It's a mere instrument — mirrors, cameras and sensors in a contraption about the size of a bus — but it changed the way we understand the universe, while revealing its mysterious beauty.

To do that, and make it to its 25th anniversary today, the Hubble space telescope also required an unusual degree of human ingenuity on the ground and courage by those astronauts who took on its repair as it hurtled about the globe at eight km per second.

Hubble was launched into space aboard the Discovery space shuttle on April 24, 1990, with great expectations of what a telescope operating above the distortions of the Earth's atmosphere would reveal.

The launch carried with it the promise of answers to the questions about the age and size of the universe, the birth of stars and black holes.

Hubble delivered on all fronts. And then revealed new mysteries still to be answered.

Its first years in space, though, were "unbelievably depressing," says astronomer Wendy Freedman, a Canadian who had joined the project before the launch and has been one of Hubble's leading scientists.

Only after astronauts on Space Shuttle Endeavour repaired Hubble in 1993, could the space telescope really do its stuff. Astronauts F. Story Musgrave, above, and Jeffrey A. Hoffman work on Hubble, Dec. 9, 1993. (NASA/Reuters)

First of all, the launch was delayed for a few years after the Challenger space shuttle exploded 73 seconds into its 1986 mission, killing its seven astronauts.

Then, once Hubble began operating, the anticipation and great hope that astronomers would soon get answers to their greatest questions were dashed as Hubble's 2.4 metre mirror couldn't focus.

While the Hubble operations team worked on a solution, Freedman says the astronomers were still getting some Hubble data about nearby galaxies.

She headed Hubble's Key Project, which would measure the universe's expansion rate. Knowing that, scientists would be able to calculate the age of the universe.

A golden era for astronomy

After determining a way to repair Hubble, in 1993 astronauts on the space shuttle Endeavour carried out the first of five servicing missions.

Hope was renewed but uncertainty remained. When Hubble's first post-repair images reached Earth, a golden era for astronomy began.

Soon Freedman and colleagues would determine the universe was 13.7 to 13.8 billion years old. Before Hubble, the range was a much broader estimate of from 10 to 20 billion years.

Hubble played an "absolutely critical" role in getting the answer, Freedman says. "There was no way we would have been able to make the measurements we did without Hubble coming along."

To determine the age of the universe, scientists focussed on the rate at which it was currently expanding. But that created a paradox: the universe appeared younger than some of its oldest stars.

But, using Hubble, scientists would soon learn that the universe hadn't been expanding at the same rate all those billions of years. It is now accelerating, expanding at a faster rate than it had previously.

A stranger universe

Hubble was helping answer some pretty big questions, but it was also revealing that the universe was stranger than most scientists thought.

Trying to find out why the universe was expanding led researchers to one of their biggest mysteries: dark energy.

Freedman says that before they began the Key Project, scientists didn't have any evidence for dark energy, which they now estimate makes up about 70 per cent of the universe.

"Dark energy is something we don't yet understand the nature of," she laments. Still, the theory is that it accounts for the universe's acceleration.

Hubble also found evidence for the existence of black holes, which led to the thinking that every galaxy has a black hole. And that raised another new mystery for astronomers, did the black hole or the galaxy come first?

Hubble captured images of supernova blowing up, of stars dying and of different types of nebulae -- "art works in the heavens," says Mario Livio, an astrophysicist with the Hubble Space Telescope Science Institute in Baltimore.

With Hubble, astronomers found Earth-like exoplanets even evidence of planets with signs of water.

Canadian astronomer Wendy Freedman, now at the University of Chicago, led the Hubble space telescope Key Project. She says Hubble changed the way we look at the universe. (University of Chicago)

Freedman, now at the University of Chicago, says for her the Hubble years have been tremendously exciting.

"Suddenly here was this magnificent tool that was giving you data with precision that had never been seen before in our field, and to be able to put an end to controversy and debate that had lasted, literally, for decades."

For non-scientists, even people who hadn't paid much attention, Hubble opened eyes to the universe. "It helped," Freedman says, "share the real excitement of discovery."

The next 20 years

In February, NASA hired science and technology historian, author and amateur astronomer Chris Gainor, who lives in Victoria, B.C., to write a scholarly history of the Hubble project, which he expects to be working on for the next five years.

Gainor describes Hubble's work to date as "an incredible change in how we view our universe," and he expects Hubble to continue making discoveries for at least another five years.

The end of the space shuttle program in 2011 means there's now no way to service or repair the telescope.

After Hubble, Gainor is looking forward to the James Webb space telescope, which is now under construction and expected to be operational in 2018.

With more sophisticated infrared technology and a light-collecting mirror twice the diameter of Hubble, the James Webb space telescope should be able to look further back into the history of the universe, Gainor says.

And while Hubble is about 500 kilometres above the Earth's surface, JWST will be 1.5 million kilometres away.

Hubble should re-enter Earth's atmosphere sometime in the 2030s. While that ought to mean it will break up and disintegrate, there's talk of a plan to guide Hubble to a splash landing that may at least save the mirror.