We are less than 24 hours from the much-awaited launch of the James Webb Space Telescope, now scheduled for Saturday Dec 25 at 7:20 a.m. ET. from Arianespace spaceport in French Guiana.
The James Webb Space Telescope (JWST) is a large infrared space telescope, a successor to the Hubble Space Telescope and the Spitzer Space Telescope, developed jointly by NASA, ESA and CSA.
JWST will be the most powerful telescope in space. It will be able to observe some of the most distant objects in the Universe, beyond the reach of current ground and space-based instruments; it will help understand the formation of galaxies, stars, exoplanets, black holes and search for light from the first stars and galaxies that formed in the Universe after the Big Bang.
The JWST sports a 6.5 m primary mirror made of 18 separate segments that unfold and adjust to shape after launch. A tennis court sized five-layer sunshield helps maintain the ultra low operating temperature below −220 °C. The telescope’s four instruments - cameras and spectrometers - have detectors that are able to record extremely faint signals. One instrument (NIRSpec) has programmable microshutters, which enable observation up to 100 objects simultaneously. JWST also has a cryocooler for cooling the mid-infrared detectors of another instrument (MIRI) to a very cold 7oK.
The NASA Goddard Space Flight Center (GSFC) in Maryland managed the development effort, and the Space Telescope Science Institute (also in Maryland) will operate Webb after launch.
JWST is named after James E. Webb, the second administrator of NASA, who played an integral role in the Apollo program.
In this diary, we will take a deep dive into some technical details about the space observatory, its science mission and its importance to humanity. This is a longish diary, so please feel free to skim over parts that are less interesting to you or come back and read it in pieces. There is plenty for everyone!
Orbit
The JWST will be located near the second Lagrange point (L2) of the Earth-Sun system, which is 1.5 million km from Earth, directly opposite to the Sun. The JWST will orbit about the L2 point in a half year period, 0.25 - 0.83 million km radius, elliptical halo orbit, around the Sun-Earth line. The halo orbit avoids the shadow of the Earth and Moon, and helps maintain a constant environment for the sunshield and the solar arrays. Since L2 is just an equilibrium point with no real object at its center, 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. Two sets of thrusters will be used to make periodic adjustments to maintain the halo orbit.
By comparison, the Hubble Space Telescope is in a Low Earth Orbit (LEO) at an altitude of ~570 km. The Sun-Earth distance is ~150 million km.
There are two other spacecraft currently at Sun-Earth L2 — the ESA Gaia probe and the joint Russian-German high-energy astrophysics observatory Spektr-RG.
The Observatory
The Observatory is comprised of three elements - the Optical Telescope Element (OTE), which includes the mirrors and backplane, the Integrated Science Instrument Module (ISIM), and the Spacecraft Element, which includes the spacecraft bus and the sunshield.
Mirrors
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 (spherical aberration, coma, and astigmatism) over a wide field. In addition, there is a fast steering mirror for image stabilization.
This animation shows the path light will follow as it hits the primary mirror, and is reflected to the secondary, and then in through the aft optics assembly where the tertiary and fine steering mirrors are. The light is then reflected and split and directed to the science instruments by pick-off mirrors.
JWST's primary mirror is a 6.5-meter-diameter gold-coated beryllium reflector with a collecting area of 25 square meters. The mirror is composed of 18 hexagonal segments, which will unfold after the telescope is launched. Very precise micro-motors will be used to position the mirror segments in space and to perform occasional adjustments every few days to retain optimal focus. WST's mirrors can be pushed and pulled a little to get the curvature right, as well as moved up, down and sideways.
The mirror construction consisted of several steps, performed across 11 different locations around the US —
- Beryllium ore mining and purification
- Pressing of Beryllium and cutting of mirror blanks
- Shaping and polishing of mirrors, to accuracies of less than one millionth of an inch.
- Cryogenic cooling and testing at -240 degrees Celsius. Needed since materials change shape with temperature.
- Gold plating, to improve the mirror's reflection of infrared light. Typical thickness of the gold is 1000 Angstroms (100 nanometers). A thin layer of amorphous SiO2 (glass) is deposited on top of the gold to protect it from scratches in case of handling or if particles get on the surface and move around (the gold is pure and very soft).
More details at jwst.nasa.gov/...
The following video shows the mirror construction process and its odyssey across the US -
Video showing mirror and instrument assembly on the “factory floor”.
ISIM and Instruments
The Integrated Science Instrument Module (ISIM) is the heart of the James Webb Space Telescope. It houses the four main instruments and other telescope electronics. The ISIM is located behind the main reflector dish.
The four instruments are shown and described below.
