Hubble Space Telescope (1990): Launch, Flaw and Discoveries

The Hubble Space Telescope was launched aboard Space Shuttle Discovery on 24 April 1990, the first large optical observatory placed in orbit above Earth’s blurring atmosphere. Soon after launch, astronomers found that its main mirror had been made to the wrong shape, an embarrassing failure that was corrected by astronauts in 1993. Repaired and upgraded, Hubble became one of the most productive scientific instruments ever built, reshaping our picture of the universe.

Background: a telescope above the atmosphere

Earth’s atmosphere absorbs much ultraviolet and infrared light and its turbulence makes stars twinkle, limiting the sharpness of ground-based telescopes. In 1946 the American astrophysicist Lyman Spitzer argued that a telescope in space would avoid these problems and see far more clearly.

NASA developed the idea in the 1960s and 1970s as the Large Space Telescope. After years of debate over its cost, the US Congress approved funding in 1977. The European Space Agency joined as a partner, supplying the Faint Object Camera and the first solar panels in return for a share of observing time.

The telescope was named after Edwin Hubble, whose observations in the 1920s showed that galaxies lie far beyond the Milky Way and that the universe is expanding. The Space Telescope Science Institute in Baltimore was set up to run its scientific programme, while NASA’s Goddard Space Flight Center managed the spacecraft.

Delays and design

Hubble was originally planned for launch in the early 1980s, but technical problems and rising costs pushed it back to 1986. The loss of the shuttle Challenger in January 1986 grounded the fleet, and the finished telescope waited in storage for several more years.

The observatory is about 13 metres long and weighs roughly 11 tonnes. At its heart is a 2.4-metre primary mirror, smaller than large ground-based mirrors but free of atmospheric blurring. It was designed to be visited and upgraded by astronauts, with instruments built as replaceable modules. By launch, its cost had risen to about 1.5 billion US dollars, several times early estimates.

How Hubble works

Hubble is a reflecting telescope: light enters the open tube, bounces off the large primary mirror to a smaller secondary mirror and is focused through a hole in the primary onto the scientific instruments behind it. These cameras and spectrographs can observe in ultraviolet, visible and near-infrared light.

The telescope circles Earth roughly every 95 minutes. To hold a target steady for long exposures, it uses gyroscopes, reaction wheels and Fine Guidance Sensors that lock onto guide stars, keeping its aim stable to a tiny fraction of an arcsecond. Its data are relayed through NASA’s tracking and data relay satellites to the ground. Astronomers from around the world compete for observing time, and each year many times more time is requested than is available.

Launch on STS-31

Discovery lifted off from Kennedy Space Center on 24 April 1990. Its crew were commander Loren Shriver, pilot Charles Bolden and mission specialists Steven Hawley, Bruce McCandless II and Kathryn Sullivan. On 25 April Hawley used the shuttle’s robotic arm to lift Hubble out of the payload bay and release it, after a tense delay when one of the solar panels was slow to unfurl.

The shuttle carried Hubble to an orbit about 600 kilometres above Earth, one of the highest altitudes a shuttle ever reached. Engineers then began weeks of testing before the first images could be taken.

The flawed mirror and its repair

When the first test images arrived in May and June 1990, they were disappointingly fuzzy. Instead of focusing light from a star into a sharp point, the telescope spread it into a blurred halo. Analysis showed that the primary mirror suffered from spherical aberration: its outer edge had been ground too flat by about 2.2 micrometres, roughly one-fiftieth of the width of a human hair.

An investigation board led by Lew Allen found that the error came from the contractor, Perkin-Elmer, which had used a wrongly assembled testing device called a reflective null corrector. One of its lenses was out of position by about 1.3 millimetres, so the mirror had been polished very precisely to the wrong shape. Other tests had hinted at the problem but were dismissed.

The flaw became a national embarrassment and a target for comedians, and it came only four years after the Challenger inquiry had criticised NASA’s management. Yet because the error was precisely measurable, engineers could design optics to cancel it, much as spectacles correct human eyesight.

