April 24 will mark a significant milestone in the life of one of mankind's greatest scientific instruments – the 25-year anniversary of the launch of the Hubble Space Telescope. This bus-sized piece of scientific equipment has become a household name, thanks to the incredible scientific insights and iconic images it has returned over the course of a quarter-century in low-Earth orbit. Join us as we celebrate the history and achievements of NASA's flagship space telescope.

Calls for the development of what would later become the Hubble Space Telescope began to be heard soon after the end of World War II, as renowned theoretical physicist and astronomer Lyman Spitzer espoused the virtues of an orbital telescope in his paper Astronomical Advantages of an Extra-Terrestrial Observatory. Spitzer argued that a space telescope would be free to gaze into the heavens without suffering the detrimental effects of Earth's atmosphere – a protective shell of gases made up predominantly of nitrogen and oxygen, which distorts or even blocks the light emitted or reflected by distant celestial objects.

The road to designing and ultimately constructing Hubble was anything but smooth. Gizmag caught up with Matthew D. Lallo, lead of the Telescopes Group at the Space Telescope Science Institute (STScI), to discuss the mission's early development. The institute, of which Lallo has been a part since before the launch of Hubble, is responsible – along with NASA's Goddard Space Flight Center – for administering and maintaining the telescope while in orbit.

"Perhaps the biggest hurdle facing its development was economic," says Lallo, discussing the early challenges faced by the program. "The period from 1977 to its launch in 1990 saw many funding and schedule problems. The costs of such a technological challenge grew over time, and the schedule slipped often."

Studies into the feasibility of creating a large space telescope began in 1971. However, funding was initially declined by the US Senate's appropriations committee four years later, as the price tag for the construction of such a telescope was deemed too steep to justify. After a series of design changes, and a call for the European Space Research Organization (later to become ESA) to develop and produce hardware for the telescope – most notably the twin solar panels – funding for the mission was finally granted in 1977.

This stunning shot of the Horse Head Nebula was captured by Hubble's high-resolution Wide Field Camera 3, which was installed on the platform in 2009 (Image: NASA, ESA, The Hubble Heritage Team)

The Perkin-Elmer Corporation was given the task of fabricating the main mirror and optics for the telescope, while the Lockheed Missiles and Space Company was tasked with developing and producing the cutting-edge spacecraft that would house the delicate equipment. In its finished form, Hubble was a technological marvel, representing the pinnacle of the agency's will to pierce the veil of deep space, and shed light on some of the most profound secrets our universe has yet to offer.

Hubble measures 13.2 m (43 ft) from end to end and weighs in at 11 tonnes (12 tons). At its core is a 2.4 m (8 ft) primary mirror, designed to reflect received light from the source onto a secondary mirror. That light is then reflected back through a hole in the primary mirror, where it finally comes to a focal point, passing to one or more of Hubble's scientific instruments.

The spacecraft is kept running by twin solar panels that feed power to six nickel-hydrogen batteries with the capacity to sustain the telescope during periods of darkness as the Earth eclipses the Sun. The observatory has no thrusters, instead using a series of reaction wheels in combination with Fine Guidance Sensors monitored by gyroscopes, to maneuver and hold a position relative to its quarry.

Space Shuttle Discovery launched on April 24 1990, carrying with it the Hubble Space Telescope (Photo: NASA)

In a forward-thinking move, Hubble was designed to allow for the scientific equipment contained in its aft section to be relatively easy to access by astronauts during servicing missions. Thanks to this flexibility, new instruments have been added to the telescope in orbit, whilst others have been replaced with more up-to-date models. There is currently a suite of six scientific instruments residing within the telescope. Together, they cover a broad range of applications, from wide field surveys with the Advanced Camera for Surveys, to observing stars that would ordinarily be shrouded by Nebulae, with the Near Infrared Camera and Multi-Object Spectrometer.

