ESA/Webb FAQs

By contacting us you can send questions to the ESA/Webb outreach team. A selection of answers to frequently asked questions are published on this page below.

Webb is an international partnership between NASA, ESA and the Canadian Space Agency. ESA’s participation in the Webb mission, formally approved by the ESA Science Programme Committee in 2003, involves four major contributions: 
  • the NIRSpec instrument 
  • 50% of the MIRI instrument, in a collaboration between the European Consortium and the University of Arizona, USA 
  • launch services on an Ariane 5 
  • a team of scientists to support mission operations 
In return for the European contributions, ESA gains full partnership in Webb and secures guaranteed access to at least 15% the time available on the Webb observatory for astronomers from ESA Member States for the duration of the mission. You can learn more about ESA’s contributions to Webb in this Space Sparks episode.

Webb will reveal the hidden Universe to our eyes: stars shrouded in clouds of dust, molecules in the atmospheres of other worlds, and light from the first stars and galaxies. With its suite of state-of-the-art instruments, Webb will push the frontiers of our knowledge of the Solar System, of how stars and planets form, and of galaxy formation and evolution, in new ways. Webb’s science goals aim to contribute to key questions such as: 
  • Where and how do planetary systems form and evolve?
  • How and where do stars form and die and how do their deaths impact the surrounding environment?
  • What did the early Universe look like? When did the first stars and galaxies emerge?
  • How did the first galaxies evolve over time? What can we learn about dark matter and dark energy?
You can learn more about Webb’s science goals on this page and in this Space Sparks episode.

The cost of Webb is split between the international partners in the observatory — NASA, ESA and CSA, with telescope being built by NASA, and ESA and CSA providing instrumentation. ESA also contributed the launch of Webb on an Ariane 5 rocket. The cost to build, test, launch and commission the observatory, as well as five years of science operations, is approximately $10 billion USD. Webb is not necessarily limited to 5 years of operations, and the observatory will contain enough fuel to potentially operate for around 20 years. The time spent building Webb is estimated to be 40 million hours in total, spread between thousands of engineers, scientists, and technicians.
 
Webb is the largest and most powerful space science telescope ever built, and will address scientific challenges that cannot be achieved by any other means. Fundamental astronomy questions drove Webb’s unique design, cutting-edge capabilities, and unparalleled infrared sensitivity – all geared to provide a new view of the universe and capture our imagination with extraordinary science discoveries. It’s a giant leap forward in our quest to understand humanity’s place in the universe. 
 
$10 billion is a large cost for a single observatory, but this money was invested over decades and has led to new and innovative space technologies that will improve future spacecraft and spur on new technological advances down on Earth. For example, technology developed for Webb now assists surgeons performing laser eye surgery. Engineers developed a technique for precisely and rapidly measuring the mirrors to guide their grinding and polishing, and this technology has since been adapted to creating high-definition maps of patients’ eyes for improved surgical precision.
 
The cost of Webb also included a painstaking seven-year integration and testing plan to incrementally validate the flight hardware, starting with individual components before testing larger assemblies and finally the fully assembled spacecraft. All flight-deployable items on Webb were tested multiple times on the ground using specially designed equipment which mimics the weightlessness that Webb will experience in space. 
 
The testing process for Webb was so meticulous and thorough partially due to the innovative design of the telescope. Flying new technologies in space entails risks, and mitigating these risks means that progress is sometimes slower and more costly than expected. Unlike Hubble, Webb cannot be visited by astronauts on a servicing mission — it has to work right the first time (and so far it has!). 
 
ESA’s contributions to Webb are aligned with ESA's purpose to provide for, and to promote, for exclusively peaceful purposes, cooperation among European States in space research and technology.

Webb will not be in orbit around the Earth, like the Hubble Space Telescope is — it will actually orbit the Sun, 1.5 million kilometers away from the Earth at the second Lagrange point or L2. Many ESA spacecrafts, like Herschel, Planck and Gaia are at L2 because of the advantages this location in space provides for astrophysical missions.
 
