The Telescope Allocation Committee: Selecting What Webb Observes Next

This week, astronomers around the world are celebrating the announcement of the next cycle of Webb observations. We asked Christine Chen, associate astronomer and JWST Science Policies Group lead at the Space Telescope Science Institute, to describe the selection process to determine the targets Webb will observe.

“On May 10, the Space Telescope Science Institute (STScI), the science operations center for NASA’s James Webb Space Telescope, announced the scientific program for Cycle 2, the second year of regular operations. This announcement was the culmination of a peer-review process to select the most scientifically compelling programs, which began with the submission of observing and archival proposals on January 27.

“For every year of regular operations, STScI plans to issue a Call for General Observer and Archival proposals from the international astronomical community to solicit ideas for new observations and archival studies to be executed in the upcoming year. Archival proposals request support to analyze already existing observations, develop theoretical models to interpret observations, and/or develop scientific software to facilitate data analysis. For Cycle 2, a record-breaking 1,600 proposals were submitted by more than 5,450 scientists from 52 countries including the United States, ESA (European Space Agency) member states, and Canada. The proposals covered all topics in astronomy and astrophysics from solar system bodies, exoplanets, supernova remnants, and merging neutron stars to nearby and distant galaxies, supermassive black holes at the centers of galaxies, and the large-scale structure of the universe. Together, the submitted proposals requested more than 35,000 hours of telescope time, far exceeding the 5,000 hours of telescope time available to be allocated.

“To select the programs that will be executed, STScI recruits hundreds of members of the international astronomical community to serve on the Telescope Allocation Committee (TAC). Each reviewer is assigned to a topical panel reflecting their scientific expertise. The peer-review process is carried out such that the proposers don’t know who is reviewing the proposals, and the reviewers don’t know who wrote the proposals, a process called Dual-Anonymous Peer Review (DAPR). STScI instituted DAPR in 2016 in support of the Hubble Space Telescope Cycle 26 TAC and has found that DAPR has decreased a previously-seen disparity in proposal selection rate for male and female investigators and has encouraged many more students to apply for telescope time.

“Once the proposals have been submitted, the STScI JWST Science Policies Group sorts the proposals by type and/or size and by scientific category. Very small proposals, are graded asynchronously by external panelists, whereas larger programs are reviewed by discussion panels. Each panel is given an allocation of telescope time, for which it can recommend observing programs.

“Reviewers are asked to grade each proposal based on three criteria: (1) impact within subfield, (2) out-of-field impact, and (3) suitability for the observatory For external panels, proposals are ranked using submitted grades. For discussion panels, proposals are first triaged using submitted grades because there is not enough time to discuss all of the submitted proposals. At the TAC meeting, the discussion panelists review the strengths and weaknesses of all of the proposals that survive triage, and regrade and re-rank the proposals. The highest ranked proposals are recommended for allocation of telescope time and/or funding. For the Large, Treasury, and Legacy Archive proposals, the panel chairs also receive and incorporate expert reviews from the community and from their discussion panels. In addition, reviewers provide feedback for the proposers detailing the perceived strengths and weaknesses.

“For this mission, the STScI director is the allocating official. Therefore, all of the recommendations from the TAC are advisory to the director. Once the director approves the programs, STScI notifies proposers of the outcome for their proposals and begins implementation of the awarded observations. The selected Cycle 2 program that was just announced contains lots of exciting and ground-breaking science. You can learn more about the breath of research areas and questions to be answered with Webb observations by reading the abstracts of the selected programs. Eventually, all of the observations in the approved programs will become publicly available in the archive, enabling additional new discoveries that may not have been foreseen by the original proposers.”

About the Author:
Christine Chen is an associate astronomer in the Science Mission Office at the Space Telescope Science Institute. She leads the JWST Science Policy Group that issues Calls for Proposals to the astronomical community to conduct research using Webb and organizes Dual Anonymous Peer Review of the proposals submitted by the astronomical community in response to these calls.


Mid-Infrared Instrument Operations Update

All 17 observing modes of the James Webb Space Telescope undergo routine performance monitoring and calibration. This month, while performing calibration by comparing the brightness of standard stars that have been well-cataloged by other observatories to what Webb’s Mid-Infrared Instrument (MIRI) was receiving, team members noticed a discrepancy in the data.

Further analysis of MIRI’s Medium Resolution Spectroscopy (MRS) mode revealed that at the longest wavelengths, the throughput, or the amount of light that is ultimately registered by MIRI’s sensors, has decreased since commissioning last year. No effect has been seen for MIRI imaging, and there is no risk to the instrument. All other observation modes – within MIRI and each of Webb’s other scientific instruments – remain unaffected.

NASA and its partners are developing a systematic plan to approach, analyze, and then explore the issue. The Webb team will continue MIRI observations as planned. The team will gather all relevant ground test and flight data to fully assess MRS performance. Further test observations will be taken to completely characterize the nature of the issue using this particular mode of observation. Next, a plan for long term-monitoring will be enacted, while the team continues to investigate the cause, identify risks, and explore mitigations that would potentially improve performance. One possible mitigation strategy includes taking slightly longer exposures at the affected wavelengths to increase the signal to noise.

Webb Shows Areas of New Star Formation and Galactic Evolution

Editor’s Note: This post highlights data from Webb science in progress, which has not yet been through the peer-review process.

