2.1.

Community Involvement

Community involvement was an intentional and important aspect of the Origins mission concept study. As noted above, Origins has roots in the NASA Astrophysics Roadmap,¹ in which a committee representing the astrophysics community envisions a far-IR Surveyor mission that offers measurement capabilities vastly superior to any prior far-IR mission. The Origins STDT, itself comprising representatives of the community, actively engaged many additional community members in developing and documenting potential science objectives and use cases for the mission. Throughout the study, the STDT’s virtual and face-to-face meetings were open and often attended by members of the wider community. Finally, if Origins is prioritized by the 2020 Decadal Survey and developed by NASA, Origins General Observers from the global scientific community will use the telescope to answer the mission-driving science questions and make unexpected, transformative discoveries.¹⁰

2.2.

Focus on Cryo-Thermal Design

Based on decades of experience with infrared space telescopes going back to the Infrared Astronomical Satellite in the early 1980s,¹² we knew that striking scientific achievements were made possible by the exceptional sensitivity afforded by cryogenically cooled telescopes. It was clear from the outset that Origins is a cryogenic mission. For this reason, we never diverted our attention from the cryo-thermal system design.¹³ No design decision was taken without understanding its impact on the system’s thermal performance. We appointed a highly experienced cryogenic physicist as chief engineer and system architect. We knew from JWST the importance of keeping warm electronics distant and well isolated from cold optical surfaces. We knew from many past infrared missions^12,14–19 that expendable cryogen is mission lifetime-limiting and consumes mass and volume that could be better used to maximize measurement capabilities if mechanical cryocoolers were employed. We were aware of the state-of-the-art and the potential for advancements in cryocooler technology.²⁰ We meticulously tracked parasitic heat loads and budgeted 100% margin in the heat-lift capacity of the cryocoolers to allow for thermal model uncertainties and permit design flexibility.¹³ Knowing that cryogenic cycling during the integration and test (I&T) phase is time-consuming and therefore expensive, we took advantage of the fact that cycling time is faster when aided by cryocoolers, as seen in JWST experience when its MIRI cryocooler was activated, and proposed a shorter I&T program than JWST. Despite its shorter overall duration, the Origins I&T plan enables cryogenic testing at a higher level of assembly.²¹ Finally, we adopted the JWST approach and allowed for primary mirror segment actuation (albeit in only three dimensions: tip, tilt, and piston), and chose thermally conducting materials for mirrors and metering structures²² to minimize temperature gradients and ensure optical component and system-level performance compliant with a preliminary optical system error budget.²³ The telescope will be diffraction limited at $30\mu \mathrm{m}$. Based on industry experience with the JWST primary mirror segments and the Origins optical error budget, we concluded that the Origins mirror segments will not require time-consuming cryo-null figuring because they will retain shapes within range of their specifications when they are cooled. Optical performance will be verified at cryogenic temperatures at the component (mirror segment) and subassembly (primary mirror wedge engineering test unit) levels.

2.3.

The Origins Study Team’s Approach to Mission Design

If it were feasible to develop a set of mission concepts that fully explore the mission design solution space, it would be possible simply to choose the design that maximizes science return for a targeted cost, or to find the sweet spot where the science per dollar is maximized. However, each design cycle is labor-intensive and takes a considerable amount of time, necessitating a surgical approach. A methodical and practical approach to cost-constrained mission design involves iteration through three design cycles, each one leading to a point design with known technical capabilities and a cost estimate, affording an opportunity to bracket the cost target with the first two cycles, and then interpolate in an effort to maximize science return at a price point.^8,24

With no NASA-imposed cost cap, the Origins study team adopted a more exploratory approach, which led to the design evolution shown in Fig. 2. The STDT initially set its sights on a far-infrared observatory that might answer a very wide range of interesting science questions. This helped the team to engage the wider community, which was eager to recommend observing programs for a telescope constrained only by physics. The result was mission “Concept 1.”

Fig. 1

Scientific goals and objectives for the Origins Space Telescope.

At its fifth face-to-face meeting in June 2017, with most aspects of the initial engineering design completed, the STDT decided to terminate the Concept 1 study and to develop a second point design. The study team decided not to allocate resources for detailed cost modeling at this juncture, as it was readily apparent that Concept 1 would be uncomfortably ambitious. Like JWST, Concept 1 required many on-orbit deployments, and JWST’s success would not be demonstrated before the Decadal Survey would have to set community priorities. The perceived risk was too great.

