The Hubble Space Telescope and Its Greatest Discoveries

Few scientific instruments have ever become household names, but the Hubble Space Telescope did.

Its images hang in classrooms and hospitals, appear on stamps and screensavers, and have shaped how millions of people imagine the universe.

Behind the pretty pictures is a remarkable story: a telescope that launched with a flaw, was rescued by astronauts,

and went on to answer questions astronomers had argued about for generations.

Here is how Hubble came to be, how it works and which of its discoveries matter most.

A Telescope Decades in the Making

The idea of a large telescope above the atmosphere was championed for decades by astrophysicist Lyman Spitzer,

and NASA and the European Space Agency eventually built it together. It was named after Edwin Hubble,

the astronomer who showed in the 1920s that many fuzzy patches in the sky were entire galaxies beyond our own,

and that the universe is expanding.

The Space Shuttle Discovery carried the telescope into orbit in April 1990.

Hubble is about the size of a school bus, roughly 13 meters (43 feet) long,

and gathers light with a main mirror 2.4 meters (7.9 feet) across.

Orbiting a few hundred kilometers above Earth and circling the planet about every 95 minutes,

it sees the sky free of the blurring effect of air.

A Blurry Start and a Daring Fix

The first images were a shock. Stars that should have been crisp points were surrounded by fuzzy halos.

Investigators discovered that the main mirror had been ground to the wrong shape.

Near its edge, it was too flat by about 2.2 micrometers, roughly one-fiftieth of the width of a human hair.

The error was tiny, but it was enough to spoil the telescope's sharpest views.

Because the flaw was so precisely measured, engineers could design a correction, much like a prescription for eyeglasses.

In December 1993, astronauts aboard the shuttle Endeavour made five spacewalks to install corrective optics and a new camera with the fix built in.

When the new images arrived, Hubble's vision was as sharp as its designers had intended, and the mission became one of NASA's proudest moments.

Five Visits From Astronauts

Hubble was designed to be serviced in orbit, which set it apart from almost every other space telescope.

Shuttle crews visited five times, each time upgrading the observatory:

• 1993: Corrected the flawed optics and installed a new wide-field camera.
• 1997: Added instruments that extended Hubble's vision into the near-infrared and improved its ability to split light into spectra.
• 1999: Replaced worn-out gyroscopes, the devices that help the telescope point accurately.
• 2002: Installed a powerful new survey camera and new solar arrays.
• 2009: In the final visit, installed a new camera and spectrograph, repaired two instruments and replaced gyroscopes and batteries.

Thanks to these upgrades, the Hubble that emerged from its final servicing mission was a far more capable observatory than the one launched in 1990.

Hubble's Greatest Discoveries

Astronomers from around the world compete for observing time on Hubble, and its data have fed many thousands of scientific papers.

A few results stand out.

Pinning Down the Age of the Universe

Before Hubble, estimates of the universe's age ranged from roughly 10 to 20 billion years, because astronomers disagreed about how fast it is expanding.

One of the telescope's main goals was to measure pulsating stars called Cepheid variables in distant galaxies, which act as cosmic mileposts.

By around 2001, a major Hubble project had measured the expansion rate to within about 10 percent.

Combined with other measurements, that work helped narrow the age of the universe to about 13.8 billion years.

A Universe That Is Speeding Up

In the late 1990s, astronomers studying distant exploding stars found that the expansion of the universe is accelerating,

a result that later earned a Nobel Prize. Hubble's observations of very faraway supernovae helped confirm the finding.

The cause, nicknamed dark energy, remains one of the biggest mysteries in science.

Black Holes in the Hearts of Galaxies

By tracking how fast stars and gas whirl around the centers of galaxies,

Hubble provided strong evidence that supermassive black holes, millions to billions of times the mass of the Sun,

sit at the hearts of most large galaxies.

It also helped show that the mass of these black holes is linked to the size of the galaxies around them, suggesting they grow together.

The Deep Fields

In December 1995, Hubble stared for about ten days at a tiny, seemingly empty patch of sky near the Big Dipper.

The resulting Hubble Deep Field revealed roughly 3,000 galaxies, some seen as they were billions of years ago.

Later, even deeper exposures found around 10,000 galaxies in a similarly small patch.

These images showed how galaxies have grown and changed over cosmic time, and they made the scale of the universe feel suddenly real.

Planets Forming and Planets Beyond

In the Orion Nebula, Hubble photographed dozens of young stars wrapped in disks of dust and gas, the raw material from which planets form.

In 2001,

it made the first detection of an atmosphere around a planet orbiting another star by noticing how starlight changed as the planet passed in front of it.

A Front-Row Seat in the Solar System

Hubble also looked closer to home.

In July 1994, it watched fragments of Comet Shoemaker-Levy 9 slam into Jupiter, leaving dark scars larger than Earth in the planet's clouds.

It discovered four small moons of Pluto and tracked storms, auroras and changing seasons on the giant planets.

The Images That Changed How We See Space

Hubble's scientific impact is matched by its cultural one. Its 1995 portrait of the Pillars of Creation,

towering columns of gas and dust where new stars are forming in the Eagle Nebula,

became one of the most recognizable astronomy images ever made.

Colorful views of colliding galaxies, dying stars shedding their outer layers and deep fields crowded with galaxies turned the telescope into a public favorite.

Many of those images are made by combining exposures taken through different filters, each assigned a color.

The results are scientifically meaningful and beautiful at the same time, which helped bring astronomy to people who had never looked through a telescope.

How Hubble Does Its Work

Light entering the telescope bounces off the main mirror to a smaller secondary mirror,

then back through a hole in the center of the main mirror to a set of cameras and spectrographs.

Together these instruments cover ultraviolet, visible and near-infrared light.

Astronomers submit proposals for observing time, and a scientific committee chooses among them, with far more requests than hours available.

Observations are sent to the ground through relay satellites, processed and stored in a public archive, where anyone can eventually use them.

A Lasting Legacy

Hubble proved that a large telescope in orbit could transform astronomy, and that astronauts could repair and upgrade one in space.

It measured the age of the universe, confirmed its accelerating expansion,

found black holes in galaxy centers and gave the public a new gallery of cosmic images.

Few machines have taught so many people so much about the universe, and its archive will keep feeding discoveries for years to come.