On Oct. 11, 2022, NASA’s James Webb Space Telescope spent over 20 hours observing the long-studied Hubble Ultra Deep Field for the first time. The general observer program (GO 1963) focused on analyzing the field in wavelengths between approximately 2 and 4 microns.

A rectangular image that appears to be two separate square images separated by a wide black gap. The gap obscures the galaxies present between the two square images. Each square image contains thousands of galaxies with many different colors. Some galaxies are shades of yellow, while others are white, blue, orange and red. Most of these galaxies appear as fuzzy ovals, but others appear thin and long. A few galaxies with distinct spiral arms are spread throughout.
This image of the Hubble Ultra Deep Field was taken by the Near-Infrared Camera on NASA’s James Webb Space Telescope. The Webb image observes the field at depths comparable to Hubble – revealing galaxies of similar faintness – in just one-tenth as much observing time. It includes 1.8-micron light shown in blue, 2.1-micron light shown in green, 4.3-micron light shown in yellow, 4.6-micron light shown in orange, and 4.8-micron light shown in red (filters F182M, F210M, F430M, F460M, and F480M). Download the full resolution from the Space Telescope Science Institute. Image Credit: NASA, ESA, CSA, Joseph DePasquale (STScI), Christina Williams (NSF’s NOIRLab).

We spoke with Christina Williams (NSF’s NOIRLab), Sandro Tacchella (University of Cambridge), and Michael Maseda (University of Wisconsin-Madison) to learn more about the first observation of the Hubble Ultra Deep Field through Webb’s eyes.

What is important for people to know about these Webb observations?

Michael Maseda: The fact that we see hot, ionized gas is telling us exactly where stars are being born in these galaxies. Now we can separate those areas from where stars already existed. That piece of information is very important because, billions of years later, we don’t exactly know how galaxies became how they are today. It’s important to note that we still haven’t seen everything there is to see. Our whole program was ~24 hours, which isn’t that much time in the grand scheme of how much time other observatories have looked at it. But, even in this relatively short amount of time, we’re starting to put together a new picture of how galaxies are growing at this really interesting point in the history of the Universe.

What are you interested in learning by exploring the Hubble Ultra Deep Field with Webb?

Christina Williams: We proposed to image the Ultra Deep Field using some of Webb’s NIRCam’s medium-band image filters, which allowed us to take images of spectral features more accurately than we could with broadband filters because medium-band filters span a shorter wavelength range. This gives us more sensitivity in measuring colors, which helps us understand the history of star formation and ionization properties of galaxies during the first billion years of the universe, like in the Reionization Era. Measuring the energy that galaxies produced in that time will help us understand how galaxies reionized the universe, reverting it from being neutral gas to once again being an ionized plasma like it was after the big bang.

Sandro Tacchella: One of the key outstanding questions in extragalactic astrophysics is how the first galaxies form. Since the medium bands cover a range of different wavelengths, we can either directly find the some of the first galaxies in the early universe, or we can age-date the stars in galaxies when the universe was about one billion years old to understand when the galaxy actually formed their stars in the past. This survey helps to pin down the formation of the first galaxies.

Michael: The capabilities that we have with Webb’s medium-band filters are actually quite new. We’re getting a sort of hybrid between imaging and spectroscopy, so we’re getting detailed information for basically all of the galaxies in the field, as opposed to traditional spectroscopy where you could only select a few galaxies in the field of view for study. It’s really a complete picture in the sense that this information complements a lot of existing data, not only from Hubble, but ground-based instrumentation like MUSE (the Multi Unit Spectroscopic Explorer) on the Very Large Telescope, where we have spectroscopy at different wavelengths for a number of these objects. MUSE is very good at finding galaxies that have Lyman-alpha emission, or light from ionized hydrogen in these galaxies, which are the type of galaxies that existed when reionization was ending. This new data is a missing piece that we did not have before in terms of understanding the full population of galaxies in this field.

A comparison between two images, one on the left and one on the right separated by a white line. The image on the left has a caption on the bottom that says, “Hubble UDF (exposure time: 11.3 days)”, while the image on the right has a caption on the bottom that says “Webb (exposure time: 0.83 days)”. The image on the left shows thousands of galaxies with many different colors. Most are orange and yellow while others are white and blue. Many of these galaxies appear as fuzzy ovals, but others look thin and long or have distinct spiral arms. Within the image is a yellow box, angled about 30 degrees off center, which contains a label on its bottom right that says “Webb’s field of view”. The image on the right shows a box with the same angle. Inside this box are many of the same galaxies seen in the box on the left in different colors. The orange galaxies on the left image are white or yellow on the right. There are a few galaxies spread throughout the right image that are clearer than those on the left.
The capabilities of NASA’s James Webb Space Telescope’s Near-Infrared Camera are on full display in this comparison between Hubble’s and Webb’s observation of the Hubble Ultra Deep Field. The left, which demonstrates Hubble’s observation with its Wide Field Camera 3, required an exposure time of 11.3 days, while the right only took 0.83 days. Several areas within the Webb image reveal previously invisible, red galaxies. Download the full resolution from the Space Telescope Science Institute. Image Credit: NASA, ESA, CSA, Joseph DePasquale (STScI).

Was there anything unexpected in these data that surprised you?

 Michael: I don’t know if I was surprised exactly, but the images were even better than I was expecting. In these images, you can actually see by eye that this is ionized gas over a fairly large area. I was expecting everything to be unresolved, but we have a high-enough resolution to actually see it. And I’m pleased to see it because it could have been a lot harder to understand what was happening.

