Best Gyroscope Usage To Maximize the HST Mission Lifetime

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 I calibrate the model to reproduce and update more accurate, but somewhat outdated, calculations by Aerospace Corporation.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. Copyright c © 2003 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.