Rarely in the history of astronomy has a single instrument done more to revolutionize humanity's view of the cosmos than the Hubble Space Telescope.
Now, with eight years left in its 20-year observing program, three spacewalking astronauts are preparing to reshape this crown jewel of US space science during a mission that has astronomers both exhilarated and anxious.
The telescope's name will be the same, but in some ways it will be a completely new astronomical tool.
A new camera will allow the observatory to survey larger patches of the sky in more detail than ever, observing objects in ultraviolet as well as visible light. The phone-booth-size camera is so sensitive that it would spot a pair of fireflies more than 6,000 miles away - and still see them as separate objects.
Astronomers hope to use the camera to view the earliest stages of star and galaxy formation in the universe, trace the effects of "dark energy," which appears to be speeding the universe's expansion rate, and hunt for planets beyond our solar system.
"In some respects, it will be like getting a brand new spacecraft," says Bruce Margon, associate director for science at the Hubble Space Telescope Institute.
The camera and several other new pieces of hardware were scheduled to launch from the Kennedy Space Center in Florida early this morning aboard the space shuttle Columbia.
Enthusiasm for the $172-million upgrade is running so high that even before astronauts loosen a bolt, the observatory has more astronomers vying for time than it can accommodate. "It's the largest oversubscription in the history of the program," Dr. Margon says.
Yet Hubble also is set to get a new, critical power-supply component that was never designed to be installed or serviced by astronauts. Before astronauts can swap the new unit for the old - which involves keeping track of where 36 cables connect to the box - controllers must cut all power to the observatory for the first time. This leaves the craft's components vulnerable to the debilitating cold of outer space. Once the operation begins, it will become a race against time.
"It kind of violates a long-standing policy in the space business that if something's working well, you don't turn it off and just hope it comes back on," acknowledges Ed Weiler, NASA's associate administrator for space science. "We're not doing it cavalierly. We fully anticipate that everything will work fine. But it's a risk that we've never taken before."
To a casual observer, it would seem hard to top Hubble's performance so far.
For astronomers, the 12-ton orbiting observatory has rewritten textbooks on cosmic evolution and yielded insights into a range of processes - from star birth to planet formation.
It gave astronomers a major leap forward on one of its primary goals: to better gauge the "Hubble constant," a key factor in estimating the size and age of a universe whose expansion was first observed by the telescope's namesake, astronomer Edwin Hubble.
In addition, Hubble has overturned the notion that only rare, distant galaxies hold a supermassive black hole at their centers. "Now it appears that it's the rare galaxy that doesn't host a supermassive black hole," Margon says.
In 1998, two teams of astronomers announced evidence not only that the universe was expanding, but that its expansion was accelerating, not slowing, as many cosmological models held. The Hubble and ground-based telescopes were used. The stunning results suggested that a "fudge factor" Einstein used to keep his universe from expanding - and later called his biggest blunder - may have a place in the cosmos after all.
Hubble also has taken the most distant views yet, offering glimpses of early galaxies and galaxy fragments.
For its sponsor, the National Aeronautics and Space Administration, Hubble has proved a public-relations bonanza, generating images of awesome spectacles - from colliding galaxies to glowing, dust-shrouded nurseries harboring young stars. Hubble images adorn neckties, leap from magazine pages, and outshine icons on computer-screen desktops.
In spacewalks next week, astronauts will not only install the camera, but also a cooling unit on a near-infrared camera and spectrometer. The old cooler's refrigerant, liquid nitrogen, leaked in 1999, rendering the camera useless.
"It was quite a loss when it went dormant," says astronomer Ray Jayawardhana, at the University of California at Berkeley. The near-infrared camera will aid the hunt for planets and the study of dust-shrouded star-formation regions.
Hubble also will get a new, more powerful pair of solar panels, and other control- and power-system improvements.
Coupled with upgrades in 2004 (installing a new wide-field camera and spectrograph), the result will be an observatory 10 to 20 times more powerful.