This point is named after Joseph-Louis Lagrange, an 18th century French-Italian mathematician, who solved the “three body problem” — is there any stable configuration in which three bodies can orbit each other, yet stay in the same position relative to one another? Lagrange found that there are five solutions to this problem, and they are known as Lagrange points after their discoverer.
 
For Webb, the three bodies in question are the Sun, Earth, and Webb itself. The combined gravitational attraction of the Sun and the Earth can just about hold Webb steady at this point, and it takes relatively little fuel to keep the spacecraft near L2.  
 
Webb’s orbit at L2 will let the telescope stay in line with the Earth as it moves around the Sun. This allows the satellite's large sunshield to protect the telescope from the light and heat of the Sun, Earth and Moon. Because Webb observes at infrared wavelengths —which are the wavelengths of radiated heat—  the telescope and its instruments must be kept extremely cold to avoid swamping very faint astronomical signals with radiated heat from the telescope. The cold and stable temperature environment of the L2 point will allow Webb to make the very sensitive infrared observations needed to achieve its science goals.
 
Webb will orbit around L2, not sit stationary precisely at L2. This “halo” orbit keeps the observatory out of the shadows of both the Earth and the Moon, allowing Webb’s solar panel to collect power.

Webb’s 5-12 year lifetime is mainly limited by the amount of fuel it carries. Webb orbits a point in space known as L2 —the second Lagrange point of the Sun-Earth system—which is approximately 1.5 million km away from the Earth. Here, the combination of the gravitational attraction of the Sun and the Earth allows Webb to stay in line with Earth as it moves around the Sun. Staying in orbit around L2 requires Webb to periodically fire its engines, and once Webb’s store of fuel is used up, the mission will end. Early estimates suggest the observatory’s fuel margins can support around 20 years of science operations.

Webb’s orbit at L2 is beyond the reach of any crewed spacecraft currently operating or planned. The Hubble Space Telescope, which was famously serviced by astronauts flying in the Space Shuttle, is much closer to Earth than Webb and therefore much more accessible. 

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). Therefore, Webb has a large shield that blocks the light from the Sun, Earth, and Moon. Without the sunshield, this light would otherwise heat up the telescope and interfere with Webb’s observations. Webb will be placed in orbit around the Sun at a special location where its sunshield can block both the Sun, Earth, and Moon all the time: the second Lagrange point (L2) of the Sun-Earth system. 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 to achieve its science goals

To turn and point at astronomical objects in space, Webb uses six reaction wheels to rotate the entire observatory towards an observational target. The reaction wheels are rapidly spinning flywheels which store angular momentum. Angular momentum is familiar from an example in everyday life — the angular momentum of bicycle wheels is what makes it easier to balance on a moving bike than on a stationary one. Slowing down or speeding up one or more of Webb's reaction wheels alters the total angular momentum of the whole observatory and causes the observatory to turn. Other space observatories such as the NASA/ESA Hubble Space Telescope also use reaction wheels to turn and point at targets.

Webb can determine its orientation in space using three star trackers and six gyroscopes which measure where the observatory is pointing and how fast it is turning. The star trackers and gyroscopes are sufficiently accurate to enable “course pointing”, meaning that they  provide sufficiently accurate readings to keep Webb’s solar array pointed at the Sun and the observatory’s high-gain antenna pointed at the Earth.

When Webb is taking observations, however, it needs more accurate “fine pointing”. Additional information for fine pointing is provided by Webb’s Fine Guidance Sensor, which is used to move the telescope's fine steering mirror to steady the beam of light coming from the telescope and going into the science instruments. 

Together, Webb's reaction wheels, star trackers, gyroscopes, Fine Guidance Sensor, and fine steering mirror work are part of the observatory's attitude control system (ACS). The ACS is what allows Webb to precisely point at astronomical objects so that the science instruments have a clear view of their target.

Webb’s operational orbit is around the second Lagrange point, around 1.5 million km away from Earth. Webb took roughly one month to travel this distance. During its journey, Webb was successfully fully deployed, which included extending its sunshield and unfolding its primary mirror. The telescope also cooled down to its operating temperature of less than -223° C, and engineers began to assess and commission the observatory and its science instruments.

