James Webb Space Telescope

The Webb team continues to make progress in aligning the observatory’s mirrors. Engineers have completed the first stage in this process, called “Segment Image Identification.” The resulting image shows that the team has moved each of Webb’s 18 primary mirror segments to bring 18 unfocused copies of a single star into a planned hexagonal formation.

This image mosaic (top), which shows 18 randomly positioned copies of the same star, served as the starting point for the alignment process. To complete the first stage of alignment, the team moved the primary mirror segments to arrange the dots of starlight into a hexagonal image array (bottom). Each dot of starlight is labeled with the corresponding mirror segment that captured it.
Credits: NASA (top); NASA/STScI/J. DePasquale (bottom)

With the image array complete, the team has now begun the second phase of alignment: “Segment Alignment.” During this stage, the team will correct large positioning errors of the mirror segments and update the alignment of the secondary mirror, making each individual dot of starlight more focused. When this “global alignment” is complete, the team will begin the third phase, called “Image Stacking,” which will bring the 18 spots of light on top of each other.

“We steer the segment dots into this array so that they have the same relative locations as the physical mirrors,” said Matthew Lallo, systems scientist and Telescopes Branch manager at the Space Telescope Science Institute. “During global alignment and Image Stacking, this familiar arrangement gives the wavefront team an intuitive and natural way of visualizing changes in the segment spots in the context of the entire primary mirror. We can now actually watch the primary mirror slowly form into its precise, intended shape!”

After starting the mirror alignment with Webb’s first detection of starlight in the Near-Infrared Camera (NIRCam), the telescope team is hard at work on the next steps for commissioning the telescope. To make more progress, the team needs to use another instrument, the Fine Guidance Sensor, to lock onto a guide star and keep the telescope pointed to high accuracy. We have asked René Doyon and Nathalie Ouellette of the Université de Montréal to explain how Webb uses its Canadian instrument in this process.

“After being powered on Jan. 28, 2022, and undergoing successful aliveness and functional tests, Webb’s Fine Guidance Sensor (FGS) has now successfully performed its very first guiding operation! Together with the Near-Infrared Imager and Slitless Spectrograph (NIRISS), the FGS is one of Canada’s contributions to the mission.

“To ensure Webb stays locked on its celestial targets, the FGS measures the exact position of a guide star in its field of view 16 times per second and sends adjustments to the telescope’s fine steering mirror about three times per second. In addition to its speed, the FGS also needs to be incredibly precise. The degree of precision with which it can detect changes in the pointing to a celestial object is the equivalent of a person in New York City being able to see the eye motion of someone blinking at the Canadian border 500 kilometers (311 miles) away!

“Webb’s 18 primary mirror segments are not yet aligned, so each star appears as 18 duplicate images. On Feb. 13, FGS successfully locked onto and tracked one of these star images for the first time. The FGS team was thrilled to see this ‘closed loop guiding’ working! From now on, most of the alignment process of the telescope mirrors will take place with FGS guiding, while NIRCam images provide the diagnostic information for mirror adjustments.”

–René Doyon, principal investigator for FGS/NIRISS, Université de Montréal; and Nathalie Ouellette, Webb outreach scientist, Université de Montréal

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By Jonathan Gardner, Webb deputy senior project scientist, NASA’s Goddard Space Flight Center

And Alexandra Lockwood, project scientist for Webb science communications, Space Telescope Science Institute

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The James Webb Space Telescope is nearing completion of the first phase of the months-long process of aligning the observatory’s primary mirror using the Near Infrared Camera (NIRCam) instrument.

The team’s challenge was twofold: confirm that NIRCam was ready to collect light from celestial objects, and then identify starlight from the same star in each of the 18 primary mirror segments. The result is an image mosaic of 18 randomly organized dots of starlight, the product of Webb’s unaligned mirror segments all reflecting light from the same star back at Webb’s secondary mirror and into NIRCam’s detectors.

What looks like a simple image of blurry starlight now becomes the foundation to align and focus the telescope in order for Webb to deliver unprecedented views of the universe this summer. Over the next month or so, the team will gradually adjust the mirror segments until the 18 images become a single star.

“The entire Webb team is ecstatic at how well the first steps of taking images and aligning the telescope are proceeding. We were so happy to see that light makes its way into NIRCam,” said Marcia Rieke, principal investigator for the NIRCam instrument and regents professor of astronomy, University of Arizona.

