On April 24, 1990 NASA launched the Hubble Space Telescope, an 11-tonne suite of instruments to observe the distant reaches of the cosmos in the visible part of the electromagnetic spectrum, as well as fringes of the UV. It was the first of the Great Observatories, an American quartet of advanced space-borne scopes, to get online on April 25 the same year. It is also the most well-known for its iconic photographs (the others are the Spitzer infrared, Chandra X-ray and Compton gamma-ray telescopes) especially of extremely distant objects, enabled by its being spared the blurring effects of Earth’s atmosphere. On the 26th anniversary of its birth, an account of its legacy follows accompanied by some of its more spectacular shots.
The Hubble Space Telescope, or just Hubble, is the heaviest space-telescope launched to date, clocking in at 11,110 kg (to compare, ISRO’s ASTROSAT weighs 1,500 kg). Hubble’s mass was the result of having to accommodate a wide variety of instruments including an eight-feet-wide primary mirror and what it has received in return is a demonstrable reputation for variety. Its five onboard instruments continue to be among the best for making observations in visible and UV light.
Within weeks of its launch, Hubble’s operators began to notice an aberration in its photographs, which they quickly realised was due to an issue with the all-important primary mirror. Though it had been ground to the necessary precision (within 10 nanometers), it had the wrong shape. As a result, light collected from different parts of the mirror were being focused at different points. Though NASA engineers developed software to correct for this aberration in the data-analysis stages, a plan was also mooted to install corrective optics on Hubble to fix the problem at the data-collection phase itself. This was executed in two phases: first, by substituting one of the instruments with a suite of corrective optics called COSTAR, and second, by fixing each instrument to cope with the aberration so that COSTAR could be removed. The entire range of operations took from 1993 to 2002 to complete.
The Hubble will stop being the most powerful telescope in its class (of imaging distant objects with high resolution) in 2018 when NASA plans to launch the James Webb Space Telescope (JWST). While Hubble sports a primary mirror eight feet wide, JWST’s will be 21 feet wide. And while Hubble has instruments to study the cosmos at wavelengths ranging from the visible to outer edges of the UV, JWST’s instruments will be logging data from the visible to the mid-infrared. It will be a successor to two of the Great Observatories – Hubble and Spitzer – in capabilities as well as cost. The US Senate has capped its budget at $8 billion, almost twice as much as it took to build the Large Hadron Collider.
The motivations for both Hubble and the JWST are rooted in the need for a Large Space Telescope, a term coined by the astrophysicist Lyman Spitzer, Jr., in a 1946 article. While Hubble had been in the works since the mid-1970s, plans for the JWST began to be drawn up in the decade leading up to Hubble’s launch. And when the optical aberrations in Hubble were detected in 1990, the JWST suffered a setback by association. It took a few more years, and multiple advancements in infrared astronomy, before work on the JWST was brought back on track.
Because of its ability to image objects with more precision than its peers across vast distances, one of the Hubble’s earliest achievements was to aid in calculations: of the rate at which the universe was expanding as well as its age. Accuracy on the former was brought from 50% down to 10% while the range of the latter was brought from 10-20 billion years to around 13 billion years. Additionally, the telescope also engaged in three famous deep-field surveys and captured three iconic images: of almost 3,000 galaxies, 10,000 galaxies and 5,500 galaxies in single images. The last one, resulting from the eXtreme Deep-field survey, also revealed galaxies from 13.2 billion years ago and in a smaller field of view than the other two.
Because of the popularity of its images, Hubble will have been humankind’s gateway to the farthest cosmic shores for the better part of three decades. That’s a great achievement in two ways. First, it familiarised the masses with images that until then had been found only in an astronomer’s imagination. Second, it colloquialised distances, bringing humankind visions from across billions of lightyears with such clarity that the intervening chasms of emptiness might as well have been absent.
By the time was launched in 1990, Hubble had cost the American government $2.5 billion. By 2010, the number stood at $10 billion. Apart from the corrective optics, one other unanticipated cost was the 1986 Challenger disaster, which pushed Hubble’s launch from the mid-1980s to 1990. In the intervening years, the telescope had to be preserved in a clean room that required $6 million a month to maintain.
Hubble is named for Edwin Hubble, a well-regarded cosmologist most famous for his rediscovery of Georges Lemaître’s finding that the universe was expanding and for his study of galaxies other than the Milky Way. The Great Observatories programme itself had been mooted by Spitzer, Jr., in 1946, after whom the programme’s infrared telescope is named. Exactly two decades later, NASA asked him to head the Large Space Telescope project. Under Spitzer, Jr.’s leadership, it would evolve to become Hubble space telescope. For this and other related ideas, he has been widely considered to be the ‘father of the space telescope’.
At no point have all four Great Observatories been in the sky together: by the time Spitzer was launched in 2003, Compton had been decommissioned in 2000. Even so, the program became emblematic of the Cosmic Frontier, the third front of scientific exploration after the Energy and Intensity Frontiers, for its ability to make for qualitatively superior observations than ground-based telescopes as well as giving astronomers the ability to study objects with multiple instruments at once, providing a more concurrent sense of underlying scientific processes.
Following a service mission in 2009 to extend its lifespan by five years, NASA has said Hubble will be alive and kicking until at least 2020 – if not until the 2030s. And at that time the atmospheric drag that it constantly experiences will get the better of it and pull it down through Earth’s atmosphere and crashing to the surface. To avoid this, and potential fatalities, NASA has been mulling taking Hubble apart through a robotic mission.