Hubble: Window to the Universe
NASA's Great Observatories

Hubble: Window to the Universe
NASA's Great Observatories

The Hubble Space Telescope is one of the greatest technological achievements in our history, and for two decades has astonished us with dynamic images of our solar system and the world beyond.

To celebrate this important twenty-year milestone, NASA looks back at the contributions of this extraordinary scientific tool, and the scientists who created it, in an engrossing documentary entitled "Hubble: Twenty Years of Discovery".

Many Hubble observations have led to breakthroughs in astrophysics, such as accurately determining the rate of expansion of the universe.

The Hubble Space Telescope (HST) is a NASA space telescope that was carried into orbit by a space shuttle in April 1990.

Although not the first space telescope, Hubble is one of the largest and most versatile, and is well-known as both a vital research tool and a public relations boon for astronomy.

The HST was built by NASA with contributions from the European Space Agency, and is operated by the Space Telescope Science Institute.

It is named after the astronomer Edwin Hubble. The HST is one of NASA's Great Observatories, along with the Compton Gamma Ray Observatory, the Chandra X-ray Observatory, and the Spitzer Space Telescope.

Space telescopes were proposed as early as 1923. Hubble was funded in the 1970s, with a proposed launch in 1983, but the project was beset by technical delays, budget problems, and the Challenger disaster.

When finally launched in 1990, scientists found that the main mirror had been ground incorrectly, severely compromising the telescope's capabilities.

However, after a servicing mission in 1993, the telescope was restored to its intended quality. Hubble's orbit outside the distortion of Earth's atmosphere allows it to take extremely sharp images with almost no background light.

Hubble's Ultra Deep Field image, for instance, is the most detailed visible-light image ever made of the universe's most distant objects. 

Hubble has helped to resolve some long-standing problems in astronomy, as well as turning up results that have required new theories to explain them.

Among its primary mission targets was to measure distances to Cepheid variable stars more accurately than ever before, and thus constrain the value of the Hubble constant, the measure of the rate at which the universe is expanding, which is also related to its age.

Hubble: 15 Years of Discovery

On the momentous occasion of the Hubble Space Telescope's 15-year anniversary, the European Space Agency released this commemorative documentary looking back of the history of discovery that the telescope has facilitated.

Retrace the story of its successes and difficulties, illustrated by state-of-the-art computer animation, as well as spellbinding footage photographs taken from Hubble, much of which has been previously unseen until now.

The film presents some of the most fascinating facts about our universe that Hubble has uncovered, and explains things like wormholes, black monsters, and event horizons.

All of this is accompanied by a score that captures the grand majesty of the film's subject matter.

Hubble Space Telescope Hot Images
Some of the Most Spectacular Images from Hubble

While Hubble helped to refine estimates of the age of the universe, it also cast doubt on theories about its future.

Astronomers from the High-z Supernova Search Team and the Supernova Cosmology Project used the telescope to observe distant supernovae and uncovered evidence that, far from decelerating under the influence of gravity, the expansion of the universe may in fact be accelerating.

This acceleration was later measured more accurately by other ground-based and space-based telescopes, confirming Hubble's finding.

The cause of this acceleration remains poorly understood; the most common cause attributed is dark energy.

The high-resolution spectra and images provided by the HST have been especially well-suited to establishing the prevalence of black holes in the nuclei of nearby galaxies.

While it had been hypothesized in the early 1960s that black holes would be found at the centers of some galaxies, and work in the 1980s identified a number of good black hole candidates, it fell to work conducted with Hubble to show that black holes are probably common to the centers of all galaxies.

The Hubble programs further established that the masses of the nuclear black holes and properties of the galaxies are closely related. The legacy of the Hubble programs on black holes in galaxies is thus to demonstrate a deep connection between galaxies and their central black holes.

The collision of Comet Shoemaker-Levy 9 with Jupiter in 1994 was fortuitously timed for astronomers, coming just a few months after Servicing Mission 1 had restored Hubble's optical performance.

Hubble images of the planet were sharper than any taken since the passage of Voyager 2 in 1979, and were crucial in studying the dynamics of the collision of a comet with Jupiter, an event believed to occur once every few centuries.

Other major discoveries made using Hubble data include proto-planetary disks (proplyds) in the Orion Nebula; evidence for the presence of extrasolar planets around sun-like stars; and the optical counterparts of the still-mysterious gamma ray bursts. HST has also been used to study objects in the outer reaches of the Solar System, including the dwarf planets Pluto and Eris.

Many objective measures show the positive impact of Hubble data on astronomy.

Over 9,000 papers based on Hubble data have been published in peer-reviewed journals, and countless more have appeared in conference proceedings.

Looking at papers several years after their publication, about one-third of all astronomy papers have no citations, while only 2% of papers based on Hubble data have no citations.

On average, a paper based on Hubble data receives about twice as many citations as papers based on non-Hubble data.

Of the 200 papers published each year that receive the most citations, about 10% are based on Hubble data. Although the HST has clearly had a significant impact on astronomical research, the financia
l cost of this impact has been large.

A study on the relative impacts on astronomy of different sizes of telescopes found that while papers based on HST data generate 15 times as many citations as a 4 m ground-based telescope such as the William Herschel Telescope, the HST costs about 100 times as much to build and maintain.

Making the decision between investing in ground-based versus space-based telescopes in the future is complex. Even before Hubble was launched, specialized ground-based techniques such as aperture masking interferometry had obtained higher-resolution optical and infrared images than Hubble would achieve, though restricted to targets about 108 times brighter than the faintest targets observed by Hubble.

NASA Poll: What Should Hubble Observe

In 2001, NASA polled internet users to find out what they would most like Hubble to observe; they overwhelmingly selected the Horsehead Nebula.

Since then, advances in adaptive optics have extended the high-resolution imaging capabilities of ground-based telescopes to the infrared imaging of faint objects. The usefulness of adaptive optics versus HST observations depends strongly on the particular details of the research questions being asked.

In the visible bands, adaptive optics can only correct a relatively small field of view, whereas HST can conduct high-resolution optical imaging over a wide field.

Only a small fraction of astronomical objects are accessible to high-resolution ground-based imaging; in contrast Hubble can perform high-resolution observations of any part of the night sky, and on objects that are extremely faint.

Many Hubble observations have led to breakthroughs in astrophysics, such as accurately determining the rate of expansion of the universe. Hubble is the only telescope ever designed to be serviced in space by astronauts.

Four servicing missions were performed from 1993–2002, but the fifth was canceled on safety grounds following the Space Shuttle Columbia disaster.

However, after spirited public discussion, NASA administrator Mike Griffin approved one final servicing mission, completed in 2009. The telescope is now expected to function until at least 2014, when its 'successor', the James Webb Space Telescope (JWST), is due to be launched.