Christina: I think that seeing how beautiful the images are and how high quality they ended up being was definitely a high point. We calculated that we would be able to do things like this, but it was different to see it and have the real data in practice.

Why did you elect to make the data immediately public?

Sandro: Galaxies are very complex systems in which a wide range of different processes work on different spatial and temporal scales, so there are many approaches that can be used to better understand the physics of galaxies. So, making it available to many different groups will facilitate the search for more insight.

Christina: Webb is still very new, and people are still learning the best practices of how to analyze data sets. So, it benefits everyone to have a few data sets that are available immediately to help people understand the best way to make use of Webb data moving forward, and to better plan programs in future cycles that are based on real experience with data.

 About the Authors:

      • Christina Williams is an assistant astronomer at the National Optical Infrared Astronomy Research Lab (NOIRLab) in Tucson, Arizona.
      • Sandro Tacchella is an assistant professor of astrophysics at the University of Cambridge in Cambridge, England.
      • Michael Maseda is an assistant professor of astronomy at University of Wisconsin-Madison in Madison, Wisconsin.

How Webb’s Coronagraphs Reveal Exoplanets in the Infrared

The study of exoplanets is a key part of the James Webb Space Telescope’s science goals. We asked Webb’s Deputy Observatory Project Scientist Christopher Stark of NASA’s Goddard Space Flight Center to tell us about one of the ways Webb studies other worlds.

NASA’s James Webb Space Telescope has many different observing modes to study planets orbiting other stars, known as exoplanets. One way in particular is that Webb can directly detect some of these planets. Directly detecting planets around other stars is no easy feat. Even the nearest stars are still so far away that their planets appear to be separated by a fraction of the width of a human hair held at arm’s length. At these tiny angular scales, the planet’s faint light is lost in the glare of its host star when trying to observe it.

Fortunately, Webb has the right tools for the job: the Near-Infrared Camera (NIRCam) and Mid-Infrared Instrument (MIRI) coronagraphic modes. Webb’s coronagraphs block the light from a distant star, while allowing the faint planet light through to reach its sensors. This is not unlike how we use our car’s visor during sunset or sunrise to see the cars in front of us, albeit Webb uses a much fancier “visor.”

Webb NIRCam and MIRI coronagraphic images of the exoplanet HIP 65426 b. The white star symbol marks the location of the star blocked out by the coronagraphs. The exoplanet does not display Webb’s hallmark six-spiked diffraction pattern due to the pupil plane coronagraph masks. Credit: NASA/ESA/CSA, A Carter (UCSC), the ERS 1386 team, and A. Pagan (STScI). Download the full image here.

Along the path light takes through Webb’s optics, there are several important locations called “planes.” The “image plane” is where the distant sky is in focus, including all astrophysical objects. The “pupil plane” allows the surface of the primary mirror to be in focus, which was used to make Webb’s “selfie.” All of Webb’s coronagraphs physically mask out unwanted starlight in both the image and pupil planes to optimize performance. Most of Webb’s image plane masks, resembling opaque spots or bars, remove starlight simply by blocking it in the image. The exception to this are MIRI’s “four-quadrant phase masks,” which shift the phase of the light so it cancels out with itself through a process called “destructive interference.”

However, due to the wave nature of light, the image plane masks can’t completely block the star. So Webb uses additional pupil plane masks, also called Lyot stops, to remove much of the remaining starlight. These pupil plane masks look very different from the hexagonal primary mirror (the telescope “pupil”). As a result, objects imaged with the coronagraphs do not exhibit Webb’s hallmark six-spiked diffraction pattern, as shown in the observations above.

Left: NIRCam’s coronagraphic image plane mask hardware, consisting of two wedge-shaped bars and three round spots (from left to right). Right: MIRI’s four coronagraphic image plane mask hardware, consisting of three phase-shifting four quadrant phase masks and one round spot (from left to right).


Illustration of NIRCam’s pupil plane mask/Lyot stop for the round image plane mask (left) and the bar image plane mask (right). Transmission through the mask is limited to the white regions. Webb’s telescope pupil is shown in gray for comparison. Credit: Mao et al. 2011

Webb’s NIRCam instrument has five coronagraphic masks, each of which can each be configured to observe at different wavelengths ranging from 1.7 to 5 microns. Webb’s MIRI instrument has four coronagraphic masks that operate at fixed wavelengths between 10 and 23 microns. The coronagraphs can observe objects as close as 0.13 arcseconds from the star, and as distant as about 30 arcseconds from the star, which roughly translates to circumstellar distances ranging from a few Astronomical Units (au) to hundreds of au around nearby stars. One AU is equivalent to the distance between the Earth and the Sun.

Despite the masks, Webb’s coronagraphs don’t perfectly remove a star’s light. To remove the last remnants of light, Webb’s astronomers will carefully use a variety of “point spread function (PSF) subtraction methods.” Simply put, this means measuring the pattern of the residual starlight, and then subtracting it from the science image. In the end, what remains is a noisy-looking pattern, which ultimately limits the faintest detectable exoplanet. This limit is expressed in terms of “contrast,” the ratio in brightness between the faintest detectable planet and the star. During commissioning, Webb’s NIRCam and MIRI coronagraphs demonstrated contrasts better than 10-5 and 10-4 at 1 arcsecond separation, respectively.


