General Questions about Webb
- What is the James Webb Space Telescope?
The James Webb Space Telescope, also called Webb or JWST, is a large,
space-based observatory, optimized for infrared wavelengths, which will
complement and extend the discoveries of the Hubble Space Telescope. It
will have longer wavelength coverage and greatly improved sensitivity. The
longer wavelengths enable Webb to look further back in time to find the
first galaxies that formed in the early Universe, and to peer inside dust
clouds where stars and planetary systems are forming today.
- What was the Webb called before it was named after James
The James Webb Space Telescope was originally called the "Next Generation
Space Telescope," or NGST. It was called "Next Generation" because Webb
will build on and continue the science exploration started by the Hubble
Space Telescope. Discoveries by Hubble and other telescopes have caused a
revolution in astronomy and have raised new questions that require a new,
different, and more powerful telescope. Webb is also a "Next Generation"
telescope in an engineering sense, introducing new technologies like the
lightweight, deployable primary mirror that will pave the way for future
missions. On 10 September 2002, the Next Generation Space Telescope was
named in honor of James E. Webb, NASA's second administrator.
- Who was James E. Webb?
This space-based observatory is named after James E. Webb (1906- 1992),
NASA's second administrator. Webb is best known for leading Apollo, a
series of lunar exploration programs that landed the first humans on the
Moon. However, he also initiated a vigorous space science program that was
responsible for more than 75 launches during his tenure, including
America's first interplanetary explorers. For more information, please
visit this page on our website. James E. Webb's
official NASA biography can be found here.
- How will Webb be better than the Hubble Space Telescope?
Webb is designed to look deeper into space to see the earliest stars and
galaxies that formed in the Universe and to look deep into nearby dust
clouds to study the formation of stars and planets. In order to do this,
Webb will have a much larger primary mirror than Hubble (2.7 times larger
in diameter, or about 6 times larger in area), giving it more
light-gathering power. It also will have infrared instruments with longer
wavelength coverage and greatly improved sensitivity than Hubble. Finally,
Webb will operate much farther from Earth, maintaining its extremely cold
operating temperature, stable pointing and higher observing efficiency than
with the Earth-orbiting Hubble. Here is a
feature that contrasts Webb with Hubble.
- When will Webb be launched?
Webb is planned to launch in October 2018.
- How will Webb be launched?
Webb will be launched on an Ariane 5 ECA rocket. The launch vehicle is part
of the European contribution to the mission. Additional information may be
- Why do we have to go to space at all? Can we not get
these data with large telescopes on the ground, using adaptive optics?
The Earth's atmosphere is nearly opaque and glows brightly at most of the
infrared wavelengths that Webb will observe, so a cold telescope in space
is required. For those wavelengths that are transmitted to the ground, the
Earth's atmosphere blurs the images and causes stars to twinkle.
Currently, adaptive optics systems can correct for this blurring only over
small fields of view near bright stars functioning as reference beacons,
allowing access to only a small fraction of the sky. Artificial light
beacons created with strong lasers may provide better access to the sky,
but the technology to provide a wide field of view is still far in the
future. Finding the earliest galaxies will require very low foreground
light levels, ultra-sharp images over large areas, and studies at many
infrared wavelengths, a combination of observing conditions only available
- How long will the Webb mission last?
Webb is designed to have a mission lifetime of not less than 5-1/2 years after launch, with the goal of having a lifetime greater than 10 years. The lifetime is ultimately limited by the amount of fuel used for maintaining the orbit, and by proper functioning in orbit of the spacecraft and instruments. Webb will carry fuel for a 10-year lifetime (with margin); the project will perform mission assurance testing of the flight system to guarantee 5 years of scientific operations starting at the end of the commissioning period 6 months after launch.
- Why is Webb not serviceable like Hubble?
Hubble is in low-Earth orbit, located approximately 375 miles (600 km) away
from the Earth, and is therefore readily accessible for servicing. Webb will be operated at the second Sun-Earth Lagrange
point, located approximately 1 million miles (1.5 million km) away from the Earth,
and will therefore be beyond the reach of any crewed vehicle currently
being planned for the next decade. In the early days of the Webb project,
studies were conducted to evaluate the benefits, practicality and cost of
servicing Webb either by human space flight, by robotic missions, or by
some combination such as retrieval to low-Earth orbit. Those studies
concluded that the potential benefits of servicing do not offset the
increases in mission complexity, mass and cost that would be required to
make Webb serviceable, or to conduct the servicing mission itself.
- How is testing different for Hubble and Webb's mirrors? What did Hubble teach us?
First, on Webb, we use completely separate sets of measurement tools and techniques for verification than what we used to guide manufacturing. This avoids one error from the Hubble experience where the same tool used for manufacturing was later used for verification.
In technical terms, specifically on Hubble, Perkin-Elmer used the same reflective null corrector to guide primary mirror final figuring and polishing that they later used to vouch for its figure. The reflective null corrector was set up wrong and furthermore was not independently checked for correct setup and thus the mirror ended up with the wrong figure. Even so, Perkin-Elmer used a separate tool--a refractive null corrector--to check focal length of the primary mirror and it could have served as an independent check of mirror figure. In fact the refractive null corrector hinted that something was wrong with mirror's figure, but this was rationalized away in favor of the reflective null corrector data because the reflective one was more precise and provided complete (albeit false) figure information, and the purpose of the focal length check was a focal length check and not a figure check.
