STScI especially seeks proposals that fall into one of several "Special Initiatives," described below. These initiatives highlight the unique science capabilities possessed by HST.

Ultraviolet Initiative

Ultraviolet GO Proposals

In recognition of the unique UV capabilities of Hubble and the finite lifetime of the mission, the UV Initiative will continue in Cycle 34. The initiative highlights programs that use the UV capabilities of Hubble and, in so doing, aims to increase the share of primary GO observing time dedicated to UV observations. A description of past programs is available on the HST UV Initiative Programs webpage.

No extra time is made available for UV Initiative proposals, and no additional weight is given in grading. However, setting the UV Initiative flag serves to highlight the unique science enabled by Hubble’s UV capabilities during the review. Proposals must still justify why Hubble is required to achieve the program's science goals, as described under the HST Proposal Selection Procedures. UV proposals recommended for acceptance must meet the usual requirement of high scientific quality set for all successful Hubble proposals. Very Small, Small, Medium, Large, and Treasury GO Proposals can benefit from the UV Initiative, in Cycle 34, as can Archival Proposals. Two conditions must be met for a GO Proposal to be eligible:

• The proposal must use the UV capabilities of Hubble. The eligible instrument modes (with central wavelength <3200 Angstroms) are ACS/SBC imaging (all filters), COS (all modes), STIS/MAMA spectroscopy and imaging (all gratings and filters), STIS/CCD spectroscopy (UV gratings G230LB and G230MB), and WFC3/UVIS imaging (UV filters F200LP, F300X, F218W, F225W, F275W, FQ232N, FQ243N, and F280N), and WFC3/UVIS G280 grism spectroscopy.
• The UV observations must be essential to the proposed science investigation. This condition will automatically be met for proposals requesting UV observations only. For proposals requesting both UV and optical/IR observations, the scientific necessity for the UV observations must be carefully justified in the Scientific Justification of the proposal.

Proposers are particularly encouraged to consider programs that will lay the groundwork for the Habitable Worlds Observatory, NASA's next UVO flagship mission.

Proposers must check the "UV Initiative" box in APT to indiciate that their proposal qualifies for this initiative.

Ultraviolet Archival Proposals

The UV Initiative also extends to Archival Proposals. STScI will ask the review panels and the TAC to give particular consideration to UV-specific archival proposals in the review process, provided they lead to UV high level data products and tools for the Hubble archive, and enable broader use of those datasets by the community, or (in the case of Theory Proposals) provide new models or theories to aid in the interpretation of UV HST data.

For Archival Programs that propose the joint analysis of UV and optical/IR datasets, the UV datasets must be essential to the scientific investigation for the UV Initiative benefit to apply. In this case, the proposers should carefully justify the importance of the UV component of their program in the Scientific Justification of the proposal.

AR proposers must check the "UV Initiative" box in APT to indicate that their proposal qualifies for this initiative.

Long-Term Monitoring (LTM) Initiative

Time-domain astronomy was highlighted as a key priority in the Astro2020 Decadal Survey. While much attention focuses on short-term transients, Hubble’s longevity provides an opportunity for investigations on substantially longer timescales, both with Hubble data alone and as a precursor to JWST follow-up through its 20-year lifetime. With that in mind, STScI constituted a Working Group to explore options for how Hubble can better support science in this area. The Working Group’s report highlights a number of areas where Hubble can make significant contributions. Consequently, STScI offers the LTM Initiative.

The community is encouraged to submit proposals that:

• Capitalize on past HST observations by providing late-epoch observations that are designed to probe long-term astrometric, photometric and/or spectroscopic variations.
• Propose first-epoch observation to lay the foundation for future time-domain work with either HST or JWST. The TAC will be instructed to assess the science impact of the full program, not just the current cycle observations. Proposers may submit joint HST-JWST proposals that request either or both HST and JWST observations in current and future cycles.
• Request observations beyond the 3-cycle limit for Future Cycle programs, up to 5 cycles. Proposers must specify the required cadence of observations in the Special Requirements section of the proposal.

LTM programs will be regularly monitored to ensure appropriate progress, but if approved, they will not need be subject to TAC review for future observations.

Proposers must check the "Long-Term Monitoring" box in APT to indicate that their proposal qualifies for this initiative.

No extra time is made available for LTM Initiative proposals, and no additional weight is given in grading. However, setting the LTM flag serves to highlight Hubble's longevity and its importance for time-domain astronomy during the review. Proposals must still justify why Hubble is required to achieve the program's science goals, as described under the HST Proposal Selection Procedures. LTM proposals recommended for acceptance must meet the usual requirement of high scientific quality set for all successful Hubble proposals.

Hubble-Roman Science (HRS) Initiative

The Nancy Grace Roman Space Telescope (Roman) will launch by May 2027, with a first call for proposals in late 2025. Roman will provide wide-field survey data including imaging and slitless spectroscopy spanning the wavelength range 0.5 to 2.3 microns, and direct imaging data from 0.5 to 0.8 microns of exoplanets using the high-contrast coronagraph instrument.

The HRS Initiative is designed to encourage observations with Hubble that complement and enhance the scientific impact of Roman observations, or that are essential to achieving critical science goals of Roman programs.

HRS Initiative proposals should comply with the following guidelines:

• Proposers must check the “Hubble-Roman Science (HRS)” box in APT to be included in this initiative.
• Proposers should use the Special Requirements section to describe the connection with specific Roman observations.
• If the Hubble observations are critical to the success of a Roman program, the science goals of the full program should be described in the Scientific Justification, including an explanation as to why Hubble observations are deemed essential to achieve those goals.
• By default, HRS proposal data will not have an Exclusive Access Period (EAP, a default of zero months). A non-zero EAP may be requested, and should be justified in the Special Requirement section. 

