Ambitious mission: hubble slated to get one heckuva tune-up. (This Week).

If all goes according to plan next week, astronauts aboard the space shuttle Columbia will embark on the fourth and most technically challenging mission to replace damaged parts and install new detectors on the Hubble Space Telescope.

After catching Hubble with the shuttle's robot arm and securing it in Columbia's payload bay, the crew will take five space walks. During the 11-day mission, astronauts will remove the European Space Agency's faint-object camera to make room for a new instrument, the Advanced Camera for Surveys (ACS).

The size of a telephone booth, ACS has twice the field of view and can detect celestial objects one-fifth as bright as Hubble's current workhorse detector, the Wide-Field and Planetary Camera 2 (WFPC-2). Featuring individual light sensors, or pixels, that are half the size of those on WFPC-2, ACS also has twice the resolution of that camera.

During its first 18 months of operation, ACS will enable Hubble to detect more faint stars and galaxies than the telescope has since the observatory's launch in 1990, says Holland Ford of Johns Hopkins University in Baltimore, who leads the ACS team.

The camera's improvements will be especially important in finding rare objects, such as distant supernovas and the earliest galaxies to have formed in the universe, Ford notes. The instrument's chronograph, which blots out the light of nearby stars, will provide new details about protoplanetary disks. These dim, doughnut-shape regions of gas and dust surround young stars and provide the material for making planets.

During one space walk, astronauts will install an experimental refrigerator in an attempt to revive Hubble's near-infrared and faint-object spectrograph. This instrument stopped working in 1999 when a heat leak caused it to prematurely run out of its nitrogen-ice coolant. The infrared camera had examined the cosmos at wavelengths longer than any other instrument aboard Hubble, enabling it to see through dusty regions that visible light can't penetrate.

But before the shuttle crew attempts to bolster Hubble's scientific capabilities, they'll have to address the basic health of the spacecraft. Their first order of business is to install a new power-control unit, which distributes current from Hubble's solar arrays and batteries. Electrical problems in the unit are now preventing Hubble from using two of its six batteries, and the trouble "doesn't have to become much worse before we could lose the ability to do any science with this observatory," says Hubble project manager Preston Burch of NASA's Goddard Space Flight Center in Greenbelt, Md.

To replace the unit, astronauts will power down the telescope for the first time since it was placed in orbit. "That scares me a lot," admits Edward J. Weiler, associate administrator for NASA's office of space science in Washington, D.C. The 160-pound control unit, he notes, wasn't designed to be replaced, so the astronauts will have to manually unscrew 36 electrical connectors. While the power is off, the crew will keep Hubble's instruments warm by wrapping them in thermal blankets.

The crew will also provide Hubble with a new set of solar arrays. The craft's third set, the arrays have only two-thirds the solar-collecting area of those now in place but will produce 20 percent more power. Before coming home, the astronauts will also replace one of the observatory's reaction wheels, which keep the observatory focused on a celestial target.