Hubble Space Telescope at 30: Our window into the universe

In a 2010 interview to the European Space Agency, John M. Grunsfeld, an American physicist and former Nasa astronaut, said that a service mission to the Hubble Space Telescope was like the “holy grail" of “going up there" and doing something that is widely regarded as extremely important. Grunsfeld has been part of three missions to the Hubble, his last one being in 2009.

This year the “holy grail" of space telescopes completes 30 years in orbit. In fact, the Hubble was deployed into low Earth orbit this week (on April 25) exactly three decades ago, after being launched on the space shuttle Discovery a day before.

Over the last three decades, the Hubble has sent back thousands of hundreds of pictures of the cosmos. That is exactly what the space telescope was built for: to study the history and evolution of the universe. Studying distant galaxies and capturing clearer images of the universe are tricky tasks for ground-based telescopes due to interference caused by the Earth’s atmosphere. When a telescope looks to the vast expanse of the cosmos to take a picture, the light it is supposed to capture must first travel through the air in our atmosphere. If the air is distorted or turbulent, an image loses its sharpness and appears hazy.

This ‘atmospheric distortion’ is the reason stars appear like they are twinkling when we look at them in the night sky. “This refers to the distortion of starlight as it passes through the air and results in fuzzy images of space. This is not fun for astronomers," says Poshak Gandhi, associate professor and astronomer at the department of physics and astronomy at the University of Southampton. “Hubble was the largest telescope operating in the visible light range to be launched into space. In space, there is no twinkling, which enabled Hubble to capture incredibly sharp images. The combination of its finesse and light-gathering power is what made Hubble unique, even amongst the other Great Observatories," adds Gandhi.

Chief among the Great Observatories

The Hubble was joined by three other magnificent astronomical telescopes as part of Nasa’s Great Observatories program: the Spitzer Space Telescope, the Compton Gamma Ray Observatory and the Chandra X-ray Observatory. Of the four, only Hubble and the Chandra X-ray Observatory remain operational today.

Vishal K Gajjar, a Templeton post-doctoral research fellow at the Berkeley SETI research center at the University of California, explains how astronomers try to look at the entire electromagnetic spectrum (which includes all wavelengths and energies) to get a good understanding of astrophysical objects and processes.

The spiral galaxy NGC 2008 in this image captured by Hubble. Credit: ESA/Hubble & NASA, A. Bellini

Gajjar, who is also a project scientist for the Breakthrough Listen scientific project aimed at finding evidence of civilizations beyond Earth, says Nasa’s Great Observatories programme provided these unique windows into the universe at different wavelengths. The Compton Gamma Ray Observatory, for instance, looked at the highest energy photons in gamma-rays while other telescopes such as the Chandra Ray and Spitzer observed cosmos at X-ray and infrared frequencies, respectively. “Hubble, on the other hand, was designed to look at the optical light along with also extending some coverage to near-ultraviolet and near-infrared. So, the Hubble was unique and different in terms of the operational waveband to other space telescopes," adds Gajjar.

These infrared capabilities, he adds, allowed the Hubble telescope to see through the dust clouds and observe new stars being born. “Hubble also observed deaths of stars by observing several supernovae events, which helped astronomers to discover the mysterious ‘dark energy’ responsible for accelerating expansion of the Universe," he adds.

The Hubble Deep Field

Among some of its biggest contributions to the scientific community, the Hubble also helped astronomers understand the correct age of the universe: around 13.8 billion years, a figure previously thought to be somewhere between 10-20 billions years. In 1995, the Hubble sent back an image that showed how small Earth was in the bigger scheme of things. Known as the Hubble Deep Field (HDF), the picture was pieced together after the telescope looked at a random part of the sky for 10 days. The results were spectacular. Gandhi says the HDF is a “striking example" of the awe-inspiring images from the Hubble that have had no equal since. This very 'deep' image of a small field of the sky, shows thousands of distant galaxies and demonstrates the vastness of the cosmos at a single glance, he adds.

The Hubble Deep Field Image shows galaxies back to the beginning of time. Credit: Nasa/JPL/STScI Hubble Deep Field Team

A January 1996 press release from Nasa called the image “one peek into a small part of the sky, one giant leap back in time." Essentially, Hubble scanned just one speck of the sky but still revealed an “assortment of at least 1,500 galaxies at various stages of evolution." As the release explained, because the light that reaches us now from distant galaxies left billions of years ago, we are viewing them at a time when they were younger.

Hubble also contributed to some of the other most important results in astronomy over the past two decades. “This includes weighing supermassive black holes in galactic nuclei, measuring the acceleration of the cosmos itself, and catching the collision of the comet Shoemaker-Levy 9 with Jupiter," adds Gandhi. Shoemaker-Levy 9’s collision with Jupiter in July 1994 was the first direct observation of an extraterrestrial collision of Solar System objects.

Ever since its mission began in 1990, the Hubble has made more than a million observations. Sitting outside the haze of Earth’s atmosphere, it can spot astronomical objects with an angular size of 0.05 arcseconds. According to Nasa, that’s like seeing a pair of fireflies in Tokyo that are less than 10 feet apart from Washington DC.

But eventually one of the most productive scientific instruments ever built, will be replaced by a successor. In this case, it’s the James Webb Space Telescope (JWST), which was scheduled to launch in March next year, but now faces a delay due to the coronavirus pandemic. While the Hubble might remain operational till 2030-40, the JWST is now expected to inspect objects and galaxies even further into the universe. “JWST will have about seven times more light-collecting power than Hubble. But its real advantage lies in the fact that it will operate primarily in the infrared range. This opens up a vast new frontier for study, from exoplanets to the very earliest galaxies formed in the universe," adds Gandhi.

Gajjar explains that since light travels at a finite speed, you can consider all telescopes as time-machines to gaze at the history of the universe. “So, while Hubble is able to see as far back in time when the very first young galaxies were around, the JWST will be able to see the birth of the very first galaxies and stars. In other words, it is designed to study the very first light in the universe," he adds.

Along its journey of exploration, the Hubble has had many stumbling blocks -- including the famous flawed mirror problem in its early days, which was eventually fixed in 1993. Funding and a huge cost burden have also been issues related to its continued working. Despite that, Gandhi believes the Hubble is ‘emperor’ amongst the ranking of telescopes and will remain so: “Hubble's real legacy has been the ability to inspire."