تلسکوپ فضایی جیمز وب
تلسکوپ فضایی جیمز وب به انگلیسی: ("James Webb Space Telescope (JWST / "Webb یک تلسکوپ فضایی است که قرار است جانشین تلسکوپ فضایی هابل شود. JWST وضوح و حساسیت بسیار بالایی را در مقایسه با هابل فراهم خواهد کرد و گستره وسیعی از تحقیقات در زمینههای نجوم و کیهانشناسی را فراهم میکند، ازجمله مشاهده برخی از وقایع و اجرام دوردست در جهان مانند شکلگیری و تکامل اولین کهکشانها. اهداف دیگر این تلسکوپ عبارتند از فهم چگونگی شکلگیری ستارگان و سیارهها، و تصویربرداری مستقیم از سیارههای فراخورشیدی و نواخترها.
آینه اصلی JWST، عنصر بصری تلسکوپ، از ۱۸ قسمت آینه شش ضلعی تشکیلشده که آینهای با قطر ۶٫۵ متر را تشکیل میدهند. این آینه بسیار بزرگتر از آینهٔ هابل با قطر ۲٫۴ متر است. برخلاف هابل که طیفهای اشعه فرابنفش، طیف مرئی و مادونقرمز (۰٫۱ تا ۱ میکرومتر) را مشاهده میکند، JWST در محدوده فرکانس پایینتری از نور مرئی با طولموج بلند از طریق مادونقرمز (۰٫۶ تا ۲۷ میکرومتر) رصد خواهد کرد، که به آن اجازه میدهد اجرام بزرگ انتقال به سرخ را مشاهده کند که برای هابل بیشازحد دور و بسیار قدیمی هستند. تلسکوپ باید بسیار سرد نگه داشته شود تا بتواند اشعه مادونقرمز را بدون تداخل دریافت کند، بدین منظور تلسکوپ در فضا در نزدیکی نقطه لاگرانژی خورشید قرار خواهد گرفت و یک آفتابگیر بزرگ آینه و دیگر قطعات تلسکوپ را زیر ۲۲۳/۲- درجه سانتیگراد نگه میدارد.
JWST توسط ناسا –با مشارکت آژانس فضایی کانادا و آژانس فضایی اروپا– توسعه دادهشدهاست و به افتخار جیمز ای. وبت، که از سال ۱۹۶۱ تا ۱۹۶۸ بهعنوان مدیر ناسا مشغول به کار بوده و نقش مهمی در برنامه فضایی آپولو داشته، نامگذاری شدهاست. توسعه تلسکوپ جیمز وب در سال ۱۹۹۶ برای پرتاب در سال ۲۰۰۷ آغاز شد اما پروژه تأخیرهای زیاد و هزینههای گزافی داشت و در سال ۲۰۰۵ طراحی دوباره شد. ساخت JWST در اواخر سال ۲۰۱۶ تکمیل شد و پس از آن مرحله آزمایشهای گسترده روی آن آغاز شد. در ماه مارس ۲۰۱۸، ناسا پس از انفجار آفتابگیر تلسکوپ در زمان شبیهسازی پرتاب ارسال را به تأخیر انداخت. پرتاب در ژوئن ۲۰۱۸ پس از توصیههای یک هیئت بررسی مستقل دوباره به تعویق افتاد و در حال حاضر برای ماه مارس ۲۰۲۱ برنامهریزی شدهاست.
JWST وزنی معادل با نصف وزن هابل خواهد داشت اما، مساحت آینه اصلی آن حدوداً پنج برابر آینه هابل خواهد بود. جیمز وب برای اخترشناسی مادونقرمز مورد استفاده قرار خواهد گرفتن اما همچنین میتواند پرتوهای نارنجی و قرمز را نیز رصد کند.