On Webb, all mirror figures have been verified individually at the mirror level, and figures will again be checked after the telescope is assembled.
Secondly, on Webb, we are performing an end-to-end optical test on the whole telescope, which was not done on Hubble. An end-to-end check of the assembled Hubble telescope should have and probably would have revealed the figure flaw in its primary mirror.
In more technical terms, the end-to-end test on Webb's telescope involves passing light through the entire assembled telescope using test point sources of light from precision-placed fiber optics and using reflecting mirrors (three auto-collimating flat test mirrors). This will re-check alignment of all the telescope optics assembled together and demonstrate that the individual primary mirror segments can be aligned to each other, and it will re-check the figures of the mirrors.
In summary, we use multiple independent testing and cross-checks with pre-defined success criteria that includes end-to-end testing, as well as comprehensive and rigorous external independent expert review as a further check.
Lee Feinberg and Paul Geithner wrote a paper (presented in 2008) on lessons learned from Hubble. (Note: You will need an SPIE account to download it.)
- Will Webb use gyroscopes for pointing?
Gyroscopes are used in combination with star trackers to sense the orientation of the observatory, but not to point it. Typically, at least three gyroscopes oriented in three different directions are needed to determine the orientation of the observatory in three dimensions (although innovative operational procedures allowed Hubble to get by with just two working gyros plus other sensors prior to its last servicing mission). Like Hubble and many other spacecraft, Webb begins life with six working gyros, so three can fail without loss of scientific capability, but unlike Hubble, Webb employs a very different kind of gyroscope.
Hubble uses traditional mechanical gyroscopes, which measure the inertia of a small spinning flywheel to sense angular motion. Mechanical flywheels require moving parts in a fluid medium, and thus are subject to wear over time. Moreover, Hubble must orient its entire body extremely precisely to point at an astronomical target, so a high degree of accuracy from the gyros is required.
Webb uses "Hemispherical Resonator Gyros" or HRGs, similar to those in use on the NASA Chandra X-Ray Observatory. Sometimes called "wine glass gyroscopes," HRGs measure the inertia of flexing vibration in a bowl-shaped stemmed crystal to sense angular motion. HRGs operate in a vacuum and have no rotating or rubbing parts, so they suffer virtually no wear. Webb's HRGs and the Fine Guidance Sensor (FGS) instrument work with the final optic in the telescope, called the fine steering mirror (FSM), to stabilize the beam of light coming from the telescope and going into the scientific instruments. The FSM can tip and tilt a minute amount very quickly to compensate for small motions or "jitter" in the light beam, thus avoiding the need to point the whole observatory extremely precisely. With this scheme, Webb can tolerate less accuracy from its gyros and still perform its science mission.
- How is Webb pointed?
To turn and point at different objects in space, Webb uses six reaction wheels to rotate the observatory. The reaction wheels are basically flywheels, which store angular momentum. The effect of angular momentum is familiar in bicycle riding. It is much easier to stay up on the bike when it is moving than when it is standing still, and the bicycle will tend to go straight in 'no hands' mode thanks to the angular momentum of the spinning wheels. Slowing down or speeding up one or more of the Webb's reaction wheels alters the total angular momentum of the whole observatory and consequently the observatory turns to conserve angular momentum. Hubble uses reaction wheels also to turn to point at different objects.
The reaction wheels work in combination with three star trackers and six gyroscopes that provide feedback on where the observatory is pointing and how fast it is turning. This enables coarse pointing sufficient to keep the solar array pointed at the Sun and the high-gain antenna pointed at the Earth. To take images and spectra of astronomical targets (i.e., galaxy, star, planet, etc.) finer pointing is needed. Additional information for finer pointing from the Fine Guidance Sensor in Webb's integrated science instrument module (ISIM) is used to move the telescope's fine steering mirror (FSM) to steady the beam of light coming from the telescope and going into the science instruments. Webb's reaction wheels, star trackers, gyroscopes, Fine Guidance Sensor, and fine steering mirror work together in the observatory's attitude control system (ACS) to precisely point and stare at targets so that the science instruments can see them and see them clearly. The system works much the same way your body uses multiple methods of differing precision -your inner ears and eyes and nervous system and muscles - to catch a baseball in the outfield.
- How big is Webb going to be?
The most important size of a telescope is the diameter of the primary
mirror, which will be about 6.5 meters (21 feet) for Webb. This is about
2.75 times larger in diameter than Hubble, or about 6 times larger in
area. The Webb will have a mass of approximately 6,500 kg, with a weight of
14,300 lbs on Earth (in orbit, everything is weightless), a little more
than half the mass of Hubble. The largest structure of Webb will be its
sunshade, which must be able to shield the deployed primary mirror and the
tower that holds the secondary mirror. The sunshade is approximately the
size of a tennis court.
- Why does the sunshield have five layers rather than one thick one?
