Astronomers around the world this weekend mark the 20th anniversary of the launching of the iconic Hubble, NASA's first orbiting space telescope that has revolutionized human understanding of the universe.
More than any other instrument, the Hubble has stimulated a modern-day infatuation with deep space, beaming to Earth the most spectacular images ever taken of faraway galaxies and the births and deaths of stars -- and along the way helping scientists make some of the most important discoveries of our time.
Hubble was launched aboard space shuttle Discovery on April 24, 1990 and deployed into orbit the following day.
Piloting the shuttle on that historic mission was astronaut Charles Bolden, who went on to become the US space agency's administrator last year.
In the two decades since, Hubble has enabled astronomers to peer through the celestial curtain to set the age of the universe at 13.7 billion years old.
"Hubble is undoubtedly one of the most recognized and successful scientific projects in history," said Ed Weiler, associate administrator for NASA's Science Mission Directorate.
From its unique perch some 570 kilometers (353 miles) above Earth, the telescope is our eye on the cosmos, snapping pictures of more than 30,000 celestial objects, some of them located on the far ends of the universe.
The quality of Hubble images is 10 times clearer than pictures from the most powerful ground-based telescopes, because it is beyond the Earth's atmosphere, which distorts the view of ground telescopes.
"It's that extreme clarity that gives us the feeling we've traveled out into space to see these objects," said Jon Grunsfeld, an astronaut who did repair work on Hubble during two shuttle missions.
"It really is our time machine."
Although Hubble is universally acknowledged today as a scientific gem, it had a rocky start: once it was deployed scientists quickly discovered that the telescope's main mirror had a slight blemish, and a shuttle mission had to be sent up to install a new mirror.
When it became fully operational in late 1993 the Hubble began transmitting pictures that stunned the world.
One of Hubble's best known images is of the vast Carina Nebula, an interstellar cloud of dust, hydrogen gas, helium gas and plasma that lies some 6,500 light years away from Earth. Each light year is about 9.5 trillion kilometers, or 5.9 trillion miles.
The Hubble also photographed supernovas, gigantic explosions that mark the death of a star.
It revealed the existence of black holes at the center of practically all galaxies. Until then scientists only suspected the existence of black holes.
Thanks to data collected by the Hubble, astronomers have learned that the universe is expanding at an accelerating rate.
This acceleration is caused by a mysterious force called dark energy, which makes up nearly 75 percent of the universe and balances the force of gravity.
Around five percent of the cosmos is made up of visible matter, while 20 percent of it is so-called dark matter.
Other discoveries credited to Hubble include the detection of the first organic molecule in the atmosphere of a planet orbiting a distant star.
It also observed small forming galaxies that emitted rays of light when the universe was less than a billion years old.
In the Earth's solar system, Hubble has observed radical changes in the direction of the winds on Saturn, and shown that Neptune has seasons.
The Hubble, the result of collaboration between the National Aeronautics and Space Administration and the European Space Agency (ESA), measures 13.2 meters (43.5 feet) long, has a maximum diameter of 4.2 meters and weighs 11,110 kilos (24,500 pounds).
A 2009 mission of shuttle Atlantis gave Hubble an extreme makeover, fitting it with a new, more powerful camera and spectrograph, sprucing up its scientific instruments, and extending Hubble's life for at least five years.
"The new and improved Hubble will continue to have a positive impact on the world for decades with many of its greatest discoveries yet to come," said Weiler, who described it as "an icon in the American psyche."
But the mission was bittersweet, as the repair job marked the end of NASA's human missions to the beloved Hubble.
Its successor is the James Webb Space Telescope, an infrared optimized space observatory scheduled to be deployed in 2014.