The Hubble Decade

December 1999, and NASA has scrambled a space shuttle for an emergency mission. It's a crack crew, on a flight that's been brought forward six months. Short of a human catastrophe in space, there's only one satellite that can command this sort of attention - the Hubble Space Telescope.

Launched almost 10 years ago, Hubble is the jewel in NASA's crown. While planetary missions fail, and endless arguments dog the International Space Station, Hubble has been consistently turning up trumps, with stunning images of deep space and new insights into the birth and evolution of the Universe.

When Sky & Telescope magazine recently polled readers for the cosmic images of the century, two came in miles ahead of the rest of the pack: the Apollo astronauts' first view of the Earth from the Moon - and the Hubble Space Telescope's glorious image of the great dark pillars in the Eagle Nebula.

On the scientific front, Hubble's achievements are also out-of-this-world. It’s an international mission - a collaboration between NASA and the European Space Agency - and literally thousands of astronomers from all over the world have used Hubble’s eagle eye to dissect over 13,000 objects in the cosmos. The data sent back to Earth would fill over ten thousand CD-Roms.

Hubble’s role in astronomy demonstrates the maxim that bigger isn’t necessarily better, even where telescopes are concerned. Astronomers have a choice of some 30 ground-based telescopes that are larger than Hubble. The space telescope's unique advantage is its crystal-clear vision. While other telescopes must look upward through the Earth's churning atmosphere, Hubble has an undistorted view of the cosmos.

In theory, that is. Soon after Hubble's launch, astronomers found that Hubble's images were blurry - due to a fault in shaping its main mirror, which threw out its intended curve by about 1/50 the diameter of a human hair. In December 1993, visiting astronauts installed 'contact lenses' - in reality, small curved mirrors - which corrected the fault. And since then, Hubble hasn't looked back.

The space telescope has looked at every type of object in the Universe, from the nearest - the Moon - to the most distant galaxies ever seen. It wasn't really designed to investigate planets, but even in the Solar System Hubble has turned up trumps. When the Mars Pathfinder mission was on its way to the Red Planet, NASA controllers used Hubble to monitor dust storms on the target world. The space telescope also observed a massive storm on Saturn, and showed - for the first time - Pluto's moon Charon clearly separate from Pluto itself. Most important, Hubble had a prime view of Comet Shoemaker-Levy-9's smash into Jupiter in 1994. The telescope saw both the giant fireballs as the comet fragments impacted the planet, and the vast 'black eyes' left on Jupiter's clouds afterwards.

In the realms of the stars, Hubble has revealed previously unsuspected aspects of the birth of stars and planets. In the great Orion Nebula, Hubble has found dark dusty discs orbiting around young stars - almost certainly on their way to condensing into systems of planets. That famous picture of the Eagle Nebula also contains more than meets the eye. Small specks on the side of the giant dust columns are similar discs surrounding young stars. But here, the ultraviolet radiation from young hot stars nearby is boiling away the dusty discs before they have a chance to condense: these planetary systems are stillborn.

At the other end of a star's life, Hubble has followed the fate of gases that erupted in the most spectacular star-explosion of recent times: Supernova 1987A. Hubble has revealed rings and loops of gas around the now-defunct star, showing that during its lifetime the star was already shedding gas in copious quantities.

But it's in the further Universe that Hubble has really come into its own. The telescope is named after American astronomer Edwin Hubble, who - in the 1920s - proved that the entire Universe is expanding. The rate of expansion is known as Hubble's Constant. If we can measure Hubble's Constant accurately, we can work out the age of the Universe. This was the prime scientific goal of eponymous space telescope.

A team of cosmologists led by Wendy Freedman, of the Carnegie Observatories, employed the Hubble Space Telescope to patiently track down the distances to more and more remote galaxies. Combine these distances with the speed these galaxies are racing away from us, and you find the Hubble Constant. In May 1999, Friedman's 27-strong international team announced the bottom line: the Universe is 12 billion years old.

The telescope's namesake would have been proud of this achievement alone. But the Hubble Space Telescope's sharp eye has revealed much on the cosmic horizon that Edwin Hubble would never even have dreamt of.

Peering right into the heart of nearby galaxies, Hubble has discovered small discs of hot gas are spinning round at a frantic rate. The high speed can only be explained if each disc is orbiting a massive black hole in the galaxy's core. These observations mean that almost all galaxies contain a central black hole.

	Deep within galaxy M87, a supermassive black hole whirls gas around, and ejects a narrow beam of electrons
	Photo - NASA

Further out, Hubble has focused on 'gravitational mirages' around whole clusters of galaxies. These ethereal arcs represent the light from more distant galaxies, bent and distorted by the gravity of the galaxy cluster. From the size and shape of the mirages, astronomers can calculate how much mass the cluster contains. In most cases, it's about ten times more than the total mass of the galaxies themselves - showing that the cluster must contain huge amounts of 'dark matter.'

And Hubble has played a key role in the latest and most exciting cosmological discovery - that the Universe is not just expanding, but accelerating. International teams of astronomers have been tracking down exploding stars - supernovae - in distant galaxies, using wide-angle telescopes on the ground. When they find a likely new dim smudge of light, they turn to Hubble to check it out. Comparing these distant supernovae with exploding stars in nearby galaxies, the astronomers discovered they are all dimmer than expected. According to theory, this can only happen if the Universe is blowing itself apart faster and faster.