Each successive layer of the sunshield is cooler than the one below. The heat radiates out from between the layers, and the vacuum between the layers is a very good insulator. One big thick sunshield would conduct the heat from the bottom to the top more than 5 layers separated by vacuum.
- How will Webb communicate with scientists at Earth?
The Webb will send science and engineering data to Earth using a high frequency radio transmitter.
Large radio antennas that are part of the NASA Deep Space Network will receive the signals and forward them to the Webb Science and Operation Center at the
Science Institute in Baltimore, Maryland, USA.
- What happens after Webb is launched?
- In the first hour: Starting at liftoff, the Ariane rocket will provide thrust for a little over 8 minutes. Webb will separate from the Ariane V launch vehicle a half hour after launch and we will deploy the solar array immediately afterward. We will also release several systems that were locked for launch.
- In the first day: Two hours after launch we will deploy the high gain antenna. About ten and a half hours after launch, JWST will pass the Moon's orbit, nearly a quarter of the way to L2. Twelve hours after launch there will be the first trajectory correction maneuver by small rocket engines aboard JWST itself.
- In the first week: The second trajectory correction maneuver will take place at 2.5 days after launch. We will start the sequence of major deployment just after that. The first deployments are the fore and aft sunshield pallets, followed by the release of remaining sub-system launch locks. The next deployment is the telescope in which the telescope and the spacecraft bus move apart from each other by about 2 meters when the deployable tower assembly extends. The full sunshield deployment with unfolding and tensioning of the membranes can then be initiated. At 6 days we deploy the secondary mirror, followed by the side wings of the primary mirror.
- In the first month: As the telescope cools down in the shade of the deployed sunshield, we will turn on the warm electronics and initialize the flight software. At the end of the first month, we will do the mid-course correction that ensures that Webb will achieve its final orbit around L2. Although the telescope cools to near its operating temperature, the ISIM is warmed with electric heaters to prevent condensation on the instruments as residual water trapped in the materials making up the observatory escapes to the vacuum of space.
- In the second month: At 33 days after launch we will turn on and operate the Fine Guidance Sensor, then NIRCam and NIRSpec. The first NIRCam image will be of a crowded star field to make sure that light gets through the telescope into the instruments. Since the primary mirror segments will not yet be aligned, the picture will still be out of focus. At 44 days after launch we will begin the process of adjusting the primary mirror segments, first identifying each mirror segment with its image of a star in the camera. We will also focus the secondary mirror.
- In the third month: From 60 to 90 days after launch we will align the primary mirror segments so that they can work together as a single optical surface. We will also turn on and operate the MIRI. By the end of the third month we will be able to take the first science-quality images. Also by this time, Webb will complete its initial orbit around L2.
- In the fourth through the sixth month: At about 85 days after launch we will have completed the optimization of the telescope image in the NIRCam. Over the next month and a half we will optimize the image for the other instruments. We will test and calibrate all of the instrument capabilities by observing representative science targets.
- After six months: Webb will begin its science mission and start to conduct routine science operations.
- How long will it take Webb to reach its orbit?
Webb will orbit around the second Lagrange (L2) point, which is about 1 million miles (1.5 million km) away from Earth, and it about a month to travel this distance. During the trip to L2, Webb will be fully deployed, will cool down to its operating temperature, and its systems will begin to be checked out and adjusted. These check-out procedures will continue until 6 months after launch, at which point routine scientific operations will begin. (See also Question 14 above and our deployment video.)
- Why does Webb have to go so much farther away from Earth
than Hubble? What is the second Lagrange point orbit?
Webb requires a distant orbit for several reasons. Webb will observe
primarily the infrared light from faint and very distant objects. Infrared
is heat radiation, so all warm things, including telescopes, emit infrared
light. To avoid swamping the very faint astronomical signals with radiation
from the telescope, the telescope and its instruments must be very cold.
Webb's operating temperature is less than 50 degrees above absolute zero:
50 Kelvin (-223° C or -370° F). Therefore, Webb has a large
shield that blocks the light from the Sun and Earth (and the Moon), which otherwise
would heat up the telescope, and interfere with the observations. Webb will be placed in orbit around the Sun at a special location where its sunshield can block both the Sun and Earth (and Moon) all the time.; the second Lagrange point
(L2) of the Sun-Earth
system has this property. L2 is a semi-stable point in the gravitational
potential around the Sun and Earth. The L2 point lies outside Earth's orbit
while it is going around the Sun, keeping all three in a line at all times.
The combined gravitational forces of the Sun and the Earth can almost hold
a spacecraft at this point, and it takes relatively little fuel to keep the
spacecraft near L2. The cold and stable temperature environment of the L2
point will allow Webb to make the very sensitive infrared observations
- How can Webb's primary mirror be six times the size of Hubble's but be less massive?