Spectrographs can analyze the spectrum of the light emitted from an object to understand its physical properties, including temperature, mass, and chemical composition.
The mid-infrared instrument (MIRI) is cooled using a helium refrigerator, or cryocooler system.
The NIRSpec instrument enables observation of up to 100 objects simultaneously. It uses a microshutter device consists of more than 62,000 individual windows with shutters arrayed in a waffle-like grid. Electronically controlled opening and closing of selected shutters enables light from specific objects to pass through for analysis.
These are delicate, fairly large and complex instruments, all of which have to survive the perilous journey from earth to L2 and to work flawlessly in the cold realm of space. Unlike Hubble, there are no repair calls. Hence, the unprecedented level of testing and uncovering/fixing problems with individual components and of the entire integrated spacecraft, which caused some of the delays in schedule.
The entire instrument assembly is mounted on the backside of the primary mirror.
The Backplane
From webb.nasa.gov/… - The backplane is the large structure that holds and supports the big hexagonal mirrors of the telescope. The backplane has an important job as it must carry not only the 6.5 meter diameter primary mirror plus other telescope optics but also the entire module of scientific instruments. All told, the backplane carries more than 2,400 kg of hardware. It is required to be essentially motionless so the mirrors can see far into deep space. To meet this requirement, the backplane was engineered to be steady down to 32 nanometers, which is 1/10,000 the diameter of a human hair! The backplane uses advanced graphite composite materials mated to titanium and invar fittings and interfaces.
The cryocooler
From jwst.nasa.gov/… - The Webb MIRI cryocooler cools the MIRI to 7 degrees above absolute zero. The Cryocooler Compressor Assembly (CCA) is a heat pump consisting of a precooler that generates about 1/4 Watt of cooling power at about 14 kelvin (using helium gas as a working fluid), and a high-efficiency pump that circulates refrigerant (also helium gas) cooled by conduction with the precooler, to MIRI. The precooler features a two-cylinder horizontally-opposed pump (that cancels vibrations) and cools helium gas using pulse tubes, which exchange heat with a regenerator acoustically. The high-efficiency pump is another two-cylinder horizontally-opposed piston device that circulates a different batch of helium gas separate from the precooler's helium.
Anyone who has worked on space systems is aware of how difficult it is to build electronics that work in the harsh environment of space. The JWST is lot more than electronics — it has mechanical, optical and electronic systems, that must operate in very cold temperatures and work without failure (there are no spares for the major instruments) for 5 to 10 years.
The Sunshield
Infrared telescopes need to stay extremely cold; the longer the wavelength of infrared, the colder they need to be, otherwise the background heat of the device itself can overwhelm the detectors.
The telescope is protected from external sources of light and heat (like the Sun, Earth, and Moon) using a 5-layer, tennis court-sized sunshield that acts like a parasol providing shade.
The sunshield allows the temperature of the telescope to be maintained below 50 Kelvin (-223°C) by passively radiating its heat into space. The sun-side of the shield will heat up to 85°C.
The sunshield consists of five layers of a material called Kapton. Each layer is coated with aluminum, and the sun-facing side of the two hottest layers also has a "doped-silicon" coating to reflect the sun's heat back into space.
Each layer of the sunshield is extremely thin. Layer 1 will face the sun and is only 0.05 millimeters (mm) thick, while the other four layers are 0.025 mm. The silicon coating is ~50 nanometers (nm) thick, while the aluminum coating is ~100 nm thick.
See jwst.nasa.gov/… for more details.
The following video shows a deployment test of the sunshield at Northrop Grumman in Redondo Beach, CA.
Infrared Astronomy
The JWST is oriented towards near-infrared astronomy, but can also see orange and red visible light, as well as the mid-infrared region. The near to mid-infrared were selected for three main reasons: distant 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 due to atmospheric absorption or by existing space telescopes such as Hubble, which is oriented towards visible and ultra-violet light.
The more distant an object is, the younger it appears: its light has taken longer to reach human observers. Because the universe is expanding (space itself is expanding), light from distant objects becomes red-shifted (its wavelength expands towards the infra-red region), and hence it is advantageous to study these objects using infrared detectors.
JWST's infrared capabilities are expected to let it see all the way to the very first galaxies, formed just a few hundred million years after the Big Bang. JWST will be a powerful time machine with infrared vision that will peer back over 13.5 billion years to see the first stars and galaxies forming out of the darkness of the early universe.
Here is a list of the major telescopes deployed to date (some of them have been retired) and the wavelengths covered by each telescope.
JWST will also study stellar evolution, penetrating the dense cold cloud cores where stars are born. It will analyze protoplanetary disks and the formation of planets. It will examine exoplanets and their atmospheres through coronagraphic imaging and transit spectroscopy and search for biologically important molecules.
