Hubble Space Telescope
Authors: Leckrone David S. 
Published: October 2003

The Hubble Space Telescope (HST) is widely viewed as one of the most important scientific and technological achievements of modern times, comparable in its impact to Galileo's first use of the telescope for fundamental astronomical research in 1610. Although it is not the first astronomical observatory to exploit the benefits of viewing the Universe from outside Earth's atmosphere, it is the first to realize fully the gain inclarityof astronomical images that results from the absence of atmospheric turbulence. Without having to contend with the atmosphere's rapidly fluctuating refraction and transmission, the HST's angular resolution is limited primarily by light diffraction at the entrance aperture of its 2.4‐meter telescope.

Earth's atmosphere glows from the emission of light by excited atoms and molecules. It is opaque at ultraviolet wavelengths below about 300 nm and strongly absorbs in broad intervals of the near‐infrared band above 1100 nm. Outside the atmosphere, the optics of the Hubble telescope and its scientific instruments provide sharply focused and remarkably stable images against a very dark sky at wavelengths that span approximately 4.5 octaves—110 to 2500 nm. The ability to concentrate light from a point or compact source into a tightly focused image superposed on a dark, low‐noise background allows the relatively small‐aperture HST to detect extremely faint astronomical objects in its direct imaging mode—fainter by as much as 1.5 stellar magnitudes (four times fainter) than current 8–10 meter mountaintop telescopes.

This unique combination of capabilities has made HST one of the most productive scientific tools of modern times—and one of the most sought after. Observing time on Hubble is allocated by a process of competitive peer review on the basis of scientific research proposals submitted yearly by astronomers from all over the world. The demand for using HST exceeds the available time typically by a factor of 6:1. The result is an almost continuous stream of amazing scientific accomplishments; many were unanticipated before Hubble's launch. These include the deepest view of the Universe ever acquired that revealed protogalaxies whose light was emitted when the Universe was less than 10% of its present age, the first demographic census of supermassive black holes at the centers of galaxies, accurate calibration of the age and expansion rate of the Universe, strong evidence acquired in partnership with ground‐based observatories that cosmic expansion is accelerating, and frequent observation of dusty disks containing complex structures of rings and gaps possibly indicative of planet formation around other stars.

The HST is essentially unique among robotic space missions because it was designed for a long lifetime in space, enabled by regular orbital visits by crews aboard the Space Shuttle who implement technological upgrades of Hubble's instruments and other systems and perform a variety of maintenance and repair tasks on the spacecraft. This concept of preplanned, periodic servicing missions by Shuttle astronauts allowed the correction of a serious optical flaw in Hubble's telescope that was discovered shortly after it was first deployed in 1990. The first HST servicing mission in 1993 demonstrated that humans can carry out arduous and complex work during a period of many days, encumbered by bulky spacesuits in the severe environment of low Earth orbit. Without the intervention of the Human Space Flight Program, the unmanned Hubble observatory would undoubtedly have come to be viewed by history as an embarrassing failure. Instead, Hubble became a national icon.