The James Webb Space Telescope

Frequently Asked Questions (FAQ)

General Questions about JWST

What is the James Webb Space Telescope?
The James Webb Space Telescope, or JWST, is a large, observatory, optimized for infrared wavelengths, that will complement and extend the discoveries of the Hubble Space Telescope, with longer wavelength coverage and greatly improved sensitivity. The longer wavelengths enable the JWST to look much closer to the beginning of time and to hunt for the unobserved formation of the first galaxies, as well as to look inside dust clouds where stars and planetary systems are forming today.
What was the JWST called before it was named after James Webb?
The James Webb Space Telescope was originally called the "Next Generation Space Telescope," or NGST. It was called "Next Generation" because the JWST is a space telescope that will build on and continue the science exploration started by the Hubble Space Telescope. Hubble has given us wonderful answers and new questions that require a new, different, and more powerful telescope. Also, JWST is a "Next Generation" telescope in an engineering sense, introducing new technologies like the lightweight, deployable primary mirror and paving 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. While Webb is best known for leading Apollo and a series of lunar exploration programs that landed the first humans on the Moon, he also initiated a vigorous space science program, 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 JWST be better than the Hubble Space Telescope?
JWST is designed to look deeper into space to see the earliest stars and galaxies form in the Universe and to look deep into nearby dust clouds to study the formation of stars and planets. In order to do this, JWST will have a much larger primary mirror than Hubble (2.5 times larger in diameter, or about 6 times larger in area), giving it much more light gathering power. It also will have infrared instruments with longer wavelength coverage and greatly improved sensitivity than Hubble, allowing it to look much closer to the beginning of time and to hunt for the unobserved formation of the first galaxies, as well as to look inside dust clouds where stars and planetary systems are forming today. Finally, JWST will operate much farther from Earth, where the cryogenic operating temperature can be easier to achieve and where giving the telescope a stable pointing is easier than with the Earth-orbiting Hubble. Here is a feature that contrasts JWST with Hubble.
When will JWST be launched?
JWST is scheduled to launch no earlier than June 2013.
How will JWST be launched?
JWST will be launched on an Ariane 5 ECA . Additional information may be obtained here.
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 JWST will observe, so a cold telescope in space is required. For those wavelengths that are transmitted, the Earth's atmosphere blurs the images seen from the ground and causes stars to twinkle. Adaptive optics can correct for this blurring currently 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 in the future. Finding the earliest star formation 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 from space.
How long will the JWST mission last?
JWST is being designed for 5 years operation at the second Lagrange (L2) point; the goal is to have a mission lifetime of 10 years.
Why is JWST not serviceable like Hubble?
Hubble is in low-Earth orbit, located approximately 600 kilometers away from the Earth, and is therefore readily accessible for servicing using the Space Shuttle. JWST will be operated at the second Sun-Earth Lagrange point, located approximately 1.5 million kilometers away from the Earth, and will therefore be far out of reach of servicing using the Space Shuttle. Studies have been conducted to evaluate the benefits, practicality and cost of servicing JWST by some means other than by using the Space Shuttle, and those studies have concluded that the potential benefits of servicing do not offset the increases in mission complexity and cost that would be required to make JWST fully serviceable.
On Hubble there is set of 6 gyroscopes to keep it at right orientation. They fail after some time and need to be replaced regularely, via shuttle mission. As JWST is going to be placed outside the reach of any service missions, how are you going to ensure the life span of gyroscopes on it, over its operational life?
The James Webb Space Telescope will employ a very different solution for gyroscopes than does the Hubble Space Telescope.
Hubble employs traditional mechanical gyroscopes, and measures the inertia of the spinning flywheel to find the orientation. The mechanical flywheel requires a fluid medium which causes a significant amount of wear on the units. Also, the Hubble Space Telescope must orient the entire spacecraft to point at an astronomical target, which means that a high degree of accuracy from the gyros is required.
JWST will employ "Hemispherical Resonator Gyros" or HRG's. HRG's rely on electromagnetism to find the orientation and operate in vacuum, so there is much less wear. JWST's steering mirror and active optics will be able to make adjustments to the pointing, so gyroscope performance, while important, is not as critical. Thus, small degredations in the JWST gyros can be accommodated without significantly impacting JWST science.
How big is JWST going to be?
The most important size of a telescope is the diameter of the primary mirror, which will be about 6 meters (20 feet) for the JWST. This is about 2.5 times larger than the diameter of Hubble, or about 6 times larger in area. The JWST will have a mass of approximately 6,200 kg, with a weight of 13,400 lbs on Earth (in orbit, everything is weightless), a little more than half the mass of Hubble. The largest structure of JWST 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.
How will JWST communicate with scientists at Earth?
The JWST will send science and engineering data to Earth using a high frequency radio transmitter. Large radio antennas (NASA Deep Space Network) will receive the signals and forward them to the JWST Science and Operation Center at the Space Telescope Science Institute in Baltimore, Maryland.