The matrix of 18 gold hexagon-shaped segments comprise Webb's primary mirror. A telescope’s primary mirror collects light. With a larger mirror, Webb can observe with more detail and of fainter objects. Webb’s primary mirror has a total diameter of 6.5 metres, making it by far the largest mirror ever to be launched into space (the next largest, ESA’s Herschel, was 3.5 metres across). Webb’s mirror has to be this large in order to collect enough light to see extremely distant galaxies, which is one of Webb’s science goals. Each individual mirror segment is 1.32 metres across (from side to side, not corner to corner). The mirror segments are gold-coloured because they are coated in a very thin layer of real gold, which optimises the mirror’s reflection of infrared light.

Significant technology had to be developed to create Webb’s 6.5 metre primary mirror. The Webb mirror is so large that, if it had been made from the same material as Hubble’s primary mirror, it would have been far too heavy to launch. Ultimately, it was decided that the mirror segments would be made out of beryllium, which is both strong and light.  Furthermore, Webb’s mirror is too large to fit into the Ariane 5 rocket (or any existing rocket) as a single unit, so the Webb Telescope team had to find a way to fold the mirror during launch.  The team decided to comprise the primary mirror of 18 hexagonal mirror segments, which were built on a folding three part structure that was unfolded in Space after launch. The development of the hexagonal segment design, the mechanism of the folding structure and the beryllium mirrors all required significant technological development.

Webb observes infrared light (or radiated heat), which is released by hot objects such as planets, stars and galaxies. This means that Webb’s mirrors need to be kept extremely cold (-220°C), otherwise infrared light from faint sources would be lost in the heat of the mirror. This means that the mirrors themselves must be able to withstand extreme cold, and a means to protect the mirrors from the heat radiated from the Sun, Earth, Moon and the telescope itself had to be devised. Therefore, Webb’s mirrors will be shielded by a five-layer, tennis-court sized sunshield (its precise measurements are 21.197 metres by 14.162 metres.) The development of both the sunshield and the technology to deploy it successfully required the development of new technology. For example, the sunshield is made of a lightweight material with special thermal properties, called Kapton, which is specially silicon-coated and arranged in an innovative five-layer structure. Each thin Kapton layer is separated from the next by a vacuum. The heat radiates out from between the layers, and the vacuum between the layers is a very good insulator, meaning that each successive layer of the sunshield is cooler than the one below.  Each layer of the sunshield had to be designed to be remarkably thin — the layer which faces the Sun directly is 0.05mm thick, while the other four layers are 0.02mm thick. That’s about the diameter of a very fine human hair.

One of the most challenging aspects of designing, testing and launching Webb was that there was no margin for error. If a ground based telescope (a telescope that is built and used on Earth) has a flaw in its design, it is relatively easy to fix — scientists and technicians can access the telescope directly to find and resolve any problems. This is much more difficult to do with space telescopes, but in some cases it is possible. Hubble famously had a flaw in its mirror that was only discovered after launch, which was caused by the malfunction of a measuring device used during the polishing of the mirror. As a result, Hubble could not achieve the best possible image quality. During the first Hubble Servicing Mission in December 1993, a crew of astronauts carried out the repairs necessary to restore the telescope to its intended level of performance. Hubble, however, orbits the Earth at a relatively accessible altitude of about 545 kilometres. In contrast, Webb is actually in orbit around the Sun, at a point in space called L2, which lies 1.5 million kilometres beyond the Earth, in the direction away from the Sun. In context, Webb is in an orbit that is over four times further from the Earth than the Moon. This is beyond the reach of any crewed vehicle currently planned. Thus, Webb had to have a design that was as close to flawless as possible. This required Webb to go through incredibly rigorous testing prior to launch. This included 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).

Astronomers from around the world will be able to submit proposals for what Webb should observe, and a team of astronomical experts will decide which proposals will be awarded observing time. In exchange for ESA’s contributions of the Ariane 5 launch vehicle, two of the four science instruments, and support operations, scientists from European member states are guaranteed access to at least 15% the time available on the Webb observatory for astronomers from ESA Member States for the duration of the mission.