There has been a lot of progress in technology since Hubble was built. The
best example of weight reduction is the primary mirror, which takes up a
considerable fraction of the total observatory mass. The mirror has to be very
accurately shaped. Any variations from the perfect shape of the mirror have
to be less than a fraction of the observing wavelengths, which start at
about 0.1 micrometer (in the ultraviolet) for Hubble and 0.6 micrometer
(gold light) for Webb. (For comparison, the average thickness of a human hair is about 100 micrometers.) To keep the mirror in such a perfect shape, Hubble
has a thick, solid glass mirror with a mass around 1000 kg (2200 lbs on
Earth). Webb's mirror will consist of 18 thin, lightweight beryllium mirror
segments, which will be kept in the right shape and place by a large number
of adjustors attached to a stiff backing frame. Including the backing
frame, the 18 segments of the Webb primary mirror total about 625 kg (1375
lb on Earth). These kinds of technologies, which were not available at the
time Hubble was built, will be used throughout Webb. Here is a pictoral
comparison of the Hubble and Webb mirrors.
- The primary mirror on Webb will be made of beryllium.
What is beryllium?
Beryllium (atomic symbol: Be) is a gray, brittle metal with an atomic
number of 4. Beryllium has a high strength per unit weight. It tarnishes
only slightly in air. The addition of beryllium to some alloys often
results in products that have high heat resistance, improved corrosion
resistance, greater hardness, greater insulating properties, and better
casting qualities. Many parts of supersonic aircraft are made of beryllium
alloys because of their lightness, stiffness, and dimensional stability.
Other applications make use of the nonmagnetic and nonsparking qualities of
beryllium and the ability of the metal to conduct electricity. Beryllium is
toxic and no attempts should be made to work with it before becoming
familiar with proper safeguards. The specific advantages to Webb are
beryllium's light weight, stiffness and its stability at very cold
- How will you protect Webb from the violent forces involved in the
Ariane rocket launch? I have read that
beryllium is relatively brittle.
Webb is not protected from the violent forces experienced during launch,
so we have to build the telescope to survive launch. This is a key
element of the design work that goes into building the telescope. We have
already constructed an engineering test mirror and demonstrated it can
survive launch with no measurable degradations. Individual elements of
the telescope are shaken with simulated launch forces to ensure that they
can survive launch. After putting together the integrated telescope
package, we will subject that to vibration testing as well.
In regards to the beryllium primary mirror, the issue of launch forces
was a consideration during selection of the material. The main concern
with beryllium mirrors is that they might change their shape very
slightly during launch and so we conducted a technology demonstration
(involving a beryllium mirror shake test) to show that the mirror will
not experience any change in shape during launch. The Webb mirror is made
from a top grade of beryllium with extensive heritage in space systems.
Concerns about beryllium mirrors being brittle are mainly an issue when
the mirrors are machined. Glass can also be pretty fragile but it is
widely used in flight mirrors so how you design, handle and support the
mirrors is what matters most.
- Will micrometeoroids damage the beryllium mirror?
We tested beryllium discs for micrometeoroids using test facilities
in the US and showed the micrometeoroids have negligible effects on the
beryllium. Cryogenic beryllium mirrors have been flown in space exposed
to micrometeoroids without problems. The Spitzer Space Telescope,
launched in 2003, has a beryllium primary mirror. All of Webb's systems
are designed to survive micrometeoroid impacts.
- Why is the mirror gold-coated and how much gold is used?
Webb's mirrors are coated with gold to optimize them for infrared light. Why does gold reflect red radiation well? Here's a scientific explanation. First, metals reflect light because they are good conductors of electricity. Electrons are widely-shared among the atoms in metals such that they form kind of a "gas" of electrons that respond very quickly to changes. It's really hard to set up an electric field in a conductor (metal) because the electrons are free to move to make it and keep it zero. Light is an electro-magnetic wave, and when it hits metal, it induces oscillations in the electrons near the surface. The electrons move to try to make the net electric field in the metal zero, so the combination of the electric field of the moving electrons and the electric field of the light adds up to zero in the metal by the light being re-emitted or bounced away in an opposite direction. Maxwell's equations can be used to explain this. Second, each element has a unique atomic structure and a different way its electrons are "arranged" and so each responds uniquely as to how well light interacts with it light and reflects it, and it varies with the wavelength of the light. Gold just happens to reflect blue light very poorly but red and infrared light extremely well. This is why it looks the color that it does to our eyes (gold colored - it reflects red light much better than blue light).
How much gold is use to coat Webb's mirrors? About a golf ball's worth. The thickness of gold coating = 100 x 10-9 meters (1000 angstroms). Surface area = 25 m2. Using these numbers plus the density of gold at room temperature (19.3 g/cm3), the coating is calculated to use 48.25g of gold, about equal to a golf ball. (A golf ball weighs 45.9 grams.)
The gold is over-coated with a thin layer of amorphous SiO2 - i.e., glass - that protects the gold.
- Why does Webb have a segmented, unfolding primary mirror?
Webb needs to have an unfolding mirror because the mirror is so large that
it otherwise cannot fit in the launch shroud of currently available
rockets. The mirror has to be large in order to see the faint light from
the first star-forming regions and to see very small details at infrared
wavelengths. Designing, building and operating a mirror that unfolds is one
of the major technological developments of Webb. Unfolding mirrors will be
necessary for future missions requiring even larger mirrors, and will find
application in other scientific, civil and military space missions.
- What is Webb's angular resolution, and how will its images compare to Hubble's? Will they be as beautiful?
