a result of performing the SM-4 mission, whereas the orange-star values associated with the robotic mission are not reset. Key results:

  • Early servicing afforded by the SM-4 shuttle mission essentially ensures 5 years of operations before other reliability factors might affect the need to suspend science operations. The specific avionics system risk factors are shown in purple on Figure 4.3 below the yellow “post-shuttle science operations” bar, with the 50 percent risk point occurring after approximately 4.5 years of operations.

  • The projected delay in the robotic mission not only results in a likely 29-month interruption of science operations (as depicted by the blue arrow on the figure) but, due to lower system reliability, is also likely to result in a shorter period of post-servicing operations. As shown in purple below the orange bar labeled “post-robotic science operations,” the projected telescope risk value is estimated to be above 50 percent (0.59) at the time of servicing and to be approximately 0.82 after 3 years.

  • The projected shuttle mission scenario results in servicing of HST prior to suspension of science operations due to gyro failure and should achieve at least 4.5 years of post-servicing operations before the avionics system risk value reaches 50 percent. Therefore, the total expected operational time in the science mode is projected to be at least 6.3 years for a shuttle servicing mission executed in July 2006 and 7.3 years for a shuttle mission executed in July 2007.

  • The projected robotic servicing mission starts with 3 years of operations prior to gyro failure followed by a 29-month suspension of science operations, at which time the projected telescope avionics system risk value will be above 50 percent.

  • Performing a direct comparison between the two servicing options, 6.3 years of SM-4-associated science operations (1.8 years prior to servicing followed by 4.5 years of post-servicing operations) is essentially equivalent to 3 years of robotics-associated science operations (all accumulated prior to servicing) at a similar level of risk.

FINDING: The projected termination in mid to late 2007 of HST science operations due to gyroscope failure and the projected readiness in early 2010 to execute the planned NASA robotic mission result in a projected 29-month interruption of science operations. No interruption of science operations is projected for a realistically scheduled SM-4 shuttle mission.

FINDING: The planned NASA robotic mission is less capable than the previously planned SM-4 shuttle astronaut mission with respect to its response to unexpected failures and its ability to perform proactive upgrades. Combined with the projected schedule for the two options, the mission risk associated with achieving at least 3 years of successful post-servicing HST science operations is significantly higher for the robotic option, with the respective risk numbers at 3 years being approximately 30 percent for the SM-4 mission and 80 percent for the robotic mission.



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