Left: Example image of residual starlight after suppression with the MIRI F1065C coronagraph. Right: The same image after PSF subtraction removing most of the remaining stellar residuals. The star is located in the center of the image. The black and yellow pattern in the center of the image set the faintest detectable planet in an observation. Credit: Boccaletti et al. (2022)

Webb’s large primary mirror and infrared capabilities mean that its coronagraphs are uniquely suited to study faint objects in the infrared and will complement other instruments currently observing at other wavelengths, including Hubble’s STIS coronagraph and multiple instruments on ground-based observatories. Exoplanet astronomers will mainly use Webb’s coronagraphs to detect giant extrasolar planets that are still warm from being formed, like those shown above, which are the first images of an exoplanet at wavelengths longer than 5 microns. Webb will also excel at imaging dense circumstellar disks of debris generated by the asteroids and comets in these exoplanetary systems, as well as protoplanetary disks in which planets are still forming. Webb’s coronagraphs can even be used for extragalactic astronomy, to study host galaxies that contain bright active galactic nuclei.

Webb’s coronagraphs won’t be able to reveal all the secrets of a planetary system. To image planets like our own around nearby Sun-like stars, we’ll need to observe even closer to the star and be able to detect planets just one ten billionth the brightness of the star. This will require a future mission fully optimized around next-generation coronagraphs. Fortunately, NASA is already looking into it. The agency’s upcoming Nancy Grace Roman Space Telescope will carry a technology demonstration instrument to test next-generation coronagraph technology. And, following the recommendations of the 2020 Astrophysics Decadal Survey, NASA is laying the groundwork for further technology development for a Habitable Worlds Observatory mission concept, a telescope that would be as large as Webb, operating in the same wavelengths as Hubble, but designed to find truly Earth-like exoplanets around other stars and search them for signs of life.  

– Christopher Stark, Webb deputy observatory project scientist, NASA Goddard






Webb Observes a Globular Cluster Sparkling with Separate Stars

A rectangular image oriented horizontally appears to be two separate square images with a wide black gap in between. The two squares do not mirror each other exactly or align perfectly together. It looks instead like they are two parts of a larger image that has been obscured in the middle by black strip. Both squares are filled with blue, white, yellow, and red points of light of different size and brightness, most of which are stars. The larger and brighter stars show Webb’s distinctive diffraction pattern consisting of eight spikes radiating from the center. Both squares show an increase in density of stars toward the central gap. Altogether, the stars appear to form a loose ball-like shape whose core is obscured by the gap.
Image of the globular cluster M92 captured by the James Webb Space Telescope’s NIRCam instrument. This image is composite of four exposures using four different filters: F090W (0.9 microns) is shown in blue; F150W (1.5 microns) in cyan; F277W (2.77 microns) in yellow; and F444W (4.44 microns) in red. The black strip in the center is a chip gap, the result of the separation between NIRCam’s two long-wavelength detectors. The gap covers the dense center of the cluster, which is too bright to capture at the same time as the fainter, less dense outskirts of the cluster. The image is about 5 arcminutes (39 light-years) across. Download the full-resolution image of M92 from the Resource Gallery at the Space Telescope Science Institute. Image credit: NASA, ESA, CSA, A. Pagan (STScI).

Editor’s Note: This post highlights data from Webb science in progress, which has not yet been through the peer-review process.

On June 20, 2022, the James Webb Space Telescope spent just over one hour staring at Messier 92 (M92), a globular cluster 27,000 light-years away in the Milky Way halo. The observation – among the very first science observations undertaken by Webb – is part of Early Release Science (ERS) program 1334, one of 13 ERS programs designed to help astronomers understand how to use Webb and make the most of its scientific capabilities.

We spoke with Matteo Correnti from the Italian Space Agency; Alessandro Savino from the University of California, Berkeley; Roger Cohen from Rutgers University; and Andy Dolphin from Raytheon Technologies to find out more about Webb’s observations of M92 and how the team is using the data to help other astronomers. (Last November, Kristen McQuinn talked with us about her work on the dwarf galaxy WLM, which is also part of this program.)

Tell us about this ERS program. What are you trying to accomplish?

Alessandro Savino: This particular program is focused on resolved stellar populations. These are large groups of stars like M92 that are very nearby – close enough that Webb can single out the individual stars in the system. Scientifically, observations like these are very exciting because it is from our cosmic neighborhood that we learn a lot of the physics of stars and galaxies that we can translate to objects that we see much farther away.

Matteo Correnti: We’re also trying to understand the telescope better. This project has been instrumental for improving the calibration (making sure all of the measurements are as accurate as possible), for improving the data for other astronomers and other similar projects.

Why did you decide to look at M92 in particular?

Savino: Globular clusters like M92 are very important for our understanding of stellar evolution. For decades they have been a primary benchmark for understanding how stars work, how stars evolve. M92 is a classic globular cluster. It’s close by; we understand it relatively well; it’s one of our references in studies of stellar evolution and stellar systems.

Correnti: Another reason M92 is important is because it is one of the oldest globular clusters in the Milky Way, if not the oldest one. We think M92 is between 12 and 13 billion years old. It contains some of the oldest stars that we can find, or at least that we can resolve and characterize well. We can use nearby clusters like this as tracers of the very ancient universe.

Roger Cohen: We also chose M92 because it is very dense: There are a lot of stars packed together very closely. (The center of the cluster is thousands of times denser than the region around the Sun.) Looking at M92 allows us to test how Webb performs in this particular regime, where we need to make measurements of stars that are very close together.

