1.1.

Current Technology Maturation Avenues for Flagship Missions

Why are the current funding and development efforts insufficient for accelerating technology development on the timescale and at the scope needed for HWO? As part of its recommendation for HWO, the Astro2020 decadal survey required that “prior to commencing mission formulation, a successful [GOMAP] program must be completed, and a review held to assess plans in light of mission budgetary needs and fiscal realities.”¹ The key to this recommendation is that critical technology and infrastructure should be matured (or have a path to maturation) ahead of the phase A start of the mission, a recommendation also made by the large UV/optical/IR mission (LUVOIR) concept study report (LUVOIR Final Report 2019).

The current paradigm, where technology is incrementally matured through a combination of Strategic Astrophysics Technology (SAT) and Astrophysics Research and Analysis (APRA) grants, is insufficient to achieve this maturation on the timescale needed for HWO. SAT awards are component-focused technology grants, in most cases designed around a limited number of formal lab-based maturation metrics that do not take into account how the hardware will be used as part of an integrated system. Furthermore, these programs often target metrics that do not derive from up-to-date mission science requirements. Because they are often not tied to specific science-driven capabilities, SAT and APRA have a disconnect with the groups defining scientific objectives and the system-level hardware testing that reveals new challenges (and opportunities) for the practical application of specific technologies.

APRA grants have led to key advancements in component-level hardware (see, e.g., the Cosmic Origins Program Analysis Group (COPAG) UV working group report³), but these grants are too small to support scaling up of basic technology (and the Astro2020 decadal report notes “basic technology grants are too small to support infrastructure”), and rocket/balloon/CubeSat missions have durations or volume allocations that are too short/small to demonstrate full prototype instruments in their relevant environments. Furthermore, APRA and SAT awards are financially insufficient to support the facility, technical, and workforce development required to rapidly advance the state of the field to support an HWO gate review later this decade. The number of 3-year grant cycles required at the current level of SAT and APRA funding makes completing HWO’s technology maturation this decade through SAT and APRA alone impossible. Finally, mid-scale flight programs are not viable pathways to major technological advancement at the scope and timescale needed for HWO because they do not have technology as part of the program goals and therefore have risk-averse technology postures (Pioneers, Explorer), long development timescales (Pioneers, Explorer), and/or sufficiently infrequent proposal opportunities (Explorers) that make these avenues infeasible for major technological advancement at the scope and timescale needed for HWO.

1.2.

Technology, Science, and Workforce Advancement with the SmallSat Technology Accelerated Maturation Platform (STAMP)

With these considerations in mind, we envision a new technology maturation concept that can be applied to all of NASA’s future Great Observatories (FGOs): the STAMP. The STAMP is a dedicated laboratory development and flight demonstration program, where accelerated facility development advances technology that feeds a parallel small satellite demonstration mission. The lessons learned from the demonstration mission inform the ongoing technology and scaling efforts such that the final technology developed through GOMAP is ready to meet all the requirements of the full HWO mission, including how the hardware would be integrated and operate in its real-world setting. The STAMP combines the three pillars of GOMAP’s charge: to advance science definition and technology for the FGOs while providing an inclusive mission framework that develops the science and engineering workforce that will lead the FGOs in the 2030s, 2040s, and beyond. We want to emphasize that the STAMP program is recommended in addition to the existing APRA and SAT program lines; APRA and SAT support the development of a range of technology at various maturation levels, and APRA supports novel science investigations and technology development for a balanced astrophysics portfolio. STAMP focuses on scaling up and systems-level validation of specific high-impact technologies; therefore, STAMP should be an augmentation of NASA’s technology development efforts to support the FGOs (ideally funded through the GOMAP; we roughly estimate the cost of the STAMP technology and workforce maturation mission outlined below, a laboratory and flight demonstration to advance ultraviolet technologies, to have a budget of $\sim \$50\mathrm{M}$, which can be compared with the anticipated $600M to $800M lifecycle GOMAP cost for HWO) and not be implemented in place of existing development programs.^4,5

A more general description of the STAMP program and its potential application to the full range of FGOs will be presented in future work. This brief focuses on a concept for a first entrant into this program, the STAMP-1 mission. As the initial prototype of this new technology development pathway, STAMP-1 advances key technologies to enable the ultraviolet capabilities of HWO, including advanced broadband optical coatings, high-sensitivity ultraviolet detector systems, and multi-object selection technology (Sec. 2). The technology maturation mission would do this while carrying out a compelling program of preparatory science for HWO: a near- and far-ultraviolet (NUV and FUV) spectral imaging survey of nearby galaxies to identify the agents of galactic feedback and quantify the relationship between galactic outflows and the surrounding circumgalactic medium. This work addresses key topics in Astro2020’s cosmic ecosystem theme (Sec. 2). In the process, STAMP-1 would feature early-career researchers (ECRs) as deputies to every major science and instrument leadership position within the mission and a science and instrument team that is comprised of more than 50% ECRs (Sec. 3). STAMP-1 engages and trains the scientists and engineers who would lead the implementation and execution of HWO as the mission moves toward phase A at the end of the current decade. Finally, STAMP-1 will include lessons about the overall viability of this development pathway for the X-ray and far-infrared flagships intended to follow HWO into GOMAP.

