degree of importance that a Hubble servicing mission would have for science, as well as some of the key factors involved in selecting a servicing mission option. Its aim is to provide useful guidance to NASA that can be utilized during the time that the committee (as well as NASA) continues to investigate the servicing options in greater detail. The work of the committee will continue during the coming weeks, and we expect to finish drafting a final report by late summer or early fall. The final report will address in detail all four of the requests in the study’s statement of task.
The Hubble Space Telescope (HST) is arguably the most important telescope in history. Much of Hubble’s extraordinary impact was foreseen when the telescope was being planned. It was predicted, for example, that the space telescope would reveal massive black holes at the centers of nearby galaxies, measure the size and age of the observable universe, probe far enough back in time to capture galaxies soon after their formation, and provide crucial keys to the evolution of chemical elements within stars.
All of these predicted advances have been realized, but the list of unforeseen Hubble accomplishments may prove even greater. Hubble did discover “adolescent” galaxies, but it also saw much farther back in time to capture galaxies on the very threshold of formation. Einstein’s theory of general relativity was bolstered by the detection of myriad gravitational lenses, each one probing the mysterious dark matter that pervades galaxies and clusters of galaxies. Gamma-ray bursts had puzzled astronomers for more than 20 years; in concert with ground and X-ray telescopes, Hubble placed them near the edge of the visible universe and established them as the universe’s brightest beacons, outshining whole galaxies for brief moments. Perhaps most spectacularly, Hubble confirmed and strengthened preliminary evidence from other telescopes for the existence of “dark energy,” a new constituent of the universe that generates a repulsive gravity whose effect is to drive galaxies apart faster over time. The resulting acceleration of universal expansion is a new development in physics, possibly as important as the landmark discoveries of quantum mechanics and general relativity near the beginning of the 20th century.
Closer to home, Hubble has zeroed in on our own cosmic past by uncovering virtual carbon copies of how the Sun and solar system formed. Dozens of protoplanetary disks have been found encircling young stars in nearby star-forming regions of the Milky Way. The sizes and densities of these disks show how surplus dust and gas collect near infant stars to form the raw material of planets. Dozens of large, Jupiter-like planets have been discovered, initially by other telescopes but recently by Hubble using a new and more precise method. Measuring the tiny drop in light as a planet transits the disk of its parent star, the new technique could lead to a method for discovering Earth-like planets—a discovery with tremendous long-term implications for the human race.
Riveting as they are, these scientific returns from Hubble are far from their natural end. With its present instruments the telescope could continue probing star formation and evolution, gathering more data on planetary systems, revealing planetary and cometary phenomena in our own solar system, and exploring the nature of the universe at much earlier times. However, two new instruments, already built for NASA’s next planned servicing mission (SM-4), would amplify the telescope’s capabilities by allowing qualitatively new observations in two underexploited spectral regions. Such rejuvenation via new instruments has occurred after every Hubble servicing mission, and the next one promises to be no different. Wide Field Camera-3 (WFC3) would increase Hubble’s discovery efficiency4 for ultraviolet and near-infrared imaging by factors of 10 to 30. The UV channel coupled with the camera’s wide field of view will image the final assembly of galaxies still taking place in the universe. The near-infrared channel of WFC3 favors discovery of the very youngest galaxies, whose light is maximally red-shifted. The available UV, visible, and near-IR channels will combine to give a sweeping, panchromatic view of objects as diverse as star clusters, interstellar gas clouds, galaxies, and planets in our own solar system.
The second new instrument, the Cosmic Origins Spectrograph (COS), will increase Hubble’s observing speed for typical medium-resolution ultraviolet spectroscopy by at least a factor of 10 to 30, and in some cases by nearly two orders of magnitude. Ultraviolet spectra carry vital clues to the nature of both the oldest and the youngest stars, yet UV rays are totally invisible from Earth’s surface. COS will fill important gaps in our understanding of the birth and death of stars in nearby galaxies. Even more impressive, COS will use the light of distant quasars to spotlight