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Webb Launch

These are exciting times for the James Webb Space Telescope mission at STScI. An Ariane 5 rocket, provided by the European Space Agency, gave astronomers worldwide a marvelous present. A flawless launch on Christmas morning, followed by two successful Mid-Course Correction (MCC1a and MCC1b) burns, sent Webb toward the L2 Lagrangian point with a nominal expenditure of fuel. This ensures that we have enough on-board fuel to support more than 10 years of science operations.

Several busy days followed launch, culminating in the critical deployment and tensioning of the five-layer sunshield, needed to keep the telescope and instruments cold. At the time of writing, the successful deployment of the secondary mirror had taken place. The deployment of the mirror and alignment of the optical telescope element will follow, which leads into instrument commissioning. These activities are planned to take a total of six months and will culminate in the release of the first public images and the start of the Cycle 1 science program. We cannot wait to see what discoveries Webb has in store for us this summer!

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Hubble Science Spotlight

The Hubble Space Telescope continues to be a distinctly powerful observational machine that enables significant advances across a broad range of astrophysics. For more than three decades, Hubble has yielded unique discoveries, contributed to multi-wavelength investigations, and campaigned to study time-variable phenomena. A few results captured in nearly 1000 refereed papers this year are mentioned here.

Time-Variable Phenomena

A major contribution of Hubble to research has been long- and short-term observations of various astrophysical objects; 31 years of operation have provided unique opportunities for such studies. The refinement of the Hubble constant, a major goal for the observatory, is a prime example benefiting from long-term, consistent, and comparable instrumentation, enabling multiple avenues for measuring the accelerating expansion of the universe, and the precision associated with Hubble observations may be revealing new physics when compared to expectations gleaned from the cosmic microwave background.

A highly visible series of investigations conducted by the OPAL program monitors the gaseous planets in the Solar System. Changes in their atmospheres have been cataloged and analyzed, allowing researchers to precisely trace the evolution of Jupiter’s Red Spot and surrounding winds, examine subtle seasonal variations on Saturn, and study storms on Neptune and Uranus. In the inner Solar System, Hubble’s insight into Martian weather dovetails with missions in situ.

Hubble’s long-term baseline enables unique insights into stellar physics and evolution, such as the 10‑year program to observe brightness variations of Eta Carina, providing evidence for the dynamic interaction of its central stars. The Stingray Nebula has been seen to morphologically change and fade, with a 20‑year baseline of data suggesting variability and outbursts of the central star. Another impressive example of the reliability and precision of Hubble data is the observation of supernovae in lensed cluster fields. In MACS J0138.0–2155, recent re‑analysis of observations of a lensed object seen in 2019 and images found in archival data from 2016 suggested that an additional instance of the object will appear in 2037. This is reminiscent of the earlier study of Frontier Fields MACS J1149.5+2223, in which a supernova, called Refsdal, was discovered and then reappeared later, providing an excellent test of lensing models, laying the ground work for ambitious additional studies.

Campaigns

Hubble has been a perfect platform for Treasury Programs and Observational Campaigns. The Director’s Discretionary program dubbed ULLYSES (Ultraviolet Legacy Library of Young Stars as Essential Standards) was designed specifically to utilize about 1,000 HST orbits to create an ultraviolet spectroscopic library of young high- and low-mass stars in the local universe. The third Data Release, or DR3, of High-Level Science Products (HLSPs) from the program is available now. This release contains 90 new targets and updated data for 137 targets. Updates include new pipeline and calibration improvements, and users are advised to replace older data with the new products available. Several additional companion targets were observed with the STIS long slit toward selected T Tauri stars. New products are also available, such as time-series spectra and drizzled images. In addition, DR3 is making data from the Las Cumbres Observatory Global Telescope (LCOGT) available for that initial subset of ULLYSES T Tauri stars.

Hubble Anomalies Get Quick Attention

The telescope performs remarkably after 31-plus years, but does experience occasional anomalies. While the pointing issues are settling down (HST Update in Newsletter v38 Issue 01), in June 2021, the payload computer in the Science Instrument Command and Data Handling unit (SI C&DH) halted. The deliberate investigation of this issue was a month long, and involved several simulations and tests (for a detailed synopsis see the July 17 press release. After many scenarios were studied and simulated, the observatory was reconfigured to use the other side of the SI C&DH unit. The telescope returned to science July 16, 2021.

In September 2021, a Single Event Upset (SEU) suspended the Cosmic Origins Spectrograph (COS). Considering that these have been experienced before, the recovery was fairly quick, and science operations resumed. In October 2021, the telescope operations stopped due to a timing synchronization issue. None of the instruments were endangered, yet all four detected the problem. As usual, the diagnosis and remedy of any event with HST is thorough and cautious, involving entire HST team. A clever idea was to use NICMOS, dormant since 2008 (before the 2009 servicing mission) to characterize this problem without affecting the active instruments; the instrument was brought to boot mode without any issues after more than a decade in safe mode. This strategy worked, yielding additional data to inform short- and long-term mitigation strategies for this apparently intermittent timing issue. Hubble returned to science on November 7, starting with the Advanced Camera for Surveys (ACS), followed by observations with WFC3. By December 6, the observatory was fully operational with all four instruments.

A Wealth of Data—The MAST Facility

As described in a recent article, MAST introduced a new improved HST data search designed with improved accessibility, including color choices, font sizes, and layout. This new form also supports data searches with a web browser or through a programming interface. Many features are available, including real-time coordinate lookups, lists, type-ahead forms, and API improvements. The user can search for both public and non-public data sets, with the latter accessible by authorized team members. The new form and the older form will co-exist for a while for user convenience.

Numerous new High-Level Science Data Products (HLSP) have been updated, including Hubble surveys of nearby spiral galaxies from PHANGS-HST, which has models for star clusters in those galaxies, numerous TESS data products, exoplanet candidate data, and the ULLYSES Data Release 3 products (described above).

On the technical side, MAST announced an encrypted FTPS-S connection, and users are urged to upgrade to a client that supports those connections for retrieving staged data. Users can retrieve both public and exclusive access via this method. Of course, the server also provides access to other MAST data from missions other than Hubble.

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Roman's Instruments

The Nancy Grace Roman Space Telescope carries two scientific instruments, the Wide Field Imager (WFI) and a coronagraph demonstration instrument.  

The WFI Instrument

The WFI instrument is designed to accomplish the surveys necessary to achieve the scientific goals for the Roman mission as defined in the Astro2010 decadal survey, and refined during the mission formulation phases. These include wide-field, multi-band, near-infrared imaging, and wide-field, slitless spectroscopic, and time-domain surveys. This instrument will address the scientific goals that motivate the Roman mission, namely precision cosmological measurements to elucidate the nature of dark energy via measurements of supernovae at cosmic distances, baryonic acoustic oscillations, weak lensing, large-scale structure, and the census of longer-period and free-floating exoplanets. Simultaneously, these surveys, combined with a robust General Investigator (GI) program, will enable a broad variety of scientific studies.

