Science Definition Team (SDT) Overview

• Science Definition Team (SDT) is a NASA-appointed expert group that refines broad astrophysical objectives into traceable mission requirements using collaborative studies and advanced modeling.
• The SDT employs rigorous performance modeling, technical trades, and risk management strategies to optimize mission configurations and secure measurable scientific returns.
• Key deliverables, such as Design Reference Missions (DRMs), establish technical, cost, and science benchmarks that guide flagship mission development and future planning.

The Science Definition Team (SDT) is an expert working group formally appointed by NASA to interpret, elaborate, and operationalize the astrophysical science goals articulated in strategic documents such as the Astro2010 Decadal Survey (“New Worlds, New Horizons,” NWNH) and to produce concrete, technically traceable mission requirements and conceptual designs for flagship missions. For the Wide-Field InfraRed Survey Telescope (WFIRST), now widely known as Roman, the SDT played a decisive role in transforming broad scientific ambitions into executable Design Reference Missions (DRMs) via a structured, collaborative engineering and science development process encompassing mission architecture, instrumentation, modeling, systematics control, and programmatic trade analysis (Green et al., 2011, Green et al., 2012, Spergel et al., 2013, Spergel et al., 2013).

1. Formation, Charter, and Organization

NASA convened the WFIRST SDT in direct response to the 2010 NWNH Decadal Survey’s prioritization of a wide-field infrared survey mission. Reporting jointly to the NASA Headquarters Astrophysics Division and the Project Office at Goddard Space Flight Center (GSFC), with coordination through the Jet Propulsion Laboratory (JPL) Program Office, SDT members were charged to:

• Refine and elaborate NWNH science requirements into concrete, hierarchically traceable mission requirements.
• Explore and optimize mission implementation options (mirror, instrument suite, orbital parameters) within programmatic constraints.
• Produce a fully documentable DRM to serve as the technical and programmatic anchor for subsequent mission phases (Green et al., 2011).

Membership was drawn from the U.S. extragalactic and instrumentation communities, including experts in dark energy probes (weak lensing, baryon acoustic oscillations, supernova physics), exoplanet microlensing, IR survey strategy, detectors, and systems engineering (Green et al., 2012, Spergel et al., 2013). The SDT operated through subgroups addressing exoplanets, dark energy, calibration, survey and instrumentation trades, and project cost. Decision-making proceeded by consensus in plenary meetings and joint teleconferences with parallel working groups at GSFC and JPL (Spergel et al., 2013).

2. SDT Charge, Objectives, and Major Milestones

The SDT’s threefold charge encompassed:

1. Translating the NWNH science objectives into mission requirements, ensuring technical traceability from top-level goals to instrument parameters.
2. Performing implementation trade studies to ensure scientific objectives could be met within feasible cost and schedule envelopes.
3. Delivering a DRM for external review and serving as the reference point for project development milestones (Green et al., 2011, Green et al., 2012).

Key milestones included:

• Formal convening: June 2010 (initial SDT), October 2012 (WFIRST-AFTA SDT for 2.4-m implementation).
• Interim DRM delivery (IDRM): June 2011, establishing technical and scientific feasibility.
• Release of finalized DRM variants—DRM1 optimized for maximal NWNH science, DRM2 for cost-reduction and non-duplication with Euclid/JWST/LSST—by July 2012 (Green et al., 2012).
• WFIRST-2.4 DRM development and report: May 2013 (Spergel et al., 2013, Spergel et al., 2013).

SDT composition included co-chairs with disciplinary breadth and voting members from major U.S. universities, national labs (STScI, GSFC, LLNL), and international collaborators, along with technical consultants and a project paper team (Green et al., 2012, Spergel et al., 2013).

3. Methodologies and Decision Framework

The SDT implemented a structured workflow:

• Science flow-down: Constructing explicit traceability matrices from high-level science requirements to survey capabilities, dataset requirements, instrument specifications, and ground-support needs.
• Mission cost modeling: Hybrid parametric, analogy-based, and bottom-up estimates, incorporating launch, spacecraft, instrument, and operations costs (C_total = C_launch + C_spacecraft + C_instrument + C_operations).
• Performance modeling and figures of merit (FoM): Quantitative simulation of science yields; e.g., for exoplanets:

$$\text{FoM}_\text{ExP} \propto (N_\oplus N_{20\%} N_\text{ff} N_\text{HZ})^{3/8}$$

where $N_\oplus$ is the yield of detected 1 $M_\oplus$ planets, $N_{20\%}$ the subset with precisely known host masses, $N_\text{ff}$ free-floating planets, and $N_\text{HZ}$ habitable-zone Earth-mass planets. For dark energy, DETF FoM $= 1/[\sigma(w_0) \times \sigma(w_a)]$ was used (Green et al., 2011, Green et al., 2012).

