How Hubble Spent 35 Years Rewriting the Story of the Universe

On April 24, 1990, the Space Shuttle Discovery carried a 13.2-meter-long cylinder into low Earth orbit and released it into the darkness above the atmosphere. The Hubble Space Telescope, named for the astronomer Edwin Hubble who proved in the 1920s that other galaxies exist beyond the Milky Way, was expected to revolutionize astronomy by observing the universe from above the blurring effects of Earth's atmosphere. Within weeks, it had become the most expensive embarrassment in NASA's history. And then, three years later, it became perhaps NASA's greatest triumph.

The Mirror Disaster and the Rescue

The problem was humiliatingly simple in its cause, if staggering in its consequence. Hubble's 2.4-meter primary mirror had been ground with exquisite precision — to within 10 nanometers of its intended shape — but to the wrong prescription. The mirror was too flat by 2.2 micrometers at its edge, an error smaller than one-fiftieth the width of a human hair. The result was severe spherical aberration: every image Hubble returned was blurred in a characteristic halo that made the telescope's photographs barely better than ground-based images.

In December 1993, Space Shuttle Endeavour carried astronauts on a servicing mission to install corrective optics — essentially a set of precisely shaped mirrors that acted as contact lenses for Hubble's flawed mirror. The repair required five spacewalks over eleven days. When the corrected images came back, astronomers wept. Stars that had been smears became pinpoints. Galaxies that had been blobs became swirls of extraordinary structure. Hubble was, finally, what it had been promised to be.

What 1.5 Million Observations Mean

Over more than three decades of operation, Hubble has accumulated a scientific record without parallel in the history of astronomy. Its more than 1.5 million observations have contributed to over 21,000 peer-reviewed scientific papers. The telescope has imaged objects ranging from asteroids in our own solar system to galaxies so distant that their light left before Earth existed.

Among its most consequential achievements was the precise measurement of the universe's expansion rate. In the late 1990s, observations of distant supernovae made using Hubble data contributed to the discovery that the universe's expansion is not slowing down as gravity would dictate — it is accelerating. This finding implied the existence of a mysterious force pushing the universe apart, which scientists named dark energy. The discovery earned its investigators the 2011 Nobel Prize in Physics.

Hubble also helped determine the age of the universe with far greater precision than previously possible. By measuring the distances to Cepheid variable stars — stars that pulse at rates directly related to their luminosity — Hubble refined the Hubble Constant, the number describing the universe's rate of expansion. The current best estimate for the universe's age, roughly 13.8 billion years, rests substantially on Hubble's data.

The Deep Fields: Looking to the Edge of Time

Some of Hubble's most iconic and scientifically transformative work came from simply staring at seemingly blank patches of sky for extended periods. In 1995, NASA pointed Hubble at a tiny region of sky near the Big Dipper — a patch that appeared essentially empty to ground-based telescopes — and exposed it for ten consecutive days. The resulting image, known as the Hubble Deep Field, revealed approximately 3,000 galaxies in a region of sky one-thirtieth the diameter of a full moon. Extrapolating from that image suggested the observable universe contains roughly 200 billion galaxies — a number that was itself later revised upward to perhaps two trillion based on further analysis.

Subsequent deep field images pushed even further, capturing galaxies whose light had been traveling for more than 13 billion years. These images provide a direct view of the early universe — galaxies as they existed when the cosmos was only a few hundred million years old — and have revealed how galaxies grow, merge, and evolve over cosmic time.

Hubble's Ongoing Relevance

Despite the launch of the James Webb Space Telescope in 2021, which observes primarily in infrared wavelengths and can peer even deeper into the universe, Hubble remains scientifically active. Its ultraviolet and visible-light capabilities complement Webb's infrared vision, and the combination of the two telescopes provides a more complete picture of the cosmos than either could alone. As of 2026, Hubble continues to operate, its gyroscopes aging but its mirror as sharp as ever, still returning data from 547 kilometers above the planet that built it.