The 1993 rescue mission

In December 1993 Space Shuttle Endeavour flew the first servicing mission, STS-61. Over five spacewalks, astronauts including Story Musgrave, Jeffrey Hoffman, Kathryn Thornton and Tom Akers replaced the solar panels and gyroscopes and installed two crucial pieces of equipment.

The first was COSTAR (Corrective Optics Space Telescope Axial Replacement), a set of small mirrors that corrected the light reaching several instruments. The second was the Wide Field and Planetary Camera 2, which had its own correction built in. In January 1994 NASA released before-and-after images showing that Hubble could now see as sharply as originally intended.

Further servicing

Astronauts returned to Hubble four more times, in 1997, 1999, 2002 and 2009, replacing worn-out parts and installing more powerful instruments such as the Advanced Camera for Surveys. Later instruments had corrections built in, and COSTAR was removed in 2009.

After the Columbia disaster in 2003, NASA cancelled the final servicing mission as too risky, provoking protests from scientists and the public. The decision was reversed, and in May 2009 the crew of Atlantis carried out the last repair, extending Hubble’s life well beyond its original 15-year design.

Major discoveries

• Age and expansion of the universe: Hubble’s Key Project measured the distances to galaxies precisely enough to fix the expansion rate, the Hubble constant, to within about 10 percent by 2001. Combined with other data, this pointed to an age of around 13.7-13.8 billion years.
• Accelerating expansion: Observations of distant supernovae, to which Hubble contributed, showed in 1998 that the expansion of the universe is speeding up, pointing to a mysterious “dark energy”. The discovery led to the 2011 Nobel Prize in Physics.
• Black holes: Hubble showed that supermassive black holes are common at the centres of large galaxies and that their masses are linked to the galaxies around them.
• Deep Fields: In December 1995 Hubble stared at a tiny, seemingly empty patch of sky for about ten days. The resulting Hubble Deep Field revealed around 3,000 galaxies, some seen as they were billions of years ago.
• Planets and stars: It observed the impacts of Comet Shoemaker-Levy 9 on Jupiter in 1994, imaged disks of dust around young stars in the Orion Nebula and made the first detection of an atmosphere on a planet orbiting another star.

Images such as the “Pillars of Creation” in the Eagle Nebula, released in 1995, became some of the most famous scientific pictures ever made.

Legacy

Hubble has made well over a million observations and supported tens of thousands of scientific papers. Its data are archived and freely available, so new discoveries continue to be made from old observations.

It also changed public engagement with astronomy, as its images appeared in newspapers, schoolbooks and on screens around the world. Its successor for infrared astronomy, the James Webb Space Telescope, launched in December 2021, while Hubble continued to operate alongside it. Since 2024 it has pointed using a single gyroscope to conserve its ageing hardware.

Myths and facts

• Myth: Hubble’s mirror was carelessly made. The mirror was one of the most precisely polished ever produced; it was simply polished very accurately to the wrong shape because of a faulty test instrument.
• Myth: Hubble’s pictures show the colours the human eye would see. Many famous images are composites of exposures through different filters, with colours assigned to show particular gases or wavelengths.
• Myth: Hubble orbits far out in space. It travels in low Earth orbit, a few hundred kilometres up, which is why space shuttle crews could reach it for repairs.

By the numbers

Timeline

• 1946: Lyman Spitzer proposes a large telescope in space.
• 1977: The US Congress approves funding for the Space Telescope.
• 1990, 24 April: Discovery launches Hubble on STS-31.
• 1990, 25 April: Hubble is released into orbit.
• 1990, June: NASA announces that the main mirror is flawed.
• 1993, December: Endeavour’s crew installs corrective optics on STS-61.
• 1995, December: Hubble records the Hubble Deep Field.
• 1998: Supernova studies reveal that the universe’s expansion is accelerating.
• 2009, May: The fifth and final servicing mission is completed.
• 2024: Hubble switches to single-gyroscope operations.

Why it matters

Hubble showed both the risks and the rewards of big science: a costly mistake was followed by a daring repair in orbit and decades of discovery. It answered long-standing questions about the age and growth of the universe and raised new ones, such as the nature of dark energy, that still drive research.