The development of the flight system, apart from the instruments, was done by contractors Ball and Lockheed following a feasibility study of the warm launch architecture done at JPL. The instrument designs were done by the Principal Investigators working with their suppliers as described above. The Spitzer design is described in detail by R. Gehrz et al. (2007, Rev. Sci. Inst., 78, 1302) and M. Werner (2006, A&G, 47, 6.11); the design and implementation incorporates a number of innovations which may be summarized as follows:
2.2.1 A Prime Directive
The Spitzer telescope and its three science instruments were cooled to their ultimate operating temperatures by liquid helium cryogen, which itself achieved a temperature of about 1.2 K under the pumping action of the vacuum of space. The cooling was necessary to minimize the intrinsic infrared radiation of the telescope structure and to reach the optimum operating point for the detectors in the instruments. Approximately 350 liters of helium were loaded into the Spitzer cryogen tank (Figure 2.3) prior to launch, and when the last helium boiled away in May 2009, the primary Spitzer cryogenic mission ended. Thus the prime directive for the design of the Spitzer mission was to minimize the unnecessary, or parasitic, heat reaching the helium tank. This dictated both the choice of Spitzer’s unusual orbit and the unusual architecture of Spitzer’s cryogenic/thermal system.
The design of the cryogenic/thermal system isolated the telescope and instrument chamber so well that after the liquid helium was depleted they remained at temperatures cold enough to still use the shortest two channels on the IRAC camera. These two channels were used for over 10 years after the cryogenic mission ended during the warm mission.
2.2.2 A Kinder, Gentler Orbit
All previous space observatories had used a low Earth orbit (e.g., IRAS and the Hubble Space Telescope) or a high Earth orbit with a period of one-to-two days (e.g., ISO and the Chandra X-Ray Observatory). Spitzer broke this pattern by utilizing an Earth-trailing heliocentric, or solar orbit with semi-major axis very slightly larger than that of the Earth’s orbit. As seen from Earth, Spitzer recedes at about 0.1 AU per year and reached a distance of 0.62 AU in five years. This orbit satisfies the Prime Directive in several ways. First, there is great benefit in getting away from the heat of the Earth. Second, the orbit allows radiative cooling into the refrigerator of deep space to play a large part in cooling Spitzer, and in keeping it cool. Third, because there are no eclipses, and the Earth and Moon are far away, the orbit permits excellent sky viewing and observing efficiency.
The orbit has one major disadvantage. As Spitzer moves away from the Earth, its radio signals become gradually weaker due to the increasing distance. The data is stored on board and radioed back to Earth through the dishes of the Deep Space Network (DSN). The nominal downlink strategy of one or two ~45 minute passes per day and a downlink data rate of 2.2 Mb/s worked for the cryogenic mission, although the largest DSN dishes (70 meters) were required for the final years of the cryogenic mission and until the spacecraft was decommissioned. During the warm mission Spitzer continued to drift further from the Earth, although this issue was partly mitigated by reduced data collection rates in this phase. At the time the spacecraft was decommissioned the downlink rate was 512 kB/s using either one 70-m or a combination of one 70-m and one 30-m DSN dish. Downlinks were 120 to 150 minutes long and typically occurred 3 to 4 times a week, and the spacecraft was 1.77 AU from Earth.
2.2.3 Flight System Architecture
Spitzer’s novel cryogenic architecture and the overall configuration of the flight system are shown in Figure 2.3. The spacecraft and solar panel were provided by Lockheed Martin. The telescope, instruments, cryostat, and associated shields and shells make up the Cryogenic Telescope Assembly (CTA), built by Ball Aerospace.
Figure 2.3: The outer shell forms the boundary of the CTA, which incorporates the telescope, the cryostat, the helium tank, and the three instruments. Principal Investigator-led teams provided the instruments, while Ball Aerospace provided the remainder of the CTA. Lockheed-Martin provided the solar panel and the spacecraft bus. The observatory is approximately 4.5m tall and 2.1m in diameter; the mass at launch was 861kg. The dust cover atop the CTA was jettisoned 5 days after launch. (Image courtesy of Ball Aerospace).
