Hubble Space Telescope (NASA/ESA, 1990)

Hubble launched in April 1990 with a mirror ground to the wrong prescription — a catastrophic flaw discovered only after launch. A 1993 Space Shuttle servicing mission installed corrective optics, and Hubble immediately became the most scientifically productive telescope in history. Its 1995 "Hubble Deep Field" image revealed approximately 3,000 galaxies in a patch of sky the size of a grain of sand held at arm's length, demonstrating that the universe contains over 100 billion galaxies. Hubble's measurements of distant supernovae, published in 1998, proved that the universe's expansion is accelerating — a discovery that earned the 2011 Nobel Prize in Physics and introduced the concept of dark energy as 68% of the universe's total content.