تلسکوپهای زمینی باید از میان اتمسفر رصد کنند که بسیاری از امواج غیرقابل مشاهده میشوند. حتی در جاهایی که اتمسفر شفاف است بسیاری از ترکیبات شیمیایی مانند آب، دیاکسید کربن و متان که در جو زمین وجود دارند کار تجزیه و تحلیل را بسیار سخت میکنند. تلسکوپهای فضایی موجود مانند هابل نمیتوانند این دسته از امواج را مطالعه کنند، زیرا آینهها به اندازه کافی خنک نیستند (آینه هابل در حدود ۱۵ درجه سانتیگراد نگهداری میشود).
JWST در نزدیکی زمین و خورشید -در نقطه L2 لاگرانژی- حدود ۱٬۵۰۰٬۰۰۰ کیلومتری مدار زمین عمل میکند. در مقایسه با هابل که در ۵۵۰ کیلومتری و ماه تقریباً ۴۰۰٬۰۰۰ سطح زمین چرخش میکنند. این فاصله میتواند تعمیرات یا ارتقاء سختافزار JWST پس از راه اندازی را عملاً غیرممکن کند. اشیاء در این فاصله میتوانند هماهنگ با زمین دور خورشید بچرخند که اجازه میدهد تلسکوپ در یک فاصله تقریباً ثابت از زمین باقی بماند و برای محافظت از گرما و نورِ خورشید و زمین از یک سپر خورشیدی استفاده کند. این باعث میشود که دمای فضاپیمای زیر ۲۲۰- درجه سانتیگراد نگه داشته شود که برای رصد امواج مادونقرمز مورد نیاز است. پیمانکار اصلی شرکت نورثروپ گرومن است.
برای رصد در طیف مادونقرمز، JWST باید بسیار سرد (زیر ۲۲۰- درجه سانتیگراد) نگه داشته شود در غیر این صورت تابش مادونقرمز اجزای تلسکوپ را در هم خواهد شکست؛ بنابراین، از یک سپر نوری بزرگ برای جلوگیری از نور و حرارتِ خورشید، زمین و ماه استفاده میشود، و موقعیت آن در نزدیکی نقطه لاکرانژی خورشید تمام این سه جسم (خورشید، زمین و ماه) را در یک طرف فضاپیما نگه خواهد داشت.
سپر خورشیدی دارای پنج لایه که از یک لایهٔ نازکی از پلیآمید ساخته شدهاست، بههمراه اندودِ آلومینیوم در یک طرف و سیلیکون در طرف دیگر سپر. اِشکال تصادفی ساختار این لایههای ظریف در طی آزمایش، یک عامل تأخیر در اجرای پروژه است.
آینه اصلی JWST یک بازتابنده از جنس بریلیم با ابعاد ۶٫۵ متری با مساحت کل ۲۵ متر مربع است. این ابعاد برای تجهیزات پرتابی موجود بسیار بزرگ است، بنابراین آینه از ۱۸ قسمت شش ضلعی تشکیل شدهاست که پس از پرتاب تلسکوپ راهاندازی میشوند.
ماژول یکپارچهٔ تجهیزات علمی (ISIM) چارچوبی است که توان الکتریکی، محاسبات منابع، قابلیت خنک سازی و همچنین پایداری ساختاری تلسکوپ وِب را فراهم میکند. مهندسان به این قسمت قلب تلسکوپ میگویند. این قسمت با ترکیب گرافیتی-اپوکسی به زیر ساختار تلسکوپ جیمز وب متصل است. ISIM دارای چهار ابزار علمی و یک دوربین راهنمای است.
مقایسه با سایر تلسکوپها[ویرایش]
تمایل به یک تلسکوپ مادونقرمز بزرگ به دههها قبل برمیگردد؛ در ایالاتمتحده آمریکا تلسکوپ مادونقرمز شاتل زمانی که شاتل فضایی در حال ساخت بود برنامهریزی شد و به پتانسیل نجوم مادونقرمز اذعان شد. در مقایسه با تلسکوپهای زمینی، رصدخانههای فضایی عاری از جذب جوی نور مادونقرمز بودند.
