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NASA's Hubble Space Telescope is one of the most powerful scientific instruments ever conceived. Hubble’s most important discoveries have provided answers to questions that astronomers did not yet know how to ask, and found objects that were not yet imagined. This installment of the "Hubble at 25" series looks at how Hubble far exceeds its original science goals and unveiled an even more complex and mysterious universe than once envisioned.

Today, 25 years ago, Hubble finally left Earth aboard the Space Shuttle Discovery after decades of planning and development. Its journey was fraught with problems and delays, but it ultimately rose above it all to bring us groundbreaking scientific discoveries and historic photos of planets, galaxies, nebulae, space dust and more. Hubble didn't only outlive its original 10-year lifespan -- it became the Michael Jordan, the Madonna of telescopes. If you're a movie fan, you've likely seen it on screen at least once. If you're a metalhead, you might have seen at least one of the photos it captured on an album cover. Even people with zero interest in space know its name.

WHAT IS HUBBLE?
The Hubble Space Telescope (HST) is an 11-ton device that's the size of a bus. It orbits the Earth at an altitude of 340 miles, or around 100 miles above the International Space Station, and reaches speeds of 17,000 miles per hour. That's enough to circle the whole planet once every 97 minutes, roughly 15 times a day. The telescope is equipped with solar-powered instruments that can capture photos of space in visible light, ultraviolet (UV) and near-infrared wavelengths. However, if Hubble were ground-based, many of its instruments would be pretty much useless. They work well because they're located beyond the Earth's atmosphere, which distorts our view of the universe.

As Dr. Patrick McCarthy, Giant Magellan Telescope's director, told us when we asked why space observatories are still important despite the ongoing construction of several gargantuan land-based ones:

From the ground we are unable to view any significant amounts of ultraviolet (UV) light because of the ozone layer. Space telescopes equipped with the right instruments can be used to study nearby star-forming regions by the UV light they emit. Certain parts of the infrared spectrum are also best studied from space, and by looking at these wavelengths astronomers can study galaxies in the very early universe.
HST is one of NASA's four Great Observatories, along with Spitzer, Compton and Chandra. Spitzer carries infrared and far-infrared equipment; Compton specializes in gamma ray; while Chandra is an X-ray observatory.

HOW HUBBLE WORKS?

The telescope carries a number of scientific instruments and cameras that analyze data and capture stunning space photos, respectively. Those cameras can't take pictures on their own, though -- similar to how ordinary cameras need lenses, the ones on Hubble need the telescope's mirrors to work.

Hubble has a big primary mirror measuring 7.9 feet in diameter -- with telescopes, the bigger the mirror, the better -- which reflects light to a secondary one. That light bounces back to the center of the main one, where there's a hole leading to the scientific instruments. The cameras then capture what the mirrors reflect in black-and-white. All the bright, colorful photos NASA and the European Space Agency (ESA) regularly release are actually composites of two or more exposures with colors added during post-processing.

WHO USES AND MAINTAINS HUBBLE?

It's not just NASA and ESA scientists that can use the telescope to study the universe. NASA gets over a thousand proposals each year from researchers all over the globe. A panel goes through that pile to determine which studies to undertake, and the chosen teams get exclusive access to the data they requested for a year before it's released.

Since NASA's space shuttle program shut down in 2012, there won't be any more servicing missions. The six instruments on board right now, including cameras, spectographs, spectrometers and sensors (all powered by sunlight), will remain with Hubble as they are until the end. There is, however, a dedicated team of engineers and computer scientists from the Goddard Space Flight Center and the Space Telescope Science Institute (STScI) that continues to monitor HST's health and performance from Earth. The folks at Goddard also get the first look at any and all data Hubble's computer beams back to the ground station, before it's forwarded to STScI. Scientists from the institute are in charge of converting data into units we can understand, such as wavelength or brightness, and uploading those details to the internet for researchers everywhere.

NOTABLE SCIENTIFIC DISCOVERIES::-

Thanks to Hubble, we're able to travel back in time in a way, as the photos it takes show the state of the universe way before our own solar system even existed. The Hubble Ultra Deep Field survey, which took a visible-light image revealing some of the most distant galaxies we've ever seen, is a favorite among scientists. To conduct the study, NASA had to locate an empty patch in the sky that was so tiny, the agency compared it to peering through an 8-foot-long soda straw. NASA periodically positioned Hubble to stare at that spot for over 10 consecutive days, several times over the years. The image above is the sharper, livelier 2014 version of the Ultra Deep Field -- the 2004 iteration looks bland in comparison -- using a composite of images taken from 2002 to 2012. Due to the galaxies' distance and the time it takes for light to reach us, the resulting image shows young galaxies as they were forming around half a billion years after the Big Bang.

Hubble's observations of a special type of star also provided the data needed for a more accurate estimate of the universe's age. It's apparently around 13.7 billion years old, instead of 15 or 16 billion years like scientists previously thought. HST found strong evidence that supermassive black holes exist in the center of galaxies and pinpointed the color of an exoplanet for the first time, as well. More recently, it helped scientists find proof that there's most likely an ocean underneath the ice crust of Jupiter's biggest moon, Ganymede. Hubble also expanded our understanding of stars' life cycles and established the amount of dark matter in the universe (three-fourths of the universe's mass!).