Space shuttle Discovery roared into orbit April 24, 1990, with a most precious cargo, NASA's Hubble Space Telescope. In the two decades since, teams of astronauts working from other shuttles repaired the orbiting eye on the universe and extended its abilities far beyond what was thought possible for longer than many thought realistic.
Hubble, named for groundbreaking astronomer Edwin Hubble, repaid the commitment with some of the most dazzling images the world has seen, along with fresh data that answered a wealth of questions and led to many new ones. The telescope's observations allowed astronomers to set the age of the universe at about 13.7 billion years with a high degree of certainty.
"I never believed in 1990 that the Hubble would end up this great," said Ed Weiler, NASA associate administrator for the Science Mission Directorate and chief scientist for the Hubble program when it launched. "It's changed a lot of thinking and it's changed a lot of what I learned 30 years ago in grad school."
Hubble's discoveries stretch over most aspects of astronomy, but its highlights include proving massive black holes exist and defining the age of the universe. It also proved the existence of something no one has seen -- dark energy.
"Nobody ever knew it existed before Hubble," said Jon Grunsfeld, an astronaut and astronomer who worked on Hubble during two shuttle missions.
The telescope's most unique element, though, is its orbit -- a perch so high above the planet that its pictures are not warped or distorted by the air currents, moisture and other effects from Earth's atmosphere.
"It's that extreme clarity that gives us the feeling we've traveled out into space to see these objects," Grunsfeld said. "It really is our time machine."
From more than 300 miles in space, Hubble looked back in time, showing astronomers what embryonic galaxies looked like almost 14 billion years ago. In some cases, Hubble's instruments picked up light that left stars only 600 million years after the Big Bang. "We're seeing the universe as it was perhaps as a toddler," Grunsfeld said.
An image that is perhaps Hubble's most famous, known as the Hubble Deep Field, was made when the telescope was pointed at a small sliver of space in the constellation Ursa Major, which appeared black and empty. Hubble found it brimming with young galaxies and stars in a kind of photographic time capsule from the universe. Astronomers called it a baby picture of space.