A web exclusive story from Astronomy magazine

How the Hubble Space Telescope changed the cosmos

Story by: David J. Eicher

“There remains only the privilege of waving the astronomers farewell on their voyage to the stars — in imagination riding with the Captain on his bridge down the bay till the pilot takes us off and puts us ashore.” So wrote journalist David Woodbury about the then latest grand astronomical project, the building of the 200-inch Hale Telescope on Palomar Mountain, California, in 1939. The vision of astronomer George Ellery Hale, the famous 200-inch instrument was, when built, the world’s largest by a factor of two and helped revolutionize human knowledge of the cosmos throughout much of the 20th century.

The same could have been written half a century later, when another grand project was about to take center stage. This year we celebrate the 25th anniversary of the launch and “first light” of the Hubble Space Telescope, the greatest scientific instrument ever produced for astrophysics and cosmology. (This year we also celebrate the 100th anniversary of Albert Einstein’s general theory of relativity.) The telescope — named for Edwin Hubble, discoverer of the expanding universe — tragically flawed and heroically fixed, has fundamentally changed our understanding of the cosmos. But the story of the Hubble telescope is not one simply of challenge, trouble, and triumph. It is one that explains why we know what we know about the universe and even our awareness of why we are here on a small planet inside one of more than 100 billion galaxies that float throughout creation.


Aiming high

Since the invention of the telescope in 1609, astronomers have been plagued by the unsteadiness of Earth’s atmosphere, which distorts starlight from cosmic objects. As early as 1923, rocketry pioneers conceptualized a telescope launched into space, thereby avoiding atmospheric turbulence, with its captured data sent via signals to the ground and assembled into information and pictures. In 1946, just as the Hale Telescope was nearing first light, Lyman Spitzer, an influential American astronomer, penned a paper in which he examined the advantages and challenges of a space-based telescope for the first time.

Is the Hubble Space Telescope really that big of a deal? “From the time it was conceptualized, it was clear that Hubble would revolutionize our view of the whole cosmos,” says Avi Mandell, a planetary scientist at NASA’s Goddard Space Flight Center who has used Hubble to study exoplanets. “The exquisite stability and clarity of Hubble’s images combined with the ability to view the universe at wavelengths of light that are unavailable to observatories on Earth make Hubble by far the most powerful telescope available — even 25 years after its launch!”

By the middle of the 20th century, astronomers realized a space-based telescope would provide enormous scientific power. Following World War II, advances in rocket technology pushed thinking on the idea further. The timing of Spitzer’s paper could not have been better. NASA launched the first orbiting observatory, a solar instrument, in 1962. Six years later, with the Apollo program in full bloom, the agency developed plans for an orbiting observatory with a mirror 3 meters across. But the funding was not yet ready, and so an original launch target of 1979 was destined to slip away.

By the mid-1970s, astronomers went on the attack. They lobbied members of Congress as the National Academy of Sciences produced a report seeking a space telescope as a key science objective. Finally, the Senate agreed to a scaled-down version of the space telescope project, limiting its cost to half that of the original request and reducing the size of the telescope’s mirror to 2.4 meters.

Edwin Hubble, the space telescope’s namesake, used the 100-inch Hooker Telescope in the 1920s to prove that the smudgy “nebulae” on photographic plates were actually galaxies outside our own and that the universe is expanding.
Courtesy of Carnegie Observatories, Carnegie Institution of Washington

NASA then secured a degree of involvement from the European Space Agency. In 1977, Congress approved funding of $36 million, and the project was off and running with a targeted launch date of 1983. That launch did not occur, as the project dragged out, but in 1983 the telescope long characterized as the Large Space Telescope was rechristened the Hubble Space Telescope. Edwin Powell Hubble, the telescope’s namesake, had been born in Missouri in 1889 and studied at the University of Chicago. Hubble became a celebrated figure in the 1920s when he was instrumental in deciphering the expansion of the universe and also helped define the cosmic distance scale. The Milky Way was but one galaxy in a countless sea of others. Hubble died in 1953, and his legacy lives on with the space observatory.

