space Agency. Formerly called the Next Generation Space Telescope (or NGST), it was renamed after NASA's second administrator, James E. Webb, in 2002. The telescope's launch is planned for no earlier than June 2013. It will be launched on an Ariane 5 rocket.

Mission

The JWST's primary scientific mission has four main components: to search for light from the first stars and galaxies which 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. Due to a combination of redshift, dust obscuration, and the intrinsically low temperatures of many of the sources to be studied, the JWST must operate at infrared wavelengths, spanning the wavelength range from 0.6 to 28 micrometres. In order to ensure that the observations are not hampered by infrared emission from the telescope and instruments themselves, the entire observatory must be cold, well-shielded from the Sun so that it can radiatively cool to roughly 40 kelvin (−233.15 °C, −387.67 °F). To this end, JWST will incorporate a large metalized fanfold sunshield, which will unfurl to block infrared radiation from the Sun, as well as from the Earth and Moon. The telescope's location at the Sun-Earth L2 Lagrange point ensures that the Earth and Sun occupy roughly the same relative position in the telescope's view, and thus make the operation of this shield possible. The observatory is due to be launched no earlier than June 2013 and is currently scheduled to be launched by an Ariane 5 from Guiana Space Centre Kourou, French Guiana, into an L2 orbit with a launch mass of approximately 6.2 t. After a commissioning period of approximately 6 months, the observatory will begin the science mission, which will be required to last a minimum of 5 years. The potential for extension of the science mission beyond this period exists, and the observatory is being designed accordingly.

Optics

Although JWST has a planned mass half that of the Hubble, its primary mirror (a 6.5 meter diameter beryllium reflector) has a collecting area which is almost 6 times larger. As this diameter is much larger than any current launch vehicle, the mirror is composed of 18 hexagonal segments, which will unfold after the telescope is launched. These mirrors are currently being developed by Axsys Technologies in Cullman, Alabama. Sensitive micromotors and a wavefront sensor will position the mirror segments in the correct location, but subsequent to this initial configuration they will only rarely be moved; this process is therefore much like an initial calibration, unlike terrestrial telescopes like the Keck which continually adjust their mirror segments using active optics to overcome the effects of gravitational and wind loading.

Ball Aerospace & Technologies Corp. is the principal optical subcontractor for the JWST program, led by prime contractor Northrop Grumman Space Technology, under a contract from the NASA Goddard Space Flight Center, in Greenbelt, Maryland. Seventeen additional primary mirror segments, secondary, and tertiary mirrors, plus flight spares, will be delivered to Ball Aerospace from its beryllium mirror manufacturing team that includes Axsys, Brush Wellman, and Tinsley Laboratories. As each additional mirror is delivered to Ball Aerospace over the next four years (to 2010), it will be mounted onto a lightweight, actuated strong-back assembly and undergo functional and environmental testing.

NASA has indicated that they will be incorporating microshutters, each about 100 by 200 micrometres, into the optics of the James Webb Space Telescope's Near InfraRed Spectrograph. An array of 62,000 of the shutters will sit in front of the spectrograph's 8 megapixel infrared detector. The microshutters will create an effect similar to a human eye squinting. When one squints, one's eyelashes block light; in the same way, the microshutters allow the telescope to focus on the faint light of stars and galaxies even if they are adjacent to brighter objects.

Current status

The JWST program is in its Final Design and Fabrication phase (Phase C). In March 2008, the project successfully completed its Preliminary Design Review (PDR). In April 2008, the project passed the Non-Advocate Review.

In January 2007 nine of the ten technology development items in the program successfully passed a non-advocate review. 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 will ultimately move the program into its detailed design phase (Phase C).

In April 2006 the program was independently reviewed following a replanning phase begun in August 2005. The review concluded the program was technically sound, but that funding phasing at NASA needed to be changed. NASA has rephased its JWST budgets accordingly. The August 2005 replanning was necessitated by the cost growth revealed in Spring 2005. The primary technical outcomes of the replanning are 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 micrometres. Other major features of the observatory are unchanged following the replanning efforts.

As of the 2005 re-plan, the life-cycle cost of the project was estimated at about US$ 4.5 billion. This is comprised of approximately $3.5 billion for design, development, launch and commissioning, and approximately $1.0 billion for ten years of operations. The ESA is contributing about 300M Euros, including the launch, and the Canadian Space Agency about $39M Canadian. As of May 2007 costs were still on target.