2.1.

Technology Demonstration Goals

For STAMP-1, there are three key enabling technologies, identified by the LUVOIR and HabEx study teams, reaffirmed by the HWO TAG, and subsequently prioritized in NASA’s biennial technology gap list, to be matured:

Fig. 1

Block diagram representation of the technology maturation path for HWO optical coatings in the STAMP framework. Unlike traditional technology maturation activities, STAMP provides direct interaction between the technology development and instrument implementation teams, providing process optimization feedback in nearly real time to mitigate cost and schedule risks associated with telescope development on HWO.

Table 1

Instrument parameter goals specified for the STAMP-1 instrument in the FUV (S1-FUV) and NUV (S1-NUV) channels. Both the spectral resolving power and angular resolution are limited by the ∼5-arcsecond pointing stability anticipated for the small satellite attitude control system. The intrinsic instrument performance specifications are listed in parentheses in these categories.

As an example of the interplay between the laboratory and flight mission development aspects of the STAMP program, we consider the maturation of broadband mirror coatings (Fig. 1). At present, advanced lithium fluoride (LiF)-based protected aluminum optics offer the best combination of UV-through-near-infrared (NIR) reflectance to achieve the full suite of HWO’s science objectives, with hot deposition and chemically catalyzed deposition techniques offering as much as 20% higher reflectance per bounce in the FUV.^6,7 However, deposition facilities and processes need to be scaled to the $\sim 2\text{-}\mathrm{m}$ size required for HWO. In addition, the uniformity and polarization properties need to be thoroughly characterized at this size scale, and optical coatings using the fabrication processes planned for HWO need to be tested in a long-term relevant environment. STAMP-1 kicks off the engineering efforts needed to address all of these issues, which require investment and schedule acceleration beyond what can be achieved in the current research opportunities in space and earth sciences (ROSES) strategic astrophysics technology (SAT) + astrophysics research and analysis (APRA) paradigm.

2.2.

Prototype Instrument Implementation

The power of the STAMP-1 instrument is in its multiplexing capability, a feature that derives directly from the combined advances in coatings (sensitivity to faint sources), high-sensitivity and low-background detectors (sensitivity to faint sources), and multi-object slitmasks (mapping large areas and different physical regions simultaneously). The STAMP-1 design maps stellar populations (by observing FUV ions that are diagnostic of the effective temperature), mass outflow rates (by observing diagnostic FUV and NUV neutral and low-ionization tracers), and FUV emission lines from the circumgalactic halo (see Sec. 2.3).

The science payload employs the highest priority enabling UV technologies for HWO to both advance their component-level TRL and demonstrate their application as an integrated system, providing a direct TRL and heritage link for their implementation on HWO (Fig. 2). The telescope is a 40-cm-diameter primary mirror Ritchey–Chrétien system. The light passes through a prototype “next-generation” microshutter array (NG-MSA) device, which creates an electrostatically actuated slit mask⁸ that allows light from targets of interest into the spectrograph while excluding the remainder of the field. The prototype NG-MSA is a single wafer of $256\times 128$ shutters (a unit cell of the $3\times 3$ grid in development for HWO), where each shutter has a $100\mu \mathrm{m}\times 200\mu \mathrm{m}$ pitch, and builds on earlier devices flight-tested on sounding rocket missions.^9,10 The light that passes through the MSA continues to a holographically ruled diffraction grating; FUV light is diffracted in the $-1$ order and directed to a photon-counting detector. The zero-order light of the FUV grating is directed to a second NUV grating that diffracts the light and directs it to the NUV focal plane array. Combining the area of the illuminated NG-MSA with the size of the FUV grating, the total field of view of the STAMP-1 instrument is $32\times 32\text{arcminutes}$ with each shutter subtending an angular size of $\sim 9\times 18\text{arcseconds}$.

Fig. 2

Schematic raytrace of the STAMP-1 instrument. Forty-centimeter primary mirror feeds a two-channel spectrograph with a next-generation microshutter array as the multi-object selection mechanism at the telescope focus.