During the image capturing process that began Feb. 2, Webb was repointed to 156 different positions around the predicted location of the star and generated 1,560 images using NIRCam’s 10 detectors, amounting to 54 gigabytes of raw data. The entire process lasted nearly 25 hours, but notedly the observatory was able to locate the target star in each of its mirror segments within the first six hours and 16 exposures. These images were then stitched together to produce a single, large mosaic that captures the signature of each primary mirror segment in one frame. The images shown here are only a center portion of that larger mosaic, a huge image with over 2 billion pixels.

“This initial search covered an area about the size of the full Moon because the segment dots could potentially have been that spread out on the sky,” said Marshall Perrin, deputy telescope scientist for Webb and astronomer at the Space Telescope Science Institute. “Taking so much data right on the first day required all of Webb’s science operations and data processing systems here on Earth working smoothly with the observatory in space right from the start. And we found light from all 18 segments very near the center early in that search! This is a great starting point for mirror alignment.”

Lee Feinberg, Webb optical telescope element manager at NASA’s Goddard Space Flight Center, explains the early stages of the mirror alignment process. 

Each unique dot visible in the image mosaic is the same star as imaged by each of Webb’s 18 primary mirror segments, a treasure trove of detail that optics experts and engineers will use to align the entire telescope. This activity determined the post-deployment alignment positions of every mirror segment, which is the critical first step in bringing the entire observatory into a functional alignment for scientific operations.

NIRCam is the observatory’s wavefront sensor and a key imager. It was intentionally selected to be used for Webb’s initial alignment steps because it has a wide field of view and the unique capability to safely operate at higher temperatures than the other instruments. It is also packed with customized components that were designed to specifically aid in the process. NIRCam will be used throughout nearly the entire alignment of the telescope’s mirrors. It is, however, important to note that NIRCam is operating far above its ideal temperature while capturing these initial engineering images, and visual artifacts can be seen in the mosaic. The impact of these artifacts will lessen significantly as Webb draws closer to its ideal cryogenic operating temperatures.

“Launching Webb to space was of course an exciting event, but for scientists and optical engineers, this is a pinnacle moment, when light from a star is successfully making its way through the system down onto a detector,” said Michael McElwain, Webb observatory project scientist, NASA’s Goddard Space Flight Center.

Moving forward, Webb’s images will only become clearer, more detail-laden, and more intricate as its other three instruments arrive at their intended cryogenic operating temperatures and begin capturing data. The first scientific images are expected to be delivered to the world in the summer. Though this is a big moment, confirming that Webb is a functional telescope, there is much ahead to be done in the coming months to prepare the observatory for full scientific operations using all four of its instruments.

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By Thaddeus Cesari, Webb science writer, NASA’s Goddard Space Flight Center, Greenbelt, Md.

While we have started the long process of aligning the telescope mirrors, almost all of the components on Webb’s cold side are still continuing to cool.

Webb’s giant sunshield keeps the telescope and cameras out of both direct sunlight and sunlight that is reflected from Earth and the Moon. Everything on the cold side of the sunshield is passively cooling, radiating heat into deep space. That will continue until the telescope and the three near-infrared (NIR) instruments reach a steady-state temperature, where the milliwatts of energy that get through the sunshield, plus heat generated by the instruments’ own electronics, exactly balances the loss of heat into space. We expect that the primary mirror will cool to below 50 kelvins (about -370 degrees Fahrenheit, or -223 degrees Celsius), and the NIR instruments will reach about 40 kelvins (about -388 degrees Fahrenheit, or -233 degrees Celsius).

Webb’s Mid-Infrared Instrument (MIRI) needs to be even colder. In addition to passive cooling, MIRI will be cooled by a closed-cycle gaseous-helium cryocooler, or refrigerator, down to a temperature below 7 kelvins (-447 degrees Fahrenheit, or -266 kdegrees Celsius). Unlike some previous cryogenic missions, which were cooled by boiling off liquid helium and venting it into space, MIRI’s cooler reuses its helium, just like the refrigerator in your kitchen continuously recycles its own coolant. The Webb team turned on the first stage of the MIRI cryocooler this week.

In the several weeks since Webb’s sunshield was deployed, Webb’s mirrors have been cooling, but they aren’t at their final temperatures yet. There is a variation of temperatures across the different segments, and the segments closer to the sunshield and spacecraft bus are warmer. We expect that these mirror segments will all cool by another 10 kelvins or so, but their final temperatures will still have a spread of 15 to 20 kelvins. The secondary mirror, hanging out on the end of its “spider” support structure, is already very cold.

