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.
Over its lifetime, HST has been an enormous scientific success, having earned extraordinary scientific and public recognition for its contributions to all areas of astronomy. Hubble is the most powerful space astronomical facility ever built, and it provides wavelength coverage and capabilities that are unmatched by any other optical telescope currently operating or planned.
The four key advantages that Hubble provides over most other optical astronomical facilities are unprecedented angular resolution over a large field, spectral coverage from the visible and the near infrared to the far ultraviolet, access to an extremely dark sky, and highly stable images that enable precision photometry. Hubble’s imaging fields of view are also considerable, permitting mapping of extended objects and significant regions of sky. In contrast, ground-based telescopes have a view that is blurred by the atmosphere,3 and they are completely blind in the ultraviolet and large portions of the near infrared. Hubble can see sharply and clearly at all wavelengths from the far ultraviolet to the near infrared. Hubble images are 5 to 20 times sharper than those obtained with standard ground-based telescopes, in effect bringing the universe that much “closer.” Image sharpness and the absence of light pollution in orbit help Hubble to see objects 10 times fainter than even the largest ground-based telescopes. Moreover, Hubble’s images are extremely stable, in contrast to those obtained with ground telescopes, whose view is continually distorted by changing atmospheric clarity and turbulence.
Singly, each of these advantages would represent a significant advance for science. Combined, they have made Hubble the most powerful optical astronomical facility in history. Hubble is a general-purpose national observatory that enables unique contributions to and insights concerning most astronomical problems of greatest current interest. Among the most profound contributions of Hubble have been the following:
Direct observation of the universe as it existed 12 billion years ago,
Measurements that helped to establish the size and age of the universe,
Discovery of massive black holes at the center of many galaxies,
Key evidence that the expansion of the universe is accelerating, which can be explained only by the existence of a fundamentally new type of energy, and therefore new physics, and
Observation of proto-solar systems in the process of formation.
In addition to its impact on science, Hubble discoveries and images have generated intense public interest. Examples of Hubble data and images that have fascinated the public (and scientists) include the big “black eye” left by comet Shoemaker-Levy’s direct hit on Jupiter’s atmosphere, which alerted the