“I find it beautifully fitting that in 2015 we will be celebrating both the 100th anniversary of the publication of Einstein’s theory of general relativity and the 25th anniversary of the Hubble Space Telescope,” says Mario Livio of the Space Telescope Science Institute, another key Hubble scientist. “Einstein’s theory transformed our perspective on space, time, gravity, and the universe. Hubble has turned those theoretical concepts into a fascinating reality through stunning images of cosmic entities that for millennia we have not been able to even imagine.”

NASA/George Ladas, base24.com (Hubble); NASA (Earth)

How to build a space telescope

NASA got on the fast track to build Hubble, frequently abbreviated HST, in the mid-1980s. The telescope’s heart, its optical system, would be a Ritchey-Chrétien design, a reflector with a hyperbolic primary mirror and a hyperbolic secondary, designed to minimize optical aberrations. The system would have a focal length of 57.6 meters and be designed with instruments that capture data in visible, infrared, near-infrared, and near-ultraviolet wavelengths. The optics would provide for outstanding imaging capabilities over a wide field of view. NASA’s Goddard Space Flight Center would take charge of the telescope’s control and data, and Marshall Space Flight Center would oversee design and construction. NASA contracted spacecraft construction to the aerospace company Lockheed and the construction of the optics, tube assembly, and guidance sensors to Perkin-Elmer. Eastman Kodak was contracted to build a backup mirror.

The space shuttle Discovery lifts off from Kennedy Space Center on April 24, 1990, carrying five astronauts and the Hubble Space Telescope.

In 1979, in Danbury, Connecticut, the engineering staff at Perkin-Elmer commenced work on the optics and finished the primary mirror some two years later. Polishing the mirror became a lengthy exercise that ran overbudget and overtime, resulting in frequent clashes between the company and NASA. And yet over the ensuing several years, the whole project came together, technicians assembled the telescope’s multitude of parts in an elaborate clean room, and a huge team of scientists made preparations to launch the first generalized space observatory, planned for the spring of 1990.

The launch went without a hitch. On April 24, STS-31 commenced as space shuttle Discovery lunged toward the sky, carrying the priceless telescope, a program that had cost up to that time about $1.5 billion. (The entire 40-year program, to the present day, has cost approximately $10 billion, a figure equal to financing the Iraqi War for two weeks.) The following day, the crew deployed the telescope and Hubble’s lifetime began. All was happy in Hubble’s universe. And then, over the ensuing weeks, came stunned shock. The images returned from the telescope indicated a serious flaw within the telescope’s optics. The triumphant launch into space of the first otherworldly observatory morphed into a disaster. Something was seriously wrong with Hubble’s mirror, and it turned out to be a severe case of spherical aberration, an easily avoidable optical flaw. A famous early image of the spiral galaxy M100 in Coma Berenices, recorded with the telescope’s Wide-Field and Planetary Camera, appeared fuzzy, as if it were shot through a thick London morning fog.

These before-and-after images of spiral galaxy M100 show the extent of Hubble’s pre-servicing blur and exactly how well the fix worked.

Hubble’s mirror had been the most precisely ground in the history of optics, with variations in the prescribed curve of only 10 nanometers — one ten-thousandth the width of a human hair — but technicians at Perkin-Elmer had ground the primary mirror to the wrong shape. And the disaster was based on a simple error: the company’s null corrector, an optical testing device, had been assembled incorrectly, leaving a lens within it 1.3 millimeters out of position. Moreover, workers had misread simple tests that could have flagged the problem.

The entire astronomy world was stunned, and Hubble was the biggest flop in the history of the business. Suddenly, not only had NASA failed to expand our view of the cosmos, but it also became the butt of a series of cultural jokes as a synonym for simple incompetence.