The observing policies for Webb will be very similar to those of other space observatories, such as Hubble. More than 80% of the observing time will be available to those submitting general observing proposals. Roughly 10% of the time will be reserved as Director's Discretionary Time, which can be used to observe unexpected or unpredicted phenomena or those which are only visible for a short period of time. Around 10% of the first three years of Webb observations will be Guaranteed Time Observations. The Guaranteed Time Observations program is designed to reward scientists who helped develop the key hardware and software components or technical and inter-disciplinary knowledge required for Webb.

It is planned that all Webb data will be released to the public and the wider astronomical community one year after the data are first available to the observer, similar to the policies for the Hubble Space Telescope. Astronomers throughout the world will be able to request data from the Webb archive through the internet. The public will also be able to view many of Webb's pictures through press releases and image releases on the internet and social media.

Webb will begin its science mission and start to conduct routine science operations six months after launch, and astronomers will receive science-ready data after this time. At this point, in order to publish a paper, astronomers need to perform their own analysis of the data, write their findings in the format of a journal paper, and have the paper peer-reviewed by other astronomers. The time it takes to complete this process can be anything from a few weeks to several years, depending on the complexity of the topic and other factors, such as the urgency of the results and the size of the team involved in the paper. The scientific community and the general public will be very eager to see the first science results from data collected by Webb, so the first papers can probably be expected within a few months after the start of routine science operations.

Webb is optimised to see deeper into the infrared than Hubble, has a much larger mirror and state of the art detectors. Its images will be detailed and spectacular. Webb's angular resolution, or sharpness of vision, will be the same as Hubble's, but at near infrared instead of optical wavelengths. This means that Webb images will appear just as sharp as Hubble's do. 

To be precise, Webb will have an angular resolution of slightly better than 0.1 arcseconds at a wavelength of 2 micrometers. Seeing at a resolution of 0.1 arcseconds means that Webb could see a regulation football at a distance of around 550 km — that’s a high enough angular resolution to watch a football match at the ESA headquarters in Paris all the way from Amsterdam!

Angular resolution is how astronomers 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. These factors are mathematically related — the resolving ability of a telescope is proportional to wavelength over diameter — so the shorter the wavelength observed and the larger the diameter, the sharper a telescope’s images can be. Hubble observes shorter wavelengths of light than Webb, but Webb has a mirror that is 2.75 times the diameter of Hubble’s. The combination of having a larger mirror but observing at longer wavelengths means that Webb will have about the same angular resolution at a wavelength of 2000 nanometres (nm) that Hubble does at a wavelength of 700 nm.

Building cutting-edge, one-of-a-kind spacecraft such as Webb is an extremely time-consuming process — the original concept for Webb was first proposed in 1989, and construction did not begin until 2004. Between 2012 and 2013, Webb’s individual pieces, constructed in a variety of locations, began to arrive at NASA’s Goddard Space Flight Center. In 2013, construction of the sunshield layers began. From 2013 to 2016, Webb’s science instruments —including those delivered by ESA—were packaged together and subjected to numerous tests at extreme temperatures and vibrations. From late 2015 to early 2016, the telescope optics and structures were assembled, featuring installation of all 18 of Webb’s individual mirrors on the telescope’s backplane structure to assemble the 6.5-metre mirror.

In 2017, the telescope assembly and the package of science instruments were integrated into one unit and subjected to mechanical integrity vibration testing at Goddard, then shipped to NASA’s Johnson Space Center in Houston, Texas, for end-to-end optical performance testing in a giant cryogenic temperature vacuum chamber.

In 2018, the performance-verified telescope plus instrument assembly was delivered to Northrop Grumman in California, where the spacecraft bus and sunshield assembly was being built and tested, and the following year these two halves of Webb were connected. Final environmental, electrical, functional, and communications testing continued until Webb was folded and stowed for the final time in 2021 before launch from French Guiana.

Despite this long process, Webb remains the most advanced space telescope ever launched, and represents the cutting edge of astronomical technology.