The WFI consists of a large focal plane covering over 0.28 square degrees—200 times the field of view of Hubble's Wide-Field Camera 3 (WFC3) infrared camera with slightly better sampling (0.11 vs. 0.129 arc seconds/pixel) providing imaging from 0.5–2.3 microns and spectroscopy from 0.8–1.9 microns. This focal plane consists of eighteen 4K × 4K Hawaii-4RG detectors, which also provide the precision-pointing control for the spacecraft. A single element wheel provides eight imaging filters, two dispersive optics (a prism and grism), and a blank for dark calibrations. The filter and spectral passbands are shown in Figure 1. The two dispersive elements provide slitless spectroscopy at low resolution (R ~ 80–180) from 0.8–1.8 microns motivated by the need to classify supernovae found in time-domain searches and at higher resolution (R ~ 500–800). A grism element covering 1.0–1.9 microns is optimized for measuring galaxy redshifts at z ~ 1.

The WFI is expected to provide ~28 magnitude 5σ point-source sensitivity in one hour (26 in the F213 filter). For spectroscopy, one hour should yield 10σ continuum-sensitivity limits of 23 and 21 magnitude for point sources at 1.2 microns, for the prism and grism, respectively.

The focal plane, and especially its detectors, have been the focus of many years of development with final deliveries, testing, and selection of the flight devices recently completed. These detectors, which represent the next generation of devices from Teledyne Imaging Systems, feature 4096 × 4096 10‑micron pixels. They represent the next generation of detectors, which build upon the 1K × 1K device flown on Hubble's WFC3 and the 2K × 2K devices on Webb's Near-Infrared Camera (NIRCam), Near-Infrared Spectrograph (NIRSPEC), and Near-Infrared Imager and Slitless Spectrograph (NIRISS). These devices have excellent quantum efficiency, low noise, and fewer pathologies compared to prior devices, thus reaching the demanding requirements expected for Roman's precise calibration. Considerable effort has been invested in the characterization of these devices by scientists and engineers at NASA's Goddard Space Flight Center, the Institute, IPAC at Caltech, and the Roman Science Investigation teams over the past several years. This is essential to the science goals of precision cosmology, which require a detailed understanding of the behavior and calibration needs of these sensors. Figure 2 shows the focal plane Engineering Test Unit (ETU).

The WFI is more than just its detectors and optical elements. Goddard and Ball Aerospace are tasked with designing, constructing, and testing the WFI collaboratively. Other key components include custom electronic controllers and signal chains for the detectors (ACADIA devices; the next generation beyond the SIDECAR devices used in Hubble's Advanced Camera for Surveys after the 2009 repair mission, Webb, and ESA's Euclid infrared instrument. Also under construction are the structural elements and the necessary cooling systems to maintain the detectors, filters, and spectral elements at their designed cryogenic temperatures. The element wheel itself (a very large component due to the optical-element size needed to achieve Roman's large field of view) has been fabricated. During 2021, the internal calibration system for the WFI was redesigned for simplicity and programmatic needs. This system provides both flat fields for internal calibration and, most importantly, precise calibration of the count rate non-linearity in the detectors via two distinct methods. Lastly, teams are defining the operations, flight software, and ground- and flight-verification and calibration plans.

Key Milestones Passed

The Roman Space Telescope has also recently passed several other key milestones. The telescope and mission-critical design reviews were held successfully in late September following a long sequence of reviews of the various components of the mission over the prior nine months. These included the ground system critical design review in July covering the relevant work at Goddard (Mission Operations Center), STScI (Science Operations Center), and IPAC (Science Support Center), and a detailed review of the calibration plans for WFI in late July. A "Request for Information" was released seeking community input on the possible early definition of a GI science program—which, if carried out, would be done with community involvement.

Formulation Science Working Group and Science Investigation Teams

The Formulation Science Working Group and associated Science Investigation Teams (SITs) have now disbanded in advance of the upcoming selection of new science teams via the NASA ROSES opportunity expected in early 2022. These teams, after six years of dedicated effort, have provided workshops during October (Roman CGI Workshop)  and November (Science with Wide-Field Instrument), documenting their work and providing tools and simulations in support of future studies.

Read more information about the WFI and the Roman Mission at NASA, STScI, and CalTech.

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Summary

NASA Astrophysics missions generally provide funding to the U.S. community to support exploitation of their scientific potential. Different missions use different mechanisms to award grants or contracts to individual programs. For Hubble Space Telescope (HST) programs, investigators construct a budget proposal that describes the work to be undertaken, including associated expenditures such as travel and publications. This provides a means to tailor the funding request to support the work undertaken by U.S. investigators, taking into account who will carry out that work (e.g., student, postdoctoral fellow, senior scientist). Budget proposals are reviewed by a Financial Review Committee (FRC) with members drawn from the user community. The FRC matches the proposed costed effort to the science goals described in the proposal reviewed by the Telescope Allocation Committee (TAC). They assess level of effort and the activities described to determine whether they are reasonable, allowable, and allocable. They flag effort that is outside the scope of the original program and/or carries excessive cost. In most cases, any adjustments are relatively small, but the review can result in significant reductions for individual proposals. The original intention was to use an FRC to determine funding for the James Webb Space Telescope (JWST) Cycle 1 General Observer/Archive Research (GO/AR) programs, but the process hit some turbulence.

The deadline to submit Cycle 1 budgets for accepted proposals was May 20, 2021. At that time, NASA had not finalized the available grant funding. Consequently, the instructions to proposers were to specify the resources needed to achieve the program science goals. The community obliged, submitting 268 proposals requesting total funding of $99.1 million. This proved to be approximately a factor of three higher than the funding level planned initially by NASA.

The Institute and the JWST Project discussed the situation with the JWST Users Committee (JSTUC) at their June 2021 meeting. Budget reductions to meet the proposed funding constraints would be extremely difficult to achieve through the standard FRC process. However, it was clearly important to use the extensive information provided by the community to verify the level of support actually required to achieve the TAC-approved science goals. The JSTUC recommended recruiting a committee of community members to assess a representative subset of the submitted budget proposals for that purpose. There were also concerns about familiarity with the process for non-HST users. Therefore, the committee should also investigate whether there were inequities in the process, particularly those stemming from inexperience with budget preparations for an FRC-style review.

The Cycle 1 Grants Task Force

STScI constituted the JWST Cycle 1 Grants Task Force, comprising 10 community members already recruited for the Cycle 1 FRC together with three representatives from the JSTUC. STScI Grants Administration identified a random subset of 25% of the proposals for review. The proposals were rank-ordered by total budget request, and every fourth proposal selected. The sample includes AR, Theory, GO, and Pure Parallel programs. The Task Force met virtually from July 20–23, 2021, reviewing individual proposals over the first three days and providing general feedback on the final day. A member of NASA's Goddard Space Flight Center's JWST Project Science team attended as an observer for NASA. The Task Force followed the standard FRC process: Each budget proposal had a primary and secondary reviewer who provided independent assessments; the Task Force discussed each proposal to arrive at a consensus assessment; if necessary, individual activities that had excessive costs or were beyond the scope of the original goals were flagged for reduction. The reductions were aggregated as a scale factor for each proposal.

