Mathematics – Probability
Scientific paper
Jan 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004acs..rept...15v&link_type=abstract
Instrument Science Report ACS 2004-03, 17 pages
Mathematics
Probability
Hubble Space Telescope, Hst, Space Telescope Science Institute, Advanced Camera For Surveys, Acs, Gyroscope
Scientific paper
Without SM4, gyroscope survival is a critical factor for the HST Mission lifetime. I present a simple Monte-Carlo model to calculate the survival probabilities for various scenarios. I calibrate the model to reproduce and update more accurate, but somewhat outdated, calcula- tions by Aerospace Corporation. Continued three-gyroscope guiding will become impossible by late January 2006. Subsequent two-gyroscope guiding will further extend the mission to late May 2007 (if no other hardware fails). I discuss the importance of alternative strategies. We can extend the HST Mission if we switch to two-gyroscope guiding sooner; this extends the lifetime of the gyroscope that is powered off. Starting two-gyroscope guiding by January 2005 could extend the HST Mission by another 10 months, to April 2008. Any 6-month implementation delay beyond January 2005 decreases the mission by 3 months. The 1-σ uncertainty in all the aforementioned 50% probability (median) dates ranges between 11-16 months. To achieve maximum lifetime it is important to guide with the gyroscopes that have the lowest failure probabilities. This strategy is important during three-gyroscope guiding as well. I recommend that: (a) More detailed gyroscope survival models should be calculated to validate these results; (b) With knowledge of the individual gyroscope failure probabilities, an effort should be made guide with the gyroscopes for which these probabilities are lowest; (c) Implementation and testing of the two-gyroscope guiding capability should be expedited as much as possible; (d) A study should be undertaken of the (net) trade-off between 13 months of three-gyroscope observations and 23 months of two-gyroscope observations; (e) any remote possibility of one-gyroscope guiding should be actively investigated, since it is expected that one functional gyroscope will be available into 2009 or beyond.
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