What are the characteristics of a globular cluster that make it useful for studying how stars evolve?

Andy Dolphin: One of the main things is that the bulk of the stars in M92 would have formed at roughly the same time and with roughly the same mix of elements, but with a wide range of masses. So we can get a really good survey of this particular population of stars.

Savino: Also, since the stars all belong to the same object (the same globular cluster, M92), we know they are all about the same distance away from us. That helps us a lot because we know that differences in brightness between the different stars must be intrinsic, instead of just related to how far away they are. It makes the comparison with models much, much easier.

This star cluster has already been studied with the Hubble Space Telescope and other telescopes. What can we see with Webb that we have not seen already?

Cohen: One of the important differences between Webb and Hubble is that Webb operates at longer wavelengths, where very cool, low-mass stars give off most of their light. Webb is well-designed to observe very cool stars. We were actually able to reach down to the lowest mass stars – stars less than 0.1 times the mass of the Sun. This is interesting because this is very close to the boundary where stars stop being stars. (Below this boundary are brown dwarfs, which are so low-mass that they’re not able to ignite hydrogen in their cores.)

Correnti: Webb is also a lot faster. To see the very faint low-mass stars with Hubble, you need hundreds of hours of telescope time. With Webb, it takes just a few hours.

Cohen: These observations weren’t actually designed to push very hard on the limits of the telescope. So it’s very encouraging to see that we were still able to detect such small, faint stars without trying really, really hard.

What’s so interesting about these low-mass stars?

Savino: First of all, they are the most numerous stars in the universe. Second, from a theoretical point of view, they are very interesting because they’ve always been very difficult to observe and characterize. Especially stars less than half the mass of the Sun, where our current understanding of stellar models is a little more uncertain.

Correnti: Studying the light these low-mass stars emit can also help us better constrain the age of the globular cluster. That helps us better understand when different parts of the Milky Way (like the halo, where M92 is located) formed. And that has implications for our understanding of cosmic history.

It looks like there’s big gap in the middle of the image you captured. What is that and why is it there?

Dolphin: This image was made using Webb’s Near-Infrared Camera (NIRCam). NIRCam has two modules, with a “chip gap” between the two. The center of the cluster is extremely crowded, extremely bright. So that would have limited the usefulness of the data from that region. The position of these images overlaps nicely with Hubble data available already.

Square image filled with blue, white, yellow, and red points of light of different size and brightness, most of which are stars. The larger and brighter stars show Webb’s distinctive diffraction pattern consisting of eight spikes radiating from the center. At the lower right is a scale bar labeled 2 light-years. The scale bar is two-ninths the width of the image, and shows that throughout the image, the distance between adjacent stars is a fraction of one light-year. The density of stars and brightness of the image is greatest in the upper left portion of the image where the stars are much closer together, and decreases gradually toward the bottom right where they are farther apart. The number of larger, brighter stars also appears to decrease from the upper left toward the lower right.
Detail of the globular cluster M92 captured by Webb’s NIRCam instrument. This field of view covers the lower left quarter of the right half of the full image. Globular clusters are dense masses of tightly packed stars that all formed around the same time. In M92, there are about 300,000 stars packed into a ball about 100 light-years across. The night sky of a planet in the middle of M92 would shine with thousands of stars that appear thousands of times brighter than those in our own sky. The image shows stars at different distances from the center, which helps astronomers understand the motion of stars in the cluster, and the physics of that motion. Download the image detail of M92 from the Space Telescope Science Institute. Image credit: NASA, ESA, CSA, A. Pagan (STScI).

One of your main goals was to provide tools for other scientists. What are you particularly excited about?

Dolphin: One of the key resources we developed and have made available to the astronomical community is something called the DOLPHOT NIRCam module. This works with an existing piece of software used to automatically detect and measure the brightness of stars and other unresolved objects (things with a star-like appearance). This was developed for cameras on Hubble. Adding this module for NIRCam (as well as one for NIRISS, another of Webb’s instruments) allows astronomers the same analysis procedure they know from Hubble, with the additional benefit of now being able to analyze Hubble and Webb data in a single pass to get combined-telescope star catalogs.

Savino: This is a really big community service component. It’s helpful for everyone. It’s making analysis much easier.

About the Authors:

        • Matteo Correnti is a research fellow at the Space Science Data Center at the Italian Space Agency and National Institute of Astrophysics in Rome, Italy.
        • Alessandro Savino is a postdoc at the University of California, Berkeley.
        • Roger Cohen is a postdoc at Rutgers University in New Brunswick, New Jersey.
        • Andy Dolphin is a technical fellow at Raytheon Technologies in Tucson, Arizona.

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Breaking the Tracking Speed Limit With Webb

In September, the James Webb Space Telescope observed as NASA’s Double Asteroid Redirection Test (DART) intentionally smashed a spacecraft into a small asteroid, in the world’s first-ever in-space test for planetary defense. Today, we hear from Stefanie Milam, Webb’s deputy project scientist for planetary science at NASA’s Goddard Space Flight Center, about how the Webb team worked to capture these one-of-a-kind observations.

