Despite advances in adaptive optics and the advent of truly huge ground-based observatories, the Hubble Space Telescope, with its relatively modest 94.5-inch mirror, will remain at the forefront of science for years to come, says John Grunsfeld, Columbia's resident astronomer-astronaut.

In fact, Grunsfeld told CBS Radio today, historians will one day view Hubble "as the most productive scientific instrument in human history. It's had that big an impact on people's lives, but also on the field of science."

Hubble is redeployed into space Saturday. Photo: NASA

Orbiting 360 miles or so above the turbulence of Earth's atmosphere, shuttle astronauts periodically equip Hubble with state-of-the-art instruments like the Advanced Camera for Surveys installed during Columbia's mission.

Adaptive optics technology is now helping much larger telescopes on the ground see clearly through Earth's atmosphere and the rapidly maturing techniques of optical interferometry promise to revolutionize ground-based astronomy. Even so, Grunsfeld said, Hubble remains in a class by itself and will continue to do so for many years to come.

"There's a couple of things that Hubble does really well that can't be done on the ground yet, one of which is that Hubble, being in Earth orbit, can observe in the daytime," he said. "And so you're able to point at a single source in the sky, say some very distant galaxy, and look at it almost around the clock. And that's something, obviously, the ground observatories can't do.

"The pointing system on Hubble is also superb, it's unrivaled in its ability to be very steady when it looks at a source. ... Now these very big telescopes like the Gemini (North and South) telescopes, the Keck (Observatory) telescopes and others that are coming on line are rapidly encroaching on Hubble's territory, and that's a good thing because the two types of technologies work very well together," Grunsfeld said. "You can do an initial discovery observation on the Hubble and then follow up with one of the big light buckets.

"The limitations on the ground telescopes now (with adaptive optics) is they really only work well in the infrared and red to make the kind of images Hubble makes, whereas Hubble works throughout the entire visible spectrum and also the infrared and ultraviolet."

But the gap is closing. The two telescopes at the W.M. Keck Observatory in Hawaii feature segmented mirrors more than 30 feet across. The European Southern Observatory operates four telescopes with 27-foot-wide mirrors while the Gemini North and South telescopes feature mirrors only a few inches smaller. And the list goes on. For comparison, the fabled Hale Telescope atop Mount Palomar in California has a mirror "just" 200 inches, or 17 feet, across. Hubble's primary mirror measures just 7.9 feet in diameter.

Such huge ground-based telescopes are now utilizing adaptive optics technology, combining high-speed computers and deformable mirrors that constantly change shape as required to counteract the effects of atmospheric turbulence on incoming light. While these instruments now rival and even exceed Hubble's powers in some areas, the space telescope still has "a very important function, in fact maybe a leading function, in helping expand the discovery space," Grunsfeld said. "The telescopes really all work together, and that's really the wonderful part of all this astronomy."

Grunsfeld and his crewmates - commander Scott Altman, pilot Duane Carey, Nancy Currie, James Newman, Michael Massimino and Richard Linnehan - pretty much took the day off today, chatting with family members via video conferencing and enjoying the view from their high-altitude perch.

Earlier this morning, the shuttle astronauts spent a few moments swapping stories with the crew of the international space station as the two spacecraft streaked along in different orbits more than 7,000 miles apart. NASA television carried live shots from inside Columbia and the space station as the two crews chatted.

"We've enjoyed following what you all have been doing on the space station," Newman told Expedition 4 commander Yuri Onufrienko, Daniel Bursch and Carl Walz. "We're very impressed. Now that we've been up here for a little over a week, we can only marvel the fact that you all have been up there for over three months and how you guys must be doing and how expert you must be at everything you do. We're still in that first week, second week learning curve and ... we can only imagine what finesse you all must have in space now."

"We've come a long way," Bursch agreed, "but every so often I have some moments, maybe right after I wake up in the morning I'm a little clumsy, but yeah, it's been a lot of fun. We were wondering what the view's like from up that high?"

Columbia is orbiting more than 100 miles higher than the station at some points in the shuttle's slightly elliptical orbit.

"We just had an incredible pass at night over the U.S. and it was so clear, we could see from Miami to Boston, we saw Chicago, we could see well past Houston," Currie replied. "Jim and I were up here in Scooter's window (Altman's set) and we just could not believe the incredibly awesome view. Up here at (360 statute) miles, you really get a lot of curvature of the Earth and it's just incredible."

"That sounds great, Nancy," Bursch said.

"And Nancy just wanted me to add we're all smiling down at you guys," joked Altman.

"Yeah, we've got some pretty big smiles ourselves up here. We were wondering, that view that you saw, it looked like there were some pretty big storms that went through the U.S. yesterday, I don't know if you guys got a chance to see them at night."

"Yeah, we're hoping that weather system is tracking to the north and out of the way," Altman replied. "We saw some of those thunder bumpers kicking off, it really is quite a show at night. But right now I'd be happy if the weather stays nice and clear for the next couple of days so we can get back and talk to you from the ground next."

The shuttle crew plans to test the ship's re-entry systems Sunday and if all goes well, glide to a Florida landing at 4:37 a.m. Tuesday to close out a remarkable successful mission to overhaul and upgrade the Hubble Space Telescope.

During five back-to-back spacewalks last Monday through Friday, Grunsfeld, Linnehan, Newman and Massimino replaced Hubble's solar arrays, its central power control unit and a motion control flywheel assembly. They also installed the Advanced Camera for Surveys and a high-tech refrigeration system to revive a dormant infrared camera-spectrometer called NICMOS. The overhaul cost taxpayers about $172 million above and beyond the cost of a shuttle flight.

A fifth and final servicing mission is scheduled for 2004, when another crew will install two more state-of-the-art instruments. Then the observatory will be left on its own until 2010 or so when its mission will be superceded by NASA's Next Generation Space Telescope. NASA managers eventually want to bring Hubble back to Earth so it can be put on display at the National Air and Space Museum.

For Grunsfeld, who has now worked on Hubble during two spacewalk repair missions, the space telescope has become an old friend and one that will always be remembered.

"I sure did enjoy the trip back to Mr. Hubble, the telescope, it really was like seeing an old friend again," he told CBS Radio. "When we ... we opened the doors to where the scientific instruments live it was like going into a shrine.

"To work on the telescope for me was just a dream come true, something I feel I've trained my whole life for. And indeed, when we set the telescope free on this mission of discovery yesterday, there was just a little bit of sadness in my heart that we weren't going to spend any more time with Hubble. But a great sense of satisfaction that we'd made it a whole lot better and I'm really excited about what's to come in the months and years in the future when we find out what the universe is made out of and other really interesting stuff."