The Hubble Space Telescope

The Hubble Space Telescope is a giant telescope that orbits the Earth above the atmosphere. Since it was launched in April 1990, Hubble has taken some of the most stunning images of the universe ever produced. Its observations have helped astronomers measure the age of the universe, discover dark energy, find planets around other stars, and watch storms on Jupiter, supernovae in distant galaxies and the births of new stars in nearby nebulae. Hubble is one of the most successful science instruments ever built.

  • Launched24 April 1990On Space Shuttle Discovery
  • Mirror size2.4 metresAcross
  • Orbit altitudeapprox. 540 kmAbout 1.5x as high as the ISS
  • Speedapprox. 27,000 km/hOne full orbit takes 95 minutes
  • Observations madeOver 1.6 millionIn 35 years
  • Servicing missions5Astronauts visited Hubble 5 times to upgrade it

Why put a telescope in space?

The Earth's atmosphere is a problem for astronomers. It is constantly moving (which makes stars twinkle and blurs telescope images), it blocks most of the ultraviolet and infrared light from space, and it scatters city light pollution all over the sky. Putting a telescope above the atmosphere solves all three problems. From orbit, the view of the universe is razor-sharp, the full range of light gets through, and the sky is genuinely dark.

How Hubble works

Hubble looks something like a giant tube about 13 metres long and 4 metres wide, weighing 11 tonnes. At one end is a 2.4-metre primary mirror that gathers light from distant objects and reflects it onto a smaller secondary mirror at the front of the tube. The light then bounces down into Hubble's instruments, which include several cameras and spectrographs. The whole telescope is wrapped in shiny silver insulation to keep its temperature stable.

Hubble orbits Earth at about 540 km altitude, completing one orbit every 95 minutes. It is solar-powered and uses gyroscopes to spin gently and point at exactly the patch of sky astronomers want to study. It can hold steady on a target so precisely that, from Earth orbit, it could keep a laser pointer aimed at a 10p coin in London from over 320 km away.

The fault that nearly killed it

When Hubble's first pictures came back in 1990, NASA had a shock: they were blurry. Investigators found that Hubble's primary mirror had been polished slightly the wrong shape: by 1/50th the width of a human hair, but enough to ruin the focus. There was no way to remove and replace the mirror in orbit.

The solution, designed and installed by astronauts on the first Hubble servicing mission in December 1993, was essentially a pair of "spectacles" for the telescope: a set of small mirrors that bent the light from the primary mirror just enough to cancel out the original error before it reached the cameras. From 1994 onwards Hubble produced the sharp images it should have produced from the start.

Famous Hubble images

Some of the most famous photographs in the history of science have come from Hubble.

  • The Pillars of Creation (1995, re-imaged 2014 and 2022): towering columns of gas and dust in the Eagle Nebula, where new stars are being born.
  • The Hubble Deep Field (1995) and Ultra Deep Field (2004): images of tiny patches of "empty" sky that revealed thousands of distant galaxies in a single shot.
  • The Sombrero Galaxy: a sharp side-on view of a spiral galaxy resembling a Mexican hat.
  • The Cat's Eye Nebula and Helix Nebula: planetary nebulae where Sun-like stars are dying.
  • Comet Shoemaker-Levy 9 hitting Jupiter: in 1994 Hubble captured the impact scars from a comet that broke apart and crashed into Jupiter.

Hubble's biggest discoveries

Hubble has done much more than take beautiful pictures. Its scientific discoveries include:

  • Measuring the age of the universe precisely: 13.8 billion years.
  • Helping discover that the expansion of the universe is accelerating, leading to the concept of dark energy.
  • Finding that almost every big galaxy has a supermassive black hole at its centre.
  • Spotting moons of Pluto that nobody knew existed.
  • Watching storms on Saturn and the changing weather on Mars.
  • Detecting the atmospheres of distant exoplanets for the first time.
Fact The famous Hubble Deep Field images came from pointing the telescope at a tiny "empty" patch of sky for hundreds of hours. The patch was so small it would be covered by a grain of sand held at arm's length. Hubble found nearly 3,000 distant galaxies in that tiny window, proving that the universe was packed with galaxies in every direction. The Hubble Ultra Deep Field (2004) extended the experiment even further and found around 10,000 galaxies in a similar postage-stamp of sky.

The servicing missions

Hubble was designed to be serviced in space by astronauts. Between 1993 and 2009, the Space Shuttle visited Hubble five times to repair instruments, replace gyroscopes and install new cameras. The 2009 mission was the last; after the Space Shuttle programme ended in 2011, no spacecraft was capable of taking astronauts to Hubble. The telescope has been operating on its own ever since, with most of its instruments still working remarkably well 35 years after launch.

Did you know? Hubble was named after the American astronomer Edwin Hubble, who in the 1920s proved that other galaxies existed beyond the Milky Way and discovered that the universe is expanding. Hubble himself never got to see his namesake telescope: he died in 1953, nearly forty years before it launched.
Deeper dive: the Hubble Deep Field, the most powerful image in modern astronomy

In December 1995, Hubble's project leaders made a bold call. They pointed the most expensive telescope in history at a tiny dark patch of sky in Ursa Major for 10 days straight, a patch chosen specifically because it appeared to be empty. Why "waste" the world's best telescope on a piece of sky with apparently nothing in it?

The reason was a thought experiment. If the universe is roughly uniform, then even an "empty" patch of sky must really be full of incredibly distant galaxies, ones too faint for any normal exposure to detect. Hubble's long stare would build up the light from those faint galaxies until they appeared. It was a bet on whether the universe really was as densely populated as it seemed.

The bet paid off spectacularly. The Hubble Deep Field image, released in January 1996, showed nearly 3,000 distinct galaxies in a region of sky barely 1/30th the diameter of the full Moon. Some of those galaxies were over 10 billion light years away: their light had been on its way to Earth since long before our Solar System existed. The image made it overwhelmingly clear that the universe contains billions or trillions of galaxies. The follow-up Hubble Ultra Deep Field in 2004 went even further, and the James Webb Space Telescope has now extended the experiment to its limit, revealing galaxies that formed within a few hundred million years of the Big Bang itself.

For Hubble's successor, see the James Webb Space Telescope. For other historic missions see Voyager 1 and 2.