Amaya Moro-Martín,, Brett Blacker,, and the Science Policy Group Team (SPG)

Twenty-six years and counting. One more year of outstanding operations has left us with an observatory in excellent condition for the next cycle—ready to deliver that impressive science we have come to expect from Hubble. This greatness stems from a diverse users’ community, with 6,623 unique investigators to date (2,295 from ESA countries). The peer-review proposal selection process plays a fundamental role in establishing a merit-based science program, and that is only possible thanks to the work and integrity of all the Time Allocation Committee (TAC) review panel members, and the external reviewers. With our sincere gratitude to all who participated in this important community service, and to all of the proposers for keeping Hubble’s scientific demand very high, we present the highlights of Cycle 24 selection process.

We received 1,094 proposals by the phase I deadline in April, including 103 in Archival Research and Archival Legacy categories, and 64 in the Theory category, requesting a total of 25,611 orbits. These proposals included investigators from 45 U.S. states (and the District of Columbia), and investigators from 44 countries. The international members of the proposal review panels and the TAC met in June to provide recommendations to the Director, who approved 245 proposals totaling 3,760 awarded orbits, which will start executing at the beginning of Cycle 24 in October. As it was for Cycle 23, up to 200 orbits will be available for Mid-Cycle GO programs targeting recently discovered, non-transient objects. These proposals may be submitted anytime between August 15, 2016 and January 31, 2017.

The Call for Proposals

The Cycle 24 Call for Proposals (CP) was released on January 13, 2016, 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). New opportunities included Webb Preparatory proposals (with a default proprietary period of zero months) to complement and enhance the scientific impact of future Webb observations. In this CP, the Institute also encouraged the community to apply for very large Treasury programs, requesting at least 350 orbits, with an orbit allocation to be shared between the Cycle 24 and Cycle 25 GO Large/Treasury allocations. Medium Proposals continued as a separate category for programs requesting between 35 and 74 orbits, to improve the success rate of programs in this historically challenging orbit range. As in previous cycles, as part of the Hubble proposal it was possible to request time on Chandra, Spitzer, XMM-Newton, and on National Optical Astronomy Observatory and National Radio Astronomy Observatory facilities.

The CP also announced opportunities to request funding for theoretical and archival research. To support the latter, it advertised the release of the Hubble Source Catalog, combining tens of thousands of single visit-based WFC3, ACS, and WFPC2 source lists from the Hubble Legacy Archive into a single master catalog with roughly 100 million individual sources. The CP also encouraged archival proposals that mine the new Hubble Spectroscopic Legacy Archive for high-level data products, containing “science grade” co-added spectra of all usable public data, combining exposures for each target from across visits.

Recognizing the unique and limited availability of Hubble’s ultraviolet (UV) capabilities, the UV Initiative was continued to encourage the community (and the TAC and panelists) to increase the fraction of time and awards dedicated to wavelengths below 3200 Å. The UV initiative applied to all Small, Medium, Large, and Treasury GO Proposals (with the exception of SNAP), as well as Archival Proposals. The available UV instrument modes include ACS/Solar Blind Channel imaging, COS spectroscopy, STIS/Multi-Anode Microchannel Array imaging and spectroscopy, STIS/Charge Coupled Device spectroscopy (UV gratings only) and WFC3/ultraviolet-visible imaging (UV filters only).

The review process

Members of the review panels and the TAC were recruited several months prior to the proposal deadline, and asked to serve on one of the 15 panels organized by science category, consisting of two panels on cosmology, three on galaxies, two panels covering active galactic nuclei and the intergalactic medium, two on stellar populations, three on stars, and two covering planets and solar-system objects. With the exception of solar system, each panel has at least one “mirror” panel, covering similar topics and expertise, allowing proposals to be transferred as needed to avoid conflicts of interest within a given panel. To accommodate the increased specialization of some of the fields, solar system and exoplanet proposals were not evaluated by the same panels, with the “Planets” mirror panels reviewing proposals covering exoplanet and debris disk science, and a separate solar system panel (meeting virtually) covering proposals for archival research/observations of solar system targets.

By the phase I deadline in April, 1,094 proposals were received electronically via the Astronomer's Proposal Tool; each was sorted by science category and organized into the review panels described above. Each review panel subsequently received between 70 to 90 Small (<35 orbits) and Medium (35 to 74 orbits) proposals to grade in preparation for the in-person discussion in June.

