An overview of the policies regarding HST proposal submissions, including deadlines, funding eligibility, and the definitions of the roles of principal investigator and co-investigator.

The Proposal Process: Phase I and Phase II

STScI manages the review of HST proposals in two phases. In Phase I, proposers submit a scientific justification and observation summary for peer review. The review panels and the Telescope Allocation Committee (TAC) recommend a list of proposals to the STScI Director for preliminary approval and implementation (see HST Proposal Selection Procedures). This Call for Proposals focuses on Phase I policies and procedures. Separate documentation is available for Phase II.

In Phase II, investigators with approved Phase I proposals must provide complete details of the observations in their proposed observing program. This allows STScI to conduct a technical feasibility review, and to schedule and obtain the actual observations. Programs are not approved fully until after submission of an acceptable Phase II program.

Eligible investigators who request funding must submit detailed budgets (see HST Grant Funding and Budget Submissions).

Proposal Deadlines

Cycle 34 has the following deadlines:

• Phase I proposals are due Thursday, April 16, 2026 at 8:00pm EDT
• Phase II proposals are due anticipated August 13, 2026 at 5:00pm EDT
• Budget proposals are due end-August 2026, for planning purposes only

Who May Submit 

Scientists of any nationality or affiliation may submit and be included on HST proposals.  Investigators may also be from any career stage, including students. Institutional endorsement is not required for the proposal submission. All proposals are reviewed without regard to the nationalities or affiliations of the investigators.

Principal Investigator and Co-Investigators

Each proposal must have a Principal Investigator (PI), who is responsible for the scientific conduct of the project. A Co-Principal Investigator (Co-PI) option is also available, allowing two or more proposers to share the scientific responsibility of the project. Any other individuals who are actively involved in the proposal should be listed as Co-Investigators (Co-Is). The proposal itself may be submitted through APT by the PI, a Co-I, or a Co-PI.

Proposals by non-U.S. PIs that have one or more U.S. Co-Is must designate one of the U.S. Co-Is as the Administrative PI (Admin PI). Some U.S. PIs, including students and postdoctoral fellows, may also need to designate an Admin PI. This person will have overall oversight and responsibility for the budget submissions of the U.S. Co-Is in Phase II (see Grant Funding for funding eligibility requirements). All proposals have the option of designating a Contact Co-I, who will serve as the contact person for that proposal. The PI remains responsible for oversight of the proposal.

ESA Scientists

An agreement between NASA and ESA states that a minimum of 15% of HST observing time (on average over the lifetime of the HST project) will be allocated to scientists from ESA member states. It is anticipated that this requirement will continue to be satisfied via the normal selection process, as it has been in previous cycles. ESA scientists will be identified automatically by APT based on the institution selected.

Institutional Endorsement

STScI does not require the signature of an Authorizing Official (AO) on HST Proposals in Phase I. However, some institutions do require AO approval of all submitted proposals. It is the responsibility of each PI to follow all applicable institutional policies concerning the submission of proposals.

Funding

Subject to availability of funds from NASA, STScI will provide financial support for U.S. PIs and Co-Is of approved Cycle 34 programs. Budgets are not due in Phase I, but are required by the budget submission deadline from successful proposers. Details of the STScI Funding Policies are outlined in HST Grant Funding and Budget Submissions.

ESA does not fund HST research programs. Therefore, successful ESA member-state proposers should seek any necessary resources from their respective home institutions or national funding agencies.

Proposal Confidentiality

Proposals submitted to STScI will be kept confidential to the extent allowed by the review process described in HST Proposal Selection Procedures. For accepted proposals, the scientific justification section of the proposal remains confidential, but other sections become publicly accessible, including PI and Co-I names, project titles, abstracts, description of observations, special scheduling requirements, and details of all targets and exposures. Phase II programs submitted for approved proposals become publicly accessible in their entirety.

Multiple In-cycle Submissions

Before the proposals are distributed to the reviewers, STScI checks all submitted proposals for multiple in-cycle submissions across scientific categories and across size categories. By multiple in-cycle submissions we refer to proposals with the same or nearly the same team, science goals, and targets. STScI reserves the right to reject all such proposals as non-compliant in the case of egregious violations, as they negatively impact the fairness of the selection process and result in unnecessary burden for the reviewers. Note that we are not referring to observation duplications (described in HST Data Rights and Duplications), or science duplications (when the scientific outcomes of two programs are very similar, and could overlap or be redundant but the programs are clearly distinct). The latter two are addressed during the Discussion panels and Executive panel meetings as part of the proposal selection process.

Next: HST Proposal Categories