بااینحال، تلسکوپهای مادونقرمز یک نقطهضعف دارند - آنها باید بسیار سرد بمانند و هرچه طولموج مادونقرمز طولانیتر شود، باید سردتر بمانند. در غیر این صورت، گرمای پسزمینه دستگاه بهخودیخود ردیابها را تحتالشعاع قرار میدهد و باعث کور شدن آن میشود. برای غلبه بر این موضوع باید تلسکوپ را بسیار دقیق طراحی کرد، بهطور خاص میتوان تلسکوپ را داخل یک محفظه عایق حرارتی ذخیرهسازی برودتی با مادهای بسیار سرد، مانند هلیوم مایع، قرارداد. این بدان معناست که بیشتر تلسکوپهای مادونقرمز طول عمر محدودی متناسب با مادهٔ سردکننده آنها دارند، بهاندازه چند ماه، شاید حداکثر چند ماه. از طریق طراحی فضاپیما میتوان دما را بهاندازه کافی پایین نگه داشت تا مشاهدات نزدیک مادونقرمز را بدون منبع خنککننده انجام داد، مانند مأموریتهای تلسکوپ فضایی اسپیتزر و کاوشگر نقشهبردار فروسرخ میدان وسیع. نمونه دیگر، ابزار NICMOS هابل است که با استفاده از بلوک یخ نیتروژن که پس از چند سال تخلیهشده بود، شروع به کار کرد، اما سپس به کریوکلر تبدیل شد که بهطور مداوم کار میکرد. جیمز وب طوری طراحیشدهاست که بتواند خودش را بدون وجود محفظه عایق حرارتی ذخیرهسازی برودتی، با استفاده از ترکیب سپر حرارتی و رادیاتور، سرد کند.
تأخیرها و افزایش هزینههای جیمز وب را میتوان با تلسکوپ هابل مقایسه کرد. وقتی پروژه هابل بهطور رسمی در سال ۱۹۷۲ شروع شد، پیشبینی میشد هزینه ساخت ۳۰۰ میلیون دلاری داشته باشد (یا ۱ میلیارد دلار در سال ۲۰۰۶)، اما زمانی که به فضا فرستاده شد، هزینهها چهار برابر شده بود. علاوه بر این، ابزارهای جدید و مأموریتهای سرویسدهی تا سال ۲۰۰۶ هزینه را به حداقل ۹ میلیارد دلار در سال ۲۰۰۶ افزایش دادند.
تحقیق و توسعه[ویرایش]
کارهای اولیه برای توسعه جانشینی برای هابل در خلال سالهای ۱۹۸۹ و ۱۹۹۴ شد که منحر به مدل مفهومی از تلسکوپی به نام تلسکوپ نسل بعدی (NGST) بود که دیافراگم ۴ متری داشت و در مدار معادل با ۴ واحد نجومی کار میکرد. این فاصله مداری از غبار بین سیارهای در امان بود. کار روی NGST در سال ۱۹۹۶ آغاز شد. این تلسکوپ در سال ۲۰۰۲، به خاطر نقش کلیدی جیمز ای. وبت در پروژه آپولو، به جیمز وب تغییر نام داد. JSWT حاصل همکاری آژانسِ فضاییِ ایالات متحده آمریکا و آژانس هوایی آمریکا با همکاریهای بینالمللی از سوی آژانس فضایی اروپا و آژانس فضایی کانادا است.
