Even before Hubble's flawed mirror was corrected -- thanks to some ingenious work by engineers and a daring repair mission by shuttle astronauts -- the telescope was sending back data that could never have been obtained from Earth. It has proven itself as an invaluable partner to ground-based instruments, zeroing in on targets that larger telescopes down here can then probe in greater detail.

Hubble's riches extend to every branch of astronomy. It has taken a "census" of stars in the crowded regions near the centers of galaxies, and in star clusters. It has found convincing evidence that super-massive black holes lurk at the heart of most galaxies. Hubble has easily tracked storms in the atmospheres of the giant outer planets, more than a billion miles away. And it has brought us closer to answering some of the most fundamental questions human beings know how to ask. Where do stars and galaxies come from? How long has the universe existed, and how long will it continue to exist?

And yet, when astronomers are asked to name Hubble's greatest achievement, it isn't science that they talk about.

Hubble "has focused all of us, not just scientists, on the enduring questions about the universe," says John Bahcall of the Institute for Advanced Studies in Princeton, New Jersey. "I think that has excited people all over the world and has enriched human life."

It isn't hard to understand what Bahcall means when you look at Hubble's spectacular images. The fact that it is even possible to take such pictures from a satellite that is whizzing around the Earth at 17,500 m.p.h. (28,165 kilometers per hour) -- a satellite that is among the most complex scientific instruments ever created -- is astonishing in itself. But even that is beside the point when we are confronted with Hubble's view of the universe.

Through Hubble's eyes, we zero in on the heart of a distant galaxy, sparkling with suns. We fly into a nebula in our own galaxy to behold pillars of gas and dust that tower a full light-year -- some 5.88 trillion miles (9.45 trillion kilometers) -- through space. And we peer into a tiny patch of black sky -- a place where there seems to be nothing at all -- to reveal a glittering array of utterly remote, utterly ancient galaxies.

The cumulative effect of all this is something like being able to stand face-to-face with Abraham Lincoln. It is as if, instead of looking at an old black-and-white photograph of Lincoln, we can see him in the flesh, study his mannerisms, hear his voice.

Hubble's images bring us face-to-face with the universe.

Although Bahcall says Hubble's power to inspire the world is its most significant accomplishment, he points out that it was an unexpected one. "It wasn't what we had in mind when we designed the telescope." In that case, we are enjoying a spectacular bonus, a kind of ongoing Christmas present.

That's why this shuttle mission is so important. One of the reasons Hubble has enjoyed such enormous acclaim is that it is an extension of ourselves, of our own curiosity. The fact that astronauts can venture into space to fix it is a reminder of the power of human ingenuity -- the same power that devised this marvelous machine.

Astronomers have big plans for Hubble in the coming decade, and they dearly want to see it returned to service. But getting Hubble back would be a Christmas gift to us all.