1.

INTRODUCTION

The Hubble Space Telescope (HST) has for decades provided some of the most valuable insights into the science of astronomy and astrophysics. Similarly the other great astronomical observatories of the past, Chandra, Spitzer, and Compton have contributed immensely to advancing science and our general knowledge of the universe. These large observatories are the some of the most productive “science machines” ever built. This tradition is expected to continue with the James Webb Space Telescope (JWST) when it is launched in 2018. Astrophysics, whether space based or ground based, is continuously driven to larger and larger apertures in order to explore the distant universe or study faint nearby objects such as exoplanets.

Unfortunately but not unexpectedly, these great observatories have also been some of the most costly observatories built by NASA. All of these observatories were threatened with cancellation during development due to cost and schedule overruns, a risk also run by NASA flagship missions outside of astrophysics. The on-orbit performance of HST and the performance expectations for JWST have also clearly established the value, scientific and otherwise, of a new and much larger space observatory covering at least the spectral region from the ultraviolet (UV) through the visible (VIS) to the near infrared (NIR). But the future great observatories will have to be built at a time when, for the foreseeable future, NASA budgets are almost certain to be flat or declining.

There have been a number of studies of the mission concepts and technologies for larger space observatories, identified as the Advanced Technology Large Aperture Space Telescope (ATLAST)^1,2 and the Modular Assembled Space Telescope (MAST)³. These studies have established the technical feasibility of a number of sizes and designs, and the science utility will be discussed in a few paragraphs. Costing factors have been addressed in other studies^4,5. The general conclusion of these studies is the fiscal incompatibility of the required large space telescope designs with the expected NASA budgets referred to in the preceding paragraph. An independent, and more detailed, analysis of this same conundrum, reaching the same conclusions regarding the incompatibility of the fiscal and the technical can be found in Arenberg et al⁶ in paper 9143-36 at this conference.

It is therefore essential to identify and implement a new development concept for large space telescopes that increases their affordability while, over time, providing the much improved resolution and light collecting power required to do the forefront science. It is our contention that, partly due to the annual funding cycle that NASA must employ, the root cause of the cancellation or delay risks run by a Flagship class mission is due to the annual cost of the mission. When this cost consumes too great a percentage of the yearly NASA budget it threatens all other missions, even those that are operating successfully and may offer an extended lifetime of great scientific merit. With multiple missions of all sizes threatened, the science community and NASA begin to consider draconic measures to reduce the annual cost of the Flagship mission: i.e., cancellation or program down-sizing and/or extension.

Potentially, it may be possible to overcome most of these programmatic risks by implementing some of their characteristics immediately in the initial stages of the program, effectively trading longer time to ultimate completion for greater affordability. Specifically, design, construction, and launch of the space telescope will be conducted in a number of stages, each producing a complete telescope fully capable of valuable scientific observations. Stage 1 will form the core of the observatory and will provide selected improvements upon systems operating at the planned launch date. Succeeding Stages will build upon the Stage 1 observatory in several year increments (nominally about 5 years between launches), and will add mirrors, structures, and instruments to the Stage 1 telescope. A full Flagship capability^† will be achieved by Stage 3 (details of one possible approach are provided in the Reference Concept section below). Future, even more capable Stages are, of course, possible, but are not addressed in this paper.

This is the concept for the Evolvable Space Telescope (EST).

2.

EST PROGRAMMATICS

As noted above, the development and deployment of EST will be conducted in a number of Stages, each of increasing capability, culminating in the full capability of an ATLAST observatory. Programmatically, this strategy will succeed if and only if the annual budget for each Stage is constrained below a reasonable fraction^‡ of the NASA astrophysics budget, and that fraction is well known to the astrophysics community. Thus, the EST programmatic strategy is, to a large extent, a design-to-annual-cost strategy combined with a “science-as-you go” strategy that is coupled with the requirements established by the principal science drivers. The annual cost target, still to be developed, will need to be based upon two principal factors: sufficiency for EST to achieve scientific viability with early flight stages so that the community will have continuing access to essential observational data, but low enough in cost to enable significant progress in astrophysics missions in other science regions.

The program will be structured with a modest cost, extended development in mind, enabling steady progress while capitalizing upon economies of scale as possible. Perhaps most importantly, maintaining a conservative but steady expenditure rate will protect EST from the effects of downsizing, which include inefficient use of personnel and equipment and increased expenditures in later program stages. As an illustrative example, if all three Stages can be acquired and deployed within a planned period of 15 years and the annual spend rate maintained at approximately $400M (current year dollars), a full ATLAST capability will be deployed for a total cost of ~$8B. Assuming that the total available for NASA astrophysics remains at about $1,200M as it has for most of the past decade, this will still leave financial room for other, extensive astrophysics systems development.

