The Hubble telescope at 25: When will Hubble II launch?

Twenty-five years ago this Friday, Nasa launched the Hubble space telescope. Within weeks, the American space agency realised that it had put the instrument into the Earth's orbit with the telescopic equivalent of a squint. It was one of the most embarrassing and expensive technological cock-ups in history.

Correcting Hubble's vision required $800m – more than half the telescope's original $1.5bn price tag – and was one of the most complex manned space missions ever flown. An American magazine ran a cartoon showing a panel of images from Hubble – a grossly distorted image of the Moon seen through Hubble, next to a grossly distorted image of Saturn, next to a grossly distorted image of angry American taxpayers waving their fists at the sky.

It was an inauspicious start. However, Hubble, in its two and a half decades of operation, has more than redeemed itself. It is generally considered to be the most important and productive telescope since the one Galileo used in Padua in 1610 to discover mountains on the Moon, the phases of Venus and the moons of Jupiter. "We've had to rewrite every astronomy textbook," says John Mather, winner of the 2006 Nobel Prize for physics and project scientist on Nasa's James Webb Space Telescope, currently awaiting launch.

The need for an optical telescope in space had long been recognised by astronomers. Observing the heavens from beneath the Earth's swirling atmosphere is like lying on the bottom of a swimming pool and looking up at the lights on the ceiling above. The constant turbulence of the atmosphere causes the images of stars to jitter, or "twinkle", smearing them out into smudges so that even the biggest Earth-based telescope affords a view hardly sharper than Galileo's pioneering instrument. (Where big telescopes come into their own is in acting as giant "light buckets", collecting more light and so allowing them to see fainter, more distant celestial objects.) Another problem with observing the universe from the ground is that some light is inevitably absorbed by its passage down through the atmosphere and some, such as ultraviolet and infrared, is cut out altogether.

It was to overcome these blurring and screening-out effects of the Earth's atmosphere that the Hubble space telescope – named after Edwin Hubble, the American astronomer who discovered that our universe is expanding in the aftermath of a "big bang"– was designed and built. It was launched into a 324 mile-high (552km) orbit around the Earth by the space shuttle Discovery on 24 April 1990.

Three and a half years later, with its disastrous squint finally corrected, it afforded the sharpest, most detailed view of the universe ever seen. It was as if the human race had always observed the heavens through frosted glass and now suddenly it was able to see the universe through a crystal-clear transparent window.

The size of a railway carriage, with a 2.4m-diameter concave primary mirror, Hubble hurtles around the Earth at 28,000 kilometres an hour, completing one orbit every 97 minutes. Despite being in constant high-speed motion, it can lock on to a target celestial object and track it continuously without its gaze wandering by more than 0.007 "arcseconds" – the width of a human hair seen at a distance of 1.5km. The whole space telescope runs on the equivalent of 20 car batteries, which are continually re-charged by 6m wing-shaped solar panels.

Crucially, any astronomer in the world can apply for time on Hubble. All they have to do is submit a proposal in which they make a good case for their particular project. Teams of experts then select those observations to be performed. Once observations are completed, the astronomers have a year to pursue their work before the data is released to the entire scientific community. This policy has contributed to Hubble being one of the most productive telescopes ever built.

Undoubtedly, it is the super-sharp and vividly coloured images from Hubble that have captured the imagination of the public: whirlpool galaxies, exploding stars and, most famously, the "Pillars of Creation", showing the birth of stars, still wreathed in their placental gas and dust. But it is not easy to pin down discoveries that are 100 per cent attributable to Hubble.

"This is not the way modern astronomy is done," says Mather. "Few breakthroughs are made by a single instrument. Most require observations from multiple instruments, operating at multiple 'wavelengths', stretching from X-rays to optical light to radio waves."

Hubble, however, is a giant among instruments and has played a key role in many discoveries that have profoundly changed our view of the universe. For instance, at the very start of its operating life, Hubble helped pin down the expansion rate of the universe by observing nearby galaxies and measuring how fast stars of standard intrinsic luminosity in such galaxies are flying away from our Milky Way. Hubble's sharp vision was able to spot such "Cepheid variables" at unprecedented distances of up to 100 million light years. (By comparison, the nearest big galaxy, Andromeda, is 2.5 million light years away.)

The rate at which the universe is expanding is related to its age – since the faster it is expanding, the shorter the time it would have taken to get to its current size. Hubble told us the universe was between 13 and 14 billion years old – only about three times the age of the Earth. "Before Hubble, the universe was known only to be between about 10 and 20 billion years old," says Mather. "Hubble – along with the Cobe satellite, which observed the 'afterglow' of the big bang – ushered in the age of precision cosmology."

