Approximately every decade the U.S. astronomical research community develops a decadal strategy for the field. A premise of the most recently developed strategy1 was that the HST SM-4 mission was an integral part of NASA’s facility planning for the future of the field and that this servicing mission would occur as planned at the time necessary to prevent the demise of the telescope. The strategy’s advisory recommendations reflect this assumption, and the committee, which was neither asked nor constituted to address any possible changes in priorities for astronomical research or research facilities, assumed that NASA would follow the decadal survey advisory recommendations. If NASA concludes that it cannot move forward with portions of the decadal survey strategy, then NASA will have to carry out an in-depth examination of priorities for the research field. The committee does not endorse such a re-examination. The committee notes, however, that if a re-examination should occur it would have to be conducted in a very timely and very expeditious fashion in order to ensure the continued operation and integrity of Hubble.

ANTICIPATED HUBBLE FAILURES

The Hubble systems with the greatest likelihood of failing and thus ending or significantly degrading Hubble science operations are the gyroscopes, the batteries, and the fine-guidance sensor (FGS) units. In addition, the HST avionics system is vulnerable to the aging of the facility.

The telescope uses three gyroscopes to provide precision attitude control. There are currently four functional gyros on HST—three in operation plus one spare. It is likely that the HST system will be reduced to two operating gyros in the latter half of 2006. The HST engineering team is currently working on approaches to sustaining useful, though potentially degraded, astronomical operations with only two gyros, and NASA expects to have that capability by the time it becomes necessary. Eventually, without servicing, the telescope will be reduced to operation with a single gyro in mid to late 2007. The spacecraft can be held in a safe configuration with one or no operating gyros, but science operations will not be possible.

Battery failures are another likely cause of loss of science operations. HST now has six batteries, of which five are necessary for full operations. If battery levels fall too low, the temperature of the structural elements in the Optical Telescope Assembly will fall below permissible levels, causing permanent damage to the facility. Recovery of scientific operations from this state is not possible.

The FGS units (in combination with their electronics subsystems) are used for precision pointing of the observatory. Two operating FGS units are required to support the HST observing program, with a third to supply redundancy. Based on recent test and performance data, one of the three currently operating FGS units is projected to fail sometime between October 2007 and October 2009, and a second is expected to fail sometime between January 2010 and January 2012.

Based on its examination of data and numerous technical reports on Hubble component operations, as well as discussions held with Hubble project personnel, the committee developed the following findings predicated on an estimated SM-4 earliest launch date of July 2006 and a most likely robotic mission launch date of February 2010.

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 responding to unexpected failures and its ability to perform proactive upgrades. Combined with the projected schedule for the two options, the mission risk2 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.

1  

National Research Council, 2001, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C.

2  

Mission risk is the risk of failing to achieve the mission objectives.



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