	Cosmic mirage: distorted images of distant galaxies form a glowing spider’s web around nearby galaxy cluster Abell 2218. False-coloured to show brightness
	Photo - NASA

Time machine

These cosmological results are all good solid science - but they are only indirect inferences from what Hubble actually sees. Hubble's real eye-opener has been its uncanny ability to see back into the past.

There's no special magic about this. Because light takes a finite amount of time to travel across space - 300,000 km every second - we see everything in the Universe as it was in the past, at the time that its light left it. We see the Sun as it was just over eight minutes ago; the nearest star four years in the past; and the Andromeda Galaxy as it was two million years ago.

But Hubble's penetrating eye peers not just millions of years into the past, but billions of years back in time. We're pretty sure that Andromeda hasn't changed much in past two million years; but look at a galaxy say six billion light years away - half the age of the Universe back in time - and we are surely seeing it at a more adolescent phase of its existence.

In 1995, astronomers pointed the Hubble Space Telescope at a spot of sky just above the stars that make up familiar pattern of the Plough. There's nothing here that our eyes can see; nothing that telescopes on Earth had revealed as interesting. Hubble was staring at a seemingly black spot of sky to see further than any other telescope had revealed.

And the Hubble Deep Field came up trumps. Even the nearest galaxy here was 500 times more remote than Andromeda. And the most distant are so far away that we see them when the Universe was only one-tenth its present age. The more distant galaxies don't look anything like the placid spirals we see around us today. Most are disrupted in some way, and many appear to comprise two or more galaxies colliding with one another.

The Hubble 'time machine' is thus showing us how galaxies were born. In the beginning, it seems there were only small galaxies. As time went by, they amalgamated into bigger galaxies, like our Milky Way.

This month's servicing mission is the third in Hubble's history. Almost four years after it April 1990 launch, the first repair crew famously fixed Hubble's faulty eyesight. They also replaced one of its bulky cameras, and changed over the solar panels. In 1997, astronauts installed a new spectrograph for splitting up light from faint stars and galaxies and an infrared camera.

Originally, the third mission was planned for next June. But Hubble's gyroscopes - essentially for pointing in the correct direction - have been suffering repeated problems. On 13 November, the fourth of Hubble's six gyroscopes failed, and the telescope had to be shut down.

So NASA has sent in the cavalry - a extra mission carrying many of the most experienced American and European astronauts. As well as replacing the gyros, the mission involved installing a new computer and replacing the tape recorder with a solid-state recorder.

But the other components scheduled for next June aren't yet ready for space. They'll be taken up on a fourth mission in spring 2001. Hubble will then boast a new camera that will map dark matter across the cosmos; a new cooler that will reactivate its infrared camera; and fresh solar arrays.

The final servicing mission is scheduled for 2003. Astronauts will install a newer version of Hubble's standard 'camera' and a spectrograph for investigating ultraviolet light, which will show signs of gases fresh from the Big Bang.

In 2008 - astronomers hope - Hubble will be succeeded by a bigger and better instrument, currently known as the Next Generation Space Telescope. It will also be cheaper: NASA is well aware that Hubble ran well over budget, costing almost $2 billion at launch. The Next Generation Space Telescope must come in at quarter of that amount.

Currently, four groups are tendering for the contract. All of them are designing a main mirror eight metres across - over three times bigger than Hubble's, which means it can see objects ten times fainter. It's impossible to put such a big mirror inside a rocket for launch, so it will be made in sections which will click together once the telescope is in space.

The new telescope will be much flimsier than Hubble, and so much lighter in weight. Instead of a space shuttle, it's need only a medium-class rocket launcher. And, instead of orbiting the Earth, the Next Generation Space Telescope will follow its own orbit around the Sun - 1.5 million km beyond Earth's orbit. At this location, the so-called Lagrangian-2 point, our planet's gravity still ensnares the telescope, so it travels with the Earth through space at a constant distance.

Huge solar panels provide the telescope with power, and - just as important - shield it from the Sun's heat. With the Next Generation Space Telescope naturally cooled down to the temperature of deep space, it can observe faint heat signals from the farthest reaches of the Universe. It will peer beyond where even Hubble can see, and explore an epoch before the youngest galaxies explored by Hubble.

With this ultimate time machine, cosmologists hope to puzzle out the last remaining major mystery of the Cosmos - how did stars and galaxies first form. The Next Generation Space Telescope's infrared cameras will reveal the very first galaxies forming after the Big Bang. The early stars in these galaxies shone in ordinary light - like the Sun - but the expanding Universe has stretched this radiation so that it arrives here as infrared.

Just as Hubble was launched to measure the age of the Universe, yet became the ultimate exploring machine in all areas of astronomy, so the Next Generation Space Telescope will undoubtedly reveal far more than anyone can currently predict.

Currently, NASA intends to close Hubble down in 2010. But it hasn't decided what to do with the venerable instrument then. One option is to raise it to a higher orbit, where it will 'hibernate' indefinitely. Another is to bring Hubble back to Earth, for a place of honour in the Air and Space Museum in Washington.

Nigel Henbest is a staff writer. His latest books (both with Heather Couper) are Universe (Channel 4 Books) and Space Encyclopedia (Dorling Kindersley).