Many proposals assessed by the Task Force were assessed as well matched to the required work. However, other proposals, particularly the large-scale proposals, were judged to require significant reductions (in some cases, a factor of two), usually reflecting duplicative or poorly justified work effort. As a matter of fairness, the Task Force volunteered to review all proposals submitting budget requests in excess of $500,000. Those assessments were completed on August 27 and September 3, 2021. In total, the Task Force reviewed 102 of the 268 Cycle 1 budget proposals (38%).

The primary conclusion reached by the Task Force is that, overall, approximately 70% of the funding requested in the proposals reviewed is justified by work required to achieve the science goals. The Task Force emphasized that funding at less than half this level would disproportionately impact the scientific productivity of U.S. researchers, with science not just deferred, but lost. Partly in response to this recommendation, NASA has increased the Cycle 1 funding allocation substantially to $60 million, approximately twice the funding available in a typical Hubble cycle.

The Task Force considered the questions on equity raised by the JSTUC. They found little evidence that proposers who lacked past experience with HST were disadvantaged significantly by the review process. Almost all proposals at least matched the scope and work efforts required to achieve the science goals. There are similar proposals that request different resources, but those differences reflect different requirements, generally who is doing the work—U.S. versus foreign investigators, senior versus junior scientists. There is no simple correlation between instrument mode and the complexity of the data reduction and analysis. There was strong endorsement of the FRC-style review as the most equitable process in accounting for the unique characteristics of each proposal.

The final report from the Task Force is available online.

Final Budget Adjustments

The Task Force assessments demonstrate that a full FRC review would not reconcile the community requests with the GO/AR funding allocation for Cycle 1; further adjustments are required. Fortunately, the Task Force provided guidance on a basis for those adjustments:

• Preserve foundational funding—cutting all programs equally could render some of the smaller programs infeasible;
• Prioritize support for personnel—travel and publication costs should be capped;
• Eliminate third year funding unless clearly justified—JWST programs are generally expected to require two years of effort; the period of performance is set at three years to allow appropriate phasing of that effort within the longer timeframe.

Those recommendations chart a course to developing a formulaic approach that can determine appropriate adjustments for each proposal.

The Task Force reviewed only a subset of the Cycle 1 budget proposals. Identifying an equitable process to define a budget that serves as the anchor for each program is an essential first step. The reference budgets were set as follows:

• Travel costs were capped at $12,000 for all programs;
• Publication costs were capped at $7,000 for all programs;
• All programs with budget requests in excess of $500,000 were assessed by the Task Force, and their scaling factor is used to determine the reference budget for those programs;
• For smaller programs with Task Force assessments, their scaling is applied if 70% or less (i.e., more than a 30% reduction in the submitted budget); for other programs, the submitted request is taken as the reference budget;
• For smaller programs not reviewed by the Task Force, work extending into a third year was eliminated unless clearly justified; otherwise, the submitted request is adopted as the reference budget.

Combined, the total reference budget is ~$75 million, ~20% above the funding level. The offset is sufficiently large that further cuts must be shared across all programs. We follow a progressive model in applying the necessary cuts (see Fig. 1):

• The initial $70,000 in each program is subject to no adjustments; this sets the foundational funding and limits the impact on the smallest programs.

• The 20 AR programs span a limited range of budget requests; all those programs are scaled to ~80% of the reference value, consistent with the overall funding offset.
• GO programs account for the vast majority of Cycle 1 programs and span a wide range of parameter space. Rather than scaling based on the total budget, we include the funding rate ($/hour, measured against the total charged time in the TAC proposal) as a parameter in scaling the budget. Those values span a wide range, as Figure 1 shows. This is consistent with experience from Hubble programs and reflects the individuality of programs, as emphasized by the Task Force. Programs at or below the average funding rate (~$14,000/hour) were scaled by a single factor; programs with above average rates had proportionately higher scaling rates.
• Pure parallel programs are considered separately from standard GO programs since parallels are implemented on a contingent basis. GO programs will be completed; parallel programs do not have that guarantee. Pure parallels require the availability of suitable scheduling opportunities from prime programs; their implementation cannot impact the prime observations; and, for JWST, they compete for those opportunities with calibration programs. Moreover, significant concerns have emerged regarding the data volume limits that will constrain for JWST observations; those limits will have a significant impact on the availability of parallel opportunities. As a consequence, there is significantly higher risk associated with pure parallel programs and the budgets are scaled accordingly. Even so, the three Cycle 1 pure parallel programs are allocated ~40% of their reference budgets for a total of approximately $1.2 million, 2% of the GO/AR budget. This is a higher proportion than in typical HST cycles.

Figure 2 shows the results of applying the adjustments, plotting the results as fractions of the reference budget for each program: the allocation for 207 of the 268 programs (77%) is at least 80% of the reference budget.

Both the Task Force recommendations and the subsequent adjustment process were discussed with the JSTUC at their October 4, 2021 meeting. The JSTUC endorsed both the recommendations and the process described above.

The JWST Project notified STScI Grants Administration of the Cycle 1 contract value on October 22, 2021, and formal notification letters were sent to Principal Investigators and/or Administrative PIs on November 1. Those letters provide a not-to-exceed value for each program; PIs have full discretion on how they allocate that funding (they are not constrained by caps on travel and publications, for example), albeit with the requirement that the expenditure must be allowable. Revised budgets are due on February 25, 2022.

Looking Forward

Determining the funding allocations for Cycle 1 GO/AR programs triggered a much more complex process than might have been anticipated, and the results evoke a mixed reaction. On the one hand, total funding for U.S. scientists to exploit JWST's potential is close to the recommendations made by the JWST Advisory Committee (JSTAC) in 2015. On the other, this level of funding may not be sufficient to cover the wide range of science investigations made possible by JWST's unparalleled capabilities. This may be the new normal. We should also acknowledge that JWST is an international mission, and funding to support research by foreign scientists generally does not come from JWST-specific sources. NASA anticipates maintaining funding at Cycle 1 levels in future cycles, so this should provide a more stable reference frame going forward.

From the STScI perspective, there is much that we can do to improve the overall process. Both the Task Force and the community have provided clear feedback in that respect.

• We need to provide better documentation for the community to help them navigate the budget process. That documentation should include examples of best practices, particularly with regard to constructing budget narratives.
• We need to set funding expectations for different proposal categories.
• We need to simplify the budget proposals themselves; for example, travel, and publications might be limited from the beginning and specified as single line items rather than requiring detailed specifications.
• We need to streamline the review process—the Task Force devoted five days to reviewing 100 proposals, and we expect ~300+ proposals in future cycles.

The Institute will work with the JSTUC to ensure smoother sailing as we approach JWST's Cycle 2.

About this Article

WASABI

The Web Application Services and Business Intelligence Branch (WASABI) in the SCOPE Engineering Division is comprised of software engineers and testers who provide software support during the proposal life cycle for the Institute's science missions. Their goal is to design, develop, and maintain web applications, providing services to groups across STScI, NASA, ADS (Astrophysics Data System), and the external science community.

The branch is responsible for multiple web applications, providing services such as investigator and account profile management; the selection of review panelists; managing the time allocation committee (TAC) proceedings; tracking and reporting science proposal submissions and TAC outcomes; the science reviews of programs prior to telescope execution; and grant management.