STAMP-1 leverages recent advances in detector process development and readout electronics to provide long-duration flight demonstrations of large-format, photon-counting detectors for both the FUV and NUV spectral regions, following the payload design originally recommended for LUVOIR.¹¹ The FUV detector is a large format ($80\mathrm{mm}\times 40\mathrm{mm}$) MCP detector employing a cross-strip anode readout system. The cross-strip readout allows the MCP gain to be dropped by factors of $\sim 10$ relative to conventional delay line readouts, providing a correspondingly longer detector lifetime and building on flight development for rocket missions. For the NUV focal plane, STAMP-1 would employ an array of $\delta$-doped CMOS detectors, including band-optimized anti-reflection coatings that suppress red response in silicon detectors. These devices leverage laboratory development through the APRA and SAT programs.¹² The peak effective areas of the STAMP-1 instrument are $\sim 120{\mathrm{cm}}^2$ (${\lambda }_{\text{peak}}\sim 110\mathrm{nm}$) and $\sim 70{\mathrm{cm}}^2$ (${\lambda }_{\text{peak}}\sim 280\mathrm{nm}$).

Upon “authority to proceed,” the facility development to process meter-class optics with the necessary overcoating would be developed (the capability for Al+LiF deposition of optics as large as 1.5 m diameter at NASA/Goddard Space Flight Center (GSFC) is being developed as part of an existing SAT). The optics demonstrated in this facility would proceed incrementally, moving from today’s maximum of a 20- to 40-cm demonstration mirror. Following laboratory reflectance and uniformity testing, the 40-cm primary mirror (as well as secondary mirror and diffraction gratings) would be delivered to the STAMP-1 instrument team where they would be integrated into the optomechanical structure of the payload ahead of instrument I&T. Mirrors developed in his program would also support testbed characterization of high-contrast imaging systems with HWO coating prescriptions. The full payload would undergo in-band performance and characterization testing, with witness samples tracked at every step to monitor coating stability. Any anomalies and lessons learned would flow back to the coating deposition team for process optimization when scaling up to the next milestone (80 cm) mirror and ultimately to a full HWO primary mirror segment demonstration (final size to be determined, pending HWO project office development). The interaction between the technology development team and the instrument development team provides the conduit to incorporate real-world lessons before the final HWO hardware is fabricated, reducing the schedule and cost risk to the full HWO implementation.

Beyond the advanced broadband coatings, spacecraft, telescope, gratings, detectors, and prototype NG-MSA at the scale and specification for STAMP-1 are part of the existing commercial or NASA-supported development efforts. We would expect telescope delivery within 12 months of order, and deposition coating runs at this scale would begin immediately. Spacecraft, detectors, grating, and MSA would be expected between 18 and 21 months after receipt of funding, leaving 9 months for instrument assembly, calibration, and test and 6 months for integration with the spacecraft, observatory I&T, and delivery to the launch service provider. Although we envision a 24-month science mission, all major technical milestones would be achieved with 6 months of in-flight operations.

2.3.

Demonstration Science Program

Astro2020 highlighted three key science themes for astronomy and astrophysics in the 2020s and 2030s: cosmic ecosystems, worlds and suns in context, and new messengers and new physics. A central element of cosmic ecosystems (NASEM Secs. 1.1.3 and 2.3)¹ is how “stellar feedback” drives the mass, energy, and chemical evolution of galaxies and the surrounding circumgalactic medium (CGM) with energetic radiation fields, stellar winds, and supernova explosions of the most massive stars. Understanding this feedback cycle was highlighted as a top priority for the cosmic ecosystem theme in the current decade and was a primary driver for the multi-object capability in the LUVOIR and Habex studies. The decadal survey specifically called for close study of the relationship between star formation and the flows of matter in/out of nearby galaxies, where these processes can be observed “in dramatic detail, revealing the full multiphase complexity of the local ecosystem” (NASEM 1-8)¹ and advance our understanding of how cool, warm, and hot gases are driven into the halos of galaxies, the properties of the gas within halos, and ultimately how that gas is recycled into future generations of stars.

While advancing the technology and workforce that will enable UV imaging and spectroscopy on HWO, STAMP-1 will map the lifecycle of galaxy feedback, producing a dataset that will complement the high-spatial-resolution local galaxy and higher-redshift halo studies to be carried out by HWO. STAMP-1 will use multi-object spectroscopy to simultaneously characterize the most massive stellar clusters with FUV photospheric and wind lines, measure the galactic outflows that drive gas into the circumgalactic medium through neutral and intermediate ionization FUV and NUV absorption lines, and trace that gas in emission in the CGM through FUV emission line mapping. The half-degree field of view of STAMP-1 enables this simultaneous mapping of nearby galaxies in a single observation, whereas HWO will likely have an angular footprint $\sim 100$ times smaller. This makes STAMP-1 an ideal tool to investigate the cycle of baryon feedback in the low-redshift universe where galaxies and galaxy halos are tens of arcminutes in angular extent. In addition, STAMP-1 will provide a precursor target list of particularly rich outflow and CGM sightlines for high sensitivity and high-angular resolution follow-up with HWO. STAMP-1 accomplishes this preparatory science program by implementing the first orbital multi-object spectrograph operating at FUV and NUV wavelengths, a pathfinder instrument for future UV imaging and spectroscopic instrument on HWO.