Construction and engineering

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 Dr. John C. Mather. Northrop Grumman Space Technology 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). Goddard Space Flight Center is also responsible for providing the Integrated Science Instrument Module (ISIM).

The ISIM contains four science instruments. NIRCam (Near InfraRed Camera) is an infrared imager which will have a spectral coverage ranging from the edge of the visible (0.6 micrometres) through the Near Infrared (5 micrometres). The NIRCam will also serve as the observatory's wavefront sensor, which is required for wavefront sensing and control activities. The NIRCam is being built by a team led by the University of Arizona, with Principal Investigator Dr. Marcia Rieke. The industrial partner is Lockheed-Martin's Advanced Technology Center located in Palo Alto, California.

In addition to the Near Infrared (NIR) imaging capabilities of the NIRCam, the observatory will also perform spectrography over this range with the NIRSpec (Near InfraRed Spectrograph). NIRSpec is being built by the European Space Agency at ESTEC in Noordwijk, the Netherlands, leading a team involving EADS Astrium, Ottobrunn and Friedrichshafen, Germany, and the Goddard Space Flight Center: the NIRSpec project scientist is Dr. Peter Jakobsen. The NIRSpec design provides 3 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. Switching of the modes is done by operating a wavelength preselection mechanism called Filter Wheel Assembly and selecting a correspondent dispersive element (prism or grating)using the Grating Wheel Assembly mechanism. Both mechanisms are based on the successful ISOPHOT wheel mechanisms of the Infrared Space Observatory. The mechanisms and their optical elements are being designed, integrated and tested by Carl Zeiss Optronics GmbH of Oberkochen, Germany, under contract from Astrium.

The mid-IR wavelength range will be measured by the MIRI (Mid InfraRed Instrument), which contains both a mid-IR camera and spectrometer that has a spectral range extending from 5 to 27 micrometres. MIRI is being developed as a collaboration between NASA and a consortium of European countries, and is led by Dr. George Rieke (University of Arizona) and Dr. Gillian Wright (UK Astronomy Technology Centre, Edinburgh, part of the Science and Technology Facilities Council (STFC)). MIRI features similar wheel mechanisms as NIRSpec which are also developed and built by Carl Zeiss Optronics GmbH under contract from the Max Planck Institute for Astronomy, Heidelberg.

The FGS (Fine Guidance Sensor), led by the Canadian Space Agency under project scientist Dr. John Hutchings (Dominion Astrophysical Observatory, Victoria), is used to stabilize the line-of-sight of the observatory during science observations and also includes a 'Tunable Filter module for astronomical narrow-band imaging in the 1.5 to 5 micrometre wavelength range. The infrared detectors for both the NIRCam and NIRSpec modules are being provided by Teledyne Imaging Sensors (formerly Rockwell Scientific Company).

NASA is considering plans to add a grapple feature so future spacecraft might visit the observatory to fix gross deployment problems, such as a stuck solar panel or antenna. However, the telescope itself would not be serviceable, so that astronauts would not be able to do things such as swapping out instruments, as has been done with the Hubble Telescope. Final approval for such an addition will be considered as part of the Preliminary Design Review in March 2008.

Most of the data processing on the telescope is done by conventional single board computers.. 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"). It is said that the SIDECAR ASIC will include all the functions of a 20-pound instrument box in a package the size of a half-dollar, and consume only 11 milliwatts of power. 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 important for maintaining the low temperature required for optimal operation of the JWST.

Ground support

The Space Telescope Science Institute (STScI) in Baltimore, MD, has been selected as the Science and Operations Center (S&OC) for JWST. In this capacity, STScI will be responsible for the scientific operation of the telescope and delivery of data products to the astronomical community.

Public displays

In May 2007 a full-scale model of the telescope was assembled for display at the Smithsonian's National Air and Space Museum on the National Mall, Washington DC. The model was intended to give the viewing public a better understanding of the size, scale and complexity of the satellite. The model is significantly different from the telescope, as the model must withstand gravity and weather, so is constructed mainly of aluminum and steel, weighs 12,000 lbs (5.5 tonnes), and is approximately long, wide and tall (24 m × 12 m × 12 m).

The model has been on display at various places since 2005: Seattle, WA; Colorado Springs, CO; Paris, France; Greenbelt, MD; Rochester, NY; Orlando, FL; Dublin, Ireland; and Montreal, Canada. The model was built by the main contractor, Northrop Grumman Space Technology.

Gallery

Bibliography

Jonathan P. Gardner et al. "The James Webb Space Telescope". Space Science Reviews 484-606. The formal case for the JWST science, plus some implementation.