About this Article

Neill Reid (inr[at)stsci.edu)

Published June 23, 2026

The STScI director may allocate time for discretionary science programs on both Hubble and Webb. Per NASA policy, those allocations are for scientific investigations received outside the normal solicitation and peer review process, and may include the study of unanticipated phenomena and high-risk programs. Most programs are small, focused requests that are submitted by community members in response to unanticipated transient events, such as supernovae, gamma-ray bursts, active comets, and interstellar interlopers. Director’s Discretionary Time (DDT) is also used to obtain images and spectra for anniversaries and other outreach events. On a larger scale, directors also have the option of initiating strategic programs that tackle high-impact scientific issues of broad interest, providing extensive datasets immediately to the community, which can serve as a catalyst for complementary and supplementary observations.

The poster child for strategic DDT programs is the original Hubble Deep Field (HDF), initiated by the then-STScI Director Dr. Bob Williams, implemented in December 1995, and processed and made available to the community in early 1996. As we mark the 30th anniversary of those ground-breaking observations, it is appropriate to consider the procession of other programs that led to the current Rocky Worlds joint DDT Hubble-Webb initiative.

What Is the Rationale for Strategic DDT Programs?

Over the years, strategic DDT programs have focused on key science areas — they are not simply an extra dollop of time for a particular science area. A key requirement is that these programs tackle high-impact projects that would find it exceedingly difficult, if not impossible, to see success through the standard Telescope Allocation Committee (TAC) process. This might be because of the scale of the observations or the scientific risk of achieving the program goals. Committees, including TACs, tend to average out to a risk-averse posture, and generally steer clear of large programs that push the envelope. Directors, on the other hand, can take risks and initiate programs that could never survive the intense competition of highly oversubscribed facilities such as Hubble and Webb. They also assume the risk and responsibility should those programs lack the anticipated impact. These characteristics are exemplified by the original HDF.

Strategic DDT programs should also be catalysts. They should break new ground and serve as force multipliers for the scientific community. They should not only attract attention in their own right, but also stimulate the community to undertake supplementary observing programs with other facilities, both in space and on the ground, and invest in theoretical analyses to exploit the new datasets. Ideally, these programs lay the foundations for innovative investigations with future facilities.

Strategic DDTs therefore support forefront community science by complementing, rather than supplementing, the annual TAC review process.

The Path to a Strategic DDT Program

Directors initiate strategic DDT programs, but those programs are developed through extensive consultation with the wider astronomical community. There is no unique path. Sometimes the topic emerges from community solicitations, sometimes the process starts with informal consultation with STScI research staff, whose interests span a wide range of science topics and regularly interact with the community. However, once the director has identified a theme, a formal working group of community scientists, U.S. and international, is usually established to flesh out the science questions and provide more specific advice. The working group is charged with soliciting wider input from the broader community and developing a conceptual science program that is likely to deliver high-impact results. The group compiles a public report for the director to summarize their recommendations. The director makes the final decision on whether the program is approved for implementation.

The working group report does not specify the detailed observing plan; that responsibility lies with STScI. Once the concept has been selected and approved, an implementation team of STScI staff prepares the APT files and works to schedule the observations. They are also responsible for producing higher-level data products and making them readily available to the community through the Barbara A. Mikulski Archive for Space Telescopes (MAST). DDT program observations do not have any proprietary time. The STScI team lead(s) continue to consult with the working group to ensure that the implementation remains true to the prime science goals outlined in their report, and additional input is often sought from the community at large. All data and higher-level science products are made available immediately to the community, who may submit archival proposals via the annual TAC to support their own analyses.

To maintain a level playing field, STScI staff members on the implementation team may not use their privileged access for personal benefit. They may not use data, data products, or newly produced software tools for their own research until those data and tools are available to the community. They are also restricted from leading archival research programs and obtaining grant funding during at least the initial year of the strategic program.

A Roll Call of Strategic DDT Programs

Hubble’s strategic DDT programs have accumulated over the past three decades, whether stand-alone or in combination with General Observer (GO) programs with Hubble and other facilities, and span a wide range of science topics, with Webb making contributions in recent years.