NASA/ESA/J. Dalcanton/B.F. Williams/L.C. Johnson (U. of Washington)/The PHAT Team/R. Gendler

A triumphant rebirth

But the story was not over. NASA had established a schedule of servicing missions for Hubble, which would be needed to maintain the telescope — to tweak, swap, and upgrade instruments over time. Now the first servicing mission would transform into a herculean effort to fix the telescope’s flawed optics. Engineers, opticians, and allied scientists began an effort to create a set of corrective optics that would be added to Hubble to make its “vision” clear, dubbed COSTAR (Corrective Optics Space Telescope Axial Replacement). During a 10-day mission near the end of 1993, astronauts employing the shuttle Endeavour installed COSTAR along with other equipment, including the Wide-Field and Planetary Camera 2, gyroscopes, and solar panels. Each change was designed to fix or idealize all systems such that Hubble would operate as well as was originally intended, or even better. (Subsequent servicing missions in 1997, 1999, 2002, and 2009 kept the telescope operating efficiently and upgraded or replaced certain instruments.)

During the first servicing mission in 1993, astronauts replaced and repaired various instruments and, more importantly, installed technology that corrected the tiny flaw in Hubble’s main mirror that distorted the telescope’s view.

By the first days of January 1994, the crisis had been surmounted and Hubble was open for business. The first science results from Hubble arrived through the campus of the Space Telescope Science Institute (STScI), an array of scientists and administrators set up by NASA to manage the telescope at the Homewood Campus of Johns Hopkins University in Baltimore. Its first director was the Italian-American astrophysicist Riccardo Giacconi, who later won the Nobel Prize in physics for his pioneering research in X-ray astronomy.

As is the case with most science projects, astronomers apply to use the Hubble Space Telescope, and a review board of scientists at STScI weigh in on the relative merits, approving or denying time to be granted on the orbiting eyes in space. Hubble peers out into the universe from so-called low-Earth orbit, about 353 miles (569 kilometers) above the planet’s surface, flies along at 17,500 mph (28,000 km/h), and orbits Earth once every 97 minutes. It’s possible to periodically see the telescope in the night sky, if it passes over your location, as a bright “star” slowly moving across the sky like other satellites do.

Jupiter appears bruised after it collided with various fragments from Comet Shoemaker-Levy 9. Hubble followed unexpected and dramatic changes in Jupiter's atmosphere caused by the event.
NASA/Hubble Space Telescope Comet Team

Soon after astronauts fixed the mammoth telescope’s optics, the instrument began to rain down impressive science results. In the summer of 1994, Hubble set its sights on a unique occurrence in the modern history of the solar system. Astronomers discovered a comet, named Shoemaker-Levy 9 after the discoverers, that was perilously close to Jupiter, had been pulled apart into a string of 21 fragments, and was destined to plunge into the giant planet. No one had ever seen a comet slam into a planet, so it was an incredible opportunity to witness what happened frequently in the early history of the solar system, when many more small bodies were flying around, accreting into larger ones. Hubble’s images and scientific observations helped write the story of this amazing event, as each of the fragments plummeted into the jovian cloud tops, creating dark plumes as they exploded like nuclear bombs.

Since that first summer of useful data, Hubble has rewritten astronomy and astrophysics, making more key observations over the ensuing 20 years than the sum of all the telescopes that came before it. In 1995, astronomer Jeff Hester of Arizona State University led a team that created the most iconic image taken with Hubble, a picture of the Eagle Nebula in Serpens, a gaseous cloud and associated star cluster decorated with tall dark nebulae that came to be called the “Pillars of Creation.” The image of stellar recycling — of gas and dust giving birth to infant stars — was so striking that it graced the cover of Time magazine and was given its own U.S. postage stamp.


