The Spitzer Space Telescope is executing the ninth year of extended operations beyond its 5.5-year prime mission. The project anticipated a maximum extended mission of about four years when the first mission extension was proposed. The robustness of the observatory hardware and the creativity of the project engineers and scientists in overcoming hurdles to operations has enabled a substantially longer mission lifetime. This has led to more challenges with an aging groundsystem due to resource reductions and decisions made early in the extended mission based on a shorter planned lifetime. We provide an overview of the extended mission phases, challenges met in maintaining and enhancing the science productivity, and what we would have done differently if the extended mission was planned from the start to be nearly twice as long as the prime mission.
Spitzer Warm Mission operations have remained robust and exceptionally efficient since the cryogenic mission ended in
mid-2009. The distance to the onow exceeds 1 AU, making telecommunications increasingly difficult; however,
analysis has shown that two-way communication could be maintained through at least 2017 with minimal loss in
observing efficiency. The science program continues to emphasize the characterization of exoplanets, time domain
studies, and deep surveys, all of which can impose interesting scheduling constraints. Recent changes have significantly
improved on-board data compression, which both enables certain high volume observations and reduces Spitzer's
demand for competitive Deep Space Network resources.
Following the successful dynamic planning and implementation of IRAC Warm Instrument Characterization activities,
transition to Spitzer Warm Mission operations has gone smoothly. Operation teams procedures and processes required
minimal adaptation and the overall composition of the Mission Operation System retained the same functionality it had
during the Cryogenic Mission. While the warm mission scheduling has been simplified because all observations are
now being made with a single instrument, several other differences have increased the complexity. The bulk of the
observations executed to date have been from ten large Exploration Science programs that, combined, have more
complex constraints, more observing requests, and more exo-planet observations with durations of up to 145 hours.
Communication with the observatory is also becoming more challenging as the Spitzer DSN antenna allocations have
been reduced from two tracking passes per day to a single pass impacting both uplink and downlink activities. While
IRAC is now operating with only two channels, the data collection rate is roughly 60% of the four-channel rate leaving a
somewhat higher average volume collected between the less frequent passes. Also, the maximum downlink data rate is
decreasing as the distance to Spitzer increases requiring longer passes. Nevertheless, with well over 90% of the time
spent on science observations, efficiency has equaled or exceeded that achieved during the cryogenic mission.
The Spitzer Space Telescope, the fourth and final of NASA's Great Observatories, was launched in August 2003. It has been a major scientific and engineering success, performing science observations at wavelengths ranging from 3.6 to 160 microns, and operating at present with a roughly 92% science duty cycle. This paper describes the essential role and procedures of the Spitzer Observatory Planning and Scheduling Team (OPST) in providing rapid rebuilds of sequences to enable the scheduling of Targets of Opportunity and to recover from anomalies. These procedures have allowed schedulers to reduce the nominal lead time for science inputs from six weeks to 2 or 3 days. We discuss procedures for modifications to sequences both before and after radiation to the spacecraft and lessons learned from their implementation.
The primary scheduling requirement for the Spitzer Space Telescope has been to maximize observing efficiency while
assuring spacecraft health and safety and meeting all observer- and project-imposed constraints. Scheduling drivers
include adhering to the given Deep Space Network (DSN) allocations for all spacecraft communications, managing data
volumes so the on-board data storage capacity is not exceeded, scheduling faint and bright objects so latent images do
not damage observations, meeting sometimes difficult observational constraints, and maintaining the appropriate
operational balance among the three instruments. The remaining flexibility is limited largely to the selection of
unconstrained observations and optimizing slews. In a few cases, the project has succeeded in negotiating DSN tracks to
accommodate very long observations of transiting planets (up to 52 hours to date with even longer requests anticipated).
Observational efficiency has been excellent with approximately 7000 hours of executed science observations per year.
Many future space missions will use cadmium zinc telluride (CdZnTe) gamma-ray detectors because their operation at room temperature makes compact, lightweight detector systems possible. Even though instruments for space using CdZnTe detectors have already been built, the effect of the high- energy particle space environment on these detectors has not been measured. To determine the effect of energetic charged particles on these detectors, we have bombarded several CdZnTe detectors with 199 MeV protons at the Indianan University Cyclotron Facility. Planar detectors of area 1 cm2 and thickness 2-3 mm from both eV products and Digirad were irradiated, along with a 2 multiplied by 2 array of proprietary design from Digirad. Using standard gamma-ray sources, the response of the detectors was measured before and after bombardment in steps up to fluences of 5 multiplied by 109 p cm-2. Significant effects from the proton irradiation were observed in the gamma-ray spectra. In particular, the peak positions of the lines in the spectrum were shifted downward proportional to the fluence. The explanation is almost certainly the production of electron traps by the high energy proton interactions, resulting in a decrease of the mobility-lifetime ((mu) (tau) ) product of the electrons. Calculations were made to model the effect of a decrease in electron trapping length on the spectrum.
The International Gamma-Ray Astrophysics Laboratory (INTEGRAL) is a proposed joint ESA/NASA/Russia gamma-ray astronomy mission which will provide both imaging and spectroscopy. It is currently at the final stages of an ESA phase-A study which it is hoped will lead to it being adopted during 1993 as the second 'medium-class' mission within ESA's Horizon 2000 plan. Launched in less than 10 years time it will be the successor to the current generation of gamma-ray spacecraft, NASA's Compton Observatory (GRO) and the Soviet- French Granat/Sigma mission. The baseline is to have two main instruments covering the photon energy range 50 keV to 10 MeV, one concentrating on high-resolution spectroscopy, the other emphasizing imaging. In addition there will be two monitors--an X-ray monitor which will extend the photon energy range continuously covered down to a few keV, and an Optical Transient Camera which will search for optical emission from gamma-ray bursts.