Following a series of setbacks involving the construction of the various pieces of the telescope, Hubble was ready and slated for launch in 1986, when the telescope was further delayed by the fallout from a national tragedy. The loss of the space shuttle Challenger grounded the entire fleet, forcing the telescope to wait in limbo for a further four years. Finally, on the 24th of April 1990, Hubble was successfully inserted into low-Earth orbit by the space shuttle Discovery, heavy with the hopes and expectations of the scientific community.

Crisis in low-Earth orbit

Hubble's most severe setback was discovered soon after launch, as the blurry images returned by the telescope hinted at a potentially devastating flaw.

Following early testing in orbit, it was discovered that the telescope had been launched with a spherical aberration in its primary mirror, caused by an improperly calibrated tool which lead to the edges of the mirror being ground too flat. The result of this manufacturing error was that, if left uncorrected, Hubble would be unable to fulfill some of its primary mission objectives, and the stunning images of the universe that it was designed to obtain would return distorted. The telescope had gone from being the epitome of a nations urge to push back the boundaries of the known universe, to potentially the greatest boondoggle in the agency's history. A mission was swiftly devised to rescue the crippled telescope.

The image on the left of galaxy M100 was taken days before the first servicing mission, the right hand image was taken soon after (Image: NASA)

Under the command of Richard O. Covey, a crew of seven astronauts launched aboard the space shuttle Endeavour on December 2, 1993. On the third day of the mission, the shuttle successfully rendezvoused with Hubble, and the crew were able to secure the massive telescope vertically in Endeavour's payload bay via the shuttle's robotic arm.

In a colossal effort, two teams of astronauts carried out five back-to-back spacewalks totaling over 25 hours, in a mission which is now considered among the crowning achievements of the space shuttle program.

EVA #4, carried out by astronauts Kathryn Thornton and Tom Akers, saw to the rectification of the fault in the primary mirror by installing the Corrective Optics Space Telescope Axial Replacement (COSTAR) unit in place of the High-Speed Photometer. COSTAR was designed to compensate for the optical error and would allow Hubble's remaining instruments to function as if the fault did not exist.

At the conclusion of the spacewalks, the crew had managed to upgrade and replace numerous elements of the telescope, including Hubble's twin solar arrays, four of the gyroscopes vital for achieving a correct orientation, upgraded the memory capacity and speed of Hubble's computer, and replaced the Wide Field/Planetary Camera instrument with the Wide Field/Planetary Camera 2.

The Hubble Space Telescope, pictured in the cargo hold of the Space Shuttle Discovery, shortly before being released into space for the first time (Photo: NASA)

On day nine of the mission, Hubble was released by Endeavour, ready to usher in a new golden age of space exploration. Over the course of the next 16 years years, four further servicing missions would tend to the telescope, installing new instruments and carrying out maintenance on the vital systems that allow the observatory to continue functioning in the harsh environment characteristic of low-Earth orbit.

"Hubble answered many of the questions that it was designed for," explains Lallo. "These were the most pressing questions astronomers knew to ask in the 70s and 80s, but Hubble’s longevity and periodic upgrades have meant that many of its most significant findings and discoveries were not even technically achievable at the time it was launched."

Maintaining a legacy

Whilst the rigors of the early days of Hubble's mission are now consigned to the history books, challenges still remain for the team operating the telescope today. Lallo, who is no stranger to the difficulties involved in maintaining such an observatory in low-Earth orbit, summarized the experience for us:

"Having Hubble in orbit is like taking the 100-inch Hooker telescope at Mt. Wilson, which was considered a very large telescope for a very long time, then polishing the mirror far more smoothly, putting it on a rocket, shaking the heck out of it, then exposing it to space vacuum and alternatively baking it and freezing it at triple-digit temperatures above and below zero every 48 minutes while keeping its optical quality extremely sharp … then getting it to point itself so stably while free-falling around the Earth that it could hold a laser beam on a dime 300 miles away for days at a time. And doing this for 25 years and counting!"