در دوران «سریعتر، بهتر و ارزانتر» در اواسط دهه ۱۹۹۰ رهبران ناسا به دنبال یک تلسکوپ فضایی کم هزینه بودند. نتیجه طرح مفهومی NGST بود که دیافراگم ۸ متری داشت و در نقطه L2 قرار داشت و تقریباً ۵۰۰ میلیون دلار تخمین زده شده بود. در سال ۱۹۹۷، ناسا با مرکز پروازهای فضایی گادرد، شرکت هوا فضا و فناوری Ball و شرکت TRW برای مطالعههایی دربارهٔ نیازهای فنی و تخمین هزینههای این پروژه وارد همکاری شد و در سال ۱۹۹۹، لاکهید مارتین و TRW را برای مطالعات اولیه انتخاب کرد. پرتاب تلسکوپ در آن زمان برای سال ۲۰۰۷ برنامهریزی شده بود اما تاریخ پرتاب متعاقباً بارها به تعویق افتاد (جدول روبرو را ببینید). در سال ۲۰۰۲، ناسا طی قراردای ۸۲۴٫۸ میلیون دلاری به TRW برای NGST، که اکنون به تلسکوپ فضایی جیمز وب تغییر نام یافتهاست، اعطا کرد. این قرارداد برای طرح یک آینه اصلی ۶٫۱ متری (۲۰ فوت) بود و تاریخ پرتاب سال ۲۰۱۰ انتخاب شد. در اواخر آن سال TRW توسط نورثروپ گرومن خریداری شد و به بخش فناوری فضایی این شرکت تبدیل شد.
مشکلات مربوط به هزینه و برنامه[ویرایش]
JWST دارای تاریخچهٔ هزینهها و تاخیرهای بسیار زیاد است که به خاطر عوامل خارجی مانند تأخیر در تصمیمگیری در مورد موشک پرتاب و اضافه کردن بودجهای به خاطر مسائل پیشبینی نشده. هزینه پروژه در ابتدا ۱٫۶ میلیارد دلار پیشبینی شده بود، اما این پیشبینی در زمانی که ساخت تلسکوپ در سال ۲۰۰۸ شروع شد به ۵ میلیارد دلار رسیده بود. در تابستان سال ۲۰۱۰ مأموریت بررسی طراحی کلیه موضوعات فنی با عالیترین نمرات انجام شد، اما تغییر هزینهها و زمان پرتاب باعث شد باربارا میکولسکی سناتور ایالت مریلند خواستار انجام تحقیقات مستقل در مورد این پروژه شد. کمیته مستقل بررسی جامع پروژه دریافت که نزدیکترین زمان ممکن برای پرتاب تلسکوپ میتواند اواخر سال ۲۰۱۵ با هزینه اضافی ۱٫۵ میلیارد دلار (کلاً ۶٫۵ میلیارد دلار) باشد. آنها همچنین خاطر نشان کردند که این امر بودجه پروژه را بالا برده و هر گونه تأخیر در پرتاب تلسکوپ باعث بالا رفتن هزینه کل پروژه میشود.
The James Webb Space Telescope (JWST or "Webb") is a space telescope that is planned to be the successor to the Hubble Space Telescope. The JWST will provide greatly improved resolution and sensitivity over the Hubble, and will enable a broad range of investigations across the fields of astronomy and cosmology, including observing some of the most distant events and objects in the universe, such as the formation of the first galaxies. Other goals include understanding the formation of stars and planets, and direct imaging of exoplanets and novas.
The primary mirror of the JWST, the Optical Telescope Element, is composed of 18 hexagonal mirror segments which combine to create a 6.5-meter (21 ft; 260 in) diameter mirror that is much larger than the Hubble's 2.4-meter (7.9 ft; 94 in) mirror. Unlike the Hubble, which observes in the near ultraviolet, visible, and near infrared (0.1 to 1 μm) spectra, the JWST will observe in a lower frequency range, from long-wavelength visible light through mid-infrared (0.6 to 27 μm), which will allow it to observe high redshift objects that are too old and too distant for the Hubble to observe. The telescope must be kept very cold in order to observe in the infrared without interference, so it will be deployed in space near the Earth–Sun L2 Lagrangian point, and a large sunshield will keep its mirror and instruments below 50 K (−220 °C; −370 °F).
