NASA photo
	Workers lower the Advanced Camera for Surveys (ACS) into the Axial Science Instrument Protective Enclosure (ASIPE) at the Kennedy Space Center in Cape Canaveral.

This will be the fourth servicing venture since the $1.5 billion Hubble went into orbit in 1990. The mission includes five extensive spacewalks by astronauts.

A fix-it crew of seven is set to blast off aboard the shuttle Columbia at 6:22 a.m., Friday with a 64 minute launch window.

``It's going to be a totally new Hubble Space Telescope,'' says project manager Frank Cepollina. ``We're installing new detectors that are 20 times more powerful than what we launched 12 years ago.''

In essence, Hubble is getting $172 million worth of fresh parts and software midway through its design life. The big upgrade will be attaching a device called the Advanced Camera for Surveys, designed to dramatically increase Hubble's vision. The phone booth-size instrument replaces the Faint Object Camera, the last of the telescope's original instruments.

``If you had two fireflies 6 feet apart in Tokyo, Hubble's vision with [the new camera] will be so fine that it will tell from Washington, D.C., that they were two different fireflies instead of one,'' says Holland Ford, an astronomer at Johns Hopkins University in Baltimore, who helped design the camera.

Astronauts also will install a new cooling system and more efficient solar arrays on the observatory, which orbits 375 miles above the Earth.

Named after the American astronomer Edwin Hubble, the telescope is the first scientific instrument designed for routine maintenance by astronauts. Its modular design allows it to be taken apart in space and upgraded with more advanced instruments, while older ones can be flown home and reconditioned for future use.

This plan turned out to be a blessing. Shortly after Hubble's launch, engineers discovered a flaw in the main light- gathering mirror, limiting its vision. During a 1993 servicing mission, astronauts fixed the problem by installing a set of ``corrective lenses.''

The National Aeronautics and Space Administration expects the mission to set new standards for other major projects, including ongoing work on the International Space Station. The agency has more spacewalks planned in 2002 than any previous year and will test new tools and techniques during the latest encounter with Hubble.

``This is the next step in our ability to do more and more in reviving and using our assets in space,'' Cepollina says. ``Hubble is the pathfinder for work on the space station and everything else that requires space walks. Hubble is setting the trend for what you can do in space.''

Not everyone believes the long-term investment in Hubble is the best route for space science. New techniques involving ground-based telescopes are rivaling Hubble's powers at far less expense. For example, a concept known as adaptive optics allows astronomers to use more than one telescope in concert. Researchers can tweak their resolving powers to compensate for atmospheric distortion and observe distant objects in the cosmos with results comparable to Hubble.

	NASA photo
	This illustration shows what astronauts will will install during the Hubble servicing mission.

HUBBLE'S SUCCESSES The Hubble has had several significant achievements. The telescope: