The Hubble Space Telescope (HST) science peer review process is organized by the Science Policy Group (SPG) of the Science Mission Office (SMO) at the Space Telescope Science Institute (STScI).

Requesting Help

Please contact the HST Help Desk (https://hsthelp.stsci.edu) for any questions that you may have, including questions about science policy or technical performance. In your correspondence, please identify yourself as a Telescope Allocation Committee (TAC) member. If your question is about a specific proposal, please state the ID for the proposal associated with your question.

Proposal Types

HST proposals may request both observing time and funding (proposers from U.S. institutions only). There are three main types:

  • General Observer (GO) proposals request observing time in orbits. GO proposals are classified as Small (1-34 orbits), Medium (>34-74 orbits), or Large (>74 orbits). US-based investigators on successful proposals are awarded funding to carry out the data reduction and analysis.
  • Snapshot (SNAP) programs use small amounts of otherwise idle observing time in the HST schedule; they are allocated a number of targets of which in practice only a fraction, ~30 - 70%, will actually be observed. US-based investigators on successful proposals are awarded funding to carry out the data reduction and analysis.
  • Archival Research (AR) programs are to carry out investigations using HST data from the Mikulski Archive for Space Telescopes (MAST) or to do theoretical work to benefit HST science (AR Theory). US-based investigators on successful proposals are awarded funding ONLY to carry out their proposed program. Reviewers will not be asked to recommend or judge the amount of funding, but they are asked to assess whether the proposal requests sufficient resources to achieve the science goals.

GO proposals include Long-Term proposals, Joint Chandra, NoirLab, NRAO, TESS or XMM proposals, Treasury proposals and Calibration proposals.

Archival proposals include Theory proposals, Cloud Computing proposals and Calibration (AR) proposals. A full description of the proposal categories is given in the Call for Proposals.

Each proposal is assigned to one of eight science categories:

  1. Solar System Astronomy,
  2. Exoplanets and Exoplanet Formation,
  3. Stellar Physics and Stellar Types,
  4. Stellar Populations and the Interstellar Medium,
  5. Galaxies,
  6. Intergalactic Medium and Circumgalactic Medium,
  7. Supermassive Black Holes and Active Galaxies,
  8. and Large-Scale Structure of the Universe.

HST Telescope Allocation Committee (TAC)

HST proposals are selected through competitive peer review. Panelists are chosen based on their scientific expertise in the areas under review by the topical panels. Each topical panel will be managed by a panel Chair and a vice-Chair (except Solar System), and the Meeting Chair overseeing the review process.

Small GO proposals requesting fewer than 16 orbits, Archival (including Theory), and Snapshot proposals are reviewed by External panelists. All other proposals are reviewed by Virtual panelists.

To assist in the review process, each panel will also be assigned a Panel Support Scientist (PSS). The role of the PSS is to ensure the process runs smoothly and act as liaison between the panel and STScI. Virtual panels will also be assigned a Leveler whose role is to ensure the discussion remains focused on the scientific merits of the proposals. Proposals will be graded on an absolute scale against the primary criteria:

  • Scientific impact within the sub-field,
  • Broader importance for astronomy,
  • Suitability of HST’s unique capabilities for achieving the scientific goals.

The TAC Chair, At-Large TAC members, and panel Chairs and vice Chairs will form the Executive Committee. The Executive Committee will review the Large GO, Treasury and AR Legacy proposals. The Executive Committee will also rank the Pure Parallel programs, and will adjudicate any cross-panel scientific issues, as needed. All recommendations for the Cycle 29 science program are advisory to the STScI Director, who is responsible for the final allocation of HST observing time and funding.

Proposal Review Process

Proposals are reviewed by Virtual or External panels depending on their size and type.

Virtual Review Process

Proposals reviewed by the Virtual panels are subject to a two-stage review process: 1) preliminary grading and triage; and 2) the review meeting.

Preliminary Grading

  • STScI assigns each proposal a Primary Reviewer, a Secondary Reviewers, and three to four additional graders.
  • The Primary and Secondary reviewer and the additional graders submit numerical scores as preliminary grades for the proposals.
  • All reviewers, but particularly the Primary and Secondary reviewers, are strongly encouraged to use the SPIRIT grading system to enter comments summarizing the strengths and weaknesses for their assigned proposals.