The JWST is being developed by NASA—with significant contributions from the European Space Agency and the Canadian Space Agency—and is named for James E. Webb, who was the administrator of NASA from 1961 to 1968 and played an integral role in the Apollo program. Development began in 1996 for a launch that was initially planned for 2007, but the project has had numerous delays and cost overruns, and underwent a major redesign in 2005. The JWST's construction was completed in late 2016, after which its extensive testing phase began. In March 2018, NASA delayed the launch after the telescope's sunshield ripped during a practice deployment. Launch was delayed again in June 2018 following recommendations from an independent review board, and is currently scheduled for March 2021. On 28 August 2019, it was confirmed by NASA that construction was completely finished.
The JWST has an expected mass about half of Hubble Space Telescope's, but its primary mirror (a 6.5 meter diameter gold-coated beryllium reflector) will have a collecting area about five times as large (25 m2 or 270 sq ft vs. 4.5 m2 or 48 sq ft). The JWST is oriented toward near-infrared astronomy, but can also see orange and red visible light, as well as the mid-infrared region, depending on the instrument. The design emphasizes the near to mid-infrared for three main reasons: High-redshift objects have their visible emissions shifted into the infrared, cold objects such as debris disks and planets emit most strongly in the infrared, and this band is difficult to study from the ground or by existing space telescopes such as Hubble. Ground-based telescopes must look through the atmosphere, which is opaque in many infrared bands (see figure of atmospheric transmission). Even where the atmosphere is transparent, many of the target chemical compounds, such as water, carbon dioxide, and methane, also exist in the Earth's atmosphere, vastly complicating analysis. Existing space telescopes such as Hubble cannot study these bands since their mirrors are not cool enough (the Hubble mirror is maintained at about 15 °C (288 K)) and hence the telescope itself radiates strongly in the infrared bands.
The JWST will operate near the Earth–Sun L2 (Lagrange) point, approximately 1,500,000 km (930,000 mi) beyond Earth's orbit. By way of comparison, Hubble orbits 550 kilometres (340 mi) above Earth's surface, and the Moon is roughly 400,000 kilometres (250,000 mi) from Earth. This distance makes post-launch repair or upgrade of the JWST hardware virtually impossible. Objects near this point can orbit the Sun in synchrony with the Earth, allowing the telescope to remain at a roughly constant distance and use a single sunshield to block heat and light from the Sun and Earth. This will keep the temperature of the spacecraft below 50 K (−220 °C; −370 °F), necessary for infrared observations. The prime contractor is Northrop Grumman.
To make observations in the infrared spectrum, the JWST must be kept very cold (under 50 K (−220 °C; −370 °F)), otherwise infrared radiation from the telescope itself would overwhelm its instruments. Therefore, it uses a large sunshield to block light and heat from the Sun, Earth, and Moon, and its position near the Earth–Sun L2 point keeps all three bodies on the same side of the spacecraft at all times. Its halo orbit around L2 avoids the shadow of the Earth and Moon, maintaining a constant environment for the sunshield and solar arrays. The shielding maintains a stable temperature throughout the structures on the dark side, which is critical to maintaining precise alignment of the primary mirror segments.
The five-layer sunshield is constructed from polyimide film, with membranes coated with aluminum on one side and silicon on the other. Accidental tears of the delicate film structure during testing are one factor delaying the project.
The sunshield is designed to be folded twelve times so it will fit within the Ariane 5 rocket's 4.57 m (5 yards) × 16.19 m (17.7 yards) payload fairing. Once deployed at the L2 point, it will unfold to 21.197 m (23.18 yards) × 14.162 m (15.55 yards). The sunshield was hand-assembled at ManTech (NeXolve) in Huntsville, Alabama, before it was delivered to Northrop Grumman in Redondo Beach, California, USA for testing.