First Light & Reionization
From webb.nasa.gov/… — After the Big Bang, the universe was like a hot soup of particles (i.e. protons, neutrons, and electrons). When the universe started cooling, the protons and neutrons began combining into ionized atoms of hydrogen (and eventually some helium). These ionized atoms of hydrogen and helium attracted electrons, turning them into neutral atoms - which allowed light to travel freely for the first time, since this light was no longer scattering off free electrons. The universe was no longer opaque! However, it would still be some time (perhaps up to a few hundred million years post-Big Bang) before the first sources of light would start to form, ending the cosmic dark ages. Exactly what the universe's first light (i.e. stars that fused the existing hydrogen atoms into more helium) looked like, and exactly when these first stars formed is not known. These are some of the questions JWST is designed to help answer.
Miscellaneous Info.
Parameter |
Value |
Primary mirror size |
6.5 m |
Wavelengths |
0.6–28.3 μm (orange to mid-infrared) |
Mass |
6,161 kg, about half of that of the Hubble telescope
Includes 167.5 kg of hydrazine and 132.5 kg of dinitrogen tetroxide for the propulsion system
|
Power |
2,000 watts from solar arrays |
Telemetry Link |
S-band: 16 kbit/s uplink, 40 kbit/s downlink |
Data Link |
Ka-band: up to 28 Mbit/s downlink (~300 Gigabytes per day) |
Solid state recorder (SSR) capacity |
64 Gbytes |
Main computer specs |
PowerPC 750, RAM = ? |
Level of effort |
40 million person-hours (about 20,000 person-years, 1,000 person average over 20 years) |
Mission duration |
5 years (after launch). Has enough fuel to operate for another 5 years |
Partners |
ESA (European Space Agency) and CSA (Canadian Space Agency) are partners with NASA in this endeavor.
ESA is providing the NIRSpec instrument, the Optical Bench Assembly of the MIRI instrument, the Ariane 5 launch vehicle, and operational support.
CSA is providing the Fine Guidance Sensor and the Near-Infrared Imager Slitless Spectrograph plus operational support.
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Why launch from French Guiana? |
ESA, as a partner, is providing/funding the launch service using the Ariane 5 launch vehicle. JWST requires a heavy launch vehicle like Ariane 5. SpaceX did not even exist when these decisions were made.
Ariane’s spaceport is in French Guiana, near the equator and near the east coast.
Launches from spaceports near the equator are advantageous, as rockets get a head start due to the higher velocity of earth’s surface near the equator
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Why is JWST so “light” at 6,161 kg? All spacecraft designs are heavily constrained by size, weight and power. A lot of design effort goes into minimizing all three. The weight is constrained by the launch vehicle and the destination of the spacecraft.
Some other proposed designs for JWST
Below are some other designs proposed for JWST during its initial conception days. TRW’s design is close to the current design; TRW got merged with Northrop Grumman, who won the contract to build JWST.
The Journey to French Guiana
The JWST was transported by ship on a 5,800-mile 16-day ocean journey from California to Port de Pariacabo on the Kourou River in French Guiana in October 2021.
The JWST was transported by road from Redondo Beach, California, to its nearby port of departure at Naval Weapons Station Seal Beach in a specially designed “suitcase” known as STTARS, short for Space Telescope Transporter for Air, Road and Sea. STTARS itself weighs about 76,000 kg. It is 18 feet (5.5 meters) high, 15 feet (4.6 meters) wide, and 110 feet (33.5 meters) long — about twice the length of a semi-trailer.
STTARS sailed to French Guiana inside MN Colibri, a special Ro-Ro (roll-on-roll-off) cargo ship designed to carry rocket components and satellites. STTARS is also a mobile clean room. A sophisticated heating, ventilation, and air-conditioning (HVAC) system built for STTARS monitored and controlled the humidity and temperature inside the container. Several accompanying trailers, loaded with dozens of pressurized bottles, provided a continuous supply of pristine, manufactured, dry air into the transporter’s interior.
In the loading video above, STTARS had to be first driven and loaded onto a barge from where it was gingerly wheeled into the cargo area of the ship. See www.nasa.gov/… for some interesting tidbits about this part of the journey.
The arrival and disembarking at Port de Pariacabo in French Guiana -
Launch
JWST will be launched on Dec 25 2021 on an Ariane 5 rocket from Arianespace's ELA-3 launch complex at European Spaceport located near Kourou, French Guiana.
