The Hubble Space Telescope: How 35 Years of Discovery Changed Our View of the Universe

The Hubble Space Telescope has delivered some of the most iconic astronomical images in history, fundamentally changing our understanding of the cosmos. Photo: NASA/Goddard Space Flight Center (Public Domain)

From Disaster to Triumph

The story of the Hubble Space Telescope is one of the most remarkable in the history of science: a billion-dollar instrument launched with a crippling defect, rescued by a daring repair mission, and transformed into the most productive scientific instrument ever built. Over more than three decades in orbit, Hubble has fundamentally changed our understanding of the age, size, and fate of the universe, produced images that have become part of our cultural consciousness, and inspired millions of people to look up and wonder.

When Hubble launched aboard the Space Shuttle Discovery on April 24, 1990, expectations were enormous. Named after Edwin Hubble, the astronomer who proved that galaxies exist beyond the Milky Way and that the universe is expanding, the telescope was designed to observe the cosmos from above Earth’s atmosphere, free from the blurring effects of turbulence that limit ground-based telescopes. With its 2.4-meter primary mirror, Hubble was expected to deliver images of unprecedented clarity.

Then came the devastating news. The first images were blurry. The primary mirror had been ground to the wrong shape, a flaw of just 2.2 micrometers (about 1/50th the thickness of a human hair), but enough to cause spherical aberration that spread starlight into fuzzy halos instead of sharp points. The most expensive telescope ever built was nearsighted.

The Repair That Saved a Mission

NASA had always planned for shuttle servicing missions to Hubble, but the urgency of the mirror problem made the first one critical. In December 1993, astronauts aboard Space Shuttle Endeavour performed one of the most complex spacewalking missions ever attempted. Over five spacewalks totaling 35 hours, they installed COSTAR (Corrective Optics Space Telescope Axial Replacement), a device containing corrective mirrors that compensated for the primary mirror’s flaw, essentially giving Hubble “glasses.”

The first images after the repair were stunning. Stars that had been fuzzy blobs snapped into sharp points. The difference was dramatic and immediate. Hubble worked. And over the next three decades, it worked far beyond anyone’s expectations.

Hubble’s Greatest Discoveries

The Age and Expansion Rate of the Universe

One of Hubble’s primary science goals was measuring the Hubble constant, the rate at which the universe is expanding. By observing Cepheid variable stars (whose pulsation periods directly indicate their luminosity) in distant galaxies, Hubble precisely calibrated the cosmic distance ladder. The result: the universe is approximately 13.8 billion years old, and it is expanding at about 73 kilometers per second per megaparsec.

Interestingly, this measurement does not perfectly agree with the expansion rate predicted from observations of the cosmic microwave background (about 67.4 km/s/Mpc). This discrepancy, called the Hubble tension, is one of the most important unsolved problems in cosmology and may point to new physics beyond our current understanding.

Dark Energy and the Accelerating Universe

In 1998, two teams of astronomers using Hubble and ground-based telescopes to study distant Type Ia supernovae made a shocking discovery: the expansion of the universe is not slowing down as expected, it is accelerating. Something is pushing the universe apart, and we do not know what it is. This mysterious force, dubbed dark energy, constitutes roughly 68% of the total energy content of the universe. The discovery earned Saul Perlmutter, Brian Schmidt, and Adam Riess the 2011 Nobel Prize in Physics, and Hubble’s observations were central to the evidence.

Supermassive Black Holes

Hubble’s sharp imaging revealed that supermassive black holes, containing millions to billions of solar masses, are not rare curiosities but common features at the centers of most galaxies. Hubble measured the velocities of stars and gas swirling around galactic centers, proving the presence of enormous concentrated masses. Even more remarkably, Hubble data revealed a tight relationship between the mass of a galaxy’s central black hole and the mass of the galaxy’s central bulge, suggesting that black holes and galaxies co-evolve.

The Deep Fields

In December 1995, Hubble pointed at an apparently empty patch of sky in the constellation Ursa Major, an area so small it could be covered by a grain of sand held at arm’s length, and stared for 10 consecutive days. The resulting image, the Hubble Deep Field, was one of the most important photographs ever taken. It revealed approximately 3,000 galaxies, many of them among the most distant ever observed, stretching back billions of years in time. A “blank” spot in the sky was actually teeming with galaxies.