In addition to developing applications, WASABI supports users and other groups at the Institute. This includes STScI's SSO (Single Sign On), the Data Management System, and additional tasks to help other teams meet the goals of STScI.

Our Web Applications

Proper/MyST

The Proposal Person and My Space Telescope (ProPer/MyST) application is responsible for maintaining information about proposals, people, and institutions, and is the authoritative source for who is an investigator on a proposal.

MyST is the external public interface for profile and account creation and maintenance, and is used by the Astronomer's Proposal Tool (APT) during the Call for Proposals submission time for inclusion of investigator data in proposals.

Other features include a tracking service performed for the Institute (e.g., TAC panelists), an interface for maintaining STScI's current and former scientific staff bibliographies and publications, an interface for the annual science staff evaluations, and support for NASA's Universe of Learning project.

ProPer strives to maintain quality-consistent data, ensuring key information appears accurately, including the correct investigators are on the appropriate program and listed with the right institution. ProPer also provides web services for other STScI applications to use for retrieval of investigator information.

ProPer maintains 25,479 unique user profiles, 2,892 institutions, and 16,369 science program details. It has been used to send more than 14 million email messages to subscribers supporting newsletters, the Call for Proposals, STScI announcements, and more. 

TPS

The TAC Panel Selection (TPS) application is used by the Science Mission Office (SMO) to select people to serve on the TAC and other committees as needed. It defines and populates the TAC, which is comprised of members of the international astronomical community.

TPS helps SMO achieve diversity and expertise goals for each panel, while working within constraints such as who is available and how recently they may have served on a previous review panel.

SPIRIT

The Science Peer Review and Ranking Tool (SPIRIT) supports the peer review of proposals by the TAC. SPIRIT is designed to support the Science Policies Group's (SPG's) goals of maximizing science output while minimizing selection bias. Each year, SPIRIT supports numerous TAC reviews with upwards of 200 to 300 panelists and other users during a given review process.

Panelists use SPIRIT to enter numeric grades and comments on the strengths and weaknesses of the proposals. SPIRIT ensures all proposals remain anonymized as needed, and prevents panelists from evaluating proposals for which they have a conflict of interest.

SPIRIT ranks the proposals in each panel based on the numeric grades. SMO and the TAC use these rankings as their basis for selecting which proposals to recommend to the Director's Office for implementation.

SPARTA

The Submitted Proposal and Reporting Application (SPARTA) is used by SMO and the Observation Planning Branch (OPB). 

SMO uses SPARTA for processing submissions in preparation for TAC proceedings, and for generating a wide variety of reports used throughout the proposal submission and selection processes.

OPB uses SPARTA to automate functions previously handled manually by SMO management, and to generate output products such as scripts for loading the Hubble and Webb front-end databases with accepted program data.

SPAR

The Science Proposal Analysis and Review (SPAR) is used by instrument scientists and contact scientists to review science and calibration proposals to help ensure the health and safety of the telescopes to maximize its science return. To date, SPAR has organized 9,990 program science reviews for both the Hubble and Webb missions.

The instrument scientists are responsible for reviewing all visits flagged by APT as needing review and must complete a review before the program is scheduled to be executed.

The SPAR web application generates numerous reports based on visit review and scheduling status. These reports help the instrument team leads monitor reviews to ensure that all visits are reviewed in a timely manner.

STGMS

The Space Telescope Grant Management System (STGMS) is used by the STScI Grants Administration Office to administer grants and conduct grantee financial and performance reporting, payments, special requests, grant amendments, and closeouts. STGMS is used by the external scientific community of principal investigators (PIs), administrators, and institutions to submit budgets, financial reports, and other reports or requests. 

The Financial Review Committee (FRC) tool manages the budget review and program funding allocation process.

The Funding Model projects future funding needs, and provides the ability to construct and try out various funding scenarios by STScI and GSFC.

Since 1985, STGMS has funded over 13,520 grants for hundreds of millions of dollars.

The WASABI Branch in the SCOPE Engineering Division provides a wide range of tools to groups across STScI, NASA, and the external science community that seamlessly enables science activities. 

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*Complete list of authors: M. Bertch (mbertch[at]stsci.edu), J. Bucklew (jbucklew[at]stsci.edu), A. Framarini (alex[at]stsci.edu), C. Hollinshead (hollinshead[at]stsci.edu), L. Nagel (nagel[at]stsci.edu), D. Paul (dpaul[at]stsci.edu), and J. Richon (richon[at]stsci.edu).

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Introduction

As the astronomical community prepares for the launch of JWST, the Institute has released a suite of analysis tools that integrates astropy software packages and libraries, a new visualization tool (Jdaviz), and Jupyter notebooks. This data-analysis tools ecosystem is intended to help astronomers with the often iterative and interactive workflow involved in converting science calibration pipeline data products into meaningful scientific results. While the software package was designed to analyze JWST data, the tools are flexible enough to support most astronomical data sets. An important strength of the project is that it is accessible to the community via GitHub, creating a collaborative environment where the whole astronomical community can interact to collectively make both the science and software better. Furthermore, the Jupyter environment and associated software ecosystem offers an advantage in terms of accessibility and training for users with different levels of experience and background. While STScI will continue to coordinate the space telescope-specific effort and provide software engineering expertise, it has reached a stage where the community can and should provide bug reports, feature suggestions, and even actual code. This article will highlight the different aspects of the JWST analysis-tool ecosystem and opportunities for users to get involved.

Astropy

The astropy package contains key functionality and common tools needed for performing astronomy and astrophysics analysis with Python, which has deep roots at the Institute. At the turn of the century, STScI first started development of Python-based utilities to substitute existing astronomical data analysis tools (i.e., IRAF, Supermongo, IDL) on a single, modern, free, object-oriented platform. It was in 2011, however, that astropy united the astronomy communities’ efforts to develop a coherent set of Python modules for astronomers. Today, Institute staff provide major contributions to the Astropy Project, including hosting several maintainers for the core astropy package, as well as key maintainers or major developers for the photutils, specutils, astroquery and regions coordinated packages, and the dust_extinction, ginga, gwcs, imexam, and synphot affiliated packages. Most major mission-specific software now depends on astropy packages. As part of the current JWST data analysis tool development efforts, STScI has focused particularly on specutils and photutils by including features both requested by the community and specific to the needs of JWST (e.g., specific readers for nearly all JWST products). In particular, for spectral analysis, specutils now includes many convenience functions to handle and manipulate 1D spectra (e.g., handling world coordinate system, splice and resample, redshift, and template fitting), and also 3D data (or cubes).

Jdaviz

As part of the effort to harness the power of Python and make it more accessible to the astronomical community, STScI has been developing a JWST Data Analysis and Visualization tool (Jdaviz). This tool provides users with a GUI-based capability for visualization and interactive analysis. One of the big advantages to the tool is that it can be accessed and manipulated directly from within a Jupyter notebook, in addition to being used as a standalone desktop application. The tool offers quick visualization of various JWST data products and analysis capabilities, such as model fitting, via plugins that utilize astropy machinery to allow for a seamless transition between code and the tool within the notebook. Jdaviz has been designed specifically to support JWST use cases, but can be used for a wide range of astronomical data. Jdaviz version 2.0 is now available, and includes five specific instances of the tool: Specviz, Specviz2D, Mosviz, Cubeviz, and Imviz. See, for example, Figure 1.