The novel features of the Spitzer cryo-thermal design and the benefits it bestows are illustrated in Figure 2.4, which compares the architecture of Spitzer as it now flies with an earlier design which resembles that of the predecessor ISO and IRAS missions. ISO and IRAS packaged the telescope inside the cryostat so that it was in close conductive contact with the liquid helium and thus cold at launch. Spitzer employed a novel design in which the greater part of the CTA was launched while at room temperature; only the science instrument cold assemblies and the superfluid helium vessel were cold within the vacuum cryostat shell. As shown in Figure 2.4, this allowed a much smaller vacuum pressure vessel and a smaller observatory mass than achieved by the cold launch architecture while maintaining telescope size and lifetime.
Figure 2.4: The warm launch architecture of Spitzer as it was flown [right] is compared with an earlier cold launch concept similar to the predecessor IRAS and ISO missions. The warm launch concept achieves the same mission lifetime and primary mirror size as the cold launch approach, but at a fraction of the cost. Some of the reduction in cost and mass was achieved by streamlining Spitzer’s measurement capabilities, but the warm launch concept retains the core functionality required for the execution of Spitzer’s most important programs.
For the longest wavelengths Spitzer required the telescope be at ~5.5K and the deep space refrigerator, together with the boil-off of the liquid helium, got us there after launch. The principles of this on-orbit cooling are deceptively simple. In the solar orbit, the spacecraft can be oriented so that the sun always falls on the solar panel, which shades the telescope outer shell. A carefully designed and fabricated system of reflective shells and shields assures that little of the solar heat reaches the outer shell to diffuse inward to heat the telescope. A similar system deflects heat from the spacecraft bus, which operates near room temperature. With no heat input, the telescope should cool very rapidly by radiating its heat to space – the anti-solar side of the outer shell is painted black to facilitate this. Prior to launch it was predicted that the telescope would achieve a temperature below ~50K through this radiative cooling, and that the evaporating helium gas, which is at a temperature of only ~1.2K, would cool the telescope to its operating temperature of ~5.5K and keep it there.
The system worked just as predicted, if not better. The initial (radiative plus gas-assisted) cooldown to ~5.5 K took about 40 days. The outermost CTA shell equilibrated at an operating temperature of 34 to 34.5 K solely by radiative cooling, so that there is very little parasitic heat leaking inwards to the helium tank. It took only about one ounce of helium per day to keep Spitzer cold. Not coincidentally, this is about the same rate at which the instruments boiled away the liquid helium by dissipating power into the helium bath. So the system was both in equilibrium and in balance thermally.
2.2.4 The Spitzer Warm Mission
In May 2009, the last of Spitzer’s helium boiled away. The cryogenic portion of the CTA started to warm up, but the passively-cooled outer shell stayed at a temperature near 35K. The telescope was cooler because it could radiate through the aperture at the top of the outer shell, and it equilibrated at 27.5 K. It was anticipated that the two shortest wavelength channels (channels 1 and 2) of the IRAC instrument at 3.6 and 4.5 microns would operate with full efficiency and sensitivity under these conditions, (although Spitzer’s other channels would be inoperative), and plans were in place to continue to operate in what is now known as the Spitzer Warm Mission. During the Warm Mission the sensitivity and image quality were virtually unchanged from the cryogenic mission. Spitzer was operated as a facility for the entire astronomical community, with all of the observing time available to the user community through the usual peer-review process. The Warm Mission lasted from July 2009 until September of 2016.
In this document the phrase ‘the warm mission’ collectively refers to the Warm and Beyond Missions.