Meanwhile, the NIR instruments are also cooling. Early in the cooldown process, the Webb team used heaters to keep the instruments warmer than the cold-side structures, to prevent water ice from forming on the optical surfaces. But that is all done now, and the instruments and their detectors are cooling nicely. Their current temperatures are about 75 kelvins (-325 degrees Fahrenheit, or -198 degrees Celsius); they will continue to cool for a few more weeks before reaching their final operating temperature.

The cooling of an infrared telescope is a precise and critical process to ensure the success of the instrumentation and, ultimately, the amazing science. We have learned from and improved upon many years of infrared missions. Webb’s historian, Robert W. Smith, explains a bit more about how Webb builds on the legacy of previous infrared observatories:

“Pioneering investigators examined various astronomical objects in the infrared from the year 1800 on. Infrared astronomy, however, began to take off only in the 1960s. Given the limitations imposed by the atmosphere, researchers experimented with telescopes on balloons and rockets.

“Nevertheless, the grand prize was an infrared telescope in space not limited to the five or so minutes observing time of a rocket flight. Efforts in the U.S., the Netherlands, and the United Kingdom led to the Infrared Astronomy Satellite (IRAS). Launched in 1983, IRAS surveyed the skies at a range of wavelengths and, during its ten-month lifetime, identified 350,000 infrared sources. The Infrared Space Observatory (ISO) followed IRAS in 1995. It became the first infrared space telescope to exploit arrays of detectors of the sort that had begun to revolutionize ground-based infrared astronomy in the years around 1990.

“Critical for the future of infrared space telescopes was the radical shift to radiative or passive cooling. The mirrors of infrared telescopes emit infrared radiation, and to observe the infrared signals emitted by astronomical sources, many of which are exceedingly faint, the mirrors need to be kept very cold. Both IRAS and ISO had kept their telescopes cold by placing them inside a Dewar filled with liquid helium. But adopting this approach seriously limited the size of the telescope that could fly. Tim Hawarden of the Royal Observatory, Edinburgh, began in the early 1980s to explore the idea of doing away with the Dewar. Instead, a telescope would be launched warm and cooled by radiating heat away to space.

“The first infrared space telescope to use passive cooling was NASA’s Spitzer Space Telescope, launched in 2003 into an Earth-trailing orbit. The primary mirror cooled passively to about 34 kelvins before liquid helium was used to get the observatory to less than 6 kelvins. The Herschel Space Observatory, an ESA (European Space Agency) project, had a passively cooled primary mirror (to 80 kelvins) with liquid helium cooled instruments. Herschel operated from 2009 to 2013 and orbited around the L2 Lagrange point, similar to Webb. Herschel’s 3.5-meter diameter mirror made it the largest infrared telescope before Webb.

“In 1989, at a workshop at the Space Telescope Science Institute, astronomers explored ideas for the ‘Next-Generation U-V-Visible-IR Telescope’ to succeed Hubble. These discussions led to the suggestion of an infrared optimized telescope, the ‘Next Generation Space Telescope,’ the vision of which was realized in the world’s largest and most powerful infrared observatory: Webb.”

-Robert W. Smith, professor of history, University of Alberta

Check back tomorrow for an exciting update about progress in the first weeks of Webb’s mirror alignment!

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By Jonathan Gardner, Webb deputy senior project scientist, NASA’s Goddard Space Flight Center

And Alexandra Lockwood, project scientist for Webb science communications, Space Telescope Science Institute

This week, the three-month process of aligning the telescope began – and over the last day, Webb team members saw the first photons of starlight that traveled through the entire telescope and were detected by the Near Infrared Camera (NIRCam) instrument. This milestone marks the first of many steps to capture images that are at first unfocused and use them to slowly fine-tune the telescope. This is the very beginning of the process, but so far the initial results match expectations and simulations.

A team of engineers and scientists from Ball Aerospace, Space Telescope Science Institute, and NASA’s Goddard Space Flight Center will now use data taken with NIRCam to progressively align the telescope. The team developed and demonstrated the algorithms using a 1/6th scale model telescope testbed. They have simulated and rehearsed the process many times and are now ready to do this with Webb. The process will take place in seven phases over the next three months, culminating in a fully aligned telescope ready for instrument commissioning. The images taken by Webb during this period will not be “pretty” images like the new views of the universe Webb will unveil later this summer. They strictly serve the purpose of preparing the telescope for science.

