How Long Can the Hubble Space Telescope Operate Reliably?–A Total Dose Perspective

The Hubble Space Telescope has been at the forefront of discoveries in the field of astronomy for more than 20 years. It was the first telescope designed to be serviced in space and the last such servicing mission occurred in May 2009. The question of how much longer this valuable resource can continue to return science data remains. In this paper a detailed analysis of the total dose exposure of electronic parts at the box level is performed using solid angle sectoring/3-dimensional ray trace and Monte Carlo radiation transport simulations. Results are related to parts that have been proposed as possible total dose concerns. The spacecraft subsystem that appears to be at the greatest risk for total dose failure is identified. This is discussed with perspective on the overall lifetime of the spacecraft.


I. INTRODUCTION
The Hubble Space Telescope (HST) was deployed from the space shuttle Discovery on April 25, 1990 into a low Earth orbit (LEO) with an approximate altitude of 569 km and inclination of 28.5 degrees. Although its primary 2.4 meter diameter mirror is not large in comparison to ground-based telescopes the advantages of being in orbit have contributed to its extraordinary scientific success. Being outside Earth's atmosphere avoids atmospheric distortions and almost all background light so that very high resolution images can be taken. In addition it allows HST to view portions of the ultraviolet and infrared spectra not observable with Earthbased telescopes.
HST's observations and discoveries have ranged from those in our own solar system to nearly the edge of the universe. They have given views of the universe within a few hundred million years of the Big Bang and helped establish its age of  13.7 billion years. Possibly the most profound conclusion has been drawn from observation of light emitted by a certain category of supernova explosions, which is that the expansion of the universe is not slowing due to gravity but accelerating. This is attributed to dark energy, an apparently dilute entity spread over all space that is significant on a cosmological scale. One of the deepest images of the universe in optical light is shown in Fig. 1. HST helped establish how galaxies are formed and evolve from the generally smaller and irregular galaxies billions of years ago to the larger and more structured galaxies of recent times such as spiral and elliptical galaxies. Within our solar system HST images of the fragmented comet Shoemaker-Levy 9 colliding with Jupiter helped raise public awareness about potential comet and asteroid collisions with Earth. Many more significant observations and discoveries by HST exist that are much too numerous to mention. Reference [1] provides an excellent overview.

II. SERVICING MISSIONS
HST was the first telescope designed to be serviced in space. The servicing missions are the primary reason that it has functioned at such a high level for a long period of time [2]. Following its deployment in April 1990 a much publicized spherical aberration was discovered in the primary mirror in June of that year. Servicing Mission 1 (SM1), which occurred in December 1993, was used to correct the mirror's flaw by installing corrective optics. It was also used to replace the wide field planetary camera with an improved one and for planned maintenance. Servicing Mission 2 (SM2) occurred in February 1997. It featured the installation , and the repair of 2 instrumentsthe Space Telescope Imaging Spectrograph (STIS) and the ACS. All 6 batteries and all 6 gyroscopes were replaced, and a refurbished Fine Guidance Sensor (FGS) was installed in addition to other activities. The James Webb Space Telescope (JWST), generally regarded as the successor to HST, will be launched no sooner than 2018. For scientific reasons it is preferable that the two telescopes operate simultaneously for at least a year or two. One of the issues that has come to the forefront in this regard is the total ionizing dose (TID) exposure of microelectronic components in HST. A 2005 report issued by the National Academy of Sciences found that "Adverse radiation effects after 2010 are more likely, with an increasing risk of avionics component failures if science operations are extended until 2014" [3]. The purpose of this paper is to evaluate the possibility of TID failures in HST until JWST is commissioned and beyond. The analysis is done in greater detail than has been considered previously and with a newer and more appropriate model for the radiation environment. These new results are useful for HST's contingency planning and life extension initiatives. Other significant factors along these lines not discussed here include battery, gyroscope, fine guidance sensor and instrument lifetimes; avionics system reliability; and orbital decay [3,4].