Webb's angular resolution, or sharpness of vision, will be the same as Hubble's, but in the near infrared. This means that Webb images will appear just as sharp as Hubble's do.
Webb will have an angular resolution of somewhat better than 0.1 arc-seconds at a wavelength of 2 micrometers (one degree = 60 arc-minutes = 3600 arc-seconds). Seeing at a resolution of 0.1 arc-second means that Webb could see details the size of a US penny at a distance of about 24 miles (40 km), or a regulation soccer ball at a distance of 340 miles (550 km).
Angular resolution is the term astronomers use to describe the "sharpness" of an image. There are two factors that affect how sharp an image is - the diameter of the mirror and the wavelength being observed. In fact they are mathematically related - resolving ability is proportional to wavelength over diameter, so the shorter the wavelength and the bigger the diameter, the sharper your images will be.
Hubble sees shorter wavelengths of light than Webb can, but Webb has a mirror that is 2.75 times larger in diameter than Hubble. If you do the math, this means Hubble has about the same angular resolution at 700 nm that Webb does at 2000 nm (a.k.a 2 microns). Hubble can see light that ranges from about 200 nm to 2.4 microns. Webb will see about 600 nm to 28 microns. (Visible light ranges about 700 - 400 nm; JWST will be able to see in the red/orange part of the visible light spectrum.)
With some infrared capability, Hubble is capturing gorgeous images, like this one of the Horsehead Nebula. Webb is optimized to see deeper into the infrared than Hubble, and has a much larger mirror, as well as state of the art detectors. Its imagery will be detailed and spectacular.
This feature explains how Hubble images are created; Hubble's detectors actually produce images in shades of black and white - color is added during image processing. The colors aren't always what we'd see if we were able to visit the imaged objects in a spacecraft. Color is a tool, which can enhance an object's detail or visualize what could never been seen by the human eye. Webb image processing will be similar.
- What kind of instruments will Webb have?
The James Webb Space Telescope includes four scientific instruments:
the Near Infrared Camera
Near-Infrared Spectrograph (NIRSpec), the Mid-Infrared Instrument (MIRI),
and the Fine Guidance Sensor/ Near InfraRed Imager and Slitless Spectrograph (FGS-NIRISS).
- What kind of detectors will Webb have?
Webb will have two types of detector arrays (SCA): visible to near-infrared
arrays with 2,048 x 2,048 pixels, and mid-infrared arrays with about 1,024
x 1,024 pixels. Several detectors will be built into mosaics to give a
larger field of view. NIRCam, NIRSpec and FGS-NIRISS will use Mercury Cadmium
Telluride (HgCdTe) detectors made by Teledyne Scientific & Imaging. MIRI
will employ arsenic doped silicon (Si:As) detectors produced by Raytheon.
- What is the operating temperature of the telescope and the
The large sunshade will protect the telescope from heating by direct
sunlight, allowing it to cool down to a temperature below 50 Kelvin (-223° C or -370° F) by passively radiating its heat into
space. The definition of the Kelvin temperature scale is that 0 K is
"absolute zero," the lowest possible temperature. Water freezes at 32 degree
F, 0 degree C or about 273 K. The near-infrared instruments (NIRCam,
NIRSpec, FGS/NIRISS) will work at about 39 K (-234° C or -389° F)
through a passive cooling system. The mid-infrared instrument (MIRI) will
work at a temperature of 7 K (-266° C or -447° F), using a helium
refrigerator, or cryocooler system.
- Why is Webb optimized for near- and mid-infrared light?
The primary goals of Webb are to study galaxy, star and planet formation in
the Universe. To see the very first stars and galaxies form in the early
Universe, we have to look deep into space to look back in time (because it
takes light time to travel from there to here, the farther out we look, the
further we look back in time). The Universe is expanding, and therefore the
farther we look, the faster objects are moving away from us, redshifting
the light. Redshift means that light that is emitted as ultraviolet or
visible light is shifted more and more to redder wavelengths, into the
near- and mid-infrared part of the light spectrum for very high redshifts.
Therefore, to study the earliest star formation in the Universe, we have to
observe infrared light and use a telescope and instruments optimized for
this light. Star and planet formation in the local Universe takes place in
the centers of dense, dusty clouds, obscured from our eyes at normal
visible wavelengths. Near-infrared light, with its longer wavelength, is
less hindered by the small dust particles, allowing near-infrared light to
escape from the dust clouds. By observing the emitted near-infrared light
we can penetrate the dust and see the processes leading to star and planet
formation. Objects of about Earth's temperature emit most of their
radiation at mid-infrared wavelengths. These temperatures are also found in
dusty regions forming stars and planets, so with mid-infrared radiation we
can see the glow of the star and planet formation taking place. An
infrared-optimized telescope allows us to penetrate dust clouds to see the
birthplaces of stars and planets.
- What about visible light?
The reflective surface on Webb's mirrors is gold. Although gold absorbs
blue light, it reflects yellow and red visible light, and Webb's cameras
will detect that visible light.
- At which wavelengths will Webb observe?