The Hubble Deep Field (1995 to 1996)

The iconic Hubble Deep Field (HDF) incorporates 150 orbits of multi-band imaging from the Wide Field and Planetary Camera 2 (WFPC2) of a high galactic latitude field in Hubble’s northern continuous viewing zone. Prompted by initial post-Servicing Mission 1 (SM1) imaging that hinted at complex structure in high-redshift galaxies (where high redshift was z ~1.2), the HDF was endorsed by a community advisory committee (even if some eminent scientists had misgivings over devoting 10 days integration to an apparently blank region of sky). The observations revealed complex, dynamical interactions, revolutionizing our understanding of galaxy formation and assembly in the early universe. STScI followed up the observing initiative with a special call for archival proposals, funding 14 (of 42) proposals, including a search for Kuiper Belt Objects and coordinated analyses with deep ground-based radio and infrared observations. The HDF served as the core of the northern field of the later Great Observatories Origins Deep Survey (GOODS) GO program.

The Hubble Deep Field South (1998)

The Hubble Deep Field South (HDF-S) doubled the areal coverage at Hubble’s depth and resolution, adding a field in the southern continuous viewing zone accessible to ground-based observatories south of the equator. The core program matched the HDF and was supplemented by 27 orbits of shallower WFPC2 imaging in flanking fields. (Only 8 orbits were devoted to this purpose for the HDF.) In addition, the HDF-S program included parallel imaging with the Near Infrared Camera and Multi-Object Spectrometer (NICMOS), and imaging and spectroscopy with the Space Telescope Imaging Spectrograph (STIS). The STIS observations centered on a z ~2.24 quasar, enabling spectroscopic observations that probe gas in galaxy halos along the line of sight.

The Hubble Ultra Deep Field, Great Observatories Origins Deep Survey (2003 to 2004)

Hubble’s Advanced Camera for Surveys (ACS) was installed as part of Servicing Mission 3B (SM3B) in March 2002, providing a larger-format detector with higher sensitivity at optical and far-red wavelengths than WFPC2. The concept of extending the HDF and HDF-S emerged through a community workshop in late 2002, and Director Steven Beckwith decided to invest over 400 orbits of multi-band imaging in the Hubble Ultra Deep Field (HUDF).

This high-latitude field built on existing deep X-ray observations, covering part of the Chandra Deep Field South (CDF-S) in Fornax, itself a > 1 megasecond DDT program initiated by Chandra’s director, Riccardo Giacconi. This field has the advantage of being accessible to both northern and southern ground-based facilities. The subsequent Great Observatories Origins Deep Survey (GOODS) GO program expanded coverage, eventually combining Hubble, Spitzer, Chandra, XMM, and Herschel data centered on the HUDF as GOODS South, and on the HDF as GOODS North. Following Servicing Mission 4 (SM4), the Wide Field Camera 3 (WFC3) infrared channel was used in a General Observer program to add deep near-infrared imaging as the HUDF09. In 2012, more than 2 million seconds were combined to produce the Hubble eXtreme Deep Field (XDF).

WFC3 Early Release Science Program’s Focus on Star Formation (2009)

The WFC3 Early Release Science (ERS) program has a distinct origins story. WFC3, installed on Hubble during SM4, was a facility instrument that was constructed by NASA’s Goddard Space Flight Center with STScI support. Scientific advice was provided by a Science Oversight Committee (SOC) made up of 21 scientists drawn from the community. Typically, when a new instrument was installed in Hubble, the Principal Investigator and their team were awarded guaranteed observing time. However, WFC3 carried no guaranteed time.

In recognition of the service of WFC3 SOC members, most of whom helped support WFC3 from its inception in 1999 through SM4 in 2009, Director Matt Mountain allocated 214 orbits of DDT with the stipulation that it should be used for a coherent set of observing. The SOC designed a two-part program focused on star formation near and far, with half the time devoted to studying galaxy assembly at 1 < z < 2 through multi-band imaging and slitless spectroscopy covering part of the GOODS South. The second part of the program targeted star-forming regions in nearby galaxies, including the Large Magellanic Cloud, M82, and M83.

Hubble Multi-Cycle Treasury Programs: CANDELS, CLASH and PHAT (2010 to 2013), CLUTCH and STELa (2024 to 2027)

The Multi-Cycle Treasury (MCT) programs present another way DDT observations have been used strategically to enhance Hubble’s scientific impact. In the run up to SM4, STScI consulted with the community on the scientific basis and appetite for very large programs ( > 500 orbits) that could tackle key questions that could not be addressed through the standard annual TAC process. Those programs would be supported partially by DDT. The community response was positive, a separate call for proposals was issued, and 39 proposals were received.