To observe NASA’s DART mission impact event, Webb had to exceed its required and tested observatory capabilities. Before launch, astronomers had plans to observe solar system objects with Webb, which meant it was designed to track targets that move with respect to stars and galaxies in the distant universe. With Webb being a large boat-shaped observatory with its large sunshield and prominent mirror standing on top resembling a sailboat, this was considered a challenge and something we wanted to both simulate before launch, as well as test rigorously during commissioning. We had to make sure that we could track these moving objects with the same precision that we have when pointing at fixed targets. Generally, using a special camera called the Fine Guidance Sensor (FGS), Webb locks onto a so-called guide star to stay pointed at its target with great precision. Moving target observations are executed by moving the guide star at the precise rate of the moving object inside the FGS field of view. This ensures the science target remains stationary in the specified science instrument. This of course also means the background field of stars and galaxies drift in the moving target images, not unlike taking a picture of a race car while turning the background spectators into a streaky blur.

Series of Webb NIRCam observations (filter: F070W) from just before DART impact.
at 7:14 p.m. EDT, Sept. 26, through five hours post-impact. The success of these observations was dependent on implementation of faster moving-target tracking rates. Image Credits: Science: NASA, ESA, CSA, Cristina Thomas (Northern Arizona University), Ian Wong (NASA-GSFC); Joseph DePasquale (STScI)

We worked diligently with the FGS team and the observatory flight software engineers to ensure that pre-flight simulations of the observatory demonstrated a tracking capability with a speed limit originally set at the rate of Mars (30 milliarcseconds per second or the width of a full moon in just under 17 hours!). Webb launched with this notional speed limit. But solar system scientists – especially those who study fast-moving small bodies like asteroids, comets, and interstellar objects – really wanted to observe objects that moved faster than Mars. Of course, we could observe these objects when they are further away from Earth and the Sun and therefore moving slower, but that is not always aligned with when they are the most interesting, e.g., when a comet undergoes an outburst.

So, we set out to not only confirm that Webb could track at the pre-flight speed limit during commissioning, but also to show that the observatory is capable of much faster tracking rates. We started out slow with our first asteroid, 6481 Tenzing, at a rate of only 5 milliarcseconds per second (mas/s) (or 18 for other asteroids. All of these tests were successful, and we confirmed that Webb was really good at tracking moving objects at a a rate of up to 67 mas/s (241ʺ/hr).

This was great news for all solar system scientists planning to use Webb!

Our next challenge was upon us when the Guaranteed Time Observation Program to study Didymos during the DART mission impact (program 1245, PI: Cristina Thomas at Northern Arizona University) was being planned. Program coordinator, Tony Roman at the Space Telescope Science Institute wanted to try to track the asteroid with Webb during the impact of the DART spacecraft (at over 100 mas/s or 360ʺ/hr) with Webb. According to Roman, “Tracking at a rate slower than Didymos’ actual motion would have resulted in blurred images. If we could track at Didymos’ actual rate, that would make the most of this unique opportunity.” This was far faster than our fastest rates from commissioning, but the project was convinced we should try this in order to support another NASA mission, in conjunction with simultaneous observations with the Hubble Space Telescope. Tests were set up on the flight simulator to verify the software and hardware could handle these super-fast tracking rates. The system was tweaked several times to optimize the performance, and to ensure that we could track an object moving that fast. Once convinced by the simulator results, we planned some engineering tests on the spacecraft with two observations of near-Earth asteroid 2010 DF1 at rates of motion that were (324 and 396ʺ/hr).  This was the fastest and brightest asteroid in Webb’s field of regard we could test on near the DART impact speed.  If we could successfully track this asteroid, we knew we could track the DART impact event. We were down to the wire with tests being done only two weeks prior to the date of the DART impact into Dimorphos!

Series of observations from Webb’s Near-Infrared Camera (NIRCam) tracking asteroid 2010 DF1 at 90 mas/s (Filter: F322W2). This is 10 images from Sept. 9, with a total exposure duration of 311 seconds. Image credit: Ian Wong (NASA-GSFC).

Observations to monitor the DART impact were prepared and uplinked to the observatory; at the scheduled time, just prior to the impact, the series of observations began executing and were successful. With data spilling into the archive for analysis, Ian Wong of NASA Goddard completed a quick, but thorough, analysis, which verified the super-fast rates were successful! We had demonstrated that we could plan for an event like the DART impact with Webb, and had beautiful results to show. While the analysis of the impact data , we have now confirmed that we can track and observe targets over 100 mas/s (360ʺ/hr) with Webb. However, we will likely not use the high rates routinely. While successful, they were challenging to plan and schedule. Guide stars only stay within the FGS field of view for a short time at those speeds. That means we would have to use multiple guide stars to support a longer observation, and changing guide stars introduces complexity and inefficiency. In the end, the new speed limit set for Webb is now 75 mas/s for future observations, but special permission may be requested for rates up to 100 mas/s.

– Stefanie Milam, Webb’s deputy project scientist for planetary science, NASA Goddard




Webb’s NIRISS Returns to Full Operations

On Jan. 15, NASA’s James Webb Space Telescope’s Near Infrared Imager and Slitless Spectrograph (NIRISS) experienced a communications delay within the science instrument, causing its flight software to time out. Following a full investigation by NASA and Canadian Space Agency (CSA) teams, the cause was determined to likely be a galactic cosmic ray, a form of high-energy radiation from outside our solar system that can sometimes disrupt electrical systems. Encountering cosmic rays is a normal and expected part of operating any spacecraft. This cosmic ray event affected logic in the solid-state circuitry of NIRISS electronics known as the Field Programmable Gate Array. Webb engineers determined that rebooting the instrument would bring it back to full functionality.