JWST's Orbit

Why will it take JWST 3 months to reach its final orbit?
JWST is going to the second Lagrange (L2) point , which is 1 million miles (1.5 million km) away from Earth, and it just takes a while to travel such a distance. Although a faster transit to orbit could be accomplished, such a direct insertion orbit would cost a great deal in extra equipment, extra fuel and a larger rocket. The JWST will be able to start operating during its trip to L2, so the time won't be "wasted".
Why does JWST have to go so much farther away from Earth than Hubble? What is the second Lagrange point orbit?
JWST requires a distant orbit for several reasons. JWST will observe primarily the infrared light from faint and very distant objects. But all objects, including telescopes, also 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 (Operating Temperature: under 50 K (-370 deg F)). Therefore, JWST has a large shield that blocks the light from the Sun, Earth, and Moon, which otherwise would heat up the telescope, and interfere with the observations. To have this work, JWST must be in an orbit where all three of these objects are in about the same direction. The most convenient point is the second Lagrange point (L2) of the Sun-Earth system, 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 rocket thrust to keep the spacecraft near L2. The cold and stable temperature environment of the L2 point will allow JWST to make the very sensitive infrared observations needed.

JWST's Mirrors

How can JWST's primary mirror be more than twice the size of Hubble's while JWST is so much less massive than Hubble?
The main reason is progress in technology since the building of Hubble. The best example of weight reduction is the primary mirror, which takes up a considerable fraction of the total mass budget. 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 shortest observing wavelength, which is about 0.1 micrometer (in the ultraviolet) for Hubble and 0.6 micrometer (green light) for JWST. 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). JWST's mirror will consist of 18 ultra-thin, beryllium, lightweight mirror segments, which will be kept in the right shape and place by a large number of adjustors attached to a stiff backing frame. These kinds of technologies, which were not available at the time Hubble was built, will be used throughout JWST. Here is a pictoral comparison of the Hubble and JWST mirrors.
The primary mirror on JWST 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.
How will you protect JWST from the violent forces involved in the Ariane rocket launch? I have read that beryllium is relatively brittle.
JWST 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 tested an engineering 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 and finally, we shake the integrated telescope package.
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. Additionally, we use a top grade of Beryllium with extensive heritage in space systems.
Finally, concerns about beryllium mirrors being brittle are mainly an issue when the mirrors are machined. Some people consider glass pretty fragile but it is widely used in flight mirrors so how you design and handle the mirrors is what matters most.
Why does JWST have a segmented, unfolding primary mirror?
JWST 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 JWST. Unfolding mirrors will be necessary for future missions requiring even larger mirrors, and will find application in other scientific, civil and military space missions.
How sharp are the images of JWST going to be?
The sharpness of images is what astronomers call angular resolution. JWST 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 we can 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).

JWST's Instruments

What kind of instruments will JWST have?
The JWST includes the Near Infrared Camera (NIRCam), the Near-Infrared Spectrograph (NIRSpec), the Mid-Infrared Instrument (MIRI), and the Fine Guidance Sensor (FGS).
What kind of detectors will JWST have?
JWST will have two types of detector sensor chip arrays (SCA): visible and near-infrared SCAs with 2,048 x 2,048 pixels, and mid-infrared SCAs with about 1,024 x 1,024 pixels. Several detectors will be mounted within instrument Focal Plane Arrays (FPA) to give a larger field of view. NIRCam, NIRSpec and FGS will use Mercury Cadmium Telluride (HgCdTe) detectors made by Rockwell Scientific. MIRI will employ arsenic doped silicon (Si:As) detectors produced by Raytheon.
What is the operating temperature of the telescope and the instruments?
The large sunshade will protect the telescope from heating by direct sunlight, allowing it to cool down to a temperature below 50 kelvin (equal to -370 degree F, or -223 degree C). The near-infrared instruments (NIRCam, NIRSpec, FGS) will work at about 39 K (-389 degree F, -234 degree C) through a passive cooling system. The mid-infrared instrument (MIRI) will work at a temperature of 7 K (-447 degree F, -266 degree C), using a cryocooler system. The definition of the kelvin temperature scale is that 0 K is "absolute zero," the lowest possible temperature. Water freezes 32 degree F or at 0 degree C or about 273 K.

