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.
Hubble's future
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.