At this point in the study, based on the NASA budget thought likely to be available for a large mission after the Roman Space Telescope, the STDT decided to set a $5B cost target (in fiscal year 2020 dollars) for Origins. Concept 1 would likely have exceeded the newly established cost target by a large but unknown factor, and thus offered little guidance when it came to choosing even the basic system architecture for what would become Concept 2. In August 2017, the Study Office convened a workshop to conceive and evaluate alternative mission architectures. Science return, complexity, and heritage were important figures of merit. Design options were presented to the STDT when it met again face-to-face the following month. The STDT adopted an architecture for Concept 2 that resembles the Spitzer Space Telescope,¹⁶ as shown in Fig. 3; the primary mirror would not be folded for launch and deployed in space, as it was in Concept 1. The team retained from Concept 1 the science priority to conduct a search for biosignatures in the atmospheres of exoplanets in transiting orbits around nearby late-type dwarf stars, and this drove the requirement for a telescope with at least a 5-m aperture diameter. Later detailed calculations yielded the more precise requirement for a 5.3-m minimum telescope size. (The STDT’s prioritized extragalactic study, which requires measurements of a statistically significant sample of galaxies at redshift $z>6$, leads to a similar requirement for a 5-m telescope, albeit with more graceful falloff in scientific capability with decreasing aperture size.¹¹) Thus, the basic parameters for Concept 2, cost target, architecture, telescope size, and science priorities,¹⁰ were defined, and the second design cycle could begin.

Fig. 2

The Origins study team took an exploratory approach to mission concept development in which two distinct architectures were studied, the second was adopted, and descopes were taken in an effort to develop a concept that maximizes science yield with an executable mission at an affordable cost.

We designed and analyzed Concept 2 in much greater detail than Concept 1. Knowing that a third full design cycle would not be achievable in the time available to complete the study, and that a high-fidelity cost estimate would only be derivable from a completed design, we used flight system mass as a proxy for cost at $1 M/kg (based on analogous missions) throughout the second design cycle, and subtracted 30% from $5B to allow for cost reserve. We paid close attention to scientific priorities, mass, and descope options during the second design cycle, but Concept 2 still came in heavy, and therefore too expensive in our rough estimation. Thus, we developed a menu of descope options and the estimated cost savings associated with each option. In August 2018, we aimed to maximize science per dollar, made the descopes we felt would be necessary to approach the cost target, based on the mass proxy for cost, and called this design (Concept 2 minus the accepted descopes) the Origins baseline design. Since the baseline design did not benefit from a third design cycle, it is sub-optimal and leaves room for further cost savings. For example, we designed Concept 2 to accommodate four studied instruments, but when one instrument was removed in the descoping process, we made no attempt to reconfigure the space allocated to science instruments. Finally, we used two approaches to derive a high-fidelity estimate of the Origins baseline mission cost.¹¹

3.1.

First Decision: Origins is a Single-Aperture Telescope

The decision process and quantitative analysis that led to selection of a single-aperture telescope are documented in Sec. 3.1.1 of the Origins Space Telescope Interim Report.²⁵ This decision was made early in the study (August 2016), based on the STDT’s 14 highest-ranked science use cases for the mission out of a total of 46 such cases developed in collaboration with the community. The process unfolded in four steps: (1) the STDT established performance requirements needed to achieve a preliminary set of scientific objectives and separated them into musts and wants; (2) four notional mission concepts were introduced, and approximate performance metrics (angular resolution and sensitivity) were calculated for each concept; (3) the STDT evaluated how well each concept would meet requirements derived from the prioritized use cases, and considered perceived risks associated with each concept; and (4) the STDT chose the best overall concept based on these considerations. The four notional mission concepts were: 5- and 15-m single-aperture telescopes; a 3.5-m dual-aperture interferometer with up to a 20-m baseline; and a 6.5-m dual-aperture interferometer with up to a 50-m baseline. Two performance metrics proved to be valuable discriminants. Based on the 14 prioritized use cases, the STDT determined that the mission should provide angular resolution better than 1.24″ at $\lambda =50\mu \mathrm{m}$ wavelength, and the total set of proposed observations should be completed in under 5 to 10 years integration time, not accounting for observing efficiency. Only the 15-m single-aperture telescope was found to satisfy these conditions. Neither the 5-m telescope nor the two interferometer configurations considered would be able to execute the entire science program in under 10 years. A single-aperture telescope was preferred over a spatial interferometer, in short, because the STDT’s favored science required extraordinary sensitivity but not sub-arc sec angular resolution.

