The James Webb Space Telescope

Frequently Asked Questions (FAQ)

This FAQ is at a general interest level. You can find a more technical FAQ here.

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 Webb?
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 (2.5 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 scheduled for launch in 2013.
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 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 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 the 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 from space.
How long will the Webb mission last?
Webb will 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 limited by the amount of fuel used for maintaining the orbit, and by the testing and redundancy that ensures that everything on the spacecraft will work. Webb will carry fuel for a 10-year lifetime; the project will do mission assurance testing 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 600 kilometers away from the Earth, and is therefore readily accessible for servicing using the Space Shuttle. Webb will be operated at the second Sun-Earth Lagrange point, located approximately 1.5 million kilometers away from the Earth, and will therefore be beyond the reach of any manned 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.
The gyroscopes used for pointing Hubble sometimes fail and need to be replaced via shuttle missions. Since Webb won't be serviceable, how are you going to ensure that its gyroscopes last for its operational life?
Gyroscopes are used to sense the orientation of the telescope. Typically, three are needed for pointing in three dimensions, although innovative operational procedures have allowed Hubble to get by with just two working gyros recently. Both Hubble and Webb start with six working gyros, so three (or even four) could fail without loss of operations. But Webb will employ a very different solution for gyroscopes than Hubble.
Hubble uses traditional mechanical gyroscopes, and measures the inertia of a spinning flywheel to find its orientation. The mechanical flywheel requires a fluid medium, which causes a significant amount of wear on the units. In addition, 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.
Webb will have "Hemispherical Resonator Gyros" or HRG's. HRG's operate in vacuum and use electromagnetism to find the orientation, so there is much less wear. Webb'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 degradations in the Webb gyros can be accommodated without significantly impacting Webb science.
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 2.7 times larger than the diameter of 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.
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 Space Telescope Science Institute in Baltimore, Maryland, USA.

Webb's Orbit

Why will it take Webb 3 months to reach its final orbit?
Webb 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. 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 commissioning procedures will continue until 6 months after launch, at which point routine scientific operations will begin.
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, (-225 Celcius, or -370 deg F). Therefore, Webb 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, Webb will be in an orbit where all three of these objects are in about the same direction; 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 needed.

Webb's Mirrors

1. 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 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 observing wavelengths, which start at about 0.1 micrometer (in the ultraviolet) for Hubble and 0.6 micrometer (gold light) for Webb. 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 temperatures.
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 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.
How sharp are the images of Webb going to be?
The sharpness of images is what astronomers call angular resolution. 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).

Webb's Instruments

What kind of instruments will Webb have?
The James Webb Space Telescope includes four scientific instruments: the Near Infrared Camera (NIRCam), the Near-Infrared Spectrograph (NIRSpec), the Mid-Infrared Instrument (MIRI), and the Fine Guidance Sensor and Tunable Filter Instrument (FGS-TFI).
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-TFI 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 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) 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 32 degree F, 0 degree C or about 273 K. The near-infrared instruments (NIRCam, NIRSpec, FGS-TFI) 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 helium refrigerator, or cryocooler system.

Webb Science

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.
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 Instrument.
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 can see.
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 below). 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 light-years.
Will Webb see planets around other stars?
The Webb 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. 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 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.
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. When the planet passes behind the star, the total brightness drops again, and by subtracting we can again determine some of the planet's characteristics.
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 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, 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 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.
NASA and the Department of Energy are planning a Joint Dark Energy Mission (JDEM), a space telescope to be launched in the middle of the coming decade. JDEM will determine 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. The observations that are needed are statistical and very subtle. Webb's role in dark energy research will be complementary to JDEM. Although the JDEM mission is not yet well defined, it is expected to have a very large field of view and to observe hundreds or thousands of supernova and millions of galaxies. In contrast, Webb will observe fewer supernova, but by observing them at higher redshift, fainter levels and further into the infrared, it will provide important calibration and confirmation of the JDEM results.

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 Space Technologies (NGST) is the main NASA industrial contractor, responsible for building the optical telescope, spacecraft bus, and sunshield and preparing the observatory for launch. NGST is leading a team including three major sub-contractors: Ball Aerospace, ITT, and Alliant Techsystems (ATK). 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:
- 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).
- 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 Tunable Filter Instrument (TFI) and Fine Guidance Sensor (FGS) are provided by the Canadian Space Agency (CSA).
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 countries are involved?
Fifteen countries are involved in building the James Webb Space Telescope: Austria, Belgium, Canada, Denmark, France, French Guyana, Germany, Ireland, Italy, the Netherlands, Spain, Sweden, Switzerland, the United Kingdom and the United States of America.
Who will be able to use Webb and how is this decided?
Webb will be a General Observatory, meaning that qualified astronomers from Universities and other research institutes around the world will write proposals to perform observational studies with Webb. These proposals will be judged by a peer review system, in which teams of independent astronomers 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 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.

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 HubbleSite website, 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?
The beauty of an astronomical image depends on two things: the resolution on the sky and the number of pixels in the camera. On both of these counts, Webb is very similar to Hubble. Hubble has a resolution of just less than 0.1 arcsec in red light. Webb has a similar resolution at 2 microns in the infrared. Hubble's main imaging camera has 16 million pixels; Webb's has twice this with 32 million pixels per image. Although Webb images will be infrared, this can be translated by computer into a visible light picture. Webb images will be different, but just as beautiful as Hubble's.

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 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 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 on sound can be clearly heard when a siren or fast train is passing by.
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 nanometer.
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 mid-infrared radiation.
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. 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.

More information

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.
Do you have anything for kids?
Check out our "For Public" 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:
- StarChild: http://starchild.gsfc.nasa.gov/ (grades K-8)
- Imagine the Universe: http://imagine.gsfc.nasa.gov/ (ages 14+)
- Amazing Space: http://amazing-space.stsci.edu/
- Cool Cosmos: http://coolcosmos.ipac.caltech.edu/
- NASA education: http://education.nasa.gov/
- For teachers: http://teachspacescience.org/
I'm a professional astronomer - how can I find out more about Webb?
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.