Users can start practicing with these tools on simulated JWST data sets either on their own or with the guidance of over 20 sample Jupyter notebooks. These notebooks implement possible scientific workflows for the various JWST observing modes, including imaging, 1D spectroscopy, integral field unit (IFU) spectroscopy, multi-object spectroscopy (MOS), time-series observations (TSO), and coronagraphy. These notebooks are designed with the community in mind, offering an environment that is both easy to share and reproduce by users at all experience levels. The notebooks integrate recommended science workflows, algorithms specific to JWST, astropy analysis packages, and the Jdaviz visualization tool. They are stored in GitHub, once again allowing for community input and development. 

Training

STScI has embarked in various efforts to train the community on JWST. The JWebbinars are just the latest of these efforts and consist of hands-on webinars to teach the community about tools and methods to analyze JWST data. Like the Master Class, this webinar series is reaching a large fraction of the scientific community and exceeded many expectations. Topics span the characteristics of the JWST data products, the calibration pipeline, analysis techniques, simulations, and example workflows with the visualization tools (Jdaviz). The synergy between the scientists, the technical staff, and the outreach team made the JWebbinars a success. The material was delivered to the community live on a dedicated cloud platform, offline in the forms of presentations and Jupyter notebooks, and as video recordings on YouTube, thus reaching people with different learning needs and schedules. We delivered nine different webinars and we have two more to go before JWST launch. During commissioning, the Early Release Science (ERS) teams will take charge and deliver three to four JWebbinars focusing on their specific science cases.

Summary

With the impending launch of JWST, the Institute has developed an ecosystem to simplify the visualization and analysis of the spectacular data sets that observers are anticipating. The design of the ecosystem, combined with the fact that it is a collaborative community effort, ensures that the tools can be adapted and further developed as we continue to discover what is needed to analyze JWST data post-launch. Ultimately, this infrastructure should serve as a model for future tool development in the astronomical community.

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Summary

The 2021 STScI Symposium, Towards the Comprehensive Characterization of Exoplanets: Science at the Interface of Multiple Measurement Techniques, was held April 19–23, 2021. The symposium was organized in a fully virtual format and had more than 400 registered participants from 43 countries.

BlueJeans was used for the talk platform, Gather.town (Figure 1) was used to host the poster sessions and social interactions, and Slack was used for interaction and questions not answered during the live sessions.

The symposium brought together the exoplanet community to address some of the most outstanding questions in the field, such as:

• How do we combine techniques to have a more complete census of exoplanets?
• Which sets of observed quantities best enable holistic planetary characterization?
• How do the observed architectures of exoplanet systems compare to our solar system?
• How does the atmospheric chemistry of exoplanets correlate with the physical properties and compositions of their host stars?
• How do planets form and evolve?
• What are emerging areas in exoplanet science?

Following the recommendation of the Science Organizing Committee of the 2020 STScI symposium, the abstract selection process was completely anonymous to avoid any potential biases the reviewers might have had. The submissions were processed through a Google form and abstracts were randomly assigned to members of the SOC for reviewing. This resulted in a diverse program with a gender balance that was representative of the submitted abstracts.

The symposium consisted of five days, with six hours each day of talks and poster sessions. There were a total of 63 talks: 5 invited (30 minutes each, including questions), 58 regular (15 minutes each, including questions), and 123 posters.

Each day had a poster session before and after the talks to facilitate participants across multiple time zones. To begin each day's talks, an invited speaker spoke on the topic of that day.

On the first day, the focus was on the current status of combining measurement techniques, including an invited talk from Matthias Nowak of the University of Cambridge about closing the gap between direct and indirect methods. Several talks were given covering various exoplanet detection techniques from transits to radial velocities and imaging to astrometry.

The second day centered on multi-techniques to understand the demographics of exoplanets, including an invited talk from Jessie Christiansen of Caltech/IPAC-NASA Exoplanet Science Institute about building an exoplanet demographics ladder. The focus of demographics was presented in the context of microlensing, astrometry, stellar hosts, and multiplicity.

The third day addressed combining characterization techniques to understand exoplanetary atmospheres, with an invited talk from Laura Kreidberg of the Max Planck Institute for Astronomy about exoplanet atmosphere characterization with multiple observatories. The studies of exoplanet and brown dwarf atmospheres were presented, including new software to analyze these observations and future prospects.

Day four was all about looking toward the future of multi-techniques, with an invited talk from Chris Stark of NASA Goddard Space Flight Center about the future overlap of exoplanet detection and characterization methods. This day focused on innovative techniques to combine various measurement methods, including probabilistic deep learning, vortex fiber nulling, and reflected-starlight observations.

The focus of the final day was future missions and upcoming facilities that will enable comprehensive characterization of exoplanets, with an invited talk from Knicole Colon of NASA Goddard Space Flight Center about the landscape of exoplanet missions in the 2020s and beyond. Expected exoplanetary science outcomes from upcoming facilities, including JWST, were presented, as well as mission concepts, upcoming surveys, and results from the Roman Exoplanet Imaging Data Challenge.

The use of Gather.town for the poster session brought a unique aspect to the virtual setup of the STScI symposium. Gather.town is an online platform that allows you to create an avatar and navigate a virtual environment designed to imitate the spontaneous interactions of an in-person conference. The virtual conference space was set up to resemble the Muller building, with five poster rooms organized by topics scattered throughout the virtual building.

At the end of the symposium, we asked the participants to fill out a survey and received 41 responses. The majority of the participants reported that the video and audio connections were excellent or good, and the schedule being an appropriate length. We also received lots of positive feedback about the use of Gather.town to interact with colleagues.

As the chair, I would like to personally thank all of the members of the SOC, the Event Planning Group, and IT staff at the Institute who made this symposium a huge success.

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Workshop

The 2021 STScI Workshop, “Multi-object Spectroscopy for Statistical Measures of Galaxy Evolution,” was held May 17–20, 2021, and had about 400 registered participants from 28 countries. This was the first time an STScI workshop was organized in a fully virtual format.

The workshop brought together the galaxy evolution community and experts in multi-object spectroscopy (MOS) to discuss some of the most interesting topics in the field, such as:

1. Chemical-dynamical evolution of galaxies
2. Star-formation timescales and environment
3. Winds in galaxies, and IGM-CGM vs. galaxies
4. Modelling spectra and simulations
5. Statistical measures for galaxies

The workshop also featured three "unconference sessions," led by six invited presenters, where participants used hands-on activities to learn about software tools used for spectroscopic data analysis and galaxy evolution studies. These sessions were:

1. Spectral analysis with Astropy and specutils
2. Grism data reduction
3. Statistical measures (SED fitting)

Originally, an on-site workshop was scheduled for two-and-a-half days of about eight hours each. Due to the virtual environment, the workshop organizing committee decided to spread out the meeting to four days of about five hours each day. BlueJeans Events was used as the talk platform and a separate Slack channel for each talk was used for most of the interactions, including questions and answers, discussions, and poster sessions. On average, 225 participants virtually attended through our BlueJeans Event platform, but more than 300 people connected through Slack.