Although the specific scientific and technical requirements for HWO’s instrument suite are still being refined by NASA’s Science, Technology, Architecture Review Team (START), TAG, and nascent HWO project office, there is already a strong case for multi-object UV spectroscopy. This capability was highlighted as a driving requirement in Astro2020’s cosmic ecosystem theme, it was recommended by the Astro2020 Electromagnetic Observations from Space-1 panel, and the HWO TAG has adopted the LUVOIR MultiObject Spectrograph (LUMOS)-B instrument^11,13 as a baseline concept for HWO’s UV capability. The STAMP-1 instrument will provide multi-object spectroscopy at a spacecraft-limited angular resolution of $\sim 7\text{arcseconds}$, enabling many tens of star-forming regions to be observed simultaneously (Fig. 3). The spectral coverage and spectral resolution of the instrument (1) allow key mass and age diagnostics of the stellar population and the properties of the associated interstellar medium to be resolved (driving the short-wavelength FUV coverage, e.g., O VI 1032, 1038 Å; S IV 1063, 1073 Å; and P V 1118, 1128 Å for stellar masses and ${\mathrm{H}}_2$ and CO, 1000 to 1120 Å, for the interstellar medium) and (2) provide direct measurements of the outflows driven by the combined effects of stellar winds and supernovae in these star-forming regions. Strong outflow lines motivate FUV spectral coverage (Si II 1190, 1193, 1526, 1533 Å) and drive the NUV spectral bandpass (rest frame and low-redshift Fe II and Mg II; 2400 to 3000 Å).

Fig. 3

Simulated overlay of the STAMP-1 MSA “footprint” on the central region of a nearby galaxy. Each shutter is represented as a yellow box, $\sim {18}^{″}$ (cross-dispersion) × 9″ (dispersion direction). This is an image of the starburst galaxy Haro 11, moved to a distance of 4.5 Mpc for this schematic figure (images reproduced with permission from Refs. 14 and 15); overlay is meant to be illustrative only. This image highlights the ability of the STAMP-1 footprint to isolate various star-forming regions within a single galaxy, but the full field of view of the STAMP-1 footprint is over 0.5 deg.

The 40-cm telescope is sized to enable the survey goal of spectrally mapping 200 nearby galaxies in a 2-year primary science mission (noting that the technical mission would be achieved after 6 months). The unique combination of multi-object spectroscopy and high-sensitivity coverage down to 100 nm is beyond the capability of Hubble or any other proposed mission. We note that neither Hubble Space Telescope (HST) nor Ultraviolet Explorer (UVEX) covers rest frame O VI in an imaging spectroscopy mode. The bandpass and spectral resolution requirements of STAMP-1 are driven by the need to cover the full set of stellar and gas diagnostics (FUV and NUV), with sufficient velocity resolution ($\sim 150\mathrm{km}/\mathrm{s}$) to resolve outflows from stellar clusters (typical outflow velocities range from 10 s to $\sim 1000\mathrm{km}/\mathrm{s}$¹⁶ and references therein) while demonstrating the technology at the key wavelengths targeted by HWO.

STAMP-1 simultaneously measures the ionization state and composition of the halo gas using low-resolution spectroscopy of wide angular fields in the halos of the target galaxies. The coverage of the most important cooling lines for gas between 10,000 and 300,000 K (O VI, Ly $\alpha$, and C IV; rest wavelengths 1032 to 1550 Å, another driver of the FUV spectral coverage) over large angular extents with high sensitivity would allow STAMP-1 to map diffuse emission from circumgalactic halos in unprecedented detail and numbers (a key component of the “Unveiling the Drivers of Galaxy Growth” science priority area, NASEM 1-8).¹ Taking into account the telescope collecting area and realistic component efficiencies, this low-redshift galaxy survey (200 nearby galaxies) would be sensitive to ($>8\sigma$) O VI surface brightness to $\sim 1\times {10}^{-18}\mathrm{erg}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}{\mathrm{arcsec}}^{-2}$ per $9^{″}\times 7^{″}$ angular bin (this is the full slit width in the dispersion direction and the observatory angular resolution in the cross-dispersion axis) in a 200-ksec observation and correspondingly lower surface brightness limits when binning over multiple shutters. This galaxy + halo survey would serve as a pathfinder scientific dataset for galaxy halo investigations over a wide range of redshifts and at much higher angular and spectral resolution with HWO. In addition, these observations will be made with an instrument similar in design and data format to the HWO UV instrument, providing a training ground to develop analysis and data calibration techniques that will ultimately be needed for HWO.