Webb will work from 0.6 to 28 micrometers, ranging from visible
gold-colored light through the invisible mid-infrared. The short wavelength
end is set by the gold coating on the primary mirror. The long wavelength
cut-off is set by the sensitivity of the detectors in the Mid-Infrared
- How faint can Webb see?
Webb is designed to discover and study the first stars and galaxies that
formed in the early Universe. To see these faint objects, it must be able
to detect things that are ten billion times as faint as the faintest stars
visible without a telescope. This is 10 to 100 times fainter than Hubble
- What are the main science goals of Webb?
Webb has four mission science goals:
- Search for the first galaxies or luminous objects that formed after the Big Bang.
- Determine how galaxies evolved from their formation until the present.
- Observe the formation of stars from the first stages to the formation of planetary systems.
- Measure the physical and chemical properties of planetary systems and investigate the potential for life in those systems.
- How far will Webb look?
One of the main goals of Webb is to detect some of the very first star
formation in the Universe. This is thought to happen somewhere between
redshift 15 and 30 (redshift explained in question 45). At those redshifts, the
Universe was only one or two percent of its current age. The Universe is
now 13.7 billion years old, and these redshifts correspond to 100 to 250
million years after the Big Bang. The light from the first galaxies has
traveled for about 13.5 billion years, over a distance of 13.5 billion
- Will Webb see planets around other stars?
The Webb will be able to detect the presence of planetary systems
around nearby stars from their infrared light (heat). It will be able to
see directly the reflected light of large planets - the size of Jupiter -
orbiting around nearby stars. It will also be possible to see very young
planets in formation, while they are still hot. Webb will have
coronagraphic capability, which blocks out the light of the parent star of
the planets. This is needed, as the parent star will be millions of times
brighter than the planets orbiting it. Webb will not have the resolution to
see any details on the planets; it will only be able to detect a faint
light speckle next to the bright parent star.
Webb will also study planets that transit across their parent star. When the planet goes between the star and Webb, the total brightness will drop slightly. The amount that the brightness drops tells us the size of the planet. Webb can even see starlight that passes through the planet's atmosphere, measure its constituent gasses and determine whether the planet has liquid water on its surface. When the planet passes behind the star, the total brightness drops, and we can again determine more of the planet's characteristics.
- Can Webb observe planets in our own Solar System?
Yes. Webb can observe everything in our Solar System that is further from the Sun than the Earth is. Webb's sensitivity will be most useful in studying the faint rocky and icy objects in the far outer Solar System, including the dwarf planet Pluto and other Kuiper Belt Objects. Webb's studies of these objects will test theories of how the Solar System formed. Webb will also observe the moons of the gas giant planets, comets and asteroids and the planets Mars, Jupiter, Saturn, Uranus and Neptune.
- What will Webb's first targets be?
The first targets for JWST will be determined through a process similar to that used for the Hubble Space Telescope and will involve NASA, ESA, CSA and scientific community participants.
The first engineering target will come before the first science target and will be used to align the mirror segments and focus the telescope. That will probably be a relatively bright star or possibly a star field.
- Will Webb contribute to the dark matter research?
Webb cannot directly see "dark matter," the unseen matter that makes up a
large fraction of the mass of galaxies and clusters of galaxies, but Webb
can measure its effects. One of the best ways to measure mass is through
the gravitational lens effect. As described by Einstein's General
Relativity theory, a light beam passing near a large mass will be slightly
deflected, because space-time is disturbed by the presence of mass. By
taking pictures of distant galaxies behind nearby galaxies, astronomers can
calculate the total amount of mass in the foreground galaxies by measuring
the disturbances in the background galaxies. Because astronomers can see
how much mass is present in stars in the foreground galaxies, they can then
calculate how much of the total mass is missing, which is presumed to be in
the dark matter. Webb will be particularly well-suited for this type of
measurement, because its very sharp images will allow very small
disturbances to be measured, and because it can see so deep into space,
giving it access to many more background galaxies to measure disturbances
caused by this gravitational lensing effect. Also, Webb will observe many
statistics of galaxy evolution and scientists can compare these
observations to theories of the role that dark matter played in that
process, leading to some understanding of the amount and nature of the dark
matter in galaxies.
- What about dark energy?
In 1998, observations of distant supernovae revealed that about 70% of the
universe consists of mysterious dark energy which is pushing on the
expansion of the universe and causing it to accelerate. Previously,
astronomers thought that the expansion would decelerate due to the gravity
of the dark matter. In 2003, observations of the cosmic microwave
background confirmed this discovery.
The 2011 Nobel Prize in Physics was awarded to Adam Riess, Brian Schmidt and Saul Perlmutter for the discovery of dark energy.
The Hubble Space Telescope has also contributed to dark energy research. At
about half the current age of the universe, the expansion rate, which had
been decelerating, changed to acceleration as the dark energy overcame the
effects of the dark matter. The deceleration in the early universe was
first seen by Hubble, which confirmed that dark energy is the best
explanation for the supernova results, rather than a change or evolution in
the supernova themselves.
