On April 24, 1990, after years of false starts and delays, the Hubble Space Telescope was launched into low-Earth orbit as part of a multi-billion-dollar project led by NASA and the European Space Agency. The bus-sized observatory is equipped with an 8-foot-wide focusing mirror and an array of cameras capable of seeing visible, infrared and ultraviolet light, and its eye-popping images have provided scientists and amateur astronomers alike with a glimpse into the inner workings of the universe. A quarter of a century after its mission began, get the lowdown on the telescope that helped revolutionize humanity’s understanding of the cosmos.

Edwin Hubble
Edwin Hubble (Credit: Underwood Archives/Getty Images)

1. It was named for a famous astronomer.
The Hubble Space Telescope takes its name from Edwin Hubble, an American astronomer whose observations helped broaden scientists’ view of the universe to include galaxies other than our own. While working at the Mount Wilson Observatory in 1923, Hubble determined that Andromeda—then considered a nebula—was actually a separate galaxy hundreds of thousands of light years away from our Milky Way. He made another groundbreaking discovery in 1929, when he helped quash the “static universe” theory by finding evidence that galaxies move away from one another at a constant rate. Hubble died in 1953, but the telescope that bears his name has since confirmed and fine-tuned many of his theories.

2. The telescope was several decades in the making.
Hubble’s early origins date back as far as 1946—more than 10 years before NASA was established. That was the year that astrophysicist Lyman Spitzer Jr. penned an influential paper discussing the merits of a space-based observatory. In it, he argued that an in-orbit telescope could view the heavens unhindered by the Earth’s atmosphere, which can blur images. Spitzer was later instrumental in developing the Orbital Astronomical Observatories, four unmanned satellites that NASA launched between 1966 and 1972, and he tirelessly lobbied the government to create a bigger and better space telescope. The project’s massive costs proved a major stumbling block, however, and it wasn’t until 1977 that the U.S. Congress finally appropriated funding for what would become Hubble.

3. The launch was delayed by the Challenger disaster.
The fledgling Hubble program suffered a massive blow in 1986, after the space shuttle Challenger exploded during liftoff, killing seven astronauts. NASA grounded its space fleet in the wake of the tragedy, leaving Hubble—which depended on the shuttle for its transport and maintenance—without a ride into orbit. Scientists made good use of the delay by upgrading the sensitivity of the telescope’s instruments and refining its ground control software, but the added years of servicing and storage in a high tech clean room sent costs soaring. By the time the space shuttle Discovery finally lifted off in 1990 with Hubble tucked into its cargo bay, the project was seven years behind schedule and more than $1 billion over budget.

Workers study Hubble's main mirror.
Workers study Hubble's main mirror.

4. A faulty mirror nearly derailed Hubble’s mission before it started.
Upon viewing Hubble’s first images in 1990, NASA scientists were devastated to discover that its main focusing mirror had been polished to the wrong specifications. This “spherical aberration” was miniscule—less than 1/50 the width of a strand of human hair—yet it was enough to blur many of its photos. In the months that followed, the telescope became something of a national joke, and a Newsweek magazine cover even branded it a “$1.5 Billion Blunder.” Redemption for NASA had to wait until December 1993, when a crew of spacewalking astronauts installed an instrument known as COSTAR, which used tiny mirrors to offset the glitch. Hubble’s “space glasses” succeeded in correcting its fuzzy vision, and it soon began transmitting jaw-droppingly detailed shots of the cosmos.

5. It’s responsible for the deepest images of the universe ever recorded.
In late-1995, Hubble’s operators allowed the telescope to stare for 10 days at a seemingly empty sliver of sky. Many doubted whether the lengthy exposure would be of value, but the resulting image, known as the “Hubble Deep Field,” was astonishing. By looking beyond the Milky Way, it revealed a plethora of never-before-seen galaxies, including some of the most distant star systems ever discovered. Astronomers repeated the experiment with 2004’s “Ultra Deep Field,” and they have since released several more pictures that use infrared and combined exposures to peer farther into space than ever before. The newest images contain more than 5,000 galaxies, some of them as far as 13.2 billion light years away. Since the light from these galaxies has taken eons to reach our solar system, it offers astronomers a window onto what the universe looked like only a short time after the Big Bang some 13.7 billion years ago.

Hubble's "Extreme Deep Field" image, 2012.
Hubble's "Extreme Deep Field" image, 2012.

6. The telescope only takes pictures in shades of black and white.
The dazzling snaps of nebulae, supernovae and galaxies captured by Hubble are usually only released after having undergone a postproduction process to add color. Hubble’s onboard digital cameras only take photos as gray-scale pixels, so astronomers make multiple exposures of the same object using different filters—typically red, blue and green—to capture various wavelengths of light. These are then overlaid to create a single color composite. Since Hubble can see in the ultraviolet and infrared ranges, scientists also occasionally insert additional color into the images to bring out details that would otherwise remain invisible to the human eye.

7. Hubble technology helped lead to better methods for detecting breast cancer.
Like many NASA programs, Hubble spawned “spinoff” technologies that proved useful in other fields. One of the most significant breakthroughs concerns the observatory’s Space Telescope Imaging Spectrograph, which Hubble used to search for supermassive black holes. NASA’s improvements to the spectrograph’s detection ability were also a boon to medical professionals, who used the same silicon chips to more effectively image women’s breast tissue and distinguish between benign and malignant tumors.

8. Its observations have settled many longstanding debates in cosmology.
Hubble’s telescope is powerful enough to spot the light of a firefly at a distance of some 7,000 miles, and scientists have used these enormous capabilities to unlock many of the secrets of the cosmos. Astronomers’ estimations of the age of the universe used to vary widely, but Hubble’s observations of ancient, burned-out stars have helped pin the date of the Big Bang at somewhere around 13.7 billion years ago. The telescope also found the first evidence of supermassive black holes at the center of neighboring galaxies, and it has been vital in hunting down extrasolar planets that may have the proper conditions to foster life. Perhaps most crucial of all, Hubble’s studies of supernovae have contributed to the theory that a mysterious force known as “dark energy” could be causing the universe to expand at an accelerated rate.

Hubble's photo of the "Pillars of Creation."
Hubble's photo of the "Pillars of Creation."

9. Anyone can make a request to use the Hubble Telescope.
Every year, Johns Hopkins University’s Space Telescope Science Institute issues an open call for proposals to conduct observations using Hubble. While there are no restrictions on who can apply, the competition is extremely tough. Hundreds of eager astronomers come forward each year, and following a review by a panel of experts, only around one-fifth of them actually receive time with the telescope. Those who fail to win a date with Hubble have to settle for perusing its extensive archive of photos, all of which are available to the public following a one-year waiting period.

10. Its successor is planned for launch in the next few years.
Five servicing mission have kept Hubble up and running well beyond its originally proposed 15-year lifespan, but it’s about to finally meet its heir in the form of the James Webb Space Telescope. The $9 billion James Webb differs from Hubble in several key ways. It is optimized for detecting infrared rather than visible light, boasts a much bigger mirror and won’t require any servicing during its 6-10 year lifespan. The Webb will also be placed well beyond the moon at a distance of some 1 million miles—Hubble is perched at an altitude of only 350 miles—which will allow it to look even deeper into the universe to see light from only a few hundred million years after the Big Bang. Though plagued by delays, the James Webb is currently set for launch in late-2018. It’s uncertain if Hubble will still be operable by then, but astronomers hope to use the two telescopes in concert with one another before Hubble is finally retired and de-orbited.