To work together as a single mirror, the telescope’s 18 primary mirror segments need to match each other to a fraction of a wavelength of light – approximately 50 nanometers. To put this in perspective, if the Webb primary mirror were the size of the United States, each segment would be the size of Texas, and the team would need to line the height of those Texas-sized segments up with each other to an accuracy of about 1.5 inches.

Scott Acton and Chanda Walker of Ball Aerospace, along with Lee Feinberg of NASA Goddard, walk through the basic steps below:

“With deployment of the mirror segments now complete, and the instruments turned on, the team has begun the numerous steps required to prepare and calibrate the telescope to do its job. The telescope commissioning process will take much longer than previous space telescopes because Webb’s primary mirror consists of 18 individual mirror segments that need to work together as a single high-precision optical surface. The steps in the commissioning process include:

1. 1. 1. Segment Image Identification
      2. Segment Alignment
      3. Image Stacking
      4. Coarse Phasing
      5. Fine Phasing
      6. Telescope Alignment Over Instrument Fields of View
      7. Iterate Alignment for Final Correction

1. Segment Image Identification

First, we need to align the telescope relative to the spacecraft. The spacecraft is capable of making extremely precise pointing moves, using “star trackers.” Think of star trackers as a GPS for spacecraft. At first, the position of the spacecraft from the star trackers does not match the position of each of the mirror segments.

We are pointing the telescope at a bright, isolated star (HD 84406) to capture a series of images that are then stitched together to form a picture of that part of the sky. But remember, we don’t have just one mirror looking at this star; we have 18 mirrors, each of which is initially tilted towards a different part of the sky. As a result, we’ll actually capture 18 slightly shifted copies of the star – each one out of focus and uniquely distorted. We refer to these initial star-copies as “segment images.” In fact, depending on the starting positions of the mirrors, it may take multiple iterations to locate all 18 segments in one image.

One by one, we will move the 18 mirror segments to determine which segment creates which segment image. After matching the mirror segments to their respective images, we can tilt the mirrors to bring all the images near a common point for further analysis. We call this arrangement an “image array.”

2. Segment Alignment

After we have the image array, we can perform Segment Alignment, which corrects most of the large positioning errors of the mirror segments.

We begin by defocusing the segment images by moving the secondary mirror slightly. Mathematical analysis, called Phase Retrieval, is applied to the defocused images to determine the precise positioning errors of the segments. Adjustments of the segments then result in 18 well-corrected “telescopes.” However, the segments still don’t work together as a single mirror.

3. Image Stacking

To put all of the light in a single place, each segment image must be stacked on top of one another. In the Image Stacking step, we move the individual segment images so that they fall precisely at the center of the field to produce one unified image. This process prepares the telescope for Coarse Phasing.

The stacking is performed sequentially in three groups (A-segments, B-segments, and C-segments).

4. Coarse Phasing

Although Image Stacking puts all the light in one place on the detector, the segments are still acting as 18 small telescopes rather than one big one. The segments need to be lined up with each other with an accuracy smaller than the wavelength of the light.

Conducted three times during the commissioning process, Coarse Phasing measures and corrects the vertical displacement (piston difference) of the mirror segments. Using a technology known as Dispersed Fringe Sensing, we use NIRCam to capture light spectra from 20 separate pairings of mirror segments. The spectrum will resemble a barber pole pattern with a slope (or angle) determined by the piston difference of the two segments in the pairing.

5. Fine Phasing

Fine Phasing is also conducted three times, directly after each round of Coarse Phasing, and then routinely throughout Webb’s lifespan. These operations measure and correct the remaining alignment errors using the same defocusing method applied during Segment Alignment. However, instead of using the secondary mirror, we use special optical elements inside the science instrument which introduce varying amounts of defocus for each image (-8, -4, +4, and +8 waves of defocus).

6. Telescope Alignment Over Instrument Fields of View

After Fine Phasing, the telescope will be well aligned at one place in the NIRCam field of view. Now we need to extend the alignment to the rest of the instruments.

In this phase of the commissioning process, we make measurements at multiple locations, or field points, across each of the science instruments, as shown below. More variation in intensity indicates larger errors at that field point. An algorithm calculates the final corrections needed to achieve a well-aligned telescope across all science instruments.