Following peer review, Director Matt Mountain selected three programs: the Panchromatic Hubble Andromeda Treasury (PHAT), providing multi-band imaging covering approximately one third of M31; the Cluster Lensing And Supernova survey with Hubble (CLASH), imaging 25 massive galaxy clusters to probe dark matter distribution; and the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS), a multi-tiered imaging survey covering five fields, including GOODS North and South, and probing galaxy evolution at cosmic dawn ( < 1 Gyr after the big bang) and cosmic high noon (2 to 4 Gyrs). Both CANDELS and CLASH included time-series components, identifying distant supernovae to probe cosmic expansion at high redshifts. DDT contributed one third of the ~2,500 orbits devoted to these programs.

The community had another opportunity to submit Hubble MCT proposals in 2023. Director Nancy Levenson made up to 750 orbits of DDT available in conjunction with up to 750 GO orbits, with the time distributed through Cycles 32 to 34. The MCT proposals were reviewed alongside Large and Treasury programs, and the Cycle 32 TAC recommended two programs: CLUTCH: The COSMOS Legacy UV-Optical Treasury Campaign with Hubble and STELa: Survey of Transiting Exoplanets in Lyman-Alpha. Both programs are currently well underway.

Frontier Fields (2013 to 2017)

The Frontier Fields program is a direct descendant of CLASH. In 2012, Director Matt Mountain chartered the HST Deep Fields Working Group to explore initiatives that would extend our knowledge of the cosmic frontier at high redshifts through the collection of data that only Hubble was capable of obtaining at that time. After extensive deliberations, the working group recommended a program of optical and near-infrared deep imaging of massive galaxy clusters, where gravitational lensing would enhance coverage of the high-redshift universe. The observations were obtained using WFC3 and ACS, simultaneously covering a field centered on the cluster and a parallel field. Applying lensing to this purpose was somewhat controversial, and STScI commissioned lens models from multiple community research groups to better establish confidence in the analyses. As implemented, Frontier Fields covers six clusters with a total of 840 orbits executed in Cycles 21 through 23. Spitzer contributed 1,000 hours of DDT for infrared imaging, while the clusters were also targeted for contemporaneous observations by X-ray (Chandra, XMM), ground-based optical (Gemini, Keck) and radio (ALMA, NRAO, SCUBA) facilities. The scientific legacy has been significant, with numerous programs on Hubble, Webb, and other facilities using cluster lensing to probe star formation in the high-redshift universe.

The Outer Planet Atmospheres Legacy (OPAL) Program (2014 to Present)

OPAL is a precursor of the long-term monitoring initiative currently in place for Hubble and Webb GO programs. Hubble provides unparalleled resolution at visible wavelengths and unique access to the ultraviolet (UV). Its observations map the evolving conditions in the atmospheres of the giant planets in the solar system. Following representation from the outer planets community, Director Matt Mountain approved ~40 orbits per cycle for an annual monitoring program. Subsequent STScI directors have continued the initiative. Initially, the targets included Jupiter and the ice giants Uranus and Neptune. Saturn was added in 2018 following the completion of the Cassini mission. The data and higher-level science products are available through MAST.

Supporting New Horizons, Identifying Arrokoth (2014)

Like the MCT programs, but on a smaller scale, the search for Arrokoth was a combined GO and DDT program. The New Horizons spacecraft’s prime goal was a Pluto flyby, which it achieved on July 14, 2015, producing spectacular imaging of the planet’s surface and the moon Charon. Once past Pluto, the mission’s secondary goal was to fly by another Kuiper Belt Object, but such objects are extremely faint and difficult to find in the crowded Sagittarius fields. Hubble’s high sensitivity, resolution, and lower background were crucial to success, and Director Matt Mountain provided a 60-orbit contribution to the 194-orbit search program using the WFC3’s UV and visible (UVIS) channel. Three candidate targets were identified, and New Horizon’s trajectory was adjusted to encounter the most favorable, a ~20-by-40 kilometer object dubbed Arrokoth. New Horizon’s close encounter on January 1, 2019 provided striking images.

