HST@STScI UpdateR. Osten (osten[at]stsci.edu) and T. Brown
Hubble continues to produce record numbers of science results and has a bright future. In that context, and to address the recent behavior of HST gyros, engineers tried out a new method that would overcome the steadily increasing value of rate bias in one of the gyroscopes used for pointing and control, but decided against implementing it at this time. The path is still clear for future implementation of this mode and there are no impacts on science data. Hubble participated in NASA's Senior Review with excellent outcomes from the evaluation.
Recent Gyro Behavior
Since October 2018, Hubble has been performing routine science operations with its three remaining gyroscopes, named Gyros 3, 4, and 6. Hubble's Gyros are presently used for both target acquisition and guiding during science observations. As described in previous newsletter articles (Volume 35 Issue 1, Volume 35 Issue 3, Volume 36 Issue 1) there was a pause in normal observatory operations last October while the anomalously large rate biases of Gyro‑3 were brought into operational range. Since that time, the rate bias of Gyro‑3 has continued a steady climb back up to larger values, and is now approaching a range that cannot be used for normal science operations. Staff at STScI and GSFC have been monitoring this bias drift, and recently developed the ability to use this gyro for regular science at these elevated bias levels. On-orbit tests indicated acceptable performance in this "high" mode for routine science operations.
Normally, all of the gyros are put in "high mode" when the telescope is slewing and "low mode" when the telescope is under fine control for science, with each mode having a distinct range for the rates that can be measured. The large rate bias on Gyro‑3 will yield rates too large to measure in "low mode" but still well within the measurable range of "high mode." This new capability enables use of "high mode" for both slewing and science, allowing all three gyros to be used for science even when the rate bias is large.
As an early test of this capability, the observatory transitioned to this "high mode" observing for one week in early September, with the expectation that science data quality would be acceptable and that the number of anomalies would be within expected values. While the one week of routine science operations did not result in excessive failures in acquisitions or re-acquisitions, roughly one sixth of the acquisitions and re-acquisitions of science targets experienced anomalies, a rate roughly triple that of the previous several uneventful weeks of observing. These losses of lock (almost all of which were recoverable) were dominated by the behavior of Gyro‑3.
Closer inspection of engineering data showed a relationship between these disturbances and a quantization effect of the Gyro‑3 step levels. These disturbances should diminish as the rate bias climbs to higher values. The median amount of jitter exhibited an expected increase, falling near 10 mas. A tail in the distribution of jitter values to higher than 15 mas, however, was deemed undesirably high, as these larger values degrade the science quality of some datasets. A review of the science observations done during the one week in high mode, along with discussion of the pointing and control behavior, resulted in a decision to step back and resume normal science operations in low mode. The option to transition to "high mode" again in a few months remains viable, when the rate bias will not alias with the Gyro‑3 step levels, and before the rate bias is outside of the normal range for science operations. In the meantime, additional work is underway to mitigate some of the issues experienced with Gyro‑3.
Each of the three remaining gyros are of "enhanced type," and are expected to last three to five times longer than prior gyros (including the three non-enhanced that have failed since SM4). One of the current gyros (Gyro‑4) has already lasted twice as long as the average lifetime for a standard gyro. The observatory is expected to remain in 3‑gyro mode for at least several more years. When the next gyro eventually fails, we can continue doing science in reduced-gyro mode. Many thanks to the staff at STScI and GSFC who continue innovating to keep the telescope operating at the forefront of science.
Hubble in the Senior Review
The NASA Astrophysics Science Division is required by Congress to review the operating missions within its portfolio that are in the extended phase of their mission operations. These independent, comparative reviews examine how to maximize the scientific return from these missions within the finite resources available. Hubble has undergone three previous Senior Reviews since Servicing Mission 4 in 2009: in 2012, 2014, and 2016. NASA uses the findings from the Senior Review to define an implementation strategy and give programmatic direction to the missions and projects concerned through the following four years. The cadence was changed from biennial to every three years in 2017 based on a National Academy Study.
The most recent review occurred in 2019, with a proposal submitted in March, panel site visit in May, and final results available in mid-July. Three panels evaluated missions: the Hubble Space Telescope and its sister Great Observatory, Chandra, each evaluated by separate panels, and the rest of the missions under review being evaluated together. The format changed slightly for this review compared to previous ones: reports from each of these panels fed into a sub-committee of the Astrophysics Program Advisory Committee, which compiled results. These reviews encompassed science results, scientific productivity, observatory operations, science operations, and budgets and staffing levels.
The final report in mid-July commended the HST program on its sustained record of impactful science, commitment to observatory and instrument support, and proactive stance towards mitigating the aging effects of long-term science operations in the harsh environment of space. The panel noted "Hubble remains the most famous telescope in the world, an icon of American scientific achievement. Its exquisite image quality and its UV imaging and spectroscopic sensitivity are unique; the UV sensitivity will not be equaled for at least the next 15 years. The 900+ peer-reviewed papers a year arising from Hubble data span the full range of astronomy and astrophysics, from Europa's water plumes to the primeval galaxies of cosmic dawn… Hubble enables a truly astonishing range of science and the Project continues to be responsive to the changing scientific landscape." We would like to thank all of those who contributed to the Senior Review preparations, as well as all those whose work enables the scientific successes of Hubble to continue.