Webb’s operational orbit at the second Lagrange point is far from the space debris populating low Earth orbit. Relatively few satellites are in operation at L2, meaning that there is relatively little space debris there. Webb’s delicate optics were only deployed once the spacecraft had passed beyond low Earth orbit in order to avoid any damage or contamination.

Webb orbits the Sun 1.5 million kilometres from Earth. At that distance, even Webb’s large 21-metre sunshield will be far too small to see with the unaided eye and would be difficult to observe with even the largest astronomical telescopes. Hubble could feasibly see Webb, but will not be capable of discerning any useful details.

The second Lagrange point, L2, is mainly used by a handful of space telescopes such as Webb and the ESA Gaia mission. These telescopes are carefully monitored and removed from the Lagrange point when their fuel is almost depleted. Furthermore, orbits around L2 are unstable, meaning that without actively adjusting their orbits spacecraft will eventually drift away from L2 into an orbit around the Sun. The combination of these factors means that it is extremely unlikely for Webb to collide with other space telescopes.

When Webb has almost depleted its reserves of fuel, spacecraft operators at NASA will command the spacecraft to fire its engines and leave L2, moving into a final orbit around the Sun. The telescope will then be passivated, meaning that all remaining fuel will be vented to space and the batteries will be drained and disconnected from the solar panels. This process means that there will be no stored energy remaining on board Webb, and will prevent the telescope posing a hazard to other satellites. There are no plans to return Webb to Earth, and there are no current or planned spacecraft capable of retrieving the telescope.

Webb requires regular adjustments to its orbit to remain operational, and these adjustments all consume fuel. When the telescope runs out of fuel it will drift away from L2 and eventually enter into orbit around the Sun. However, engineers will deliberately put Webb into a carefully chosen final orbit before it completely runs out of fuel.

Unless explicitly stated otherwise, all of our images and videos are available for use without prior permission and without charge. You must however include a credit — the credit required for each image or video is listed along with the respective caption. Any text used from this website should be credited as ‘ESA/Webb’. Our full copyright policy is here. Please contact us for specific copyright questions.

No, it will not. In fact, the 18 segments will work in concert to form one perfect primary mirror

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. Science data transfer will be done via the Ka-Band Science Link, with 7, 14 or 28 Mbps options. The default speed is 28 Mbps i.e. 3.5 MB/s.

The telescope's sunshield and mirrors are expected to be bombarded with tiny meteorites (sand-like grains) in space, and their design and construction have taken that into account. 

The study of supermassive black holes is part of Webb’s science objectives. In particular, Webb will look at how supermassive black holes and their host galaxies influence each other. Webb’s data will also answer the compelling questions of how black holes formed and grew early on, and what influence they had on the formation and evolution of the early Universe.

We are looking forward to Webb surprising us. Some of the most interesting Hubble discoveries were unexpected, and with a factor 100 greater in sensitivity than Hubble, we EXPECT Webb to surprise us. Thanks to its powerful capabilities at infrared wavelengths, Webb will offer a unique view of the outer planets in our own magnificent Solar System. Looking beyond, Webb will also study in detail the atmospheres of a wide diversity of known exoplanets. It will search for atmospheres similar to Earth’s, and for the signatures of key substances such as methane, water, oxygen, carbon dioxide and complex organic molecules, in the exciting hope of finding the building blocks of life. Stay tuned for exciting Webb discoveries in the years to come!

These patterns are called diffraction spikes. All telescopes which use a mirror to collect light, like Webb, have this form of distortion which arises from the design of the telescope. In Webb's case, the six large "starburst" spikes appear because of the hexagonal symmetry of Webb's 18 primary mirror segments. The two small, horizontal spikes to the left and right of stars are caused by light bending very slightly around the vertical strut that supports the secondary mirror. The other two struts supporting the secondary were cleverly designed so that their diffraction spikes line-up with those from the mirror segments.
Diffraction patterns like these ones are only noticeable for very bright, point-like objects like stars where all the light comes from the same place. Extended objects like galaxies, even though they appear very small to our eyes, are darker and more spread out, and therefore don't show this pattern. The pattern is not beneficial for astronomers, but it's easy to correct for when using Webb data for research.