7. Iterate Alignment for Final Correction

After applying the Field of View correction, the key thing left to address is the removal of any small, residual positioning errors in the primary mirror segments. We measure and make corrections using the Fine Phasing process. We will do a final check of the image quality across each of the science instruments; once this is verified, the wavefront sensing and controls process will be complete.

As we go through the seven steps, we may find that we need to iterate earlier steps as well. The process is flexible and modular to allow for iteration. After roughly three months of aligning the telescope, we will be ready to proceed to commissioning the instruments.”

—Scott Acton, Webb lead wavefront sensing and control scientist, Ball Aerospace; Chanda Walker, Webb wavefront sensing and control scientist, Ball Aerospace; and Lee Feinberg, Webb optical telescope element manager, NASA’s Goddard Space Flight Center

Following Webb’s arrival at its orbital destination around Lagrange Point 2 (L2) on Jan. 24, the mission operations team began working its way through a critical series of steps: powering on all the science instruments, turning off heaters to begin a long cooldown process, and ultimately capturing the first photons on Webb’s primary camera to enable a months-long alignment of the telescope.

While the MIRI instrument and some instrument components were powered on in the weeks after Webb’s Dec. 25 launch, the team didn’t finish turning on the remaining three instruments – NIRCam, NIRSpec, and FGS/NIRISS – until the past few days.

The mission operations team’s next major step is to turn off instrument heaters. The heaters were necessary to keep critical optics warm to prevent the risk of water and ice condensation. As the instruments meet pre-defined criteria for overall temperatures, the team is shutting off these heaters to allow the instruments to restart the months-long process of cooling to final temperatures.

When NIRCam reaches 120 kelvins (approximately -244 degrees Fahrenheit, or -153 degrees Celsius), Webb’s optics team will be ready to begin meticulously moving the 18 primary mirror segments to form a single mirror surface. The team has selected the star HD 84406 as its target to begin this process. It will be the first object NIRCam “sees” when photons of light hit the instrument’s powered-on detectors. The process will essentially create an image of 18 random, blurry points of light. For the first few weeks of mirror alignment, the team will keep the instrument trained on the star while they make microscopic adjustments to the mirror segments; ultimately that collection of 18 blurry dots will become a focused image of a single star. Cooling of the telescope and instruments will also continue over the next month, with the near-infrared instruments ultimately reaching 37-39 kelvins. The cryocooler will cool MIRI to 6 kelvins in the following months.

The big news for Webb this week was the final insertion into orbit around the second Lagrange point. The team also turned on the High-Gain Antenna, enabling downlink to Earth through the Deep Space Network using the Ka radio band. The Ka-band provides a much higher data rate than the S-band that Webb has been using for communications up until now. The Ka-band and the High-Gain Antenna will eventually allow the observatory to send all of the science images and data down to the ground for astronomers around the world to analyze and make discoveries.

As we look back on Webb’s first month in space, we asked Bill Ochs, Webb project manager, to share with us his thoughts on the mission so far:

“It has been about a month since launch, and it has been an unbelievable ride. I am so filled with pride for our team. The deployments could not have gone more perfectly. After the last wing deployment, more than one person made comments like, ‘it seemed so simple; did we overstate the complexity and difficulty of the deployments?’ The perfection of the deployment execution and the subsequent activities reflects directly on how hard everyone worked and the diligence and sacrifice it took on the part so many people. The fact that it looked simple is a tribute to all those over the years who have worked towards Webb mission success. We are now ready to align mirrors and commission instruments and have already proven that all the hardware associated with the optics (including 132 actuators) is working beautifully. By the time we get to instrument commissioning, we are going to have one hell of a telescope. Finally, our ops team and ground system have done a fantastic job of executing the commissioning timeline, and all those rehearsals have paid off. As folks on Webb know, I’m a pretty big Jimmy Buffett fan (understatement – I’m a parrot head). Driving around today, the song ‘Book On The Shelf’ came on, and this verse (particularly the third line, and I changed the fifth line) just shouted the JWST team to me.

And I know these stories were sailed way before me
Toss a note in a bottle and hope that it helps
I’m so damn lucky to have an all-star crew
Some stoic, some crazy, some just passin’ through
I know I’m so privileged to work with the best and
I’m all done explaining or passin’ some test
So pour me another, it’s good for my health
I’m not ready to put the book on the shelf

“Thanks to the Webb team for everything you have done and continue to do.”

