Astronauts Finish Work on Hubble

Two astronauts ventured out on a Christmas Eve walk in space Friday and finished fixing the Hubble Space Telescope by installing a new radio transmitter, digital recorder and sun shades.

Already smarter and steadier after repairs earlier in the week, the $3-billion telescope will be cut loose from space shuttle Discovery today and resume its survey of the heavens two weeks later. Its observations ceased in mid-November after a series of breakdowns.

In the third and final spacewalk of the mission--and NASA’s last opportunity to work on Hubble until 2001--Steven Smith and John Grunsfeld quickly hooked up an electronic unit to the pointing system, then replaced Hubble’s broken radio transmitter. The instrument stopped working last year, and scientists had to rely on a backup transmitter to relay data.

NASA never expected the radio transmitters to break, so the connectors were not made to be handled with the astronauts’ bulky gloves. Engineers redesigned the replacement unit and devised a special tool to remove and install the small connectors.

Even with the tool, it was tedious work. Grunsfeld had to rest his hands while unplugging the old transmitter.

Hubble’s old reel-to-reel recorder was meant to be replaced so it came out much easier. The new solid state recorder has no reels, no tapes and no moving parts, and can hold 10 times as much data.

Despite the serious business going on more than 370 miles up, Christmas was in the air inside Mission Control, where stockings and Santa caps decorated computer consoles. Shortly after sunset, NASA interrupted its TV programming to broadcast the landing of Santa’s sleigh. The videotaped spoof had flight controllers giving the final “go” for Santa’s touchdown in Houston.

Besides installing the two instruments, Smith and Grunsfeld hung thin stainless steel sheets, measuring 5 feet by 4 feet, at the base of the 9-year-old telescope to guard against the sun’s damaging rays. During the last service call, in 1997, astronauts reported that Hubble’s thermal insulation was cracked and peeling on the side facing the sun.

First, the astronauts had to pull off some of the tattered insulation.

“I feel like a kid going in and getting toys out of Santa’s bag,” Grunsfeld said as he stuffed the old foil into a sack.

The men did not have time to install all six shades, but managed to cover the two worst areas. Despite the unfinished work, NASA declared the mission a full success.

The tailoring should help maintain a constant room temperature inside the telescope as the observatory passes from day into night with each orbit of Earth--which brings temperature swings of 300 degrees--said Hubble program manager John Campbell.

Before ending their eight-hour spacewalk, Smith and Grunsfeld sent back holiday greetings and a wish for peace on Earth. “Merry Christmas!” Smith said, waving.

During their outing Wednesday, Smith and Grunsfeld installed six new gyroscopes, restoring the telescope’s ability to aim steadily at stars, galaxies and other celestial objects. The observatory shut down Nov. 13 when a fourth gyroscope failed. The men also added voltage regulators to Hubble’s six batteries to prevent overheating.

On Thursday, Michael Foale and Claude Nicollier put in a new guidance sensor and replaced the telescope’s old-fashioned computer with a newer, faster model. “We have a brainier Hubble Space Telescope than we have ever had before,” declared David Leckrone, a senior Hubble scientist.

All the equipment installed on those first two spacewalks has tested well. As a result, NASA is confident that when Hubble’s eye opens and begins scrutinizing the universe again--hopefully, by the second week of January--everything will work just fine.

NASA had to put off covering the upper portion of the telescope with wallpaper-like strips of sun-resistant material. The work, part of a canceled fourth spacewalk, will be performed when astronauts return in 2001.

Because of the mission’s late start, managers shortened Discovery’s flight from 10 to eight days to get the shuttle back with a few days to spare before New Year’s Eve. Although engineers believe NASA’s computers are Y2K-compatible, they do not want to take any chances.

A Monday landing is planned.