black holes in galaxies is thus to demonstrate a deep connection between galaxies and their central black holes.Main article: Comet Shoemaker-Levy 9The collision of Comet Shoemaker-Levy 9 with Jupiter in 1994 was fortuitously timed for astronomers, coming just a few months after Servicing Mission 1 had restored Hubble's optical performance. Hubble images of the planet were sharper than any taken since the passage of Voyager 2 in 1979, and were crucial in studying the dynamics of the collision of a comet with Jupiter, an event believed to occur once every few centuries.Other major discoveries made using Hubble data include proto-planetary disks (proplyds) in the Orion Nebula;[94] evidence for the presence of extrasolar planets around sun-like stars;[95] and the optical counterparts of the still-mysterious gamma ray bursts.[96] HST has also been used to study objects in the outer reaches of the Solar System, including the dwarf planets Pluto[97] and Eris.[98]Main articles: Hubble Deep Field and Hubble Ultra-Deep FieldA unique legacy of Hubble are the Hubble Deep Field and Hubble Ultra-Deep Field images, which utilized Hubble's unmatched sensitivity at visible wavelengths to create images of small patches of sky that are the deepest ever obtained at optical wavelengths. The images reveal galaxies billions of light years away, and have generated a wealth of scientific papers, providing a new window on the early Universe.The non-standard object SCP 06F6 was discovered by the Hubble Space Telescope (HST) in February 2006.[99][100][edit] Impact on astronomyDistant galaxies in a Hubble Ultra-Deep Field photographMany objective measures show the positive impact of Hubble data on astronomy. Over 9,000 papers based on Hubble data have been published in peer-reviewed journals,[101] and countless more have appeared in conference proceedings. Looking at papers several years after their publication, about one-third of all astronomy papers have no citations, while only 2% of papers based on Hubble data have no citations. On average, a paper based on Hubble data receives about twice as many citations as papers based on non-Hubble data. Of the 200 papers published each year that receive the most citations, about 10% are based on Hubble data.[102]Although the HST has clearly had a significant impact on astronomical research, the financial cost of this impact has been large. A study on the relative impacts on astronomy of different sizes of telescopes found that while papers based on HST data generate 15 times as many citations as a 4 m ground-based telescope such as the William Herschel Telescope, the HST costs about 100 times as much to build and maintain.[103]Making the decision between investing in ground-based versus space-based telescopes in the future is complex. Even before Hubble was launched, specialized ground-based techniques such as aperture masking interferometry had obtained higher-resolution optical and infrared images than Hubble would achieve, though restricted to targets about 108 times brighter than the faintest targets observed by Hubble.[104][105] Since then, advances in adaptive optics have extended the high-resolution imaging capabilities of ground-based telescopes to the infrared imaging of faint objects. The usefulness of adaptive optics versus HST observations depends strongly on the particular details of the research questions being asked.