  • STScI compiles a ranked list using the averaged preliminary grades and performs a "triage". The top ~60% of proposals are flagged for further discussion during the review meeting, and the rest are declined.
  • At the panel meeting, each panelist has the option of resurrecting one proposal that has been declined.

Preparation for the Discussion Topical Panel Meetings

  • Panelists review proposals that have been selected for further discussion at the panel meeting. Each panelist has already submitted grades and comments for some but not all of the selected proposals. Panelists should make sure to review all the proposals and can use the SPIRIT system to enter comments for those proposals prior to the meeting.

Discussion Panel Meetings

  • All proposals that pass triage are discussed and re-scored by the relevant topical panel during the panel meeting. For non-Treasury Small, Medium, and Archive proposals, the ranked list produced by the panels will be used to determine resource awards. 
  • The topical panels provide advice to their Panel Chair and vice-Chair on the strengths and weaknesses of the Large and/or Treasury programs in their area of expertise.
  • The Primary Reviewer is responsible for leading the proposal discussion and writing the consensus report that will be sent to the PI. The Secondary Reviewer(s) is responsible for supporting the proposal discussion and contributing to the consensus report.

Executive Committee Meeting

  • The Executive Committee reviews the larger-scale programs and recommends which should be scheduled.

After the Discussion Panel Meetings

  • During the week following the panel meeting, the Panel Chairs review the panel reports for the proposals assigned to their panel for correctness and consistency.

External Review Process

Proposals reviewed by the External panels are subject to a one-stage review process.

  • STScI assigns each proposal five reviewers.
  • The reviewers submit numerical scores as grades for the proposals.
  • All reviewers to enter comments summarizing the strengths and weaknesses for their assigned proposals.

Final Steps

Finalizing the Science Program

  • The STScI Director reviews the recommendations made by the TAC and approves the final selection.
  • STScI uses the selected proposals to generate the Long Range Plan.

Dissemination of Results

  • The results of the proposal review process are sent to the PIs, including the quintile ranking and the comments from the panels.

Roles and Responsibilities of the TAC Participants

The roles and responsibilities for review members are summarized as follows.

TAC Meeting Chair

  • Oversee the proposal review process.
  • Moderate the discussion of the Large programs at the Executive Committee meeting.
  • Review the consensus reports for the Large Programs.

At-Large Members

  • Participate in the panel discussion when their expertise is needed.

Panel Chairs and Vice-Chairs

  • Carefully read all Large Programs, regardless of category, and provide written comments on the strengths and weaknesses of the proposals.
  • Read and score all proposals assigned.
  • Moderate the discussion of the proposals in their panel.
  • Discuss the scientific merits of the Large Programs in the Executive Committee meeting and recommend which Large Programs should be awarded telescope time.
  • Write consensus reports for Large Programs for which they are assigned Primary Reviewer.
  • Review the consensus reports for the proposals in their panel.

Virtual Panel Members

  • Read and score assigned proposals and provide written comments on the strengths and weaknesses of the proposals.
  • Discuss the scientific merits of the proposals with other panel members at the topical panel meetings and re-score the proposals.
  • Produce final consensus reports for proposals for which they are assigned Primary Reviewer.

External Panel Members

  • Read and score assigned proposals.
  • Provide a written report summarising the strengths and weaknesses of their assigned proposals.

Panel Support Scientists (STScI Staff Members)

  • Serve as intermediaries between Panel Members and STScI, answering any questions that Panel Members might have.
  • Record minutes of the panel discussions.
  • Bring critical issues to the attention of the Science Policies Group (SPG) and/or relevant STScI technical staff.

Levelers (STScI Staff Members)

  • Focus proposal discussions on the science presented in the proposal.
  • Remind panelists of criteria for recommending disqualifications during team expertise discussions.

Software Tools

STScI has developed the web-based SPIRIT tool to facilitate preliminary grading of the proposals before the TAC meeting and final grading of the proposals during the TAC meeting. Grading assignments can be viewed in SPIRIT (https://spirit.stsci.edu). A Quick Guide to SPIRIT (Reviewer Tool) can be downloaded here.

Next: Selection Criteria and Scoring System