ULLYSES: The Ultraviolet Legacy Library of Young Stars as Essential Standards (2020 to 2023)

To date, ULLYSES represents the single largest investment of time in a strategic DDT program. Following discussions with STScI staff, Director Ken Sembach commissioned a community working group to develop a UV legacy program focused on stars and star formation. The working group recommended constructing a reference library of UV stellar spectra spanning a broad mass range. The final program includes COS and STIS spectra of massive stars in the Milky Way and nearby galaxies, including the Small Magellanic Cloud, Large Magellanic Cloud, and low-metallicity dwarf galaxies such as Sextans A. The program also obtained spectra of 71 K and M dwarfs, including time-series observations of T Tauri stars. As with other strategic DDT programs, the Hubble observations are enhanced through complementary observations with other facilities, including photometric monitoring with Las Cumbres Observatory.

Rocky Worlds (2024 to 2027)

Rocky Worlds is the first explicit multi-mission DDT program, with complementary contributions from Hubble and Webb identified at the outset. In 2023, Director Nancy Levenson constituted two community working groups. The first was asked to provide recommendations for strategic exoplanet initiatives with Hubble and Webb. Their report highlighted the key opportunity to use 15-micron photometry with Webb to probe the potential presence of atmospheres around rocky worlds orbiting in the habitable zone of M dwarfs. Ultraviolet spectra with Hubble provide contemporaneous insight into the high-energy environment around these active stars, and their potential to disrupt primordial exoplanet atmospheres. A total of 500 hours on Webb and 250 orbits on Hubble will be devoted to these observations. The program is currently underway, with Webb having taken observations of the first of nine targets straddling the cosmic shoreline, the region of parameter space that marks the balance between gravitational ability to retain atmospheres and external excitation.

A High-z Transient Program with Webb

The second working group constituted by Director Levenson was chartered to provide guidance on optimal strategies for maximizing the scientific return from Hubble and Webb time-domain observations. As part of the process, the working group was asked to consider a potential DDT program that could take advantage of Webb’s unparalleled infrared sensitivity to probe transient phenomena in the early universe. The long-term monitoring working group presented a number of options without converging on a consensus program. Director Jennifer Lotz has since constituted a follow-up working group to examine the feasibility of the various options and determine if there is a program offering quantifiable scientific gains. The feasibility working group’s deliberations are currently in progress, with recommendations scheduled to be submitted later this year.

The Science Impact of DDT Programs

STScI directors have worked with the user community to apply their discretionary time in a variety of ways to push scientific boundaries and enable consequential investigations. The table lists the number of refereed publications associated with each program by MAST as of May 12, 2026, but that statistic provides only part of the story. For example, the Arrokoth program cross-references to only three publications, but the New Horizons Extended Mission would not have been possible without those Hubble observations. OPAL is broadening our understanding of multi-year variations in gas giant atmospheres and demonstrating the continued value of near-Earth observations as a complement to in-situ probes.

The cumulative impact of the deep field programs from HDF through Frontier Fields is difficult to overestimate. The suite of HST observations not only sparked contemporary observations with other facilities (Chandra, XMM, Spitzer, Herschel, Keck) of those same fields but also prompted complementary GO programs with Hubble (GOODS, HUDF09, XDF, CANDELS) and, later, follow-up observations with cutting-edge facilities such as Webb (JADES, PRIMER, UNCOVER), ESO-VLT (MUSE-HUDF), and ALMA (ALMA-CRISTAL, ASPECS). CLASH and the Frontier Fields revolutionized our understanding of how strong gravitational lensing could be used to probe the far universe, allowing Hubble to sample redshifts z > 8 prior to Webb’s launch. PHAT mapped the stellar populations in Andromeda’s disk, prompting similar observations of M33 and supplementary coverage of M31, and pointing the way toward Roman’s large-scale surveys. The ULLYSES spectra represent a reference library of UV stellar properties that is already being applied in many areas, including enhancing stellar population models of star-forming high-redshift galaxies.

Rocky Worlds has the potential to achieve similar impacts. The overall sample is relatively small, but each target requires a substantial investment of telescope time, particularly the cooler exoplanets that have the highest chance of retaining an atmosphere. The ensemble of results from this program, characterizing host stars and either detecting or excluding atmospheres, will be crucial in guiding how we should marshal future resources in the search for habitable exoplanets.

With the prospect of Webb operating for more than 15 more years and Hubble operating into the 2030s, we anticipate future opportunities to respond to key scientific questions and challenges that arise from the astronomical community.