2.2.5 The Spitzer Beyond Mission
The Spitzer Beyond Mission lasted from October of 2016 until the telescope was decommissioned on January 30, 2020. Because of Spitzer's orbit and age, the Beyond phase presented a variety of new engineering challenges. Spitzer trails Earth in its journey around the sun, but because the spacecraft travels slower than Earth, the distance between Spitzer and Earth has widened over time (Figure 2.5). Spitzer communicated with Earth through an antenna fixed on the bottom of the spacecraft, so entire spacecraft had to be properly oriented to transmit and receive data. Early on, this could be done while keeping the sunlight incident on the solar panel within the original design limit of 30 degrees. In the Beyond Mission (Figure 2.5), the orbital geometry required that this incidence angle be as large as 55 degrees, so that the battery was used to help to power the downlinks, and had to be recharged during operations following the downlink session. At the same time, in these extreme orientations, sunlight was incident through the back of the spacecraft on structures which had been intended to be permanently shadowed, which might have impacted the system’s pointing performance. Fortunately, these considerations had minimal impacts on Spitzer’s scientific performance, and Spitzer returned excellent data right up to the end of the mission.
Figure 2.5: The phases of the Spitzer Mission from launch to decommissioning. In this diagram Spitzer is shown at the pitch angle needed to communicate with Earth. As the mission progressed, this angle increased. In this document the phrase ‘the warm mission’ collectively refers to the Warm and Beyond Missions
2.2.6 The Rest of the Story
Additional innovations as well as careful design, fabrication, and testing characterize the rest of the Spitzer spacecraft. Key components, described in detail in R. Gehrz et al. (2007, Rev. Sci. Inst., 78, 1302), include:
1. An all-Beryllium telescope, with an 85 cm primary mirror. Beryllium was selected because of its high strength-to-weight ratio and reproducible cryogenic behavior. The secondary mirror of the telescope could be moved in an axial direction to focus the telescope on-orbit. Pre-launch test and analysis determined the proper setting of the secondary. Only a small adjustment (less than .01 mm) in the position of the secondary was made after on-orbit cooldown to bring the telescope into proper focus. The telescope achieved diffraction-limited performance at all wavelengths longward of 5.5 microns across its entire focal plane, which had a radius of about 15 arcminutes. Spitzer achieved an image size of ~2 arcseconds at its shortest operating wavelengths.
2. An excellent pointing and control system. This system was built around a high performance autonomous star-tracker – which pointed the telescope to the desired star-field by using a pattern recognition algorithm based on an internal star catalog. In addition, the system included gyroscopes, reaction wheels to turn the spacecraft, and a visible light sensor in the telescope focal plane to establish and track the line of sight of the cold telescope relative to that of the warm star tracker. The overall performance was excellent; Spitzer could be pointed to an accuracy of less than one-half arcsecond and achieved short-term stability much less than 0.1 arcseconds.
3. A robust spacecraft. The spacecraft bus shown in Figure 2.3 provided the Spitzer telescope and instruments with a robust and reliable support system. The spacecraft performed a variety of functions including pointing Spitzer; managing execution of the science program by clocking through a series of pre-loaded commands; storing, compressing and telemetering the science data to Earth; and monitoring its own health as well as that of the telescope and instruments. The Spitzer spacecraft resembles other unsung heroes in that when it operates smoothly (which it did essentially 100% of the time) we were unaware of its existence. After almost 16.5 years in orbit, the spacecraft remained fully redundant (it had two fully functional independent sides of electronics). The Spitzer spacecraft included one expendable substance, high-pressure nitrogen gas, used to spin down the reaction wheel assemblies when they start to accumulate too much angular momentum. At the time of decommissioning, enough nitrogen remained on board for Spitzer to control its attitude for several years.
4. Detector Arrays. The properties and power of Spitzer’s detector arrays are well known to users of the observatory, and, by implication, to anyone who has seen some of Spitzer’s spectacular images or spectra. The arrays and their applications in the Spitzer instruments are discussed in detail in subsequent sections of this documentation. Here we merely emphasize that Spitzer’s great scientific power follows directly from the size and quality of the arrays used at all wavelengths and throughout the focal plane – both for imaging and spectroscopy. It is here that Spitzer achieved its gain over previous cryogenic observatories: IRAS flew single detectors, and ISO’s few small arrays covered only a small fraction of its entire wavelength range. Spitzer as a cryogenic observatory in space achieved a thousand-fold increase in sensitivity at wavelengths beyond 3 microns over what could be done from within the atmosphere. When this increase in sensitivity is exploited by arrays with tens of thousands of pixels, the scientific grasp and the gain over alternative facilities are measured in factors of millions.
