Scheduling Hubble in the Era of Webb

I. Neill Reid, inr[at]

In less than two years, the James Webb Space Telescope will be launched on an Ariane rocket from the European Space Agency’s (ESA’s) space site at Kourou, French Guyana. Webb will provide the worldwide community with access to near- and mid-infrared wavelengths at unparalleled sensitivity and resolution. Meantime, Hubble continues to stand as the prime space observatory at visual and ultraviolet wavelengths. Hubble is currently in excellent health, and NASA has indicated that it will continue to operate the observatory as long as it is scientifically productive, as assessed by the periodic Senior Review process. Institute staff members have been working with the Hubble and Webb Project teams at Goddard Space Flight Center and with the Space Telescope Users Committee (STUC) to optimize community access and logistical support for both observatories.

Programmatic synergies

Hubble science operations are highly likely to overlap with Webb; however, there are no guarantees regarding the operational characteristics of any particular Hubble science instrument more than three years in the future. Consequently, if there are key science investigations that require Hubble's unique capabilities, it is essential that the community act sooner, rather than later. To that end, we created the category of Webb Preparatory Proposals for Cycle 24, and we are continuing that category in Cycle 25.

Time allocation committees always struggle in dealing with the tension between "science now" and "science later." That is particularly the case with Webb preparatory proposals, where the full science potential of the program may only be realized with the Webb observations; indeed, the Hubble observations alone may provide little new scientific insight, even if they are crucial to the overall investigation. We are addressing this tension by emphasizing that the TAC must focus their assessment on the full program; that is, the key parameter is "science later"—what is the impact and importance of the science enabled by the combined Hubble and Webb datasets?

Hubble has established a number of joint programs with other observatories, notably Chandra, XMM, NOAO, and NRAO. Those programs mitigate the double jeopardy inherent to supporting multi-wavelength programs through multiple TAC processes. Indeed, the Institute and Chandra X-ray Center are extending the joint Hubble–Chandra program to include large programs, requiring at least 75 orbits of Hubble time and 400 Msecs of Chandra observations. Joint Hubble–Webb observations will not be available for Webb Cycle 1, although they may be introduced in later cycles. However, modifications to the schedule for Hubble’s Cycle 26 will provide the community with opportunities for observational synergies.

Proposal schedule

Hubble typically follows an annual proposal cycle. In recent years, the Call for Proposals has been issued in early January with the Phase I proposal deadline in mid-April, the Telescope Allocation Committee (TAC) meeting in early June and the Phase II proposals and budgets due in early to mid-July. The observing cycle runs from October 1 through to September 30 in the following year. Webb will also follow an annual proposal cycle, with the anticipated observing for Cycle 1 starting in mid-April 2019 and running through to April the following year. Starting in Cycle 2 (2020/2021), the Webb General Observer (GO) Call will be issued in early September, with the proposal deadline in early December and the TAC meeting in February.

The Webb Cycle 2 (and beyond) schedule integrates well with the current Hubble schedule, easing the community’s workload. That is not the case, however, for the Webb Cycle 1 proposal schedule. The Call for Proposals for Cycle 1 GO proposals will be issued in late November 2017, with the proposal deadline set for March 3, 2018 and the TAC meeting in mid-May 2018. Clearly, this is uncomfortably close to the nominal Hubble Cycle 26 schedule. In addition, we should note that the TAC for Chandra’s Cycle 20 will meet in mid-June 2018, while the ALMA Cycle 6 TAC will meet in late June 2018. Combining all of these activities represents a significant stress on the worldwide community, both with regard to developing and submitting observing proposals and participating in the review process.

Recognizing the impact on the community, the Institute has worked with its partners and stakeholders to develop a revised schedule for the Hubble Cycle 26 proposal ingestion and review that will mitigate the impact on the community. The resulting implementation plan can be summarized as follows:

  • The Cycle 25 TAC will pre-allocate a subset of Cycle 26 programs, allowing us to allocate the remainder of Cycle 26 orbits in a ∆Call and ∆TAC scheduled later in 2018.
  • Hubble will continue to offer mid-cycle opportunities, with two deadlines prior to and one after the Cycle 26 deadline.
  • Hubble will resume the current proposal schedule in 2019 for Cycle 27.

The overall schedule is shown in Figure 1.

Figure 1: The Hubble proposal schedules for Cycles 25, 26, and 27.

Expanding on the schedule, the Cycle 25 TAC will allocate a total of ∼4750 orbits; this compares with ∼3200 orbits available in a typical year. Table 1 shows the distribution by proposal size. Note that ∼250 orbits in Cycle 25 are already allocated in support of the Panchromatic Comparative Exoplanetary Treasury Program, the very large program selected by the Cycle 24 TAC; moreover, the Institute is reserving a further 150 orbits for the Chandra TAC to allocate in support of Large Joint Hubble–Chandra programs. The Cycle 25 TAC will also have a larger SNAP allocation, and accepted proposals will carry the same priority level in Cycles 25 and 26. Accepted GO programs will be designated as either Cycle 25 or Cycle 26 based on their scheduling profile within the Long Range Plan, i.e., subsequent to the Phase II and budget submissions. Up to 200 Cycle 25 orbits will be reserved for mid-cycle proposals. Those proposals, limited to no more than 10 orbits, provide an opportunity to respond to discoveries made subsequent to the Cycle 25 deadline.

Table 1: Orbit allocation for the Cycle 25 TAC.

The majority of the remaining orbits in Cycle 26, ∼1500, will be allocated by the ∆TAC, scheduled for October 2018. The Cycle 1 Webb GO programs will be known at least six weeks in advance of the Phase I proposal deadline, in mid-July 2018, providing the community with an opportunity to propose complementary Hubble programs. Given the relatively limited orbit allocation, the Cycle 26 ∆TAC will only consider proposals in the following categories:

  • Medium, Large and Treasury GO proposals;
  • Joint Hubble–Chandra, Hubble–NRAO, and Hubble–XMM proposals; and
  • AR Legacy proposals, including Legacy Theory proposals.

The Cycle 26 ∆TAC will not include an opportunity to apply for small Hubble proposals (<35 orbits) except as joint observatory proposals. This approach decreases the workload for the community, with regard to both proposal preparation and review, in a year where the Cycle 1 Webb call is likely to attract substantial attention. We will reserve approximately 300 orbits for smaller-scale Cycle 26 mid-cycle proposals, with a deadline of January 31, 2019.

Our current plans are to return to the "standard" Hubble proposal schedule for Cycle 27, with the Phase I deadline in early April and the TAC meeting in June. However, we are considering modifications to the review process. Specifically, we are considering a conceptual process where external reviews (à la mid-cycle process) are used to assess regular GO and AR proposals. The on-site TAC will review the highest ranked regular proposals for overall topical balance, and will directly assess the medium and Large/Treasury programs. The goal in adopting this revised process is to reduce the overall workload for the community. The detailed mechanisms for achieving this goal will be developed in consultation with the Hubble Project and the STUC.


Hubble and Webb will provide the astronomical community with complementary capabilities at UV/optical and near/mid-IR wavelengths. We have been working with the Hubble and Webb projects at GSFC and with the STUC to develop formalisms to enable the community to take full advantage of those capabilities. We are modifying the proposal schedule for Hubble’s Cycles 25 and 26 to mitigate the workload on the community, while preserving opportunities for synergies between the observatories.