Spitzer is unique in many ways; the cryostat and mission design allow a very long cryogenic lifetime (5.7 years) with a relatively small amount of cryogen (~360 liters of superfluid helium). [For comparison, IRAS had a 10-month lifetime using 560 liters of liquid helium, and ISO had a 28-month lifetime using 2140 liters of cryogen.] During the prime mission the helium bath temperature was maintained at 1.24 K, the telescope temperature could be ≤ 6 K, and the outer shell temperature is approximately 34 K. Some of Spitzer’s unique features are its Earth-trailing solar orbit and the fact that the telescope was launched at ambient temperature. This “warm launch” architecture allows Spitzer to achieve its lifetime goal with a smaller, lighter cryostat than would otherwise be required.
A basic external view of Spitzer is shown in Figure 3.2. The spacecraft provides structural support, pointing control and telecommunications and command/data handling for the entire observatory. The spacecraft, including the spacecraft bus, the solar panel and PCS (including the Pointing Control Reference Sensors, or PCRSs, which are located in the focal plane) were provided by Lockheed Martin Space Systems Company.
The CTA, shown in Figure 3.3, consists of the telescope, the superfluid helium cryostat, the outer shell group and the Multi-Instrument Chamber (MIC), which hosts the cold portions of the IRAC, IRS, and MIPS science instruments and the PCRS. The telescope assembly, including the primary and secondary mirrors, metering tower, mounting bulkhead (all made of beryllium) and the focus mechanism, is mounted and thermally connected to the cryostat vacuum shell. The barrel baffle of the telescope assembly is separately attached to the vacuum shell at its flange. The MIC, an aluminum enclosure containing the instrument cold assemblies, is mounted on top of the helium tank. The vapor-cooled shields, the vacuum shell and the outer shields are connected to the cryostat by a series of low connectivity struts. The CTA was provided by Ball Aerospace and Technologies Corporation.
The spacecraft and CTA (and science instruments) together comprise the observatory, which is ~4 m tall, ~2 m in diameter, and has a mass of ~900 kg.
Figure 3.2: Basic external view of Spitzer. The observatory coordinate system XYZ (shown in Figure 3.4) is an orthogonal right-hand body-fixed frame of reference. The X-axis passes through the geometric center of the top surface of the spacecraft, is parallel to the CTA optical axis (which passes through the primary and secondary mirror vertices), and is positive looking out of the telescope. The Z-axis intersects the line forming the apex of the two surfaces of the solar panel. The Y-axis completes the right hand orthogonal frame. The X-axis origin is defined such that the on-axis point between the CTA support truss and the spacecraft bus mounting surface is located at X = +200 cm, in order to maintain positive X values throughout the observatory. The Sun always lies within 2º of the XY plane (i.e., the roll angle is constrained to ±2º).
Spitzer executes autonomously a pre-planned, typically week-long, schedule of science observations, calibrations and routine engineering activities, which has been uploaded in advance and stored on board. This “master sequence” might have 14 different 12–24 hour Periods of Autonomous Operation (PAOs), containing observations and calibration activities, each followed by a 30-60 minute period spent re-orienting the spacecraft for downlink and transmitting the data to the ground. For IRS campaigns, we routinely had a 24-hour PAO with one downlink lasting longer than 40 minutes. After the downlink, Spitzer returns to the pre-planned sequence of observations and calibrations. Because efficient communication with the ground requires use of the high-gain antenna mounted on the bottom of the spacecraft (Figure 3.2), executing a downlink of the collected data requires slewing the spacecraft to orient the high gain antenna toward one of the Deep Space Network (DSN) stations on Earth. Any of the three DSN sites (Canberra, Madrid and Goldstone) can be used when visible. During the time that data are being transmitted to the ground, no science data can be collected. In extreme circumstances, Spitzer is designed to survive for up to a week with no ground contact at all.
Figure 3.3: Cryogenic Telescope Assembly.
Figure 3.4: Observatory Coordinate System.
3.4.1 The Warm Launch and Telescope Temperature Management
One of the unique aspects of Spitzer is the warm launch concept. The telescope was at ambient temperature at the time of launch and gradually reached its lowest operating temperature (≤ 6K) approximately 45 days after launch, through passive cooling and by vapor vented from the cryostat. The science instruments are in contact with the helium bath at all times and were thus cold at the time of launch. The on-orbit performance confirms the soundness of the Spitzer thermal design principles.