After completing the reboot, NIRISS telemetry data demonstrated normal timing, and to fully confirm, the team scheduled a test observation. On Jan. 28, the Webb team sent commands to the instrument to perform the observation, and the results confirmed on Jan. 30 NIRISS is back to full scientific operations.

“NASA and CSA partnered to approach the problem as technically possible, using a detailed consideration of all areas of operation of the instrument. They analyzed all possible methods to safely recover the electronics. When performing the operation, reviews were held at each intermediate step. We are now happy to report that Webb’s NIRISS instrument is back online, and is performing optimally,” said Julie Van Campen, Webb Integrated Science Instrument Module (ISIM) systems engineer at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

Webb Spies Chariklo Ring System With High-Precision Technique

Editor’s Note: This post highlights data from Webb science in progress, which has not yet been through the peer-review process.

In an observational feat of high precision, scientists used a new technique with NASA’s James Webb Space Telescope to capture the shadows of starlight cast by the thin rings of Chariklo. Chariklo is an icy, small body, but the largest of the known Centaur population, located more than 2 billion miles away beyond the orbit of Saturn. Chariklo is only 160 miles (250 kilometers) or ~51 times smaller than Earth in diameter, and its rings orbit at a distance of about 250 miles (400 kilometers) from the center of the body.

We asked members of the science team observing Chariklo to tell us more about this unique system, the occultation technique, and what they learned from their Webb observations.

In 2013, Felipe Braga-Ribas and collaborators, using ground-based telescopes, discovered that Chariklo hosts a system of two thin rings. Such rings had been expected only around large planets such as Jupiter and Neptune. The astronomers had been watching a star as Chariklo passed in front of it, blocking the starlight as they had predicted. Astronomers call this phenomenon an occultation. To their surprise, the star blinked off and on again twice before disappearing behind Chariklo, and double-blinked again after the star reemerged. The blinking was caused by two thin rings – the first rings ever detected around a small solar system object.

Pablo Santos-Sanz, from Instituto de Astrofísica de Andalucía in Granada, Spain, has an approved “Target of Opportunity” program (program 1271) to attempt an occultation observation as part of Webb’s solar system Guaranteed Time Observations (GTO) led by Heidi Hammel from the Association of Universities for Research in Astronomy. By remarkable good luck, we discovered that Chariklo was on track for just such an occultation event in October 2022. This was the first stellar occultation attempted with Webb. A lot of hard work went into identifying and refining the predictions for this unusual event.

On Oct. 18, we used Webb’s Near-Infrared Camera (NIRCam) instrument to closely monitor the star Gaia DR3 6873519665992128512, and watch for the tell-tale dips in brightness indicating an occultation had taken place. The shadows produced by Chariklo’s rings were clearly detected, demonstrating a new way of using Webb to explore solar system objects. The star shadow due to Chariklo itself tracked just out of Webb’s view. This appulse (the technical name for a close pass with no occultation) was exactly as had been predicted after the last Webb course trajectory maneuver.

In the middle of a black background, a glowing white object seems to pulse with each image frame. The object, a star, is mostly round with tiny juts of light pulsing out from its edges. Another glowing white object, Chariklo, travels in toward the center from the 11 o’clock position. Chariklo is smaller and slightly less bright than the star. It crosses diagonally across the image, passing directly in front of its star and moving toward the 5 o’clock position.
This video shows observations taken by NASA’s James Webb Space Telescope of a star (fixed in the center of the video) as Chariklo passes in front of it. The video is composed of 63 individual observations with Webb’s Near-infrared Camera Instrument’s view at 1.5 microns wavelength (F150W) obtained over ~1 hour on Oct. 18. Careful analysis of the star’s brightness reveals that the rings of the Chariklo system were clearly detected. Credit: NASA, ESA, CSA, Nicolás Morales (IAA/CSIC)

The Webb occultation light curve, a graph of an object’s brightness over time, revealed that the observations were successful! The rings were captured exactly as predicted. The occultation light curves will yield interesting new science for Chariklo’s rings. Santos-Sanz explained: “As we delve deeper into the data, we will explore whether we cleanly resolve the two rings. From the shapes of rings’ occultation light curves, we also will explore the rings’ thickness, the sizes and colors of the ring particles, and more. We hope gain insight into why this small body even has rings at all, and perhaps detect new fainter rings.”

The rings are probably composed of small particles of water ice mixed with dark material, debris from an icy body that collided with Chariklo in the past. Chariklo is too small and too far away for even Webb to directly image the rings separated from the main body, so occultations are the only tool to characterize the rings by themselves.

Graphic titled “Centaur Chariklo Occultation Light Curve: NIRCam Filter F150W.” At the top is a diagram showing the change in relative position of a background star with respect to an icy body and its rings. The star appears to move behind the rings at two points along the path. Below the diagram is a graph showing the change in relative brightness of the star between 9:33 a.m. and 9:41 a.m. in Baltimore, Maryland, on October 18, 2022. The diagram and graph are aligned vertically to show the relationship between the relative position of the background star and the object and rings, and the measurements on the graph. The graphic shows that the brightness of the star is constant except when it appears to pass behind the rings, at which point it dips sharply. The graph shows two deep, narrow valleys when the star is partially blocked by the rings. For a full description, download the Text Description PDF.
An occultation light curve from Webb’s Near-infrared Camera (NIRCam) Instrument at 1.5 microns wavelength (F150W) shows the dips in brightness of the star (Gaia DR3 6873519665992128512) as Chariklo’s rings passed in front of it on Oct. 18. As seen in the illustration of the occultation event, the star did not pass behind Chariklo from Webb’s viewpoint, but it did pass behind its rings. Each dip actually corresponds to the shadows of two rings around Chariklo, which are ~4 miles (6-7 kilometers) and ~2 miles (2-4 kilometers) wide, and separated by a gap of 5.5 miles (9 kilometers). The two individual rings are not fully resolved in each dip in this light curve. Image credit: NASA, ESA, CSA, Leah Hustak (STScI). Science: Pablo Santos-Sanz (IAA/CSIC), Nicolás Morales (IAA/CSIC), Bruno Morgado (UFRJ, ON/MCTI, LIneA). Download the full-resolution version from the Space Telescope Science Institute.