As a successor to Webb, NASA is planning a Wide-Field Infrared Survey Telescope (WFIRST), a space observatory designed to settle essential questions in both exoplanet and dark energy research, and which will advance topics ranging from galaxy evolution to the study of objects within our own galaxy. WFIRST will be a wide-field-of-view near-infrared space telescope that will observe hundreds or thousands of supernova and millions of galaxies. It will make the subtle statistical measurements that reveal the properties of the dark energy and could find out what the eventual future fate of the universe: collapsing into a big crunch or expanding forever in a big rip. In contrast, Webb will observe fewer supernovae, but by observing them at higher redshift, fainter levels and further into the infrared, it will provide complementary information to WFIRST.
Building and Using Webb
- Who are the partners in the Webb project?
NASA is the lead partner in
Webb, with significant contributions from the European Space Agency (ESA) and the Canadian Space Agency (CSA). Northrop Grumman Aerospace Systems (NGAS) is the main NASA industrial
contractor, responsible for building the optical telescope, spacecraft bus,
and sunshield, and for preparing the observatory for launch. NGAS is leading a
team including three major sub-contractors: Ball Aerospace, Harris (formerly ITT-Exelis), and Orbital-ATK (formerly ATK).
The three principal beryllium mirror
subcontractors to Ball Aerospace are Coherant (formerly Tinsley Laboratories), General Dynamics Global Imaging Technologies (formerly Axsys
Technologies), and Materion (formerly Brush Wellman Inc.) The instrument complement is provided
- The Mid-Infrared Instrument (MIRI) is provided by a consortium of European countries and the European Space Agency (ESA) and the NASA Jet Propulsion Laboratory (JPL) with detectors from Raytheon Vision Systems.
- The Near-Infrared Spectrograph (NIRSpec) is provided by ESA.
- The Near-Infrared Camera (NIRCam) is built by the University of Arizona working with Lockheed-Martin.
- The Near-Infrared Imager and Slitless Spectrograph (NIRISS) are provided by the Canadian Space Agency (CSA).
- All of the near-infrared detectors are supplied by Teledyne Technologies, Inc.
The launch vehicle and launch services are provided by ESA. The Science and
Operations Center will be at the Space Telescope Science Institute (STScI).
- Which states are involved?
The Webb project has partners or contractors in 27 states and the District of Columbia. In addition, in a program with the Girl Scouts of the USA, Webb has Education and Public Outreach activities in 41 states, the District of Columbia, Guam and a US Air Force Base in Japan.
- Which countries
Fourteen countries are involved in building the James Webb Space Telescope:
Austria, Belgium, Canada, Denmark, France, Germany, Ireland,
Italy, the Netherlands, Spain, Sweden, Switzerland, the United Kingdom and
the United States of America. The launch of JWST will take place in French Guiana, an overseas department of France located in South America.
- Who will be able to use Webb and how is this decided?
Webb will be a General Observatory, meaning that competitively selected proposals from around the world will be used to develop the observing plans. These proposals will be judged by a peer review system, in which teams of independent scientists will rank the observing proposals according to scientific merit, and the highest ranked proposals will be selected. The results of these studies will be published in scientific journals, and the data will be made available through the Internet to other scientists and the general public for further studies. This is the same system that is used to schedule the Hubble Space Telescope and many other space and ground-based observatories.
Webb and the Public
- Will I be able to see Webb pictures?
The public has access to many of the beautiful images of the sky that
Hubble has taken through the
which is maintained by the Space Telescope Science Institute (STScI). The
images in the gallery and the scientific results are also packaged into
products for use by museums and by teachers. Hubble's scientific
discoveries are explained in press releases. Webb images and discoveries
will be made available to the public, to teachers and to the press in the
same way, and the same Outreach team at the STScI will begin supporting
Webb in addition to Hubble after launch.
- Will Webb images look as good as Hubble's?
Yes! For a more detailed answer, see this question in our FAQ.
- What will the first galaxies that formed after the Big Bang
Current theories of galaxy formation suggest that the birth process for
these vast systems of stars may be very violent events, and will be
billions of times brighter than our Sun. Such events may remain visible at
highly redshifted wavelengths. That is, although much of the energy
produced is emitted in the ultraviolet, it will be redshifted into the
infrared by the time it gets to us because of the extreme distance (in
space and time) from the present.
- What is redshift and how do you measure it?
Redshift is a special astronomical case of a physical phenomenon called the
Doppler effect (after Christian Doppler [1803-1853]). The Doppler effect
occurs when a source sending out waves (either sound or light) is moving
with respect to an observer. When the source is moving toward the observer,
waves arrive earlier than they would in the stationary case and the wave
peaks arrive closer together (the sound is higher pitch or the light is
bluer). If the source is moving away from the observer, the waves get more
stretched out (the sound is lower pitch or the light is redder). The
Doppler effect on sound can be clearly heard when a siren or fast train is
In astronomy, most galaxies are moving away from us because the Universe is
expanding, so the light from the galaxies is redshifted. The farther the
galaxy is away from us, the faster it is moving, and the larger the
redshift. How redshift is connected to the distance of an object depends on
the expansion rate of the Universe, the geometry of the Universe and the
energy content of the Universe (slowing down or accelerating the
expansion). Determining these values is an important subject of
investigation of current-day astronomy. Redshifts are measured by taking
spectra of the electromagnetic radiation (X-rays, ultra-violet, visible and
infrared light, microwaves, radio waves, etc.) of astronomical objects.