There were a total of 32 talks: 10 invited (25 minutes each, including questions), 22 contributed (15 minutes each), and ~50 poster talks (one minute each). There were more than 100 posters presented during the workshop. Each poster had its dedicated Slack channel, where participants actively joined and chatted with the presenter. During each of the five science sessions (Monday–Thursday), 25 minutes were dedicated to short poster talks and about 40 minutes were dedicated to open discussion sessions. Slack was also used to foster discussion among the participants through different channels, and/or through small group calls. This method of combining two platforms to support talks, posters, questions, and discussions worked extremely well and most of the participants were satisfied, according to our symposium survey. A general Slack channel was available for participants to contact the organizing committee, and a private organizing committee channel helped greatly in coordinating and managing the meeting. These separate Slack channels freed participants from using the BlueJeans chat, ensuring they did not compete with the presentations on the same platform. Additionally, we also organized few "wonder.me" rooms for informal discussions and social gathering.

We received 173 abstracts spread across the five science topics. We could only accommodate four contributed talks per science topic and an additional two contributed talks to accompany the review on future MOS missions. The committee graded and ranked the abstracts following a rubric, which was designed to address the relevance of the topic to the workshop, the clarity of the pitch, and the attractiveness of the result with four levels for each rubric item. Each committee member graded about 65 abstracts to obtain an average of seven grades per abstract. The grading process was blind to the name, gender, and institution of the contributor, which resulted in good gender and career-stage balance for the selected abstracts. Out of the 22 contributed speakers, 10 identified as women and 18 were either students or early-career researchers (less than 10 years past the highest degree). The time slots were then assigned, considering the time zone of the speaker as much as possible.

The first session was an introduction and overview session, beginning with two talks covering a short history of MOS and how MOS has changed our understanding of galaxies through cosmic time. Also included was an overview talk of the James Webb Space Telescope (JWST) Early Release Science (ERS) and Guaranteed Time Observations (GTO) programs focused on galaxy evolution. The session ended with two talks on future MOS facilities, highlighting the infrared capabilities of JWST from space and the Magellan Infrared Multi-Object Spectrograph (MIRMOS) on the ground-based Magellan telescope, as infrared observations are crucial for identifying and putting strong constraints on stellar population properties (e.g., dust, ages, masses) of intermediate- and high-redshift galaxies.

All the invited and contributed talks in the subsequent sessions corresponded to the science topics and presented interesting results about the properties of galaxies and their surrounding medium [1], and how they evolve with time. These studies used powerful ground- and space-based spectroscopic and imaging surveys, and state-of-the-art analysis techniques. The results clearly demonstrated the importance of large, statistical, MOS-enabled samples for gaining better insight into the processes of galaxy formation and evolution. Though many observational results combined some kind of theoretical background, the workshop also had a focused session on simulations and modeling, as those play a key role in interpreting observations and understanding the underlying physics. 

On the last day, the concluding talk gave a synopsis of the workshop by summarizing the current state of the field, and calling attention to an excellent suite of upcoming facilities, which will not only bring significant technical and instrumentation improvement, but will also take a big step forward toward better understanding galaxies and their properties across cosmic time.

The workshop was designed to include hands-on sessions to introduce new and state-of-the-art tools for spectroscopic data analysis and spectral energy distribution (SED) fitting. Three parallel sessions were run on the second and fourth afternoons to allow people to join at least two of the three sessions. The first unconference session covered options for spectral analysis with astropy and specutils. The presenter worked through a Jupyter notebook with the attendees and showed how to load, manipulate, and analyze 1D spectra. The second unconference session covered everything grism. The presenters showed demos of three tools for the analysis of slitless spectra: hstaxe, grizli, and the JWST Calibration Pipeline. The third session (see the dedicated webpage) was designed to reach the users of SED fitting tools on the second afternoon and the developers of SED fitting tools on the fourth afternoon. On the second afternoon, the presenters walked the attendees through demos of two state-of-the-art SED fitting tools, Bagpipes and dense_basis, and moderated a free-form discussion about controversial questions in galaxy spectral fitting. On the fourth afternoon, the presenters discussed sampling methods for spectral fitting and ways to perform full spectral fitting. The free-form discussion covered machine learning for spectral fitting.

All sessions were recorded and shared with the attendees and available online. All the hands-on material was also shared in the form of detailed installation instructions and Jupyter notebooks. This allowed the attendees to keep the material they worked with and use it as a reference for their future analysis.

On the last day of the workshop, we asked the participants to complete a survey and received 62 responses. The majority of participants reported that the video and audio connections were satisfactory, and that the five‑hour daily schedule worked fine. Noteworthily, the participants found that talks, including short poster talks, discussion sessions, and unconference sessions, were the most effective and enjoyable portions of the workshop. They were mostly happy about using Slack as the platform for the discussions and poster presentations.

The chairs would like to thank all the members of the organizing committee, STScI’s Events Planning Group, IT department, Office of Public Outreach, and all the participants who helped us make this workshop a big success!

[1] including chemical, dynamical, environmental, star-formation, dust attenuation, stellar population, gas inflows/outflows, circumgalactic, or intergalactic medium.

About this Article

Status Update

Commissioning of the James Webb Space Telescope continues to be on track. This week, 132 actuators moved all 18 primary mirror segments and the secondary mirror out of their launch restraints and into their nominal positions, ready for subsequent activities to align the JWST optics. The instruments and the telescope continue to cool as JWST arrived at its L2 orbit insertion point and we head toward the completion of the first month of commissioning. In the near future, once the temperatures are right, all the instruments will be turned on and we will begin a three-month-long period to align the primary mirror. Science operations are still five months away, but every week we get closer to that goal.

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Community Events

The launch and commissioning of the James Webb Space Telescope is a monumental achievement that has taken the skill and perseverance of devoted professionals across the globe to accomplish. With this new observatory, we seek to answer questions of our common origin: the beginnings of stars, planets, and galaxies themselves.  STScI's Office of Public Outreach has partnered with NASA to share this achievement with the public and to meet them where they are in their own exploration of the universe. We are working with communities across the country to support celebrations of Webb's launch and first images through the Webb Community Events initiative. We want to empower all people to look up and explore with Webb.

Modeled after the extremely successful science engagement campaign held for the 2017 Solar Eclipse, Webb Community Events are locally organized by partners who best know the needs of their own communities. These partners include libraries, museums, science centers, amateur astronomy clubs, Girl Scout troops, classrooms, national parks, and others. Because each community has different needs, we have worked with partner networks across NASA's Science Activation program to provide a plethora of resources and guides on how to use each. NASA's Universe of Learning, led by STScI's Office of Public Outreach, is just one of many partners in this program, who reach a variety of audiences nationwide and have all contributed enormously to the success of the initiative.

