Hubble by either a crewed or robotic mission.[82] NASA is currently (as of 2011) studying the best method for deorbiting Hubble, projected to be needed around 2025.[83][edit] Scientific results[edit] Key ProjectsIn the early 1980s, NASA and StScI convened four panels to discuss Key Projects. These were projects that were both scientifically important and would require significant telescope time, which would be explicitly dedicated to each project. This guaranteed that these particular projects would be completed early, in case the telescope failed sooner than expected. The panels identified three such projects: (1) a study of the nearby intergalactic medium using quasar absorption lines to determine the properties of the intergalactic medium and the gaseous content of galaxies and groups of galaxies;[84] (2) a medium deep survey using the Wide Field Camera to take data whenever one of the other instruments was being used[85] and (3) a project to determine the Hubble Constant within ten percent by reducing the errors, both external and internal, in the calibration of the distance scale.[86][edit] Important discoveriesOne of Hubble's most famous images, Pillars of Creation shows stars forming in the Eagle NebulaThe Hubble has helped to resolve some long-standing problems in astronomy, as well as turning up results that have required new theories to explain them. Among its primary mission targets was to measure distances to Cepheid variable stars more accurately than ever before, and thus constrain the value of the Hubble constant, the measure of the rate at which the universe is expanding, which is also related to its age. Before the launch of HST, estimates of the Hubble constant typically had errors of up to 50%, but Hubble measurements of Cepheid variables in the Virgo Cluster and other distant galaxy clusters provided a measured value with an accuracy of 10%, which is consistent with other more accurate measurements made since Hubble's launch using other techniques.[87]While Hubble helped to refine estimates of the age of the universe, it also cast doubt on theories about its future. Astronomers from the High-z Supernova Search Team and the Supernova Cosmology Project[88] used the telescope to observe distant supernovae and uncovered evidence that, far from decelerating under the influence of gravity, the expansion of the universe may in fact be accelerating. This acceleration was later measured more accurately by other ground-based and space-based telescopes, confirming Hubble's finding. The cause of this acceleration remains poorly understood;[89] the most common cause attributed is dark energy.[90]The high-resolution spectra and images provided by the HST have been especially well-suited to establishing the prevalence of black holes in the nuclei of nearby galaxies. While it had been hypothesized in the early 1960s that black holes would be found at the centers of some galaxies, and work in the 1980s identified a number of good black hole candidates, it fell to work conducted with Hubble to show that black holes are probably common to the centers of all galaxies.[91][92][93] The Hubble programs further established that the masses of the nuclear black holes and properties of the galaxies are closely related. The legacy of the Hubble programs on