Physical processes within the atoms and molecules that make up stars and
galaxies cause the spectra to have certain recognizable features at very
specific wavelengths. The wavelengths of these atomic and molecular
absorption or emission lines can be measured very accurately. By measuring
the observed wavelength of a feature in the spectrum of a galaxy, and
comparing it to the known emitted wavelength, astronomers can measure the
Doppler shift of the galaxy. Galaxies are said to have a redshift of 1 if
their spectral features have shifted to twice as long a wavelength. If
their features have shifted to 3 times longer wavelength they have redshift
2, and so on. Webb is designed to see galaxies at redshifts of 15 or more,
where the ultraviolet light is redshifted into the infrared.
- What is a light-year? And what is a parsec?
A light-year is the distance traveled by light in one year, about
5,880,000,000,000 miles (9,460,000,000,000 kilometers). Since it takes
light as long to travel from there to here as the distance in light-years,
we can say that when we look at an object that is a million light-years
away, we see it now here as it was a million years ago there. Looking deep
into space is looking far back into time. Astronomers generally use the
unit "parsec" to measure distances. One parsec is equal to about 3.26
light-years. Distances between galaxies are measured in Megaparsecs (Mpc),
or millions of parsecs.
- What is a micrometer? What is a micron?
A micrometer, also called a micron, is a millionth of a meter, or a
thousandth of a millimeter. As a reference, the diameter of a human hair is
about 100 micrometers. Wavelengths of infrared radiation are typically
expressed in micrometers. A thousandth of a micrometer is called a
- What is an arc-minute? What is an arc-second?
Arc-seconds and arc-minutes are used to measure very small angles. An
arc-minute is 1/60 of a degree, and an arc-second is 1/60 of an arc-minute,
or 1/3600 of a degree.
- What is infrared radiation?
Infrared radiation is one of the many types of 'light' that comprise the
electromagnetic spectrum. Infrared light is characterized by wavelengths
that are longer than visible light (400-700 nanometers, or 0.4-0.7
micrometers; also denoted as microns). Astronomers generally divide the
infrared portion of the electromagnetic spectrum into three regions:
near-infrared (0.7-5 micrometers), mid-infrared (5-30 micrometers) and far
infrared (30-1000 micrometers). Webb will be sensitive to near-infrared and
- What is the electromagnetic spectrum?
Much of the information we have from the universe comes from light.
Sunlight (and starlight) is made up of many different colors. We can see
this by holding a prism up to the sunlight. The prism separates the light
into the individual colors of the rainbow - the visible light spectrum. Yet
the light we can see represents only a very small portion of the
electromagnetic spectrum. Just beyond the violet light is light with an even shorter wavelength called "ultraviolet", and beyond that X-ray light and gamma rays, with wavelengths millions of times shorter than those of visible light. Likewise, just beyond the red is light we call "infrared," and beyond that microwaves and radio waves having wavelengths millions of times longer than those of visible light. The wavelength is directly related to
the amount of energy the waves carry per photon. A photon is a fundamental
particle of electromagnetic energy. The shorter the radiation's wavelength,
the higher is the energy of each photon. Although the photon energy carried
by each wavelength differs, all forms of electromagnetic radiation travel
at the speed of light - about 186,000 miles (300,000 km) per second in a
- How does our atmosphere block infrared radiation from
Only certain parts of the electromagnetic spectrum (all light ranging from
gamma ray to radio waves) can make it to the Earth's surface. Our
atmosphere absorbs much of this light. Visible light, radio waves and a few
small ranges of infrared wavelengths do make it through. Gamma rays, X-rays
and most of the ultraviolet rays and infrared rays do not. This is why
infrared telescopes are placed on high, dry mountains (like Mauna Kea in
Hawaii) so that they can observe more infrared radiation. The only way to
study the entire range of infrared (as well as gamma ray, x-rays,
ultra-violet) is to place telescopes in space well above the atmosphere.
Only some (not all) of the infrared radiation between 1 and 40 micrometers
makes it to the Earth's surface. Water vapor in our atmosphere absorbs most
of the rest. Infrared radiation is also absorbed to a lesser degree by
carbon dioxide, ozone, and oxygen molecules.
- How can I find out more about Webb?
Browse the various pages on our website to find out more about the James Webb Space Telescope.
See also the website maintained by the Space Telescope Science Institute.
- Do you have anything for kids?
Check out our "Features" and "For Educators" pages for products and programs suitable for
kids. The Astrophysics
Division at NASA's Goddard Space Flight Center also has various education and outreach programs that may of interest. In
addition, NASA has lots of great websites about astronomy for kids (and teachers!) Here are
just a few:
- I'm a professional astronomer - how can I find out more about
The science goals and planned implementation of the observatory were
published by Gardner et al. 2006, Space Science Reviews, 123/4, 485-606,
available by clicking here. Learn about recent progress by signing up for
our email newsletter, by
looking through our website, or by attending the JWST town hall at the
winter meetings of the American Astronomical Society.
You can also find a more technical FAQ here.