The Hubble Space Telescope is a school bus-sized telescope orbiting the Earth at an altitude of about 353 miles. After its launch in 1990, Hubble made news for returning out-of-focus images, the result of spherical aberration in its primary mirror that was scattering the light about.
Hubble has since been serviced five times, beginning in 1993 with a mission to install COSTAR (the Corrective Optics Space Telescope Axial Replacement), which corrected the telescope's fuzzy vision. The last servicing mission was flown in May of 2009, when space shuttle Atlantis replaced two failed instruments and installed two new ones.
Over the course of 21 years, Hubble has returned hundreds of images of the cosmos around Earth, deepening scientists' understanding of the universe around us. The telescope is run 24 hours a day by four flight teams based at Goddard Space Flight Center in Maryland.More on Hubble