Hubble’s amazing legacy

The depth of Hubble’s data, however, has touched or rewritten nearly every area of astrophysics. Ever since the discovery of the expanding universe in the 1920s, astronomers had struggled with the rate of expansion and what it means. The so-called Hubble constant, the universal rate of expansion, was much in doubt, with two factions arguing very different conclusions from the data. The Hubble constant is also inversely proportional to the age of the universe, another key holy grail of science. One of the primary goals of Hubble was to measure the Hubble constant accurately, using a variety of distance indicators, and by the turn of the 21st century, this helped define a relatively accurate Hubble constant of 72±8 and an age of the universe, which the more recent European Planck satellite has refined further to 13.8±0.04 billion years.

Hubble, with the help of other telescopes, has mapped the dark matter distribution in various galaxy clusters, like Pandora’s Cluster seen here. In this image, dark matter is represented in blue.
NASA/ESA/J. Merten (Institute for Theoretical Astrophysics, Heidelberg/Astronomical Observatory of Bologna)/D. Coe (STScI)

Hubble also was instrumental in the 1998 discovery and subsequent analysis of distant supernovae by two research teams, a finding that showed the universe’s expansion is accelerating. This accelerating universe gave rise to the term dark energy to explain the force that is speeding up the universal expansion. The astronomers measured the brightnesses of type Ia supernovae at great distances. They found the light measurements would be explained if a repulsive force were being exerted on space. The nature of dark energy remains a mystery, but its effects are clear, and the race to define exactly what it is and how it works is the hottest area of cosmology. The answer also will help define the nature of the fate of the universe — whether the cosmos will end in a “big freeze” or in some other way.

Hubble played a key role in exploring another mysterious aspect of the universe, dark matter. We can see that much of the matter in the universe is invisible — in fact far more than the bright, normal matter that makes up stars, galaxies, planets, people, trees, and cats. Hubble investigated dark matter, which has been inferred for decades, by looking at gravitational lensing of distant galaxies to create a three-dimensional map of dark matter. The maps of dark matter within individual clusters of galaxies created by Hubble have helped define how dark matter behaves, as with the famous Bullet Cluster.

BoldlyGo and the ASTRO–1 Telescope
Veteran Hubble Space Telescope scientist Jon Morse is chief executive officer and chair of the board of the BoldlyGo Institute, which plans on building and launching a space telescope that will add to Hubble’s legacy. Called ASTRO–1, the 1.8-meter scope will continue and enhance Hubble's visible light and ultraviolet observations. Read Morse’s thoughts on Hubble and more.

Studies of black holes in the centers of galaxies with Hubble have determined much of what we know about these central engines, monsters hiding deep in the centers of nearly all galaxies. Black holes, even supermassive ones, were known to exist before Hubble, but the space telescope enabled astronomers to survey large numbers of galaxies to show that every galaxy with a bright central stellar bulge contains a supermassive black hole in its center. Many dwarf galaxies lack these central black holes, but most galaxies contain one with a mass of millions to billions of times that of the Sun. Hubble results also showed that the central bulges of stars in galaxies and their attendant black holes evolve together over time; the growth of the central bulge is linked to the growth of the black hole.

Astronomers also used Hubble to record a historic series of so-called deep fields to study distant galaxies and the star formation rate in the universe in unprecedented detail. These long exposures of small areas of sky recorded thousands of distant, young, blotchy protogalaxies and demonstrated that as far away as astronomers can see, the universe is the same smooth, familiar place everywhere and in every direction, involving more than 100 billion galaxies total. Not only have these surveys shown astronomers that stars formed at different rates over cosmic time, with star formation peaking some 7 billion years ago, but they also show that galaxies were small and irregular in the early universe and came together gravitationally to form larger, normal galaxies through the accretion of many small precursors.

