About this Article

Published September 17, 2025

STScI recently completed the proposal review and telescope allocation process for Hubble Cycle 33. This was a hybrid review comprised of External panels that were fully asynchronous, Discussion panels that met virtually, and an Executive Committee (EC) that also met virtually this year due to budgetary considerations. As always, we were impressed by the extremely high quality of the proposals, the compelling science — and the dedication, commitment, and integrity of all those involved in the process, both as reviewers and behind the scenes.

Here, we summarize the review process and the results from the Hubble Cycle 33 proposal review and discuss upcoming changes for Cycle 34 and beyond.

Proposal Submissions

The Cycle 33 Call for Proposals was released on December 18, 2024, announcing observing opportunities with Hubble’s current instrumentation — the Advanced Camera for Surveys (ACS), the Cosmic Origins Spectrograph (COS), the Fine Guidance Sensors (FGS), the Space Telescope Imaging Spectrograph (STIS), and the Wide Field Camera 3 (WFC3) — along with opportunities to request funding for theoretical and archival research.

There were a few major items of note:

• Due to actual and anticipated budget cuts:
  • WFC3/IR and ACS/WFC were offered as shared risk.
  • Legacy Archival programs were not offered (though regular Archival programs were offered).
• Following feedback from past HST and JWST reviewers, and the Users Committees, page limits were changed substantially, in many cases halved.
• Due to a decline in submissions in the Large-Scale Structure science area, there was no separate review panel for these proposals. Instead, these were distributed among panels in other science areas. Proposals in this area are still welcomed and encouraged.

A number of special initiatives from previous years were continued to encourage proposals in special-interest areas, including: Ultraviolet (UV) Initiative proposals, Transiting Exoplanet Survey Satellite (TESS) coordinated proposals, Archival Cloud Computation Studies proposals, Calibration proposals, and UV Legacy Library of Young Stars as Essential Standards (ULLYSES) proposals. In addition, the new Rocky Worlds Director’s Discretionary Time (DDT) Program offered opportunities to submit proposals that exploit or have synergy this rich new dataset. We also offered two new initiatives: the Long-Term Monitoring (LTM) Initiative, and the Roman Preparatory Science (RPS) Initiative. We discuss these below.

As in previous cycles, it was possible to request joint time on JWST, Chandra, XMM-Newton, and NOIRLab and NRAO facilities alongside HST observations.

The Phase I deadline was on April 10, 2025. As always, extensions were possible for those with last-minute emergencies, however no extensions requests were received this cycle.

At the deadline, we received a total of 833 proposals. Of these, 33 proposals were disqualified for page limit, formatting, policy, or anonymity violations. This left a total of 800 proposals reviewed:

• 682 General Observer (GO) programs, requesting a total of 17,647 orbits
• 38 Snapshot (SNAP) programs, requesting 1,279 targets
• 80 Archival Research (AR) programs, of which 26 were in the Theory category

These proposals included investigators from 44 U.S. states and territories, and investigators from 48 countries.

This number of submitted proposals was approximately 14% lower than Cycle 32, but the GO orbit request was approximately the same as for Cycle 32 (not including the Multi-Cycle Treasury programs offered last cycle). We have seen the largest reductions in the Exoplanets and Galaxies science areas, which we attribute to the availability of JWST opportunities and proposal fatigue, both in the astronomy community overall and in these science areas specifically. The unchanging orbit request indicates a shift to larger orbit requests per proposal.

Proposal Review

Approximately 40% of the proposals were sent for asynchronous External review by 179 reviewers separated by subject area; 55% of the proposals were reviewed by 86 reviewers in Discussion panels separated by subject area; and the remaining 5% were reviewed by an Executive Committee (EC) of 17 members covering all subject areas. Due to proposal numbers in each science area, the type of review panel varied for different proposal types, as described in the table below.

The External review process and the preliminary phases of the Discussion and EC processes were conducted in May. The virtual Discussion panel meetings took place in June, and the virtual EC meeting took place in July. Due to the asynchronous nature of the review, the External panelists spanned the globe. For the synchronous component, Discussion panelists spanned from Eastern Europe to Hawaii. To accommodate the wide range of time zones, the Discussion panel meetings ran across four six-hour days to allow plenty of time for comprehensive discussions and writing feedback during the reviews.

We continue to strive for impartiality and fairness in the HST review process. Conflicts of interest for each reviewer were identified, including competing proposals, mentorship relationships, family members, and close collaborations. Panelists were not permitted to review, grade, or discuss proposals for which they were conflicted. As with most NASA programs, this was a Dual-Anonymous Review.

A total of 45 proposals were flagged for compliance issues, of which 33 were judged to be severe violations and were disqualified from the review, and 12 were judged to be minor violations and were reviewed but issued warnings for future cycles. The nature of the compliance issues included:

• 23 proposals were found to exceed the new page limits (20 disqualifications, 3 warnings).
• 7 proposals were noncompliant or only in marginal compliance with our Dual-Anonymous Peer Review (DAPR) guidelines (5 disqualifications, 2 warnings).
• 9 proposals were found to violate our formatting requirements and had changed the font, font-size, or margins of the proposal templates (5 disqualifications, 4 warnings).
• 3 proposals were joint programs that received a no-go from a partner observatory for their joint component (3 disqualifications).
• 3 proposals were flagged for other policy issues (3 warnings).