III. METHODS
Due to the longevity of HST the main radiation concern at this point is a hard failure due to TID or displacement damage that could bring science operations to a halt. The potential for TID failures in HST has been investigated on several occasions internally at GSFC with limited detail about shielding. Consequently worst case assumptions were typically made in these analyses [5][6][7]. In an effort to provide a more robust analysis an extensive review of the HST mechanical design was undertaken, particularly the subsystem and instrument dimensions and wall thicknesses, masses and placement within the spacecraft.
The Numerical Optimizations, Visualizations, and Integrations on Computer Aided Design (CAD)/Constructive Solid Geometry (CSG) Edifices (NOVICE) code [8] was used for analysis by solid angle sectoring/3-dimensional ray trace and Monte Carlo radiation transport. This code has two main advantages. First it interfaces with CAD models, allowing complex shielding geometries such as spacecraft to be analyzed. Second, it runs in an adjoint mode, as opposed to a forward mode, which greatly increases the calculation efficiency. A detailed CAD model of the HST spacecraft (not including the subsystems and instruments) was obtained from Lockheed Martin and converted to the NOVICE radiation model shown in Fig.2. Layout and box locations were provided and were used to cross-check information in the mechanical design. Instruments are generally placed toward the bottom (left side of Fig.2) of the spacecraft behind the primary mirror. Subsystems are generally more toward the periphery in bay regions for the optical telescope assembly and support systems. The interiors of the spacecraft top door, bottom and walls were converted to honeycomb material to match their known composition. The total mass of each box and instrument is accurately known but the internal mass distribution within each unit was not readily available for this analysis. It was therefore assumed that except for electronics boards the interior mass was uniformly distributed within each unit and was given a density such that the total mass equaled the known mass of the unit. Electronics boards were placed in units based on the position of connectors shown in mechanical drawings. Two 3 x 3 arrays of virtual radiation dose detectors were placed on the top and bottom sides of each board to evaluate the dose variation. The reported doses are the mean, maximum and minimum values seen in the virtual detectors for each unit. The total mass accounted for in these simulations was 96% of the actual HST mass of 12,218 kg. This seems quite reasonable considering the simulation does not include items such as cabling.
The five servicing missions add a degree of complexity to the TID analysis. In some cases components are exposed to TID from launch to the end of mission. In other cases they are exposed from a servicing mission to mission end or even from one servicing to a subsequent servicing. Thus, similar to Poivey [6] a procedure was implemented to track the start and end dates of TID exposure of all components.
Next the choice of radiation environment models is considered. At the electronics shielding levels of HST, TID is mainly due to trapped protons with a smaller contribution from trapped electrons. The dose due to solar events in this low Earth, low inclination orbit is very small by comparison because of geomagnetic shielding. The trapped proton flux in LEO is anisotropic. However, HST has been maneuvered many times to focus on objects and regions of space in varying directions. The proton flux for the purpose of TID calculations is therefore assumed to be isotropically incident. The longtime standard Aerospace Proton-8 (AP-8) model [9] for trapped protons is now out of date and known to have shortcomings [10]. Consequently there have been a number of notable efforts to develop new trapped proton models [11][12][13][14]. In principle the environment for HST from its 1990 launch to the present is known so it is preferable to use a trapped proton model that is calibrated to environmental parameters during these times. This should include a description of solar cycle dependence because the HST servicing missions occurred during different phases of the solar cycle. The only trapped proton model that satisfies these criteria is the Boeing Trapped Proton Model-1 (TPM-1) [11], a model based largely on the long-term Television Infrared The new AP-9 model, version 1.2, was therefore used to extrapolate the energy spectra out to 2 GeV. This was done by normalizing each energy spectrum to the average fluence of the two models at 81.3 MeV. The resulting average value is in close agreement with trapped proton data measured by the Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX) instrumentation [12]. The utility of AP-9 for this study is limited to this because it does not contain an explicit solar cycle dependence. The choice of a trapped electron model has little influence on the final results because the TID due to trapped electrons is substantially less than that due to trapped protons. The Aerospace Electron-8 (AE8) model [17] was chosen on the basis that it contains approximate solar cycle dependence.