The Hubble eXtreme Deep Field (XDF) shows galaxies out to the most distant reaches of the cosmos.
NASA/ESA/G. Illingworth/D. Magee/P. Oesch (UCSC)/R. Bouwens (Leiden U)/The HUDF09 Team

In addition, scientists using Hubble have shone light onto the earliest days of the cosmos in their quest to understand how the Big Bang transformed into the universe we now know. Soon after the Big Bang, the cosmos was an opaque sea of plasma, and only some 380,000 years later did it cool sufficiently to undergo a so-called phase transition — protons and electrons combined to form neutral hydrogen atoms. The universe suddenly became transparent, and the cosmic microwave background radiation, so famous for being discovered in 1964 and for proving the Big Bang theory, was born. The Cosmic Dark Ages began, and some 100 million or more years later, the first stars formed. These produced ultraviolet radiation, which reionized the medium making up the cosmos. By roughly a billion years after the Big Bang, this reionization was complete. Hubble enabled astronomers to explore this era of reionization to determine that in order to reionize the universe, more galaxies must have existed than those that Hubble can see. So an intriguing question is left for the James Webb Space Telescope, the next great space telescope, which will be able to detect young galaxies better than Hubble.

No area of astronomy has been hotter or more rapidly changing over the past few years than the discovery of exoplanets, planets orbiting stars other than the Sun. Hubble has played a key role here, too, enabling planetary scientists to study the atmospheres of exoplanets. Scientists hope that increasing sensitivities will enable future detections of biosignatures on other worlds, marking the first discoveries of the evidence of life elsewhere in the cosmos. For now, Hubble has detected elements and molecules in the atmospheres of “hot Jupiters” orbiting other stars and has unveiled evidence of absorption lines in the spectra of several exoplanets indicating water vapor in their atmospheres. Hubble and its sister telescope Spitzer, the infrared instrument, have detected clouds in the atmospheres of several exoplanets.

Reflections on Hubble from a veteran astronomer
For years, Garth Illingworth of the University of California Observatories and Lick Observatory has been a senior user of the Hubble Space Telescope. He was deputy principal investigator of the Advanced Camera for Surveys that was launched in 2002 and has been able to study galaxies just a few hundred million years after the Big Bang. Read Illingworth’s reflections on Hubble’s legacy.

And these are but a few areas, important as they are, touched or redefined by the existence of the world’s greatest telescope. The effects, the influence of Hubble, are woven like a dominant thread throughout the last quarter-century of astrophysics, of cosmology, of planetary science. In 2011, NASA’s star instrument passed a milestone when the ten-thousandth scientific paper using its data was published. Each year, about 10 percent of the most cited papers published are based on Hubble data. Many thousands of images of all manner of astronomical objects have poured out of Hubble, redefining how the public sees the cosmos.

In an age when science funding and support are often lacking, when a public basks in the benefits of science and technology mostly without providing for their futures, Hubble is a standout. Memories of a forgotten age of Moon exploration linger as moonwalking astronauts age into old men; an abandoned $2 billion supercollider stands amid wild grass in Texas; Americans seem content to watch reality shows as other nations push science and technology forward. And yet the Hubble Space Telescope, now 25 years into its spectacular run, stands triumphant as the instrument that explored and redefined the cosmos. This magnificent telescope has allowed, for a time, astronomers to take off on a new and unprecedented voyage to the stars.

Gallery: A peek into the Hubble vault

Seyfert’s Sextet

NASA/J. English (U. of Manitoba)/S. Hunsberger, S. Zonak, J. Charlton, and S. Gallagher (PSU)/L. Frattare (STScI)

NGC 3314

NASA/ESA/The Hubble Heritage Team (STScI/AURA)/W. Keel (U. of Alabama)

The Mice

NASA/ESA/H. Ford (JHU)/The ACS Science Team

Spiral galaxy M83


The Tarantula Nebula (NGC 2070)


The Monkey Head Nebula (NGC 2174)


Variable star RS Puppis


The Crab Nebula (M1)

NASA/ESA/J. Hester and A. Loll (ASU)

Planetary Nebula NGC 2440


Planetary Nebula IC 4593


The Ring Nebula (M57)


Star-forming region NGC 3603

David J. Eicher is Editor of Astronomy magazine. He is author of 18 books on science and history, including the forthcoming The New Cosmos: Answering Astronomy’s Big Questions (Cambridge University Press, 2015). Follow him on Twitter at @deicherstar.