It should also be noted that the only costs that will be addressed in the proposed EST study will be those to be financed directly and managed by NASA. This rule is intended primarily to reduce program turbulence over its long lifetime. Other organizations, national or international, public or private, may be able to make major contributions to EST once it enters development, but a central management will be essential to maintaining the viability of the program.

3.

SCIENCE DRIVERS

A fundamental design rule for EST will be the requirement that each Stage, beginning with Stage 1 itself, will be designed to contribute significant science beyond that which constitutes the then current state-of-knowledge in important sub-fields of astronomy. The specifics of this requirement will be detailed in the design studies for the separate Stages but, as a minimum, will be based upon the science drivers used in the ATLAST studies by Green, Postman and others^1,7,8,9 and a few examples are shown in Figure 1. It will be necessary to derive intermediate science requirements that contribute to the attainment of these goals using the lesser capabilities of the intermediate stages of EST.

Figure 1:

Example Science programs for EST

These science drivers, at all levels of the EST program, will address multiple questions, many of which are organized around three topics which are paraphrased here and may be reviewed in detail in the cited references for ATLAST and MAST.

Other topics, such as solar system physics (planetary and small body) may be addressed by EST as well. Indeed, EST can be expected to follow the path of almost all early astronomical observatories, uncovering and resolving new issues that cannot be foreseen at this time and generating needs for more and different observations from still newer observatories over time.

4.

MISSION CONCEPT

The preceding introductory sections defined the Evolvable Space Telescope (EST), outlined its value, rationale, and timeliness (scientific and otherwise), and sketched the approach and expected products of the feasibility study needed as the first step towards an EST. This section will outline important aspects of the programmatic approach and environment, while later sections will present more details of the trade space accessible to the general concept, outline near term studies needed to begin assessment of the concept, provide an example of a generic design to suggest the sort of detailed study that will be needed, and describe the in-space servicing capabilities and technologies expected to be available at least through Stage 1 of EST.

5.

EST TRADE SPACE

The preliminary concept study will need to make a large number of technical tradeoffs in order to define a basic reference design for future in-depth study. Although a totally complete set cannot be established in the early stages, these tradeoffs will necessarily include those listed in the following table. Some tradeoffs that would be included in a totally bottoms-up study have already been made, and their results are included in the assumptions. Two of these are:

Note that this table does not address tradeoffs among instruments, which will have to be considered in later work once the telescope (and spacecraft) designs are more settled for each Stage. These analyses will be largely based upon detailed considerations of the primary science drivers, with allowances made for the flexibility needed to address unexpected scientific questions and opportunities.

6.

THE INITIAL EST STUDY

As with any study of a new mission concept, the first step must be to decide precisely what to study in detail and how to proceed with those detailed assessments. There is a natural flow down, possibly involving one or more iterative cycles as later steps encounter flaws in earlier conclusions that this study will follow.

As the EST Mission Concept Study develops, it should provide a source of information and insight that will help to both build community interest (including the scientific, government, and public communities) and as a construct for all of these communities to discuss, debate, and refine. Much of the discussion can be expected to revolve around evolving Design Reference Concepts. A simplified, preliminary concept is outlined in the following section.

7.

DESIGN REFERENCE CONCEPT

This paper is not intended to provide an extensive list of design concepts that could be used to implement the Evolvable Space Telescope architectural philosophy. However, the authors believe that comprehension of this design philosophy will be assisted through discussion of at least one elementary example as a reference.

The details of the assembly of the EST Primary Mirror Assembly (PMA) are represented in Figure 2. For comparative purposes, this paper assumes that all the hexagonal segments shown are ~4 meters flat-to-flat, or approximately 4.5 meters point-to-point, although these dimensions are purely illustrative. The EST approach can accommodate any segment size, but the telescope aperture at each stage will of course scale with the segment size.

Figure 2:

EST Reference Concept. See paragraph below for details.

Figure 2 is intended to represent only the underlying approach to be employed in EST, and not a specific implementation of that approach. The details of possible implementations will be worked out in detailed studies to follow this initial work, but some specifics are included to provide a relatively intuitive insight into the EST concept.

This discussion is based upon Figure 2, which represents one possible configuration of the primary mirror in each of the three Stages:

8.

SERVICING CONSIDERATIONS

As implied under the discussion of the principle measures of merit in the Mission Concept section, serviceability will be a key to the success of the Evolvable Space Telescope, particularly given the extended operational lifetime expected of the observatory. Two principal technologies are developing rapidly that will enable this critical capability: robotics/telerobotics and multiple space platforms that can form the basis for servicing operations of all types.

9.

SUMMARY