Together, the reviewers provided recommendations to the Director, who approved 151 regular GO, SNAP, and AR proposals totaling 2,731 awarded prime orbits and 1,279 SNAP targets. Programs will start executing at the beginning of Cycle 33 on November 1, 2025. Notifications were sent on July 18, 2025.

Note: Since concluding the TAC process, one approved proposal was withdrawn due to duplications issues. It is included in all stats of reviewed and approved programs but is not listed in the approved proposals list. One joint program disqualified following a no-go from a partner observatory was reinstated but ultimately rejected. It is not included in the stats of reviewed programs below.

Approved Science Programs

Overall, 800 GO, SNAP, and AR proposals were reviewed. Of these, 151 proposals were approved. This includes 130 GO programs for a total of 2,731 orbits (75 Very Small for 520 orbits, 36 Small for 844 orbits, 12 Medium for 659 orbits, 7 Large and Treasury for 708 orbits), 10 Snapshot programs for 1,279 targets, and 11 Archival Research programs (8 Regular and 3 Theory).

The approved programs included 63 UV Initiative programs, 14 Long-Term Monitoring programs, 6 Roman Preparatory Science programs, 1 ULLYSES program, 1 Rocky Worlds program, and 1 Data Science Software program. A number of joint programs were also approved, including 2 joint HST-Chandra programs, 6 joint HST-JWST programs, 2 joint HST-NRAO programs, and 1 joint HST-XMM-Newton program.

In addition, 29 Target of Opportunity (ToO) activations were approved across 11 programs: 8 disruptive activations of 2 to 21 days, and 21 non-disruptive activations of more than 21 days. No ultra-rapid activations of less than 2 days and no Flexible Thursday activations were approved. 5 of the approved ToO programs requested Carry-Over status to Cycle 34.

View a List of the Cycle 33 Approved Programs

The overall success rate for Cycle 33 proposals was 18.9%, compared to 17.4% in Cycle 32 and 16.7% in Cycle 31. PIs from ESA member countries led 23% of the accepted Cycle 33 programs (compared to 30% in Cycle 32 and 17% in Cycle 31). Additional statistics appear in Table 1.

Due to the shared risk of ACS/WFC and WFC3/IR, we saw significant changes in the instrument usage for approved programs compared to Cycle 32. WFC3 remains the most-awarded instrument, with 55% of the allocated time (38% in Cycle 32). A further 17% (15% in Cycle 32) was allocated for COS, 17% (24% in Cycle 32) was allocated for STIS, and the remaining 6% (23% in Cycle 32) was allocated for ACS. Overall, 63% of the allocated observations are for imaging and 37% are for spectroscopy. A further breakdown of observing modes is in Table 2. Overall, 52% of the approved orbits are UV orbits (1,424 of 2,731).

One major shift we have seen since the introduction of Dual Anonymous Peer Review (DAPR) is the increased success of first-time PIs. In Cycle 33, 30% of PIs are first-time PIs (41 of 136 unique PIs). This continues the trend we have seen since the introduction of DAPR (around 30%), and in stark contrast to earlier cycles (around 5%), which did not.

The Cycle 33 proposal/orbit distribution per science category is as follows:

• 9.9% / 13.0% for Exoplanets and Planet Formation (relative to 10.5% / 12.2% submitted)
• 18.5% / 28.0% for Galaxies (22.4% / 26.0% submitted)
• 6.6% / 13.3% for Intergalactic Medium and Circumgalactic Medium (6.5% / 9.8% submitted)
• 9.3% / 3.4% for Solar System (6.8% / 3.7% submitted)
• 26.5% / 14.4% for Stellar Physics (26.8% / 19.3% submitted)
• 14.6% / 18.6% for Stellar Populations (16.4% / 19.4% submitted)
• 14.6% / 9.4% for Supermassive Black Holes (10.8% / 9.6% submitted)

New Initiatives

Two new initiatives were offered in Cycle 33:

The Long-Term Monitoring (LTM) Initiative highlights Hubble’s longevity and importance for time-domain science, encourages proposals that exploit this rich heritage, and allows proposers to submit Future-Cycle programs over 5 cycles instead of the standard 3 cycles. We received 28 LTM proposals, of which 14 were approved, including 1 program that used the new 5-cycle Future-Cycle opportunity. Combined, these proposals represented 3.5% of the submissions and saw an impressive success rate of 50%.

With the upcoming launch of Roman Space Telescope, the Roman Preparatory Science (RPS) Initiative encourages proposals that complement and enhance the scientific impact of Roman observations, or that are essential to achieving critical science goals of future Roman programs. We received 21 RPS proposals, of which 6 were accepted. They represented 2.6% of submitted proposals and saw a better-than-average success rate of 29%.