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. 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, 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 project, led by prime contractor Northrop Grumman Aerospace Systems, under a contract from the NASA Goddard Space Flight Center, in Greenbelt, Maryland. 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 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.
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.
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. Along with the Sunshield, it forms the Spacecraft Element of the space telescope. The other two major elements of the JWST are the Integrated Science Instrument Module (ISIM) and the Optical Telescope Element (OTE). Region 3 of ISIM is also inside the Spacecraft Bus; region 3 includes ISIM Command and Data Handling subsystem and the MIRI cryocooler.
The structure of the Spacecraft Bus must support the 6.5-ton space telescope, while it itself weighs 350 kg (about 770 lb). It is made primarily of graphite composite material. 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 2021 launch. The bus can provide pointing of one-arcsecond and isolates vibration down to two milliarcseconds.
The Spacecraft Bus is on the Sun-facing "warm" side and operates at a temperature of about 300 K. 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 the shade of the spacecraft sunshield.
Another important aspect of the Spacecraft Bus is the central computing, memory storage, and communications equipment. 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. 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.
Comparison with other telescopes
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. Compared to ground telescopes, space observatories were free from atmospheric absorption of infrared light; this would be a whole "new sky" for astronomers.
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. If not, the background heat of the device itself overwhelms the detectors, making it effectively blind. 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. 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. 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 and NEOWISE. Another example is Hubble's 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 sunshield and radiators with the mid-infrared instrument using an additional cryocooler.
The telescope's delays and cost increases can be compared to the Hubble Space Telescope. When Hubble formally started in 1972, it had an estimated development cost of $300 million (or about $1 billion in 2006 constant dollars), but by the time it was sent into orbit in 1990, the cost was about four times that. In addition new instruments and servicing missions increased the cost to at least $9 billion by 2006.
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), International X-ray Observatory (2011), MAXIM (Microarcsecond X-ray Imaging Mission), SAFIR (Single Aperture Far-Infrared Observatory), SUVO (Space Ultraviolet-Visible Observatory), and the SPECS (Submillimeter Probe of the Evolution of Cosmic Structure), the JWST is the last big NASA astrophysics mission of its generation to be built.
Development and construction
Early development work for a Hubble successor between 1989 and 1994 led to the Hi-Z telescope concept, a fully baffled[Note 1] 4-meter aperture infrared telescope that would recede to an orbit at 3 AU. This distant orbit would have benefited from reduced light noise from zodiacal dust. Other early plans called for a NEXUS precursor telescope mission.
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. 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.
In the "faster, better, cheaper" era in the mid-1990s, NASA leaders pushed for a low-cost space telescope. The result was the NGST concept, with an 8-meter aperture and located at L2, roughly estimated to cost $500 million. In 1997, NASA worked with the Goddard Space Flight Center, Ball Aerospace, and TRW to conduct technical requirement and cost studies, and in 1999 selected Lockheed Martin and TRW for preliminary concept studies. Launch was at that time planned for 2007, but the launch date has subsequently been pushed back many times (see table further down).
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. Later that year, TRW was acquired by Northrop Grumman in a hostile bid and became Northrop Grumman Space Technology.
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 and the Mid Boom Assembly (MBA) which helps to deploy the large sunshields on orbit. Goddard Space Flight Center is also responsible for providing the Integrated Science Instrument Module (ISIM).
Cost growth revealed in spring 2005 led to an August 2005 re-planning. 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 project was independently reviewed in April 2006. The review concluded the project 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. ESA is contributing about €300 million, including the launch, and the Canadian Space Agency about $39M Canadian.
In January 2007, nine of the ten technology development items in the project successfully passed a non-advocate review. 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. 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. 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.
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 have been pioneered by the project, and it has passed its design reviews. In the 1990s it was unknown if a telescope so large and low mass was possible.