For its next trick, in December 1995, only 10 months after it obtained its first sharp image, Hubble spent 10 consecutive days drinking in the light from an apparently empty square of the sky less than a tenth the width of the full Moon. The resulting image, known as the "Hubble Deep Field", revealed about 3,000 galaxies from the dawn of time – galaxies that had existed long before the Earth was born and whose light had been travelling to us for most of the 13.82 billion years the universe has been in existence. "Seeing those galaxies was a big shock," says Mather. The reason was that everyone had thought galaxies like our own Milky Way formed relatively recently in the lifetime of the universe.

However, the Hubble Deep Field showed that the universe contained fully-fledged galaxies when it was barely a billion years old. "We had expected that Hubble would show us galaxies being born. But it turned out it couldn't," says Mather. "It simply wasn't powerful enough to see that far away and that far back in time." Another of Hubble's great discoveries was of supermassive black holes in the hearts of galaxies. In 1963, the astronomer Martin Schmidt had discovered "quasars" – the cores of galaxies which are pumping out hundreds of times the light of a normal galaxy from a tiny volume smaller than our solar system.

Astronomers quickly realised that the only possible energy source is matter heated to millions of degrees as it swirls down on to a black hole as much as tens of billions of times as massive as the Sun. Until Hubble, galaxies with supermassive black holes in their hearts had been believed to be rare, accounting for only about one per cent of galaxies. Hubble showed that this was wrong. It could see and measure the speed of stars swirling round and round in the hearts of hundreds of galaxies.

It was able to show that supermassive black holes are doing the swirling – not just in one per cent of galaxies but in pretty much all of them. It is just that most supermassive black holes have run out of fuel and so are slumbering. That includes Sagittarius A*, the 4.3 million solar mass supermassive black hole lurking in the dark heart of our very own Milky Way.

Arguably, however, Hubble's most important contribution was its role in a 1998 discovery that transformed our picture of the universe: the discovery of "dark energy". Hubble was able to spot the most distant Type Ia supernova – a type of exploding star believed to always have the same intrinsic luminosity. The supernova appeared fainter than it should be. Along with evidence from other Type Ia supernovae, it indicated that the expansion of the universe, contrary to all expectations, was speeding up, pushing the supernovae further away than expected. Astronomers explained the observation by postulating the existence of mysterious dark energy – the invisible stuff that fills the empty space between galaxies and whose repulsive gravity is speeding up the expansion of the universe.

Dark energy accounts for a whopping 68.3 per cent of the "mass-energy" of the universe and nobody knows what it is. In fact, when our very best theory of physics, quantum theory, is used to predict the energy of the vacuum – that is, dark energy – physicists get a number which is 1 followed by 120 zeroes bigger than what is observed. This is the biggest discrepancy between a prediction and an observation in the history of science. It is safe to say that something is seriously wrong with our understanding of the universe!

The Hubble space telescope, in 2015, is still working as well as ever. "However, the odds of it making another equally surprising discovery are slim," says Mather. With the retirement of Nasa's space shuttles, there are no longer any vehicles to go up and repair and replace Hubble's instruments – the last servicing mission was in May 2009. "Another 25 years of operation is unlikely, though it could probably do good science for another 10 years," says Mather. The retirement of the space shuttle fleet also means that there is currently no way to bring the telescope back down to the ground. When its time is up, it is expected that a robotic mission will help "de-orbit" Hubble, guiding it so that it plunges through the atmosphere and into the ocean.

Mather thinks it makes more sense not to extend the life of Hubble but to build a bigger and better space telescope – a Hubble II. He envisages an orbiting optical telescope with a light-collecting mirror five times as big as Hubble, making it capable of seeing detail in the heavens five times sharper. "Can you imagine what the universe would look like with an instrument like that?" he enthuses. "One possibility is that we might even be able to see planets around other stars directly."

Actually, there is a de facto Hubble II currently awaiting launch. It is the telescope for which Mather is principal investigator. The James Webb Space Telescope will have a mirror 6.5m across, more than twice the diameter of the Hubble primary mirror. It is made of 18 hexagons which will unfold in space like a piece of telescopic origami. "They're all finished – they've spent 18 months in storage jars," says Mather. "The instruments are finished, too."

The James Webb, due for launch on a European Ariane 5 rocket in October 2018, will not only see red optical light but also infrared light screened out by the atmosphere – a kind of light so far unexplored by a big telescope. "Hubble had lots of spy satellite predecessors – they had looked down at the ground from space. It simply looked up," says Mather. "The James Webb, on the other hand, has no predecessors." Will it change our picture of the universe as profoundly as Hubble? "Yes, I think it will," says Mather. "I am hoping for some lovely surprises."

Marcus Chown's book, 'What a Wonderful World: Life, the Universe and Everything in a Nutshell' (Faber & Faber, £9.99) is out now