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When Galileo Galilei peered through his small telescope 400 years ago, it resulted in an unprecedented period of astronomical discovery. There is no doubt that the Hubble Space Telescope (HST; Fig. 1) in its first 13 years of operation has had a similarly profound impact on astronomical research. But the Hubble Space Telescope did much more that that. It literally brought a glimpse of the wonders of the universe into millions of homes worldwide, thereby inspiring an unprecedented public curiosity and interest in science.

Figure 1

Figure 1. Hubble Space Telescope in orbit photographed by the crew of the Space Shuttle Columbia, just after being released during mission STS-109 to service and upgrade Hubble. March 9, 2002. Credit: NASA.

The Hubble Space Telescope has seen farther and sharper than any optical/UV/IR telescope before it. Unlike astronomical experiments that were dedicated to a single, very specific goal (like the Cosmic Background Explorer or the Wilkinson Microwave Anisotropy Probe), the Hubble Space Telescope's achievements are generally not of the type of singular discoveries. More often, the Hubble Space Telescope has taken what were existing hints and suspicions from ground-based observations and has turned them into certainty. In other cases, the level of detail that HST has provided forced theorists to re-think previous broad-brush models and to construct new ones that would be consistent with the superior emerging data. In a few instances, the availability of HST's razor-sharp resoution at critical events provided unique insights into inividual phenomena. In total, by observing tens of thousands of astronomical targets, the Hubble Space Telescope has contributed significantly to essentially all the topics of current astronomical resarch, covering objects from our own solar system to the most distant galaxies.

The 12.5-ton orbiting observatory was launched into orbit on April 24, 1990. It circles the Earth every 90 minutes in a 330 nmi (607 km) orbit and operates around the clock above all but the thinnest remnants of the Earth's atmosphere. The telescope provides information to many individual astronomers and teams of scientists worldwide, and it is engaged in the study of virtually all the astronomical constituents of the universe.

Crucial to fulfilling the objective of a 20-year mission is a series of on-orbit manned servicing missions. The first servicing mission (SM1) took place in 1993, the second (SM2) was flown in 1997, and the third was separated into two parts, one (SM3A) in 1999 and the second (SM3B) in 2002. A fourth mission is currently planned for the end of 2004 (pending the investigation into the shuttle Columbia disaster). During these missions, shuttle astronauts upgrade the the observatory's capabilities (by installing new instruments) and perform planned maintenance activities and necessary repairs. To facilitate this process, the telescope was designed so that its science instruments and several vital engineering subsystems have been configured as modular packages with standardized fittings accessible to astronauts in pressurized suits.

Hubble was designed to provide three basic capabilities, all of which are essential for innovative astronomical research:

  1. High angular resolution - the ability to image fine details. The spatial resolution of HST in the optical regime is about 0.05 arcseconds.
  2. Ultraviolet performance - using the fact that the observatory is above the Earth's atmosphere to produce ultraviolet images and spectra. The Hubble Space Telescope is sensitive down to a wavelength of 1150 Å (below the Lyman Alpha line of hydrogen).
  3. High sensitivity - the ability to detect and take spectra of even very faint objects. The Hubble Space Telescope is about a hundred to a thousand times more sensitive in the ultraviolet than the previous space observatory (the International Ultraviolet Explorer).

Astronomers and astrophysicists using HST data have published over 3000 scientific papers to date. Hubble's findings are thus far too numerous to be described even briefly in one article. The following sections therefore simply highlight a few of the astronomical discoveries which, in my clearly biased view, have significantly advanced our understanding of the cosmos. In the spirit of progressing by "powers of ten" in cosmic distance, I shall start with the solar system and finish with cosmological distances. I apologize in advance to the many astronomers whose important work will not be presented in this article. I also regretfully acknowledge the incompleteness of the list of references. My list should be regarded as representative rather than comprehensive. Finally, the different sections are not intended to be exhaustive reviews - they are concise summaries of HST contributions to major astrophysical poblems.

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