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. Final construction of the Webb telescope was completed in November 2016, after which extensive testing procedures began. In March 2018, NASA delayed the JWST's launch an additional year 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 JWST's launch an additional 10 months to March 2021, based on the assessment of the independent review board convened after the failed March 2018 test deployment. In August 2019, the mechanical integration of the telescope was completed, something that was scheduled to be done 12 years ago in 2007. Following this engineers now are working to add a five layer sunshield in place to prevent damage to telescope parts from infra red rays of the sun.
Cost and schedule issues
The 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, $1 billion had been spent on developing JWST, with the budget at about $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 $4 billion (about $7 billion in 2006 dollars).
The telescope was originally estimated to cost $1.6 billion, but the cost estimate grew throughout the early development and had reached about $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 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.
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. $3 billion had been spent and 75% of its hardware was in production. 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". In response, the American Astronomical Society issued a statement in support of JWST, as did Maryland US Senator Barbara Mikulski. A number of editorials supporting JWST appeared in the international press during 2011 as well. In November 2011, Congress reversed plans to cancel the JWST and instead capped additional funding to complete the project at $8 billion.
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. Because the runaway budget diverted funding from other research, a 2010 Nature article described the JWST as "the telescope that ate astronomy".
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.
One reason for the early cost growth is that it is difficult to forecast the cost of development, and in general budget predictability improved when initial development milestones were achieved. By the mid-2010s, the U.S. contribution was still expected to cost $8.8 billion. In 2007, the expected ESA contribution was about €350 million. With the U.S. and international funding combined, the overall cost not including extended operations is projected to be over $10 billion when completed. On 27 March 2018, NASA officials announced that JWST's launch would be pushed back to May 2020 or later, and admitted that the project's costs might exceed the $8.8 billion price tag. In the 27 March press release announcing the latest delay, NASA said that it will release a revised cost estimate after a new launch window is determined in cooperation with the ESA. If this cost estimate exceeds the $8 billion cap Congress put in place in 2011, as is considered unavoidable, NASA will have to have the mission re-authorized by the legislature. In February 2019, despite expressing criticism over cost growth, Congress increased the mission's cost cap by $800 million.
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. The CSA will provide the Fine Guidance Sensor and the Near-Infrared Imager Slitless Spectrograph plus manpower to support operations.
Public displays and outreach
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, Canada; Hatfield, United Kingdom; and Munich, Germany. The model was built by the main contractor, Northrop Grumman Aerospace Systems.
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; 6.1 short tons).
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.
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. 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)).
Launch and mission length
As of July 2019, launch is planned 30 March 2021, 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. Its nominal mission time is five years, with a goal of ten years. 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. The planned five year science mission begins after a 6-month commissioning phase. An L2 orbit is only meta-stable so it requires orbital station-keeping or an object will drift away from this orbital configuration.
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. 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. This requires some station-keeping: around 2–4 m/s per year from the total budget of 150 m/s. Two sets of thrusters constitute the observatory's propulsion system.
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. Observing in the infrared 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, often overwhelming light from the object being observed. This makes a space telescope preferable for infrared observation.
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. 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.
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, such as the molecular clouds where stars are born, the circumstellar disks that give rise to planets, and the cores of active galaxies.
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. 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 cryocooler.
Some of the missions in infrared astronomy that impacted JWST development were Spitzer and also the WMAP probe. Spitzer showed the importance of mid-infrared, such as in its observing dust disks around stars. Also, the WMAP probe showed the universe was "lit up" at redshift 17, further underscoring the importance of the mid-infrared. Both these missions launched in the early 2000s, in time to influence JWST development.
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. 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 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.
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). 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. 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.
Allocation of observation times
JWST observing time will be allocated through a Director's Discretionary Early Release Science (DD-ERS) Program, a Guaranteed Time Observations (GTO) Program, and a General Observers (GO) Program. 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. GO programs will be selected through peer review by a Time Allocation Committee (TAC), similar to the proposal review process used for the Hubble Space Telescope. JWST observing time is expected to be highly oversubscribed.
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. 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.