• Technical trades: Parameter studies on aperture (1.1 m vs 1.3 m vs 2.4 m), detector technology (H2RG/18 vs H4RG/10), IR cutoff, focal plane layout, mission duration, launch vehicle, and orbit.
• Risk management: Active “risk burn-down” plans to retire the highest-impact technical uncertainties first (Green et al., 2011).

SDT subgroups coordinated with engineering teams to refine mechanical/thermal/optical allocations (STOP analyses), detector packaging, fine-guidance requirements, and survey cadences.

4. Deliverables and Major Technical Recommendations

The SDT’s principal deliverables were:

• Interim DRM (IDRM): 1.3 m off-axis TMA, three focal planes (one imaging, two spectroscopy), L2 orbit, five filter bands, dual spectrometer channels, and guest-observer program (Green et al., 2011, Green et al., 2012).
• DRM1: Single focal plane, insertable disperser wheel (prisms for spectroscopy), 2.4 μm detector cutoff, 5-year mission, cost reductions of 10–15% over IDRM (Green et al., 2012).
• DRM2: 1.1 m TMA, 3-year duration, H4RG-10 detectors, reduced redundancy, narrower GRS bandpass to avoid duplication with Euclid, ≥75% science yield of DRM1 for dark energy and exoplanets, with ~25–30% cost reduction (Green et al., 2012).

For WFIRST-2.4 (AFTA), the SDT recommended:

• Utilizing the 2.4-m on-axis Hubble-quality mirror to increase light-gathering by >3x and PSF sharpness ~1.9x over earlier designs, for major cosmological and exoplanet performance gains (Spergel et al., 2013, Spergel et al., 2013).
• Incorporating an optional high-contrast coronagraph for direct exoplanet imaging and debris disk studies.
• Integrating an IFU spectrometer to reduce supernova systematic uncertainties, boost DETF FoM by ≳20%, and enable more robust Type Ia standardization (Green et al., 2012, Spergel et al., 2013, Spergel et al., 2013).
• Specifying systematics control: Additive shear error ≤3×10⁻⁴, multiplicative ≤2×10⁻³ per tomographic bin, photo-z offset ≤0.002(1+z); supernova distance modulus error budget per bin σ_stat² = σ_meas² + σ_int² + σ_lens² with σ_sys = 0.01(1+z)/1.8 mag (Spergel et al., 2013).

5. Interactions, Community Engagement, and Agency Coordination

The SDT–Project Office workflow featured regular monthly plenary SDT meetings and weekly telecons to propagate science requirement updates, with technical feedback from instrument and mission concept teams (Green et al., 2011). Science working groups acted in concert with systems-engineering teams to address calibration, survey design, and figure-of-merit analyses. All major trades were subject to SDT approval.

The SDT solicited broad community input via workshops, white paper calls, and engagement with the Euclid, LSST, and JWST communities, enabling coordinated delineation of unique infrared and survey capabilities (Green et al., 2012). External reviews (e.g., CATE), NASA–ESA coordination, cross-checks with Decadal/Advisory panels, and the inclusion of a ≥10–25% Guest Observer program were integral to ensuring scientific robustness and non-duplication (Green et al., 2012, Spergel et al., 2013).

6. Legacy, Implementation Impact, and Forward Path

By mid-2013, the SDT had delivered a robust, quantitatively justified DRM framework for WFIRST. The 2.4-m AFTA configuration adopted by NASA incorporated all core SDT scientific and technical recommendations, positioning the mission for high-impact IR surveys, precision dark energy science, definitive exoplanet microlensing census, and direct exoplanet imaging (Spergel et al., 2013, Spergel et al., 2013).

The SDT’s processes and deliverables established the technical and programmatic template for subsequent large survey missions. Extensions such as the IFU spectrometer, advanced coronagraph, and adoption of H4RG detector technology exemplified science-driven but risk-aware innovation. The SDT’s engagement model and systematics emphasis have been influential for subsequent mission studies, with a plausible implication of shaping community expectations for traceability and rigor in future flagship mission planning (Green et al., 2012, Spergel et al., 2013).