to get too cold. Batteries are not predicted to fail until mid 2011 (Chapter 4), but the battery model involves considerable uncertainty, and Hubble’s batteries could fail sooner. Fourth, the failure model for Hubble’s avionics (Chapter 4) predicts an increasing number of component failures with time. A robotic servicing mission lacks the flexibility to deal with these. A shuttle mission has the required flexibility but might not have the capacity to deal with the added number of problems that a servicing delay might give rise to. Finally, it is a fact that all predictions for spacecraft longevity are just that, predictions. Components might start degrading sooner than expected, or the telescope could be hit by space debris, or some other unexpected event might occur. For all these reasons, it is prudent to get the maximum science out of the telescope in the shortest time possible, which points to servicing as soon as can reasonably be managed.

FINDING: Servicing Hubble expeditiously is highly desirable.

REHOSTING

A number of studies are underway to examine the possibility of rehosting WFC3 and/or COS on a new spacecraft(s). The alternatives being studied range from a full Hubble replacement, including a lighter mirror but with the same aperture and diffraction-limited performance in the UV and optical domain, to a smaller single-purpose spacecraft to carry one or the other of these two instruments. There was not time to explore the various possible options thoroughly, and most of them are still undefined in any case. The conclusions here are therefore very general.

It is possible that these studies, when completed, may result in a mission design that essentially replaces Hubble with a new spacecraft and a new mirror of equal performance to be launched as a replacement. The committee notes, however, that this approach would require a mirror that is at least 2.4 meters in diameter with diffraction-limited performance down to the ultraviolet, along with a very accurate pointing and guiding system consistent with HST’s capabilities. If all this could be done at a cost competitive with that of a servicing mission, still taking into account provisions for Hubble reentry, it would be scientifically attractive. However, preliminary cost information provided to the committee suggested that the savings would not be large.

Moreover, all options for rehosting take time to evaluate, select, and develop, and all options carry the risk that a new spacecraft may ultimately fail to operate to specifications. By contrast, Hubble is a proven platform on orbit now, to which several successful servicing missions have already been sent.



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