With the Faint Object Camera temporarily attached to fixture on side of payload bay, the Advanced Camera for Surveys is installed into Hubble as shown in animation. Photo: NASA TV/Spaceflight Now

Astronauts James Newman and Michael Massimino plan to float out of Columbia's airlock around 3:27 a.m. to begin a six-and-a-half hour spacewalk to install the Advanced Camera for Surveys, or ACS, a 16-megapixel digital camera built around an ultra-sensitive CCD detector about the size of a paperback novel.

Newman and Massimino also will install an electronics package that will work in concert with an experimental refrigerator scheduled for installation Friday that is designed to revive a now-dormant infrared camera-spectrometer known as NICMOS.

About the size of a phone booth, the Advanced Camera for Surveys will be installed in place of a no-longer-used instrument called the Faint Object Camera. Newman will ride on the end of Columbia's robot arm for the camera installation while Massimino will be the designated "free floater."

Before the camera is installed, Massimino will float into FOC/ACS instrument bay to install a cable harness needed by the NICMOS electronics module. Newman then will slide the ACS into place with guidance cues from Massimino.

After stowing the Faint Object Camera in a cargo bay carrier, Massimino will take Newman's place on the robot arm and the astronauts will install the NICMOS electronics module on the floor of the aft instrument bay just in front of the advanced camera.

The ACS actually includes three cameras sensitive to a broad range of wavelengths, from the ultraviolet to the far infrared. The 870-pound camera has twice the resolution and five times the sensitivity of the upgraded Wide Field-Planetary Camera - WFPC-2 - that currently is in place.

"With ACS, Hubble will detect more faint stars and galaxies during its first 18 months than have been detected with all of the previous Hubble instruments," said principal investigator Holland Ford of Johns Hopkins University. "For astronomers, those stars and galaxies in the data archive are money in the bank."

To visualize the power of the ACS, it helps to recall one of the telescope's most famous photographs, the so-called "Hubble Deep Field," one of the most remarkable images ever produced by the space telescope.

In December 1995, Hubble was aimed at a presumably empty patch of sky near the Big Dipper about the size of a rice grain held at arm's length. The spot was chosen specifically because it appeared, for all practical purposes, to be devoid of stars and galaxies.

Over the next 10 days, WFPC-2 took 342 images that later were digitally combined. In the resulting Hubble Deep Field image, amazed astronomers counted some 1,500 discernible galaxies, or fragments of galaxies, dating back to a billion years or so of the big bang.

"The Hubble Deep Field, one of humanity's and Hubble's singular achievements, can be done in two days instead of 10 days," said Ford. "A ten-fold increase is especially important for finding rare objects such as the first galaxies and distant supernovae."

Working in tandem with the revived NICMOS, the Advanced Camera for Surveys also will play a major role in one of the hottest fields in modern astronomy, the ongoing search for exploding stars called type 1A supernovae.

Consider a binary star system that includes a compact white dwarf. The smaller white dwarf's gravity may pull in gas and dust from the companion star. If the white dwarf's mass builds up to about 1.4 times that of the sun, catastrophic fusion reactions begin and the star explodes in a type 1A supernova.

By definition, all type 1A supernovae involve stars of roughly the same mass and the intensity of the emitted light follows a well-defined "light curve," brightening and then fading away. The intensity of the light can be used to infer the supernova's distance from Earth and spectroscopic analysis can provide a measure of its recession velocity.

Astronomers have long assumed the expansion of the universe is slowing down. And based on the presumed rate of that deceleration, one would expect to find certain brightness levels for type 1A supernovae at various points in time and space after the big bang.

But to the surprise of everyone in the astronomical community, researchers in the late 1990s discovered type 1A supernovae at extreme distances appeared dimmer than one would expect based on their observed recession velocities.

As it turned out, the only reasonable, if counter intuitive, explanation was to assume the expansion of the universe is actually accelerating. The nature of the "dark energy" powering that acceleration is a complete unknown and determining its nature is a top astronomical priority.

"This is an amazing time for both physics and astronomy," said David Leckrone, Hubble project scientist at the Goddard Space Flight Center. "We've come to start to realize over the past few years that we do not understand 95 percent of the content of the universe in which we live.

"Between dark matter and dark energy, those two things together constitute approximately 95 percent of the total energy and mass balance, or budget, of the universe," he said. "These are very challenging physical problems.

"What is the nature of dark energy, that may well be the most important question in the physical sciences today. The beauty of (ACS and NICMOS) is they give us very powerful tools for beginning to address these very fundamental and revolutionary new issues in physics."

Installation of the ACS is relatively straight forward, according to payload commander John Grunsfeld, an astronomer who is making his second visit to Hubble.

"There are four scientific instrument bays where we can put refrigerator-size instruments," he said in a NASA interview. "We have some experience doing that. Nevertheless, it's kind of a tricky task. The scientific instruments have to be kind of threaded into the telescope.

"The first thing that they're going to do is to take out the Faint Object Camera," he said. "This is the last of (Hubble's) original scientific instruments. Jim Newman will take that out and put it onto a special fixture hanging over the left side of the space shuttle. And then, he and Mike will take out the Advanced Camera for Surveys and put it into the telescope.

"Once the Faint Object Camera is out, Mike has one of the best opportunities of anybody on the crew because he gets to go inside the telescope, and his whole body basically - he's a big guy, almost his whole body, maybe his feet will be hanging out - he gets to go inside the telescope and look around to make sure that there isn't any insulation or tape or anything that's hanging in the way before we put the Advanced Camera for Surveys in.

"His head will basically be, you know, at the heart, at the soul of Hubble because this is where all the light from the telescope comes through into the scientific instruments," Grunsfeld said. "The sad thing is the cover on the telescope will be closed, so he won't actually be able to look up and, you know, see galaxies or anything like that. But, you know, for me as an astronomer, that would be a very exciting place to be."