3.2.

Mission Concept Evolution

Figure 6 shows a side-by-side comparison of Concept 1, Concept 2, and the baseline mission concept. We never actually studied a telescope as large as 15 m. Based on earlier studies, we knew that a UV/optical telescope with a primary mirror as large as 9 m could be folded to fit into a 5-m diameter launch vehicle fairing, so we strove first to find a solution for Origins in that size range.

Concept 1 has a 9.1-m diameter off-axis telescope cryocooled to 4 K and five science instruments, a medium resolution survey spectrometer (MRSS), a high-resolution spectrometer (HRS), a mid-infrared imager, spectrometer, coronagraph (MISC), a far-infrared imager/polarimeter (FIP), and the Heterodyne Receiver for Origins (HERO).²⁵ Collectively, these instruments and this telescope were designed to satisfy the measurement requirements of the fourteen highest-ranked observing programs mentioned in Sec 3.1, and could do a good deal more. The primary mirror, a large Instrument Accommodation Module (IAM), and a five-layer sunshade were stowed for launch and would be deployed on-orbit. Long-wavelength instruments are large, and we learned that the telescope, IAM, sunshield and spacecraft bus would not fit into a 5-m diameter fairing. Thus, we were forced to adopt NASA’s Space Launch System (SLS) with an 8.4-m diameter fairing as the launch vehicle.

Concept 2 has very few and only straightforward deployments, as shown in Fig. 7, and like Concept 1, takes advantage of the large launch vehicles under development for human space exploration (the SLS with an 8.4-m diameter fairing, or a similar commercial vehicle). Study engineers determined that a 5.9-m diameter telescope would leave room in the fairing for a two-layer Spitzer-like sunshield, and the STDT decided to adopt this size, as it would provide “science margin” over the 5.3-m minimum required aperture diameter mentioned in Sec. 2.3. The STDT settled on three main science goals for the mission, and for each goal established three well-defined scientific objectives (Fig. 1).¹⁰ Working again in consultation with the community, the STDT developed 25 use cases for a notional 5.9 m, 4 K telescope, nine in support of the primary objectives and others that were deemed meritorious. To maximize the mission’s scientific potential, all of these use cases were allowed to influence instrument choices for Concept 2, and the measurement requirements flowing from the nine primary objectives were treated as fundamental design drivers. We extended the short wavelength cutoff to 2.8 from $5\mu \mathrm{m}$ to capture an important methane band at $3.3\mu \mathrm{m}$ in support of the search for biosignatures. The study team developed a new instrument for Concept 2, the Origins Spectrometer for Surveys (OSS),²⁶ which combined MRSS and HRS far-IR spectroscopic capabilities, and we truncated the long wavelength cutoff at $588\mu \mathrm{m}$ because the atmosphere is relatively transparent at longer wavelengths, enabling complementary measurements with ground-based sub-millimeter and millimeter-wave telescopes. The MISC,²⁷ FIP,²⁸ and HERO²⁹ instrument designs were further developed. Based on its mass and the fiducial $1M/kg cost factor, the total cost of Concept 2 was estimated to be roughly $8B, indicating that we should seek ∼$3B in cost savings to reach the target cost.

Fig. 4

A single-aperture telescope is best suited to achieve the STDT’s prioritized science goals. Subsequent trades look at aperture size, packaging and deployment schemes, materials, and instrument selection. Figure 5 shows the section of this paper in which each trade is discussed.

Fig. 5

Origins Space Telescope major trades and decisions.