A shot of Hubble in orbit taken in the course of Servicing Mission 4 (Photo: NASA)

Lallo went on to explain that the true contemporary challenge for Hubble operators comes in the form of calibrating and fine-tuning the giant telescope to the many disparate roles required of it. He said that the level of understanding obtained of the telescope, and the calibration capabilities afforded by an upgraded Hubble, are beyond anything that the designers of the observatory would have thought possible. The dedication of those who worked to design, build, and then maintain the telescope has been repaid many times over. Hubble has gifted us with countless insights into the nature of our universe. We have been allowed to observe how galaxies such as our own Milky Way come to form, and observed the death of stars in events known as supernovae, with which it is possible to measure the vast distances between ancient galaxies.

Hubble's Ultra Deep Field survey is proving instrumental in unlocking some of the secrets of the early universe (Image: NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI))

Hubble has studied the nature of black holes and examined the characteristics of the atmospheres of exoplanets hundreds of light years away, even managing to capture the first-ever visible light image of an extrasolar planet, namely Fomalhaut b. Hubble's Ultra Deep Field survey encompassed an astounding 10,000 galaxies in its scope, allowing scientists to observe the development of our universe, as well as how massive galaxies interact and co-exist with on another. These are just a few examples of the many ways that Hubble has contributed, and indeed will continue to contribute to mankind's knowledge of the universe in which we live.

The passing of a titan

As unpalatable as the thought might be, we have to accept that Hubble's mission must one day come to an end. May 2009 saw the launch of the final telescope servicing mission of the US Space Shuttle Program, and with no further manned or automated missions to the telescope in the pipeline, it is an inevitable fact that the great adventure of the Hubble Space Telescope is inexorably approaching the final years of its life.

The iconic telescope was first intended to be returned to Earth in the cargo hold of a space shuttle. However, with the final flight of the program touching down in 2011, it is likely that when Hubble is finally decommissioned, it will be allowed either to de-orbit naturally or in a more controlled fashion. The latter would involve the aide of an unmanned spacecraft attaching to the telescope, and then manipulating a re-entry into Earth's atmosphere over an unpopulated area.

Shot of Hubble in orbit taken by Servicing Mission 1 (Photo: NASA)

But for those like myself, who would be devastated at the loss of so venerated a telescope, there exists a slim ray of hope, and the possibility of a future unmanned mission to Hubble that would make use of a relatively new feature on the observatory – the Soft Capture Mechanism (SCM) – which could potentially prolong the telescope's life.

Lallo put it best, stating, "The SCM is part of a system designed to support the rendezvous and docking of other types of potential future robotic spacecraft, so who knows? Might we one day see R2D2 servicing Hubble? We humans get attached to things and always like to stay hopeful!".

"Hubble is currently doing some of its best science ever. Since its last Servicing Mission in 2009, its systems have been performing incredibly well. Our goal and everyone’s hope is to enjoy a period of overlap between missions, when we have James Webb and Hubble active and productive at the same time … Of course without any further servicing, its lifetime is limited. For now, we and the world’s astronomers are still making the most of this incredible resource while it’s still with us."

Artist's rendering of Hubble's successor, the James Webb Space Telescope (Image: Northrop Grumman)

However, when the unthinkable does come to pass, there is some comfort in knowing that its legacy will live on in the minds of the thousands that have been inspired by its exploits, and in the inevitable achievements of its successor – the James Webb Space Telescope. Lallo, who has tended to the telescope throughout its stay in low-Earth orbit, offered this conclusion:

"The word 'transformative' gets used a lot these days, but I think Hubble has been truly transformative. Hubble must be the best-known scientific instrument in history. It is recognized by the general public throughout the world. After 25 years, its name alone conjures images of the spectacular. It’s also become a symbol for human ingenuity in our quest for understanding. It is a source of pride for not only Americans or Europeans, but for everyone, for humankind."

For a selection of some of the most striking Hubble images, as well as shots of the telescope in orbit, head to the gallery.

View gallery - 34 images