The James Webb Space Telescope (JWST) is a large infrared space telescope under construction, to be launched in 2018, as a successor to the Hubble Space Telescope and the Spitzer Space Telescope.
The JWST sports a 6.4 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 7 K so they can work.
JWST 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 and planets.
JWST is named after James E. Webb, the second administrator of NASA, who played an integral role in the Apollo program.
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 circle about the L2 point in a half year period, 0.8 million km radius, 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.
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, which can adjust its position many times per second to provide 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 ISIM is the heart of the James Webb Space Telescope, what engineers call the main payload. This is the unit that 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.
Sunshield
The telescope is protected from external sources of light and heat (like the Sun, Earth, and Moon) as well as from heat emitted by the observatory itself, 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 (-370°F, or -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 (designated Layer 1 and Layer 2) also have a "doped-silicon" (or treated silicon) coating to reflect the sun's heat back into space.
Each layer of the sunshield is incredibly thin. Layer 1 will face the sun and is only 0.05 millimeters (0.002 inches) thick, while the other four layers are 0.025 mm (0.001 inches). The thickness of the aluminum and silicon coatings are even smaller. The silicon coating is ~50 nanometers (nm) (1.9 microinches) thick, while the aluminum coating is ~100 nm (3.93 microinches) thick.
More details at jwst.nasa.gov/...
The following video shows the sunshield deployment tests.
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.
First Light & Reionization
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.
Mission duration: 5.5 years (after launch). Has enough fuel to operate for another 5 years.
Mass - 6,500 kg, about half of that of the Hubble telescope
Power - 2,000 watts
Telemetry Link — S-band: 16 kbit/s uplink, 40 kbit/s downlink
Data Link - Ka-band: up to 28 Mbit/s downlink (~2.4 Terabytes per day)
Launch and Deployment
JWST will be launched in Oct 2018 on an Ariane 5 rocket from Arianespace's ELA-3 launch complex at European Spaceport located near Kourou, French Guiana.
Here is a NASA video illustrating the deployment of the JWST.
It will take approximately 6 months after deployment for JWST to be commissioned and the instruments to be calibrated. Around April 2019, 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.
Challenges
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, 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.
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. Overall, the U.S. space agency received $17.8 billion for the 2012 fiscal year - $924 million less than the White House requested and $684 million less than the previous year.
Beyond JWST
NASA is already studying the next generation space telescope, that may be launched in the 2025–2035 period. The Advanced Technology Large-Aperture Space Telescope (ATLAST) will have an 8 to 16.8-meter mirror operating at ultraviolet, optical, and infrared wavelengths, with substantially better resolution than HST and JWST.
References
- NASA JWST web site — www.jwst.nasa.gov
- JWST FAQ — jwst.nasa.gov/…
- JWST Wiki — en.wikipedia.org/...
- FAST, the World's Largest Radio Telescope — www.dailykos.com/…
- Asteroids and Planetary Defense — www.dailykos.com/...
All of the images (except the budget graph) are from the NASA JWST web site.