Shortly after the occultation, Webb targeted Chariklo again, this time to collect observations of the sunlight reflected by Chariklo and its rings (GTO Program 1272). The spectrum of the system shows three absorption bands of water ice in the Chariklo system. Noemí Pinilla-Alonso, who led Webb’s spectroscopic observations of Chariklo, explained: “Spectra from ground-based telescopes had hinted at this ice (Duffard et al. 2014), but the exquisite quality of the Webb spectrum revealed the clear signature of crystalline ice for the first time.” Dean Hines, the principal investigator of this second GTO program, added: “Because high-energy particles transform ice from crystalline into amorphous states, detection of crystalline ice indicates that the Chariklo system experiences continuous micro-collisions that either expose pristine material or trigger crystallization processes.”

Graphic titled “Centaur Chariklo: Surface Composition; NIRSpec PRISM.” The graphic shows a reflectance spectrum in the form of a graph of the Brightness of Light (relative reflectance) on the vertical y-axis versus Wavelength of Light in microns on the horizontal x-axis.
Webb captured a spectrum with its Near-infrared Spectrograph (NIRSpec) of the Chariklo system on Oct. 31, shortly after the occultation. This spectrum shows clear evidence for crystalline water ice, which was only hinted at by past ground-based observations. Image credit: NASA, ESA, CSA, Leah Hustak (STScI). Science: Noemí Pinilla-Alonso (FSI/UCF), Ian Wong (STScI), Javier Licandro (IAC). Download the full-resolution version from the Space Telescope Science Institute.

Most of the reflected light in the spectrum is from Chariklo itself: Models suggest the observed ring area as seen from Webb during these observations is likely one-fifth the area of the body itself. Webb’s high sensitivity, in combination with detailed models, may permit us to tease out the signature of the ring material distinct from that of Chariklo. Pinilla-Alonso commented that “by observing Chariklo with Webb over several years as the viewing angle of the rings changes, we may be able to isolate the contribution from the rings themselves.”

Our successful Webb occultation light curve and spectroscopic observations of Chariklo open the door to a new means of characterizing small objects in the distant solar system in the coming years. With Webb’s high sensitivity and infrared capability, scientists can use the unique science return offered by occultations, and enhance these measurements with near-contemporaneous spectra. Such tools will be tremendous assets to the scientists studying distant small bodies in our solar system.

About the authors:

        • Heidi B. Hammel is a Webb interdisciplinary scientist leading Webb’s Cycle 1 Guaranteed Time Observations (GTO) of the solar system, including Program 1271 as highlighted here. She is the vice president for science at the Association of Universities for Research in Astronomy (AURA) in Washington, D.C.
        • Dean Hines is an observatory scientist at the Space Telescope Science Institute (STScI) in Baltimore, Maryland and part of Webb’s Mid-infrared Instrument Team. He is the principal investigator for Webb’s Guaranteed Time Observations Program 1272 “Kuiper Belt Science with JWST.”
        • Noemí Pinilla-Alonso is an associate scientist in planetary science at the Florida Space Institute at the University of Central Florida and deputy principal scientist for the Arecibo Observatory. She is leading the science analysis of the Chariklo system’s spectrum obtained by Webb’s Near-infrared Spectrograph.
        • Pablo Santos-Sanz is a planetary scientist at the Instituto de Astrofísica de Andalucía (CSIC) and director of the Sierra Nevada Observatory in Granada, Spain. He is the principal investigator for Webb’s Guaranteed Time Observations Program 1271 “ToO TNOs: Unveiling the Kuiper belt by stellar occultations.”

Near Infrared Imager and Slitless Spectrograph Operations Update

On Sunday, Jan. 15, the James Webb Space Telescope’s Near Infrared Imager and Slitless Spectrograph (NIRISS) experienced a communications delay within the instrument, causing its flight software to time out. The instrument is currently unavailable for science observations while NASA and the Canadian Space Agency (CSA) work together to determine and correct the root cause of the delay. There is no indication of any danger to the hardware, and the observatory and other instruments are all in good health. The affected science observations will be rescheduled.

James Webb Space Telescope Operations Update

The James Webb Space Telescope resumed science operations Dec. 20, after Webb’s instruments intermittently went into safe mode beginning Dec. 7 due to a software fault triggered in the attitude control system, which controls the pointing of the observatory. During a safe mode, the observatory’s nonessential systems are automatically turned off, placing it in a protected state until the problem can be fixed. This event resulted in several pauses to science operations totaling a few days over that time period. Science proceeded otherwise during that time. The Webb team adjusted the commanding system, and science has now fully resumed.

The observatory and instruments are all in good health, and were not in any danger while Webb’s onboard fault management system worked as expected to keep the hardware safe. The team is working to reschedule the affected observations.