The Hubble: a mission of discovery

Deploying telescope to require microscopic precision

The shuttle Discovery will soar toward a record altitude this week with NASA's $1.55 billion Hubble Space Telescope and a crew of five veteran astronauts, two of them prepared for spacewalks if necessary to successfully deploy the observatory.

The ship is scheduled to lift off on its five-day mission from the Kennedy Space Center in Florida on Tuesday at 7:47 a.m. CDT, the opening of a 2 1/2-hour launch period.

The long countdown was scheduled to begin Saturday afternoon as the five shuttle fliers arrived at Cape Canaveral to begin the final preparations for their flight.

If all goes well, Discovery's crew will begin the lengthy deployment of the 25,000-pound space telescope early Wednesday, raising it from the shuttle's cargo bay with the orbiter's 50-foot robot arm.

The delicate maneuver commanded from inside Discovery by astronaut Steve Hawley, the arm operator - and fittingly, an astronomer - will initiate a series of crucial steps to unfurl the telescope's antennae and its power-generating solar arrays.

Then, with a command from Hawley, the space telescope will drift free of the arm by midafternoon.

A 38-year-old astrophysicist, Hawley conceded he is more nervous about this mission than either of his two previous spaceflights because of his personal and professional association with many of the astronomers who will use the telescope.

"I know those guys, and they know me," said Hawley, who also serves as the deputy chief of NASA's astronaut office. "I know they will be watching.' In order to place the telescope high above the distortions and pollution of the Earth's atmosphere, Discovery will race toward an altitude of 380 miles, a record during the 9-year-old shuttle program and twice as high as the shuttle ships typically operate.

U.S. astronauts have not been so far from their home planet since the final Apollo moon mission in 1972.

The space agency is prepared to deploy the telescope as low as 327 miles, and possibly even lower, if propulsion problems develop.

However, that contingency would likely send NASA scrambling to prepare another shuttle mission within the next year to boost the fragile telescope, which has no maneuvering engines of its own.

The mission is commanded by Air Force Col. Loren Shriver, 45, and piloted by Marine Col. Charles Bolden, 43. Hawley serves as the flight engineer as well as the arm operator.

If a spacewalk becomes necessary, the risky task will fall to Navy Capt. Bruce McCandless II, 52, and geologist Kathryn Sullivan, 38, both veterans of past spacewalks.

"We are going to great extremes on this flight to take advantage of what crewmen can do in orbit to help us," explained Bill Reeves of the Johnson Space Center, NASA's lead flight director for the Hubble mission. "We are taking every possible precaution to be able to back up any potential problem we could have with the deployment of the telescope.' A NASA veteran whose association with the space program dates to the Apollo program, McCandless has spent nearly half of his 24 years with the space agency as an astronaut tracking the development of the complex space telescope.

Dressed in bulky space suits and working in huge NASA-owned water tanks to simulate weightlessness, he and Sullivan have outtrained previous U.S. spacewalkers for the key repair tasks that might confront them on this high profile mission.

To cut the normal four-hour preparation time for a spacewalk to just two hours, the pressure in Discovery's crew cabin will be lowered from 14.7 to 10.2 pounds per square inch once the shuttle achieves orbit.

That drop will sharply reduce the time McCandless and Sullivan must breath pure oxygen if they are called upon to venture outside the spacecraft. The "prebreath" step is necessary for them to avoid a painful condition called "the bends," the release of nitrogen gases within the body tissues and blood stream while they are in their space suits.

One of the most critical tasks the two spacewalkers could be asked to perform involves the telescope's vital electrical supply.

The Hubble's power needs will be supplied by six nickle hydrogen batteries until its twin 40-foot long, electricity-generating solar arrays are unfurled during the early stages of the telescope's deployment.

The batteries were charged last week and they will not be recharged again unless consecutive launch attempts over five days are unsuccessful.

The recharging process takes nearly a week.

While aboard Discovery, the Hubble's status is monitored through a long "umbilical" data line that connects the observatory to the shuttle orbiter. The umbilical also provides the space telescope with some electrical power generated by Discovery's fuel cells.

Once that line is disconnected during the deployment, ground controllers estimate they have seven hours at most to extend the winglike solar panels so that they can begin generating electricity before the telescope's battery power lapses.

If the panels don't extend on command, McCandless and Sullivan will begin the final preparations for a spacewalk to unfurl the panels manually.

Among the other tasks the two astronauts could be called on to perform manually:

Unlocking the latching system on Discovery that holds the telescope to the shuttle's payload bay, or to repair the robot arm if it fails to respond to commands.

Extending the two antennas on Hubble that allow it to communicate with ground controllers once it is in free drift. The antennas are a crucial link in pointing the telescope in the proper direction and transmitting the signals that will be translated into images of the celestial objects it observes.

Opening the 10-foot wide aperture door that covers the opening of the telescope, protecting it from harmful bright sunlight or contamination from the exhaust of the space shuttle.

If the door failed to open on command, the Hubble would be useless.

Once the telescope is free of the space shuttle, Shriver will gradually move Discovery to a point nearly 50 miles behind the Hubble, while ground controllers test the responsiveness of the observatory to their commands.

Late Thursday, ground controllers will command the aperture door to swing open. If it doesn't, the Discovery astronauts will return to the telescope, and McCandless and Sullivan will don their gear for a spacewalk to open it.

Once the door is open, the cabin pressure on Discovery will be raised and preparations will begin for landing.

In addition to the deployment of the telescope, the shuttle fliers plan to conduct nearly two dozen science and medical experiments.

Discovery's return to Edwards Air Force Base, Calif., is planned for Sunday morning, April 15. The mission would be extended by a day if the astronauts must rendezvous with the telescope to open the aperture door.

Getting the picture Functioning like a high-flying video camera, the Hubble Space Telescope will beam images of its celestial observations to the Space Telescope Science Institute at Johns Hopkins University in Baltimore, Md. over a network of communications satellites and ground stations. Teams of engineers based at NASA's Goddard Space Flight Center in Greenbelt, Md., will send commands over the network to control the telescope.

TDRS stands for NASA's Tracking and Data Relay Satellite. A DOMSAT is a domestic satellite. Both are communications satellites.