The strong response and excellent success rate for both of these initiatives demonstrates both their value to the community and to Hubble science, and the relevance and uniqueness of Hubble’s outstanding capabilities in the era of JWST and Roman.

Cycle 34 Dates

We anticipate that the Cycle 34 Call for Proposals will be released in mid-December 2025 and that the Cycle 34 Phase I deadline will be Thursday April 16, 2025. The review panels will take place in late June to early July 2026. Cycle 34 will run November 1, 2026 through October 31, 2027.

Serve on Upcoming TACs

STScI runs two TACs each year, one for HST (typically in the April to July timeframe) and one for JWST (typically in the November to February timeframe). We know that they require a lot of work from the community, and that you have many other commitments and demands on your time. We very much appreciate all who consider our invitations to serve. We cannot run these reviews without your expertise and your support, and we are incredibly grateful for the time and energy committed by all to making these reviews a success.

Anyone interested in serving on a TAC panel, either as a Discussion panelist or an External panelist, may fill out the HST Review Volunteer Form and/or the JWST Review Volunteer Form to be considered. Typically, postdocs with at least two years of experience, and those more senior are eligible, but in exceptional cases, more junior researchers could be considered.

Users Committees

The Hubble Space Telescope Users Committee (STUC) is responsible for advising STScI and the HST Project on all aspects of observatory operations. Membership for the STUC is drawn from the U.S. and ESA communities. Membership lasts for three years, and the committees meet twice a year, typically in the spring and fall, with additional communications as warranted. The next STUC meeting is in mid-November 2025.

We encourage users to reach out to current STUC members with any feedback ahead of the next meeting. Further information, including presentations from past meetings, reports from the committees, current committee members, and contact forms, may be found on the STUC website.

There is an equivalent JWST Users Committee (JSTUC) whose members are drawn from the U.S., ESA, and CSA communities. The next JSTUC meeting is in early December 2025.

Serve on the Space Telescope Users Committee (STUC)

Would you like to serve on the STUC? We welcome nominations for STUC membership at any time by email: stuc-nominations@stsci.edu. Self-nominations are permitted but not required. Submissions are encouraged, but not required, to include a brief cover letter and curriculum vitae summarizing the nominee’s relevant background and HST-related interests. While members must be affiliated with institutions in the U.S. or ESA countries, there is no restriction on citizenship. Nominations are considered on a rolling basis, as several members rotate off after every meeting, approximately every 6 months.

JSTUC nominations may be submitted to jstuc-nominations@stsci.edu. 

Acknowledgements

We are extremely grateful to everyone who was involved in this process at any stage. It takes an enormous amount of time and energy from a huge number of people to see the review through, from start to finish.

First and foremost, we thank the Hubble TAC Chair, Panel Chairs, Panel Vice Chairs, At-Large members, Discussion panelists, and External panelists for their service on the Hubble Cycle 33 TAC.

View the List of the Cycle 33 Panelists 

The overall TAC process was managed by Claus Leitherer and Laura Watkins. The TAC logistics were coordinated by Aleksandra Hamanowicz and Brett Blacker. In addition, numerous STScI staff supported the successful completion of the review process:

Technical Managers: Aleksandra Hamanowicz and Brett Blacker received, organized, and distributed the proposals, oversaw the proposal database, and announced the results. Amber Armstrong provided additional technical support.

Science Policy: Daniel D’Orazio, Amy Jones, Claus Leitherer, Molly Peeples, and Laura Watkins were responsible for panelist selection, assigning proposals for review based on expertise while avoiding conflicts of interest, managing the panels, and coordinating and processing the reviews. Mercedes López-Morales and Neill Reid assisted with policy coordination, and oversight before and during the panel meeting. Andy Fruchter, Becca Levy, Amaya Moro-Martin, Jamila Pegues, and Linda Smith provided additional policy support during the panel meetings.

WASABI Support: Doug Paul, Matej Bludsky, Jeff Bucklew, Aasif Chanandin, Alex Framarini, Craig Hollinshead, Kyla Miller, and Lauretta Nagel

AV Support: Kevin Flinn and Gary Gilbert

Panel Support: Amber Armstrong, Tri Astraatmadja, Macarena Garcia del Valle-Espinosa, Calum Hawcroft, Amy Jones, Aiden Kovacs, Mariarosa Marinelli, Sapna Mishra, Carol Rodriguez, Wilson Joy Skipper, and Christian Soto

Instrument and Telescope Support: Annalisa Calamida, Joleen Carlberg, Marco Chiaberge, Leo Dos Santos, Travis Fischer, Joel Green, Norman Grogin, Svea Hernandez, TalaWanda Monroe, and Marc Rafelski

Planning and Scheduling Support: James Caplinger, William Januszewski, Brigette Hesman, and Bill Workman

ESA Observer: Chris Evans

HSTMO Observers: Helmut Jenkner, John MacKenty, Marc Rafelski, and Julia Roman-Duval

NASA’s Goddard Space Flight Center and HQ Observers: Ken Carpenter, Doris Daou, Andrew Ptak, and Jennifer Wiseman