Encapsulation of the observatory inside the Ariane 5 rocket …
… and the start of the rollout -
The Ariane 5 rocket has been rolled out to the launch pad -
Deployment
The journey from earth to the L2 orbit will take more than 2 weeks; at various stages in this journey, the solar arrays, antennas, sunshield and mirrors will be deployed and tested. It will be a nail-biting two weeks as plenty can go wrong in the complex chain of deployment events.
Here is a video illustrating the deployment steps of the JWST.
JWST instruments will start to cool following sunshield deployment. The NIRCam instrument will take a week after achieving its L2 orbit, to cool down to a point where its can start operating to support telescope alignment. It will take another three weeks to get the shaded portion of the observatory down to its operating temperatures of less than 40oK.
The MIRI instrument, which uses a cryocooler to reach its operating temperature 7oK, will achieve its full cooldown about 100 days after launch.
Operations
It will take approximately 6 months after deployment for JWST to be commissioned and the instruments to be calibrated. Around June 2022, the Early Release Science program will be initiated. followed by science missions executed by various researchers around the world. JWST is expected to look into the universe’s infancy, when the very first galaxies were forming; study the birth of stars and their planetary systems; and analyze the atmospheres of exoplanets, perhaps even detecting signs of life.
Funding
The JWST has a history of major cost overruns and delays. The JWST was originally estimated to cost $1.6 billion for a launch in 2011. In 2011, among severe cuts to the NASA budget, 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. Funding was restored in a compromise spending plan and capped at $8 billion.
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. Representatives from ESA and CSA stated their project contributions amount to approximately €700 million and CA$200 million, respectively. en.wikipedia.org/…
NASA’s total budget was $23.2 billion for 2021, less than 0.5% of the Federal budget.
Beyond JWST
NASA is already studying the next generation space telescope, to be launched in the next two decades.
The Nancy Grace Roman Space Telescope (formerly the Wide-Field Infrared Survey Telescope or WFIRST) is a NASA infrared space telescope scheduled to launch in 2027. It sports a 2.4 m primary mirror, similar to Hubble, operates in the 0.48 to 2.0 micrometers spectrum and can taken images as sharp as Hubble but with a wide field of view, 100 times larger than Hubble. Its science mission will be focused on dark energy and exoplanets. Similar to JWST, it will be located at L2.
The Large Ultraviolet Optical Infrared Surveyor (LUVOIR) telescopes will have 8 and 15.1 meter mirror resp. operating at ultraviolet, optical, and infrared wavelengths, with substantially better resolution than JWST. Planned launch year is 2039.
Epilogue
JWST is the biggest, most complex, and most expensive science mission that NASA has ever attempted, and expectations among astronomers and the public are huge.
The engineering challenges to build something this complex, that can deploy and operate flawlessly in space, are enormous. Plenty can go wrong between now and the time when JWST becomes operational. The possibility of failure lurks everywhere — from launch, to the intricate unfurling of its mirror and sunshield and the possibility of failure in its many cutting-edge technologies. Unlike Hubble, which was saved by a space shuttle mission that repaired its faulty optics, JWST is too far from Earth to fix. And not just the future of space-based astronomy, but also NASA’s ability to build complex science missions, depends on its success.
Like NASA/ESA/CSA engineers, we all share the excitement and the anxiety of this launch and we pray that all goes well.
We send our best wishes to the telescope and the mission and hope to see the fruits of the labor of thousands of engineers, scientists and support staff, spent over many many years. We look forward to the JWST serving humanity for many years to come and to advance our understanding of the Universe and our place in it.
And to all us here, here’s wishing a happy and safe holiday season. May our days be filled with love, joy, inspiration and the yearning to learn more and to do more for humanity.
References
- NASA JWST web site — webb.nasa.gov/...
- Media Kit — webb.nasa.gov/...
- JWST FAQ — jwst.nasa.gov/…
- JWST Wiki — en.wikipedia.org/…
- JWST — directory.eoportal.org/…
- JWST images — www.flickr.com/…
- James Webb Space Telescope - Launch Timeline — planet4589.org/...
- The James Webb Space Telescope (2016) — www.dailykos.com/… — a diary written over 5 years ago!
- The James Webb Space Telescope arrives in French Guiana after a 5,800 mile journey — www.dailykos.com/…
- ArianeSpace press kit — www.arianespace.com/...
- Large Ultraviolet Optical Infrared Surveyor — en.wikipedia.org/…
- List of proposed space observatories — en.wikipedia.org/…
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List of objects at Lagrange points — en.wikipedia.org/...
P.S. Most of the images and much of the text are taken from the NASA JWST web site.
YouTube link for launch webcast. Coverage starts at 6:00 a.m. ET, Dec 25. Another link -jwst.nasa.gov/...