Over 450 Community Events were held to celebrate Webb's launch, with events in all 50 US states and Puerto Rico. Every community host was given access to one-on-one communication with a point of contact on our team, press and social media kits, print and virtual materials, event examples, activity guides, a slide deck written for public audiences about Webb science and launch, multiple trainings on Webb and Webb science, and the opportunity to be matched with an astronomy subject matter expert in their region.

In total, 186 scientists and engineers across North America were successfully matched with over 250 community host sites to bring Webb science to their communities to celebrate launch. The support from the astronomical community in supporting events near them has been critically important. Post-event evaluations have shown that event participants who have an opportunity to directly interact with a scientist report a significant increase in interest in science, as well as an understanding of the science content. Direct outreach efforts from scientists help humanize science for an audience, and can more firmly establish a sense of identity as people who enjoy and follow science. The relationships established between event hosts and local scientists can also serve as springboards to future work in their communities.

Just as launch is only the beginning of Webb's work, it is also only the beginning of community engagement efforts around Webb science. Planning has already begun for Community Events celebrating the end of commissioning and the Early Release Observations, around six months after launch, and to nurture the relationships established in these milestone events into long-lasting partnerships.

Any scientists or engineers who want to be involved in future outreach efforts can get involved by joining NASA’s Universe of Learning as an expert!

About this Article

Department of Physics and Astronomy, The Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland 21218, USA

Editor’s Note: Part of the article below is an abridged and updated version of a paper originally published in a Conference Proceedings: From Giotto to Rosetta: 30 Years of Cometary Science from Space and Ground, eds. Cesare Barbieri and Carlo Giacomo Someda, Galilean Academy of the Sciences, Humanities, and Arts and CISAS University of Padua, held 27–29 October 2016 in Padua, Italy.

1. Introduction

Early discussions about the Space Telescope (the name Hubble was added later), originally scheduled for launch in 1983, featured detailed plans for the observation of comet 1P/Halley in 1986 (Morrison 1979; Brandt 1982), including the spacecraft’s maximum moving target tracking rate that was based on the motion of Halley on the sky at its closest approach to Earth in April 1986. Regrettably, various problems led to multiple postponements and HST was finally launched in April 1990, albeit with a primary mirror defect of spherical aberration, which was later corrected. Nevertheless, the nearly 30 years of Hubble’s far ultraviolet (FUV) spectroscopy of comets provides an enduring legacy of probing cometary composition, which we summarize here.

The original complement of instruments on board Hubble included two with FUV capability, the Goddard High Resolution Spectrograph (GHRS) and the Faint Object Spectrograph (FOS). The latter suffered from the use of visible light sensitive detectors that made it less sensitive in the far-ultraviolet than the GHRS. In 1997, these were replaced by the Space Telescope Imaging Spectrograph (STIS), and in 2009 the Cosmic Origins Spectrograph was installed in HST. We will limit our discussion to the wavelength range 1150–3100 Å covered by the Hubble instruments. Comets observed spectroscopically by HST are listed in Table 1. In the following sections, we will highlight some of the main results by instrument.

2. Faint Object Spectrograph (FOS)

The first comet to be observed by the FOS, in September 1991, was 103P/Hartley, a Jupiter family comet that was to be observed on successive apparitions by later instruments and was the target of NASA’s EPOXI fly-by mission in 2010. The first detection of CO Cameron band (a³Π−X¹Σ⁺) emission was made in this comet (Weaver et al. 1994), and also subsequently observed by FOS in comets C/1991 T2 (Shoemaker-Levy) and C/1996 B2 (Hyakutake), the latter at higher spectral resolution (McPhate et al. 1996). The primary source of CO in the metastable a³Π state is photodissociation of CO₂, so this detection in 103P allowed a means of determining the relative CO₂ coma abundance, which relative to that of water, was found to be comparable to that derived for 1P/Halley from in situ mass spectrometer measurements made during the Giotto encounter in March 1986. However, CO Fourth Positive Group (A¹Π−X¹Σ⁺, 4PG) fluorescence, a measure of the CO abundance in the coma, was not seen, and the derived 3σ upper limit, ∼1%, indicated that CO was significantly depleted relative to Halley. This conclusion was later borne out by more sensitive COS observations of 103P in 2010 (Weaver et al. 2011), which are discussed below.

The close passage of C/1996 B2 (Hyakutake) to Earth in April 1996 (0.10 AU) allowed the FOS detection of S₂, which had only been seen once before, in comet C/1983 H1 (IRAS-Araki-Alcock) with IUE under similar observing geometry (A'Hearn et al. 1983). The detection of this species in 67P/Churyumov-Gerasimenko by the mass spectrometer ROSINA on Rosetta has stimulated renewed interest in its formation process in cometary ice (Mousis et al. 2017). S₂ was subsequently also observed by STIS in comets C/1999 H1 (Lee) and 153P/Ikeya-Zhang, taking advantage of the very high spatial resolution of this instrument.

3. Goddard High Resolution Spectrograph (GHRS)

The close approach of C/Hyakutake also allowed for significant measurements by the GHRS. The H Ⅰ Lyman-α line profile was used to probe the dissociation dynamics of H₂O in the coma (Combi et al. 1998). The D Ⅰ Lyman-α line, shifted 0.3 Å shortward of the H Ⅰ line, one of the initial objectives of the GHRS (Brandt 1982), was not detected, and it remained for STIS, using an echelle mode with a resolving power of ∼110,000, to make this detection in comet C/2001 Q4 (NEAT) in 2004 (Weaver et al. 2004). CO and H₂ fluorescence induced by solar Lyman-α and O Ⅰλ1304 were also detected in C/Hyakutake, primarily because of the high CO abundance in this comet (Wolven & Feldman 1998; Lupu et al. 2007).

4. Space Telescope Imaging Spectrograph (STIS)

STIS is an almost ideal instrument for long-slit spectroscopy of extended sources such as comets. In addition to visible channels using CCD detectors, STIS includes two UV channels, FUV and NUV, together spanning 1150–3100 Å, with photon counting detectors and multiple gratings for a range of spectral resolving powers. With detector pixels of ∼0.025" square, the spatial resolution along the 25" long slit translates to 35 km for a comet at a geocentric distance of 1 AU. The long wavelength end of the NUV spectral range includes the strong (0,0) band of the OH A²Σ⁺−X²Π system centered near 3090 Å, and which is used for a surrogate for the H₂O abundance in the comet (Schleicher & A'Hearn 1988). Models used to determine the H₂O production rate, Q(H₂O) (Festou 1981), can be compared to the observed spatial profiles, often showing asymmetrical outflow of the OH dissociation product. Nevertheless, the determination of the nearly simultaneous H₂O production rate allows for the abundances of the other detected species to be given in terms of abundance relative to H₂O.

The NUV range also allows for the detection of S₂, which because of its short photodissociation lifetime (∼300 s at 1 AU) and relatively low abundance requires the high spatial resolution of STIS to be seen. As noted above, S₂ has been detected in only two of the recent comets observed by STIS, moderately active ones that can be defined as having Q(H₂O) ≥ 10²⁹ molecules s^–1. The same holds for the detection of CO Cameron bands, which despite the usually large relative abundance of CO₂ remain weak features of the coma spectrum because of their high rotational temperature that distributes the photons of a single band over multiple detector pixels. We note also that most comets observed by Hubble are roughly 1 AU from the Sun, due in part to the HST solar exclusion zone of 50º that limits the ability to observe closer to the Sun where the solar fluorescence is higher.

