A failing guidance sensor aboard the Hubble Space Telescope was to be the primary target tonight for two astro-handymen aboard the shuttle Discovery.
Gregory J. Harbaugh and Joseph R. Tanner are to don their spacesuits and step into the shuttle's payload bay at 11: 21 p.m. for a 6-hour and 15-minute spacewalk -- the mission's second.
After a flawless launch Tuesday, the Discovery astronauts chased down and captured the $2 billion telescope on the first try early yesterday and installed instruments that will allow astronomers to look deeper in space.
By Tuesday, Hubble should have two new scientific instruments and replacements for its failing hardware. It will then be set free for another two years. Discovery is due back next Friday from the $795 million mission.
Tonight, after installing the handrails and foot restraints, Harbaugh and Tanner will slide the failing Fine Guidance Sensor (FGS) from the side of the telescope like a large drawer, and slip in its $8 million replacement as they move around carefully.
The sensor is one of three whose primary job is to guide the telescope to its targets by finding and locking onto a set of pre-selected "guide stars."
But they also make scientific observations called "astrometry." By precisely measuring the sight angles to nearby stars, scientists can use parallax calculations to determine their distance -- the same way the brain uses sight angles from each eye to estimate distances. Scientists use the data to infer the distances to similar, but much more distant stars.
Astrometry can also detect small wobbles in the motions of stars that may reveal unseen companion stars or large planets.
The new FGS unit should improve Hubble's astrometry because it contains new optics to correct for the manufacturing flaw in the telescope's main mirror.
Later in the spacewalk, Harbaugh and Tanner are to replace one of Hubble's two engineering and science data tape recorders with a refurbished spare. They will also install a black box called the Optical Controls Electronics Enhancement Kit.
It will be one of the mission's more difficult tasks, with six bolts and 18 separate electrical connections to be secured while working alongside Hubble's vulnerable solar arrays.
Astronauts say hand work is very tiring in spacesuits. Harbaugh compared the suit's gloves to ski gloves. "They're fairly rigid and require a fair amount of effort to close your grip. Handling screws is not easy to do," he said.
The astronauts shape up before their missions by working with small weights and doing exercises designed for rock climbers.
Late last night, astronauts Steven L. Smith and Mark C. Lee, were preparing for the first of the mission's four spacewalks. Overnight, they were to remove two of Hubble's original 1970s-design spectrographs and replace them with two 1990s instruments.
As good as Hubble's work has been since its 1993 repairs, it's "nothing compared to what you're going to see three or four weeks from now" if these instruments work as designed, said Frank Cepollina. Servicing project manager at the Goddard Space Flight Center, Cepollina said the data and pictures would be "astonishing."
The instruments installed yesterday included the $125 million Space Telescope Imaging Spectrograph, and the $105 million Near Infrared Camera and Multi-Object Spectrometer.
Discovery's crew captured the Hubble telescope at 3: 33 a.m. yesterday at the end of a two-day orbital chase.
When Discovery was within 35 feet of Hubble -- in darkness over the southwest coast of Mexico -- flight engineer Dr. Steven Hawley moved the end of the shuttle's remote manipulator arm to one of the telescope's two grapple fixtures and commanded it to grasp.
"Looks like you just shook hands with an old friend," Commander Kenneth Bowersox said.
Bowersox, who piloted the shuttle on the 1993 repair mission, said the telescope looked "just the way we left it the last time."
A video survey, however, revealed a hole in one of Hubble's two dish-shaped high-gain antennas. The "ding," probably caused by micrometeor or a bit of orbital junk, appeared to be about the size of a fingernail. The damage has not affected communications with the telescope.
Pub Date: 2/14/97