IV. RESULTS Fig. 4 shows results for 6 TID vs. shielding depth curves from launch and each servicing mission to the start of calendar year 2020. The curves were calculated using the methods described above. In order to obtain an initial assessment of the situation the shielding geometry was first assumed to be a solid aluminum sphere. The sharp fall-off of the curves for shielding thickness < 25 mils is due to the relative ease at which electrons and protons with energy less than about 10 MeV are attenuated by shielding. This result verifies that the dose due to high energy protons is the main contribution for shielding relevant for the spacecraft electronics.   5 shows the expected dose for each instrument and subsystem unit currently onboard HST using the model shown in Fig.2. Acronyms are defined in Appendix I. Calculations were done from the time the unit was inserted until 1/1/2020. The doses range from about 2.5 to 11 krad(Si). As described in section III the error bars represent the maximum and minimum dose values within each unit. The TID requirements for HST parts range from 5 to 15 krad(Si). However, conflicting reports exist as to how these numbers were derived. The HST Parts and Control Plan specifies this should depend on whether test data for parts are generic or flight lot specific. On the other hand a Lockheed Martin report specifies the requirement is 3 times the calculated solar minimum dose for a 5 year period where the dose is calculated inside specific areas of the spacecraft [18]. In any event both specifications produce a range of approximately 5 to 15 krad(Si). From the box level perspective shown in Fig.5 the exposure of the majority of units falls within the 5 to 15 krad(Si) dose requirement range, indicating potential problems. However, these are the minimal TID requirements for electronics and a review of a large number of parts showed that they were often selected to significantly exceed these requirements [18]. Examination of available parts lists showed many parts were procured to be TID hard to 100 krad(Si).
There are over 14,000 electronic parts procured for HST dating back to pre-launch so an exhaustive parts analysis is no longer realistic. Parts analyses have been done in the past that have identified key components as potential concerns. The most extensive of these, reference [18], grouped parts into families of technologies, ranging from the CD4000 series Complementary Metal Oxide Semiconductor (CMOS), the S series Schottky bipolar, the LS series low power Schottky bipolar and the L series low power bipolar. In addition there were the LM series bipolar linears. Within each family of technology an attempt was made to obtain radiation data and if the technology appeared to be fairly robust, i.e., tolerant to at least 50 krad(Si), spot checks of a few part types within a family were made. Attention was paid to bipolar parts in light of the fact that the Enhanced Low Dose Rate Sensitivity (ELDRS) effect was not discovered until after the launch of HST. Data were mainly obtained from the DoD Nuclear Information Analysis Center (DASIAC), which housed data going as far back as 1976 but is no longer available for use. These results are summarized in Appendix II. HST was initially developed in the 1980s when bipolar technologies were generally more advanced and TID hard than CMOS technology. As a result the parts listed turned out to be all CMOS. Inspection of the appendix indicates that the multiple access transponder (MAT) units are a concern going forward because they contain microprocessors, Random Access Memory (RAM) and Read Only Memory (ROM) with low TID hardness that has already been significantly exceeded beyond the uncertainty in the simulation. The build-up of dose in MAT-1 over the course of the mission and up to 2020 is shown in Fig.6 along with the TID exposure for each year. However, there are several factors that work in favor of the continued operation of these units. Annealing of the parts is not accounted for in the simulation and this is likely significant considering the length of time these parts have been in orbit. In addition the specified TID hardness could be conservative due to the test methodology chosen such as bias conditions and dose rates. Finally, the parts may still operate satisfactorily even if their parameters begin to go out of the manufacturers specifications. The latter appears to be the case for the Field Programmable Gate Arrays (FPGAs) in the solid state recorders shown in Appendix II. These arrays develop increased leakage current in the 5 -10 krad(Si) dose range but otherwise perform well out to 15 krad(Si) [19]. It was reported that a radiation failure was believed to have occurred in a GaAs LED used in an optical encoder of a fine guidance sensor (FGS) [3]. The LED was characterized by a reduced light output over time and is no longer onboard HST. However, a thorough investigation of the flight lot LEDs at GSFC indicated the most likely cause of the failure was a degraded solder joint [20]. Our radiation simulations support this as well. If the LED failure was due to radiation it would be a result of displacement damage. The exposure of the failed unit was calculated using nonionizing energy loss [21] to determine equivalent fluences of 1 MeV neutrons and 10 MeV protons in GaAs. These results are shown in Table I for the failed unit, FGS-0, and three other units currently onboard HST. All LEDs are from the same flight lot. Since the performance of the other three LEDs has not deteriorated substantially in spite of their greater exposure our simulations are not consistent with a radiation failure of FGS-0.  The Hubble Space Telescope has been in orbit for over 24 years. As a result of its longevity, potential total dose failures have become an important consideration for the mission's continuation. A complete TID analysis of HST has been performed at the box level and compared to electronic and photonic parts that are potential problems. From this analysis the biggest radiation concern is the performance of several parts in the transponder units although the parts analysis is not a complete one. Calculation of nonionizing dose exposure of FGS units indicated that LED degradation in optical encoders due to displacement damage should not limit their performance. The results of this analysis are beneficial to the HST Project in their contingency planning and prioritization of life extension initiatives.

Appendix I
Mean doses expected within units currently onboard HST at the start of calendar year 2020. *FGS-2 was taken out of HST during SM3A and re-inserted during SM4 **RSU-1 was taken out of HST during SM1 and re-inserted during SM4