To decrease the burden on the panelists, each was only assigned about two-thirds of the proposals in their panel. These grades were collected a few weeks before the meeting, and sorted into a preliminary rank order within each panel. Proposals ranking in the bottom 40% were triaged, and generally not discussed further in the TAC process unless raised for discussion by a non-conflicted panelist. The Large and Treasury proposals (>75 orbits) were reviewed by the TAC members for discussion in their meeting following the panel reviews.

The review panels met over three days in Baltimore, MD, to deliberate and re-grade the proposals, and produce a final rank order for the non-triaged proposals in each panel. Members of the TAC were also assigned to these panels to serve as non-voting chairs, guiding the discussion and carrying forward opinions (should they be necessary) from the panels to the TAC. Each panel was provided a nominal orbit allocation to help guide decisions, especially for proposals critically ranked at or near the potential award boundary.

The Medium proposals were ranked amongst the Small proposals, allowing a gauge of their relative importance in the competition for the pool set aside for the Medium category, nominally 650 orbits. In a change from Cycle 23, each Medium proposal was assigned to one of the existing panels for review and each panel had an allocation of no more than one Medium proposal. A panel could choose to identify an additional Medium proposal to award from its nominal allocation, ensuring the proposal’s success, albeit at the expense of a large fraction of that panel’s awardable time. Panelists were also asked to review the Large and Treasury proposals pertinent to their panel’s science category. Comments on the Large proposals were provided to the panel chairs for the TAC review.

Immediately following the panel review, the TAC met for an additional two days to review the panels’ recommendations, and to decide the final rank orders for the Large and Treasury programs, within those respective orbit pools. Prof. Catherine Pilachowski of Indiana University served as chair of the Cycle 24 TAC, and Prof. John Gallagher of University of Wisconsin-Madison, Dr. Karl Stapelfeldt of the Jet Propulsion Laboratory, and Prof. Nicholas Suntzeff of Texas A&M University served as TAC members at large. The Institute Director completed the final review of the TAC recommendations in the week following the TAC meeting, and the Cycle 24 results were announced shortly thereafter.

Ensuring an impartial review

We continue to strive for impartiality and fairness in the Hubble review process. Conflicts of interest for each reviewer are identified based on institution and publication record, and mirror panels are used to avoid conflicts when possible. Once the proposals are initially distributed to the panel, each panelist must identify any remaining strong conflicts of interest, including competing proposals, mentorship relationships, and close collaborations. Panelists are not permitted to grade proposals for which they are conflicted, and for strong conflicts, e.g., any in which they themselves or their institutions would directly benefit from, panelists are not permitted to participate in the discussion.

Additionally, the Institute has taken steps to address the unconscious gender bias of the Hubble TAC process, which has resulted in small but statistically significant over-representation of male PIs relative to female PIs in each of the last 23 Hubble cycles. Following the recommendation of the Space Telescope Users Committee, printed copies of proposals, used for evaluation by the review panels, listed investigators alphabetically without identifying the Principal Investigator. Most panelists and TAC members welcomed this change and recommended making the proposals completely anonymous, regardless of proposal size. The orientation given to all panel and TAC members also included a presentation by Dr. Karoline Gilbert (STScI) on unconscious/implicit bias and best practices for reviewers.


With 261 of 1,115 proposals accepted, the average Hubble Cycle 24 acceptance rate was 22.4%, negligibly lower than the 23.4% acceptance rate from the last cycle. The oversubscription ratio for all General Observer (GO) programs was 6.8:1 by orbit and 4.5:1 by proposal (compared to 5.4:1 and 4.4:1, respectively, for the previous cycle). The estimated oversubscription of Archival and Theory proposals by nominal funding was 4.0:1, compared to 3.7:1 from the previous cycle. PIs from ESA-member countries lead 25.6% of the accepted Cycle 24 programs, slightly higher than in the last cycle. The success rate of Medium proposals was 15% by proposal (14 out of 93), for a total of 666 orbits, representing a slight decrease with respect to Cycle 23 (with a success rate of 19%), but still higher than the previous cycles.