An FUV spectral image of comet 153P/Ikeya-Zhang, obtained on 2002 April 20, is shown in Figure 1 (from Lupu et al. 2007). The geocentric distance of the comet was 0.43 AU so that the image, top to bottom, spans 2,800 km on either side of the nucleus. The brightest spectral features are C Ⅰ λ1561 and C Ⅰλ1657, which are dissociation products and extend fairly uniformly far into the coma. The others are primarily bands of the CO 4PG that peak near the nucleus (the zero-offset position) and then decrease due to the radial outflow of CO from the nucleus. Analysis of the individual bands by Lupu et al. showed that the strongest bands were optically thick near the nucleus due to saturation of the solar pumping radiation, which was then modeled to give a good fit to the observed spectra. These models have been widely used in the interpretation of subsequent observations of this band system, both by COS, and by the Alice far-ultraviolet spectrograph on Rosetta.

5. Cosmic Origins Spectrograph (COS)

COS, installed in Hubble in 2009, is exclusively an ultraviolet spectrograph comprising both FUV and NUV channels. The FUV channel is complementary to that of STIS in that its sensitivity and spectral resolution (in a standard observing mode) are significantly better, but without any spatial resolution that makes STIS valuable for extended source observations. With a 2.5" diameter entrance aperture, spectral resolution of ∼1.0 Å is achieved for an extended source, making it possible to resolve multiplet structure and molecular band structure. The small aperture also favors species originating on or near the nucleus.

As noted above, the first comet observations using COS were made of the Jupiter family comet 103P/Hartley before, during and after the fly-by of this comet by the EPOXI spacecraft in November 2010. The high FUV sensitivity and small field of view permitted several bands of the CO 4PG to be detected, leading to a derived relative CO/H₂O abundance of 0.15%–0.45% (Weaver et al. 2011), consistent with the previous upper limit from 1991 (Weaver et al. 2004), even though the H₂O production rate had decreased by a factor of ∼5 from 1991. The strongest features in the spectrum, however, were partially resolved multiplets of S Ⅰ, centered at 1429 Å and 1479 Å, and multiplets of C Ⅰ at 1561 Å and 1657 Å. These are characteristic of all of the comets observed by COS, and are discussed further below.

Comets C/2009 P1 (Garradd) and C/2014 Q2 (Lovejoy) were significantly more active and more abundant in CO than 103P so that COS was able to detect nineteen individual bands of the CO 4PG system in these comets. As an example, the spectrum of C/Garradd is shown in Figure 2, together with a synthetic CO 4PG spectrum using the model of Lupu et al. (2007). Nearly simultaneous STIS spectra were used to obtain the H₂O production rate from the observed OH emission so that the relative abundance of CO to H₂O, often found to be temporally variable, could be determined. C/Garradd was particularly interesting in this respect as due to favorable viewing geometry, it was observed over a long period of time and was found to exhibit very asymmetric outgassing around perihelion. Following perihelion, while the visual magnitude remained relatively constant, the H₂O production rate declined rapidly with time, while that of CO increased rapidly (Feaga et al. 2014).

Long-slit spectroscopy has demonstrated that all the S I emissions observed in comets are very extended, confirming that they were produced by resonance scattering of S atoms following photodissociation of short-lived sulfur-bearing molecules and not by direct excitation of these same molecules. The only sulfur-bearing molecules detected in the UV are CS (a dissociation product of CS₂) and S₂, and these are not sufficient to produce the amount of atomic sulfur inferred from the S Ⅰ emissions (Meier & A'Hearn 1997). Several other sulfur-bearing molecules were detected through millimeter wave spectroscopy (Bockelée-Morvan et al. 2004), but the full complement of sulfur-bearing molecules was not revealed until mass spectroscopic measurements by the ROSINA instrument on Rosetta (Calmonte et al. 2016). They confirmed earlier results that H₂S was the most abundant of these species in 67P, and it is probable that this is the case for most comets, including long-period comets. The short photodissociation lifetime of H₂S can account for the relatively high atomic sulfur column densities derived from the COS spectra.

Remarkably, two objects on clearly interstellar orbits (i.e., with hyperbolic heliocentric orbital elements) have been discovered during the last several years: 1I/'Oumuamua in 2017 and 2I/Borisov in 2019. Although 'Oumuamua exhibited non-gravitational accelerations usually associated with mass ejection and its reaction force on the nucleus, neither dust nor gas emissions were ever detected (ISSI Team 2019), which suggests that 'Oumuamua was unlike typical comets. In contrast, 2I/Borisov displayed both dust and gas emissions and had other comet-like properties. Hubble COS observations of 2I/Borisov detected CO 4PG emissions during four different sets of observations taken in December 2019 and January 2020 (Bodewits et al. 2020). The estimated CO/H₂O ratio of ~1.7 was unusually large, more than three times larger than the largest value measured at a similar heliocentric distance (2 AU) in solar system comets, which suggests that 2I/Borisov formed outside the CO snow line in an extrasolar planetary system.

6. JWST Investigations of Cometary Composition

HST's FUV spectroscopic investigations are sensitive to atoms and small molecules. The only "parent" species (i.e., those sublimating directly from the nucleus) probed by Hubble are CO and S₂. Moderate resolution ( l/dl ~ 2700) infrared (IR) spectroscopy of comets by Webb can provide direct measurements of the key parent molecules H₂O, CO, and CO₂, which usually are responsible for driving cometary activity. The JWST spectral range also provides access to the fundamental vibrational transitions of many organic species, including those that don’t have allowed radio/mm/sub-mm transitions (e.g., CO₂, CH₄, C₂H₆, C₂H₂,…).

The high sensitivity of Webb can extend IR compositional measurements to larger heliocentric distances than previously possible. For example, JWST monitoring of a single comet starting at ~10 AU pre-perihelion (where both CO and CO₂ are active) and extending to ~10 AU post-perihelion could provide a unique and uniform set of data that could be used to investigate in detail the various processes and conditions that affect cometary outgassing behavior (e.g., sublimation temperature, exposed active area, seasonal effects, etc.). Further details on potential Webb spectroscopic investigations of comets can be found in Kelley et al. (2016) and Milam et al. (2016), with the latter also providing information on the various operational issues associated with JWST observations of moving objects (e.g., targets that require non-sidereal tracking).

With the successful launch of JWST on Christmas Day 2021, we eagerly anticipate the initial spectroscopic observations of a comet and trust that the compositional diversity of comets will be revealed by subsequent observations of multiple comets over Webb's lifetime.

Acknowledgements

The author wishes to thank many students and collaborators, and in particular Hal Weaver, over the past 30~years. Thanks are also due to the operations teams at STScI for the superb planning and execution of difficult observations of moving targets. This work is based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. Financial support was provided by NASA through grants from the Space Telescope Science Institute.

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