The team then considered 32 descope options and strove to maximize science per dollar in a less expensive mission, which was to become the Origins baseline concept. The STDT considered but rejected a reduction in telescope size; the baseline telescope is still 5.9 m. The MISC Transit (MISC-T) spectrometer retained all three of its spectral channels, and it offers continuous spectral coverage from 2.8 to $20\mu \mathrm{m}$, an excellent wavelength range in which to search for biosignatures. However, we removed the MISC camera, which was designed to provide wide-field imaging and low-resolution spectroscopic capability with filters and grisms in the 5 to $28\mu \mathrm{m}$ range. This change leaves a small gap in Origins’ wavelength coverage from 20 to $25\mu \mathrm{m}$. All six wavelength bands of OSS were retained, giving this instrument continuous spectral coverage from 25 to $588\mu \mathrm{m}$, an ideal range in which to measure the physical properties of distant galaxies and look for water in protoplanetary disks. OSS keeps its high and ultra-high spectral resolution modes. However, the number of detector pixels was halved, resulting in a factor of two reduction in mapping speed. Only two of FIP’s original four spectral channels were retained: the 50 and $250\mu \mathrm{m}$ bands, and FIP can still make polarimetric measurements. The FIP pixel count was also reduced. The telescope temperature was adjusted from 4 to 4.5 K to take advantage of a stage common to existing cryocoolers.²⁰ In the descope, we gave up the MISC camera, the 100 and $500\mu \mathrm{m}$ channels of FIP, and the HERO instrument. Even with their reduced pixel counts, OSS and FIP offer spatial-spectral and photometric survey speeds, respectively, more than six orders of magnitude faster than Herschel, and several orders of magnitude faster than JWST and ALMA, which were not designed for wide-area surveys (see Fig. 11 in Ref. 11). By far the greatest cost savings, summing to an estimated $1.7B, came from the elimination of the MISC camera and HERO. The tabulated savings account for estimated system-level cost impacts, which go beyond the hardware costs of the instruments themselves. The sacrificed capabilities are visible in the Origins focal plane (Fig. 8). In total, the accepted descopes are expected to reduce the mission cost by ∼$2B. The STDT estimated qualitatively that, with these changes, the Origins baseline mission would still be able to accomplish 80% of the science represented by the 25 use cases adopted for Concept 2, without significantly compromising the nine primary scientific objectives shown in Fig. 1. When carefully modeled, the baseline mission cost ($6.7 to 7.3B at the 50% and 70% confidence levels, respectively)¹¹ turned out to be greater than but in reasonable agreement with $8B to $2B, considering that $8B and $2B were rough estimates.

Fig. 6

Three concepts for the Origins Space Telescope showing the design evolution.

3.3.

Trades and Decisions

Figure 5 summarizes the major trades and decisions reached during the Origins mission concept study. Experience with Spitzer,¹⁶ the Herschel Space Observatory,¹⁹ and JWST,³ and Concept 1, strongly influenced our decisions.

Fig. 7

On-orbit deployment of the Origins Space Telescope. This deployment scheme was adopted for Concept 2 and the baseline design. Origins relies on three simple and standard deployments for its communication antenna, solar array and protective cover ejection, and only one new but simple approach to deploying a two-layer sun shade. Much like a “pop-up tent,” the shade is spring loaded and unfurls, then translates to separate the layers. For comparison, to fit into a 5-m fairing, JWST relies on 22 deployment events. (Video 1, MP4, 8 MB [URL: https://doi.org/10.1117/1.JATIS.JATIS.7.1.011014.1]).

3.3.3.

Operating temperature

To suppress the observatory’s thermal self-emission and approach the natural astrophysical background photon noise level—essential to achieving Origins’ science goals—the telescope’s optical elements must be cooled to $<6\mathrm{K}$.¹¹ At wavelengths $>200\mu \mathrm{m}$, the background noise level is extremely sensitive to telescope temperature: 4 K is much better than 6 K. Past far-infrared missions, including Spitzer, were cooled with expendable cryogens, but that approach requires a large, massive cryostat and limits the life of the mission. Advances in cryocooler technology for JWST, Hitomi, and other missions enabled us to adopt mechanical cryocoolers rather than expendable cryogens for Origins.¹³ As shown in Fig. 6, we adopted 4 K as the operating temperature in Concept 1, but later relaxed that temperature to 4.5 K to take advantage of current-state-of-the-art cryocooler technology.¹³ The quoted operating temperature is the temperature of the telescope, including the baffle, mirrors, and support structure, and the volume that houses the instruments.

Fig. 8

Concept 2 instrument footprints in the Origins focal plane. Items with a dashed line border were descoped to derive the baseline mission concept.