There's no doubt among scientists and engineers about the heart-stopping moment during HST's upcoming makeover.

Hubble's central controller is to be shut down and replaced with fresh hardware. The HST's current Power Control Unit (PCU) regulates and distributes critical energy the telescope needs to be the powerful light bucket it is. The PCU also safeguards Hubble from power spikes and controls battery charging. All telescope subsystems, including thermal control, will be in the "off" position.

"I'm nervous as hell" about shutting off the PCU, said Anne Kinney, director NASA's astronomy and physics division at NASA Headquarters in Washington, D.C.

Powering down Hubble's PCU "kind of violates the long-standing policy in the space business. That is, if something is working well, you don't turn it off and just hope it comes back on," said Ed Weiler, NASA head of space science. "Turning this telescope off for the first time in 11 years…that scares me a lot," he said.

The new PCU allows astronomers to make full use of additional power from a set of new rigid solar arrays to be outfitted on Hubble. These arrays are smaller but yield 20 percent more energy than HST's current set of solar cell-laden panels. These newer solar arrays are stronger, more stable, and are rigid. They are built to fold rather than roll up. Lastly, the smaller size arrays decreases on-orbit drag and retards the rate at which Hubble's orbit decays.

The Hubble program procured the new solar panels from the commercial vendor involved in cranking out Iridium telecommunications satellites.

At the end of the upcoming servicing mission, the total investment in Hubble, including shuttle hops to the telescope, is in the neighborhood of $6.9 billion, said Preston Burch, HST program manager at NASA's Goddard center.

Plans for HST's fourth and final full-service mission in 2004 are already underway. The current plan is to operate Hubble until 2010, then use a shuttle to bring the orbiting observatory back to Earth, said NASA's Kinney.

Once plucked from space, talk has it of stuffing Hubble into the Smithsonian Air and Space Museum in Washington, D.C. - to be seen but never heard from again.

On tap to replace HST is the high-tech Next Generation Space Telescope (NGST). It is slated for launch in 2009. To be hurled into orbit by an Atlas booster, NGST will park itself near a Lagrange point locale (L-2), orbiting the Sun about 1 million miles (1.5 million kilometers) further out than the Earth. That spot allows NGST to keep pace with the Earth as they go around the Sun each year.

From that position, NGST cameras and spectrographs sensitive to infrared radiation are to observe the farthest reaches of the universe. But it's also a distance from Earth where stopover, fixer-up'er visits by astronauts are not on the books, at least for right now.

Given a slew of technical and budget hurdles the project faces, can NGST be ready in time to pick up observational work as Hubble departs the space scene?

"My only concern is with the vagaries of program management in programs. I'd like to make sure that we continue to plan to keep the Hubble Space Telescope at its maximum capability until such time as it really is replaced,"
said Bruce McCandless II, a former astronaut and an early member of the Hubble servicing club. He is now chief scientist for reusable space transportation systems at Lockheed Martin Astronautics in Denver, Colorado.

Setting up a Hubble servicing mission in 2007 -- one that is not now on the books -- means getting underway with planning soon. Input from the astronomical community is key in judging instrument upgrades or plugging in new devices. Also needed is a careful look at what Hubble components may need replacing to assure protracted use of the observatory.

"There's a tremendous interest in the scientific community in using Hubble," said Volmer of Ball Aerospace, adding that the telescope has only observed a small part of the sky. "I'd like to see Hubble continue for a long time," he said.

The idea of keeping both NGST and HST humming along simultaneously is clearly a matter of money, McCandless told SPACE.com.

"Money management is a concern, particularly now within NASA," McCandless added. "All that I'm suggesting is we should make sure we don't have a period of several years with no astronomical imaging capability…that is, taking pictures."