The connection between the proposed Hubble and Roman observations will be considered as part of the review process. Where the Hubble observations are deemed essential to achieving the overall science goals, the proposal will be assessed based on the science expectations for the full program including both the Hubble and Roman observations. Requests for non-zero EAP data will also be assessed by the TAC.
 
No extra time will be made available for HRS Initiative proposals, and no additional weight will be given in grading. However, setting the HRS flag serves to highlight Hubble's broad impact and its importance for future science with Roman during the review. Proposals must still justify why Hubble is required to achieve the program's science goals, as described under the HST Proposal Selection Procedures. HRS Initiative proposals recommended for acceptance must meet the usual requirement of high scientific quality set for all successful Hubble proposals.

Habitable Worlds Observatory Precursor Science (HPS) Initiative 

The Habitable Worlds Observatory (HWO) is a large infrared/optical/ultraviolet space telescope recommended by the National Academies’ Pathways to Discovery in Astronomy and Astrophysics for the 2020s. HWO will be the first telescope designed specifically to search for signs of life outside our solar system, however its powerful capabilities will transform a broad range of fields across astrophysics, as reflected in the HWO Community Science Case Development Documents.

The HPS Initiative is intended for investigations with Hubble that will support HWO science planning to inform mission architectures and trades, with the goal of reducing the cost and risk of HWO’s future design and development. Hubble’s unique ultraviolet capabilities make it an especially critical asset: no other current or planned observatory provides comparable UV access before HWO. Leveraging Hubble’s capabilities now is therefore essential to inform key aspects of HWO science planning and maximize the eventual return of the mission.

HPS programs are expected to generate knowledge, methods, or data that:

• constrain astrophysical parameters critical for HWO target selection, instrument design, or observational strategies,
• provide essential empirical inputs for science programs that HWO aims to address, or
• develop analysis techniques and data products that can be applied or scaled to HWO observations.

HPS Proposal Guidelines

• Proposers must check the “HWO Precursor Science (HPS)” box in APT to be included in this initiative.
• The Special Requirements section should explicitly describe the link to specific HWO precursor studies, Science Case Development Documents, anticipated HWO observations, or technical trades.
• If Hubble observations are critical to HWO planning or its future success, proposals should describe the relevant HWO science goals and explain why Hubble data are essential for de-risking or enabling those goals.
• By default, HPS proposal data will not have an Exclusive Access Period (EAP, a default of zero months). A non-zero EAP may be requested, and should be justified in the Special Requirements section. 

Review of HPS Proposals

• The strength of the connection to HWO planning and development needs will be explicitly considered during the review. Where the Hubble observations are deemed essential to achieving long-term HWO science goals, review panels will evaluate the proposal in the context of the combined Hubble + HWO programmatic impact.

• Proposals must still demonstrate why Hubble is required to achieve the program's science goals, as described under the HST Proposal Selection Procedures.

• No extra time will be made available for HPS Initiative proposals, and no additional weight will be given in grading.

• Requests for non-zero EAP data will be assessed by the TAC.

HPS Initiative proposals recommended for acceptance must meet the same high standards of scientific excellence expected of all Hubble programs. Highlighting them within the HPS framework will underscore Hubble’s unique role—particularly in the ultraviolet—to inform planning, reduce risk, and maximize the eventual scientific return of HWO.

HST-TESS Exoplanet Initiative

NASA’s Transiting Exoplanet Survey Satellite has discovered a wide range of planetary systems, notably small exoplanets (mini-Neptunes and super-Earths) around nearby stars.  The HST-TESS Advisory Committee (AC) was constituted by the STScI Director to provide guidance on optimal strategies for maximizing the scientific return from HST observations of TESS exoplanet targets. Following extensive consultation with the community, the HST-TESS AC final report highlights the vital role that HST can play in characterizing small exoplanets and identifying high priority targets for subsequent JWST observations.

Specifically, the committee noted that to maximize the science return, it is crucial that TESS targets have well determined periods and masses. Proceeding in a linear fashion, however, will lead to significant delays in obtaining follow-up HST observations of sufficient systems. Moreover, working on a target-by-target, proposal-by-proposal basis is unlikely to optimally sample the exoplanet population. Based on those considerations, the Space Telescope Users Committee has recommended the HST-TESS Exoplanet Initiative (HTEI) to provide the community with an opportunity to propose for observations of a well-characterized, representative sample.

Exoplanet Initiative proposals should

• Focus on mini-Neptunes and super-Earths
• Be sufficiently comprehensive in scope to address demographic questions
• Characterize the atmospheric properties as a function of size and equilibrium temperature
• Lay the foundations for subsequent observations with JWST

HST-TESS Exoplanet Initiative programs are Treasury programs and must meet the requirements for those programs. They are anticipated as long period (multi-cycle) programs that can capitalize quickly on the ongoing characterization of TESS exoplanet discoveries. The HST-TESS AC also recommended strong community participation in these programs, particularly with regard to target selection.

All HTEI exoplanet targets must have reliable mass determinations. Since an appropriately characterized sample of TESS targets is not available at the present time, HTEI proposals should identify specific targets that could be observed in Cycle 34 but may list generic targets for future cycles. The proposal must specify the quantitative criteria (such as mass, density and separation) that will be used to define the full sample. In addition, the proposals must describe appropriate mechanisms for building community consensus on how new targets will be added in future cycles.

HTEI programs will be assessed by the TAC along with other Large and Treasury programs. There is no specific orbit allocation for this initiative.

As with all HST proposals, HTEI proposals must conform with the dual anonymous guidelines.

Next: HST Observation Types