JWST Science

Why is JWST optimized for near- and mid-infrared light?
The primary goals of JWST 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, creating redshift. Redshift means that light we normally see and study 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.
At which wavelengths will JWST observe?
JWST will work from 0.6 to 28 micrometers, ranging from visible green light through the invisible mid-infrared.
How faint can JWST see?
JWST 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 can see.
What are the main science goals of JWST?
JWST has four mission goals:
- Search for the first galaxies or luminous objects formed after the Big Bang
- Determine how galaxies evolved from their formation until now
- 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 JWST look?
One of the main goals of JWST 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 below). At those redshifts, the Universe was only one or two percent of its current age. This figure is uncertain because we do not know how much the expansion of the universe has been speeding up or slowing down since the beginning. We also do not know the exact age of the Universe, but it is about 13 to 14 billion years. So if the Universe is exactly 13 billion years old now, the first star formation would have occurred about 12.7 to 12.9 billion years ago, and JWST must see light that has traveled for this length of time, and hence this distance expressed in light years, to observe this first star formation.
Will JWST see planets around other stars?
The JWST will be able to detect the likely presence of planetary systems around nearby stars from their infrared radiation. It may even 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. JWST will also detect planets that transit across their parent star. JWST 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. JWST 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. JWST can only see large planets orbiting at relatively large distances from the parent star. To see small Earth-like planets, which are billions of time fainter than their parent star, a space telescope capable of seeing at even higher angular resolution will be required. NASA is studying such a space mission, the Terrestrial Planet Finder.
Will JWST contribute to the dark matter research?
JWST can not directly see "dark matter," the unseen matter that makes up a large fraction of the mass of galaxies and clusters of galaxies, but JWST 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. JWST will be particularly well-suited for this type of measurement, because of its very sharp images which 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, JWST will observe the epoch of galaxy formation 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.

JWST and the Scientific Community

Who are the partners in the JWST project? Which countries are involved?
NASA is the main partner in JWST, with significant contributions from the European Space Agency (ESA) and the Canadian Space Agency (CSA). The main NASA industrial partner, responsible for building the optical telescope, spacecraft bus, and sunshield and preparing the observatory for launch is Northrop Grumman Space Technologies (NGST). NGST is leading a team including three major sub-contractors: Ball Aerospace, ITT, and Alliant Techsystems. The three principal beryllium mirror subcontractors to Ball Aerospace are Tinsley Laboratories, Axsys Technologies, and Brush Wellman Inc. The instrument complement is provided as follows: Mid-Infrared Instrument (MIRI) - provided by the European Consortium (EC) (with the European Space Agency (ESA)) and the NASA Jet Propulsion Laboratory (JPL), Near-Infrared Spectrograph (NIRSpec) - provided by ESA, Near-Infrared Camera (NIRCam) - provided by the University of Arizona, Fine Guidance Sensor (FGS) - provided by the Canadian Space Agency (CSA). The launch vehicle/launch services is provided by ESA.
Who will be able to use JWST and how is this decided?
JWST will be a General Observatory, meaning that qualified astronomers from Universities and other research institutes can write proposals to perform observational studies with JWST. These proposals will be judged by a peer review system. Teams of independent astronomers will rank the observing proposals according to scientific merit. The results of these studies will be published in scientific journals, and the data will be made available through the web to other astronomers 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.

Basic Science

What will the first galaxies that formed after the Big Bang look like?
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 comes out in the ultraviolet (for an observer nearby), it will be redshifted into the infrared for 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 Johann 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 can be clearly observed when a siren or fast train is passing by.
In an astronomy context, 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 in laboratories. 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. JWST 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.
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). JWST will be sensitive to near-infrared and mid-infrared radiation.
What is the electromagnetic spectrum?
Much of the information we have from the universe comes from light. Sunlight (and all 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. On one end are gamma rays, with wavelengths millions of times shorter than those of visible light. On the other end of the spectrum are radio waves having wavelengths millions of times longer than those of visible light. In between we have X-ray, ultraviolet, visible and infrared light, and microwaves. 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 vacuum.
How does our atmosphere block infrared radiation from space?
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.