—Bill Ochs, Webb project manager, NASA’s Goddard Space Flight Center

Next up: HD 84406! That is the first star Webb will point at to gather engineering data to start the mirror alignment process. The team chose a bright star (magnitude 6.7 at a distance of about 260 light-years, as measured by Gaia). The star is a sun-like G star in the Ursa Major constellation, which can be seen by Webb at this time of the year. This is just the first step; HD 84406 will be too bright to study with Webb once the telescope starts to come into focus. But for now, it is the perfect target to begin our search for photons, a search that will lead us to the distant universe.

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By Jonathan Gardner, Webb deputy senior project scientist, NASA’s Goddard Space Flight Center

And Alexandra Lockwood, project scientist for Webb science communications, Space Telescope Science Institute

Today, at 2 p.m. EST, Webb fired its onboard thrusters for nearly five minutes (297 seconds) to complete the final postlaunch course correction to Webb’s trajectory. This mid-course correction burn inserted Webb toward its final orbit around the second Sun-Earth Lagrange point, or L2, nearly 1 million miles away from the Earth.

The final mid-course burn added only about 3.6 miles per hour (1.6 meters per second) – a mere walking pace – to Webb’s speed, which was all that was needed to send it to its preferred “halo” orbit around the L2 point.

“Webb, welcome home!” said NASA Administrator Bill Nelson. “Congratulations to the team for all of their hard work ensuring Webb’s safe arrival at L2 today. We’re one step closer to uncovering the mysteries of the universe. And I can’t wait to see Webb’s first new views of the universe this summer!”

Webb’s orbit will allow it a wide view of the cosmos at any given moment, as well as the opportunity for its telescope optics and scientific instruments to get cold enough to function and perform optimal science. Webb has used as little propellant as possible for course corrections while it travels out to the realm of L2, to leave as much remaining propellant as possible for Webb’s ordinary operations over its lifetime: station-keeping (small adjustments to keep Webb in its desired orbit) and momentum unloading (to counteract the effects of solar radiation pressure on the huge sunshield).

“During the past month, JWST has achieved amazing success and is a tribute to all the folks who spent many years and even decades to ensure mission success,” said Bill Ochs, Webb project manager at NASA’s Goddard Space Flight Center. “We are now on the verge of aligning the mirrors, instrument activation and commissioning, and the start of wondrous and astonishing discoveries.”

Now that Webb’s primary mirror segments and secondary mirror have been deployed from their launch positions, engineers will begin the sophisticated three-month process of aligning the telescope’s optics to nearly nanometer precision.

Watch a special episode of NASA Science Live at 3 p.m. EST today to learn more about what’s next for the James Webb Space Telescope.

On Monday, Jan. 24, engineers plan to instruct NASA’s James Webb Space Telescope to complete a final correction burn that will place it into its desired orbit, nearly 1 million miles away from the Earth at what is called the second Sun-Earth Lagrange point, or “L2” for short.

Mathematically, Lagrange points are solutions to what is called the “restricted three-body problem.” Any two massive, gravitationally significant objects in space generate five specific locations – Lagrange points – where their gravitational forces and the centrifugal force of the motion of a small, third body such as a spacecraft are in equilibrium. Lagrange points are labeled L1 through L5 and are preceded by the names of the two gravitational bodies that generate them (the big one first).

While all Lagrange points are gravitational balance points, not all are completely stable. L1, L2, and L3 are “meta-stable’ locations with saddle-shaped gravity gradients, like a point on the middle of a ridgeline between two slightly higher peaks wherein it is the low, stable point between the two peaks, but it is still a high, unstable point relative to the valleys on either side of the ridge. L4 and L5 are stable in that each location is like a shallow depression or bowl atop the middle of a long, tall ridge or hill.

So why send Webb to orbit Sun-Earth L2? Because it is an ideal location for an infrared observatory. At Sun-Earth L2, the Sun and Earth (and Moon, too) are always on one side of space, allowing Webb to keep its telescope optics and instruments perpetually shaded. This enables them to get cold for infrared sensitivity, yet still access nearly half the sky at any given moment for observations. To view any and every point in the sky over the course of time requires merely waiting a few months to travel farther around the Sun and reveal more of the sky that was previously “behind” the Sun.