WFC3 remained the most requested instrument, with 37.9% of the awarded time utilizing this instrument in its various modes on primary targets (11.0% WFC3/IR imaging, 8.3% WFC3/IR spectroscopy, 18.4% WFC3/UVIS imaging, and 0.1% WFC3/UVIS spectroscopy). STIS is the second-most requested instrument, with 27.6% of the awarded time almost evenly split across the spectroscopic modes. COS was awarded 20.5% of available orbits, divided into FUV (18.6%) and NUV (1.9%) spectroscopy. ACS completes the allocation, with 13.9% of the time going to the WFC (12.5%) and SBC (1.2%) imaging modes. The success rate for proposals under the UV Initiative was 36% by proposal (9 out of 42 for archival research and 78 out of 196 for GO), and 54% by orbit (2,041 orbits out of 3,760 requested).

The Cycle 24 orbit distribution per science category was as follows: 24.3% for Exoplanets (of 16.5% submitted), 18.1% for Galaxies (of 34.2% submitted), 16.9% for Stellar Physics (of 12.5% submitted), 15% for Stellar Populations (of 10.9% submitted), 11.1% for IGM and Cosmology (of 16.3% submitted), 7.6% for Solar System (of 3.4% submitted), and 7% for Black Holes (of 6.1% submitted).


We thank all of the Hubble TAC members, review panelists, and external reviewers for their service on the Hubble Cycle 24 TAC. Numerous Institute personnel contributed to the support of review process.

Science Policies Group astronomers Andy Fruchter, Janice Lee, Claus Leitherer, Jennifer Lotz, Amaya Moro-Martín, Neill Reid, and Lou Strolger were responsible for selecting the panelists, distributing the workload among the panelists according to expertise, while taking into account the conflicts, coordinating policy, providing oversight during the review process, and checking for duplications within the recommended proposal pool. Antonella Nota and Loretta Willers oversaw the invitations and logistics arrangement for all the ESA-supported panel members and Chairs.

Technical Manager Brett Blacker received, organized, and distributed the proposals, oversaw the proposal database, announced the results, and prepared the statistical summaries and figures provided here.

The TAC logistics were devised and coordinated by Sherita Hanna with Brett Blacker providing technical assistance. Administrative support came from Jody Charles, Martha Devaud, Sarah Flores, Flory Hill, Linda Kaiser, Tracy Lamb, Kari Marzola, Alisa Meizlish, Kim Oyler, David Patrick, Karen Petro, Karyn Poletis, Michele Sharko, Rickell Sheppard, Darlene Spencer, Rolanda Taylor, Ana-Maria Valenzuela, and Loretta Willers.

Amber Armstrong, Varun Bajaj, Mia Bovill, Meredith Durbin, Nick Earl, Deatrick Foster, Lisa Frattare, Benjamin Gompertz, Crystal Mannfolk, Catherine Martlin, Johan Mazoyer, Tala Monroe, Tony Roman, David Sahnow, Laura Watkins, and Marie Ygouf provided panel support.

Instrument expertise was provided by Marco Chiaberge, John Debes, Linda Dressel, Norman Grogin, Matt Lallo, John MacKenty, Ed Nelan, Cristina Oliveira, Molly Peeples, Julia Roman-Duval, Elena Sabbi, and Nolan Walborn.

IT support was provided by Val Ausherman, Romeo Gourgue, Jay Grimes, Craig Hollinshead, Craig Levy, Jessica Lynch, Thomas Marufu, Greg Masci, Glenn Miller, Corey Richardson, Patrick Taylor, Calvin Tullos, Shane Wolfe, and other members of ITSD.

Vickie Bowersox, Margie Cook, Roosevelt Davis, Karen Debelius, Cathy Donellan, Adia Jones, Lisa Kleinwort, Mimi Lazar, Amy Power, Ninel Serebreni, Paula Sessa, and Sarah Shin provided support from the Business Resources Center. Pam Jeffries provided support from the Office of Public Outreach, and Zak Concannon and Marcellous Grant provided assistance from the Copy Center.

Finally, for facilities we thank Andre Deshazo, Rob Franklin, Tiffany Lallo, Rob Levine, Glenn Martin, Greg Pabst, Kevin Powell, Frankie Schultz, Mike Sharpe, LaMont Strong, Mike Venturella, G Williams, and the Bloomberg facilities staff.