Because the telescope is cooled by vapor vented from the cryostat, the telescope temperature depends upon how much power is dissipated in the cryostat. Because the cryogenic design and performance result in near-zero parasitic heat loads into the cryostat, the power entering the helium bath is essentially that of the instrument in use. Due to variations in the power dissipation of the instruments, the telescope temperature would naturally vary from ≤ 6 K to ~12 K. A “make-up” heater is available to add additional heat when needed to lower the telescope temperature. Since very low background is required for 160 µm observations, make-up heater operations are always planned to cool the telescope to ≤ 6 K and maintain it at that temperature during MIPS campaigns that are obtaining 160 microns observations. This is accomplished using a heat pulse that has been optimized to use the least amount of cryogen, while still guaranteeing a cold telescope for MIPS; it uses considerably less cryogen than holding the temperature constant all the time. The heat pulse is typically ~10 hours in duration and takes place about 10 days prior to the start of the MIPS campaign. The details of pulse size and duration are optimized to minimize cryogen usage and maximize scheduling efficiency.
In order to maximize Spitzer’s cryogenic lifetime, starting in Cycle 2, MIPS campaigns were organized into “warm” (telescope cooled to ~8.5 K) or “cold” (~5.5 K) campaigns. AORs requiring 160 microns were scheduled only in cold (~5.5 K) MIPS campaigns. MIPS observations not requiring 160 microns were scheduled in warm (~8.5 K) or cold (~5.5 K) campaigns.
3.4.2 Data Storage
The Spitzer spacecraft includes the command and data handling subsystem (C&DH) which shares the flight computer (a RAD 6000) with the pointing control subsystem (PCS). The C&DH validates and executes either previously stored or real time commands, receives and compresses data, and writes the compressed data into the mass memory. The C&DH also provides a stable clock to correlate data and events.
During normal science operations, the observatory collects, compresses, and stores 12 to 24 hours’ worth of science data prior to downlinking it. Spitzer has 8 Gbits of solid state memory available for each of two redundant flight computers, for a total of 16 Gbits of storage (for both science and engineering data). During each 12 to 24-hour PAO, up to 6 Gbits of the memory are filled. Enough additional storage capacity is left unused during that 12 to 24-hour PAO to permit missing a downlink opportunity (e.g., due to a problem at the ground station) and continuing to observe without overwriting previously collected science data. Since the full mass memory (16 Gbits) is available to both redundant computers, in general multiple passes would have to be missed before the risk of losing science data that had not been transmitted to the ground becomes significant. However, e.g., MIPS sometimes generates enough data that a single missed pass could become important.
3.4.3 Data Transmission
Spitzer has two antenna systems for data uplink and downlink. The high gain antenna (HGA) supported a maximum downlink rate of 2.2 Mbit/s for the first 2.6 years of the Spitzer cyrogenic mission. Since Spitzer drifts away from the Earth at about 0.12 AU per year, the data transmission rate on the HGA will have to decrease as a function of time, and late in the mission, we will have to change our downlink strategy to accommodate this. The two low gain antennae (LGA) give wide angle coverage, but only supported downlink at 44 kb/s at the beginning of routine observing, and decreased to 40 kb/s after 3 years. The LGA are used in safe mode, but are not normally used to transmit science data during routine science operations.
Because of the location of the HGA, it is necessary to stop observing and point the telescope in the anti-Earth direction to downlink the science data. Depending upon the details of the observations that are scheduled, Spitzer produces ~1 to 6 Gbits of (compressed) science data during 12–24 hours of observing. Initially, Spitzer collects data for about 12–24 hours and then spends 0.5–1 hour downlinking the data. The scheduling system predicts the compressed data volume that is generated during each downlink and schedules the downlink contact time accordingly. The downlink periods are also used for some spacecraft maintenance activities (such as dumping angular momentum) and for uplinking commands. If low volumes of data are being generated, longer periods of time between downlinks may be used.
At each downlink opportunity, Spitzer attempts to transmit all the data in the mass memory. Any data that the ground has not confirmed as received will be retransmitted at the next downlink pass. It is anticipated that some data could be missed at any pass, and it may take several passes before the ground receives all the science data for a given observation.
Whenever the HGA is used for downlink, it is also possible to uplink commands and files to Spitzer. Normally, up to ~5 communication periods per week are used to send up all the sequences and information needed to support the next upcoming week of observations. No communications with Spitzer are planned outside the scheduled downlink sessions.
The downlink strategy has little impact on observation planning with Spitzer. It is one component in determining the maximum length of an AOR, but the necessity for instrument maintenance operations and pointing system calibrations on shorter than 12 hour time scales constrains the maximum AOR length more tightly than the downlink schedule1. A 12 to 24 hour interval between downlinks does imply that it can be a day or longer before science data are available on the ground after an observation is complete.
The uplink strategy also does not directly affect observation planning, but it drives the time scale on which changes to the stored science schedule and its contents can be made (e.g., for a ToO).