Moreover, at L2, Earth is far enough away that the roughly room-temperature heat radiating from it won’t warm up Webb. And because L2 is a location of gravitational equilibrium, it is easy for Webb to maintain an orbit there. Note that it is simpler, easier, and more efficient to orbit around L2 than to dwell precisely at L2. Furthermore, by orbiting rather than being exactly at L2, Webb will never have the Sun eclipsed by Earth, which is necessary for Webb’s thermal stability and for power generation. In fact, Webb’s orbit around L2 is larger in size than the Moon’s orbit around Earth! L2 is also convenient for always maintaining contact with the Mission Operations Center on Earth through the Deep Space Network. Other space-based observatories including WMAP, Herschel, and Planck orbit Sun-Earth L2 for the same reasons.

Generally speaking, getting a spacecraft to Sun-Earth L2 is fairly straightforward, but Webb’s architecture added a wrinkle. Karen Richon, Webb’s Flight Dynamics lead engineer, describes getting Webb to L2 and keeping it there:

“Think about throwing a ball straight up in the air, as hard as you can; it starts out very fast, but slows down as gravity pulls it back towards Earth, eventually stopping at its peak and then returning to the ground. Similar to your arm giving the ball energy to go up a few meters from the Earth’s surface, the Ariane 5 rocket gave Webb energy to go the great distance of 1.1 million kilometers, but not quite enough energy to escape Earth’s gravity. Just like the ball, Webb is slowing down, and, if we allowed it, would eventually stop and fall back towards Earth. Unlike the ball, Webb wouldn’t return to the Earth’s surface, but would be in an extremely elliptical orbit, with a perigee altitude of 300 kilometers and an apogee altitude of 1,300,000 kilometers. Utilizing thrust every three weeks or so from small rocket engines aboard Webb will keep it orbiting L2, looping around it in a halo orbit once every six months.

“So, why did the Ariane not give Webb more energy and why did Webb need course correction? If the Ariane had given Webb even a little bit too much energy than needed to get it to L2, it would be going too fast when it got there and would overshoot its desired science orbit. Webb would have to do a significant braking maneuver by thrusting toward the Sun to slow down. Not only would that big burn cost a lot of propellant, it would be impossible because it would require Webb to turn 180 degrees in order to thrust toward the Sun, which would have exposed its telescope optics and instruments directly to the Sun, thus overheating their structures and literally melting the glue that holds them together. Mounting thrusters on the telescope as a way to direct braking thrust was infeasible for a number of reasons and was never a design option.

“Therefore, Webb requested just enough energy from the Ariane rocket to ensure that we would never have to do a retro burn, but would always require a burn from the observatory to precisely make up the difference and place it in the desired orbit. The Ariane 5 targeted Webb so accurately that our first and most critical burn was smaller than we had to plan and design for, leaving more fuel for an extended mission!”

—Karen Richon, Webb Flight Dynamics lead engineer, NASA’s Goddard Space Flight Center

A detailed breakdown of Webb’s orbit can be found here.

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By Thaddeus Cesari, Webb science writer, NASA’s Goddard Space Flight Center, Greenbelt, Md.

Editor’s Note: This post was updated to clarify the strength vs. stiffness of beryllium.

Last week, the Webb team began moving the observatory’s individual mirror segments out of their launch positions. Today, we hear from Erin Wolf, Webb program manager at Ball Aerospace, about the completion of that process:

“Today, the James Webb Space Telescope team completed the mirror segment deployments. As part of this effort, the motors made over a million revolutions this week, controlled through 20 cryogenic electronics boxes on the telescope. The mirror deployment team incrementally moved all 132 actuators located on the back of the primary mirror segments and secondary mirror. The primary mirror segments were driven 12.5 millimeters away from the telescope structure. Using six motors that deploy each segment approximately half the length of a paper clip, these actuators clear the mirrors from their launch restraints and give each segment enough space to later be adjusted in other directions to the optical starting position for the upcoming wavefront alignment process. The 18 radius of curvature (ROC) actuators were moved from their launch position as well. Even against beryllium’s bending stiffness per weight, which is six times greater than that of ordinary steel, these ROC actuators individually shape the curvature of each mirror segment to set the initial parabolic shape of the primary mirror.

“Next up in the wavefront process, we will be moving mirrors in the micron and nanometer ranges to reach the final optical positions for an aligned telescope. The process of telescope alignment will take approximately three months.”

—Erin Wolf, James Webb Space Telescope Program Manager, Ball Aerospace