Hubble followed this with the Hubble Ultra Deep Field (2004), an even deeper image that captured about 10,000 galaxies in an even smaller patch of sky, some dating to just 400 million years after the Big Bang. These images transformed our understanding of galaxy formation and evolution and demonstrated that the observable universe contains at least 200 billion galaxies (a number later revised upward to 2 trillion based on additional analysis).

Iconic Images That Changed Culture

Hubble has not only advanced science; it has changed how humanity sees the universe. Several of its images have become cultural icons:

Pillars of Creation (1995, re-imaged 2014): Towering columns of gas and dust in the Eagle Nebula (M16) where new stars are being born. To observe Messier objects yourself, see our Messier catalog guide. This image may be the most famous astronomical photograph ever taken. It has appeared on everything from T-shirts to postage stamps and has become a visual shorthand for the beauty of the cosmos.

Hubble Deep Field: The image that showed us just how many galaxies exist in what appeared to be empty space.

Sombrero Galaxy (M104): A stunning edge-on spiral galaxy with a prominent dust lane and bright nucleus, showcasing Hubble’s ability to reveal galactic structure in exquisite detail.

Crab Nebula (M1): The remnant of the supernova observed in 1054 AD, showing intricate filaments of gas expanding at thousands of kilometers per second, powered by a rapidly spinning neutron star at its center.

Gravitational lensing images: Hubble images of massive galaxy clusters bending and distorting the light of more distant galaxies behind them, creating arcs and rings of light that dramatically demonstrate Einstein’s general relativity.

The Servicing Missions

One of Hubble’s unique advantages was its design for in-orbit servicing. Over five shuttle missions spanning 16 years, astronauts visited Hubble to replace failed components, install new instruments, and upgrade its capabilities:

• SM1 (1993): Installed COSTAR corrective optics, fixed the mirror problem
• SM2 (1997): Installed NICMOS (near-infrared camera) and STIS (imaging spectrograph)
• SM3A (1999): Replaced failed gyroscopes and installed a new computer
• SM3B (2002): Installed the Advanced Camera for Surveys (ACS), dramatically improving imaging capability
• SM4 (2009): The final and most ambitious mission. Installed Wide Field Camera 3 (WFC3) and the Cosmic Origins Spectrograph (COS), repaired ACS and STIS, and replaced all six gyroscopes and both batteries. SM4 was expected to extend Hubble’s life through at least 2014; as of 2026, it is still operating.

Each servicing mission essentially created a new telescope, with improved instruments that kept Hubble at the cutting edge of astronomical research for over three decades.

Hubble and JWST: Complementary Observatories

When the James Webb Space Telescope launched in December 2021, some asked whether Hubble was obsolete. The answer is definitively no. Hubble and JWST complement each other:

• Hubble observes primarily in visible and ultraviolet light, wavelengths where many astrophysical processes are best studied (hot stars, ionized gas, stellar atmospheres, supernova remnants).
• JWST observes in infrared light, penetrating dust clouds and detecting the most distant, red-shifted objects in the universe.

Many astronomical objects are best understood by combining observations from both telescopes. Hubble shows the hot, energetic components while JWST reveals the cold, dusty, and distant components. Together, they provide a more complete picture than either could alone.

Hubble’s Current Status and Legacy

As of 2026, Hubble is still operating and producing science. For the broader history of space exploration, see our space exploration milestones guide, though it faces challenges. Several of its six gyroscopes have failed over the years, and NASA has switched to a one-gyroscope mode to conserve the remaining units and extend the mission. The telescope’s orbit is slowly decaying due to atmospheric drag, and without a reboost mission, it will eventually re-enter the atmosphere, likely in the early-to-mid 2030s. NASA and SpaceX have studied the feasibility of a private mission to reboost Hubble to a higher orbit, which would extend its operational lifetime significantly.

Hubble has produced over 1.5 million observations, generated more than 19,000 peer-reviewed scientific papers, and been used by astronomers in over 50 countries. It is the most scientifically productive telescope in history.

But perhaps Hubble’s greatest legacy is not scientific but cultural. It showed humanity the beauty of the universe in a way that no previous instrument had. It made astronomy visual, visceral, and accessible to everyone. The Pillars of Creation, the Deep Fields, the galaxy collisions, and the nebulae are not just data. They are art, evidence that the universe is beautiful, complex, and worth understanding.

Hubble transformed us from observers of points of light into witnesses of cosmic majesty. Whatever eventually happens to the telescope, its images and discoveries will endure as long as humans look up and wonder. To explore more of what space science has revealed, see our article on the search for exoplanets.