A file picture taken from space station Mir shows the space shuttle's dish-shaped Ku-band antenna extending over the right-hand side of the forward payload bay. Photo: NASA

The failure today of the shuttle Discovery's primary television and rendezvous radar antenna will not prevent the crew from safely docking with the international space station Friday as planned, officials say.

But the loss of rendezvous radar data will make the final rendezvous sequence slightly more complex for commander Brian Duffy while the loss of KU-band television coverage will prevent flight controllers - and the public - from watching the linkup live.

If the problem cannot be corrected - and engineers are not optimistic - the only television from Discovery for the remainder of the mission will consist of still images and brief video downlinks during infrequent passes over U.S. ground stations.

"Right now, I would not bet we're going to get real-time TV back," said lead flight director Chuck Shaw. "The signature is puzzling because it just quit.

"But again, I've got to tell you, people want to make it into a big deal," he said. "From an operations impact standpoint, it's not a big deal. I would put it in the irritant category more than an impact category. And that's true."

He said television is not required for safety or mission success and that other sensors - laser range finders and the shuttle's optical star trackers - are available to replace the radar system if necessary during Friday's rendezvous.

"The loss of the TV is going to be an irritant and it's disappointing, because it's fun to watch the TV of these events," Shaw said. "But it is no way an impact to either safety or mission success."

In the meantime, the astronauts are beaming down still images through the shuttle's S-band communications system at about one frame every 10 seconds of so.

The high-speed KU-band antenna system is used to beam data and television back to Earth via NASA's globe-spanning set of communications satellites. The antenna also acts as a radar system during the final phases of rendezvous with the space station.

When deployed from the forward right side of the shuttle's cargo bay, the 180-pound antenna assembly measures seven feet long and one foot wide. A motor-driven three-foot-wide parabolic dish antenna is mounted on one end of the assembly.

As the shuttle orbits the Earth, the graphite-epoxy dish antenna locks onto and tracks one NASA communications satellite after another, moving about as required to maintain a solid link, even when the shuttle maneuvers or changes its orientation.

The antenna's forward link, that is, the shuttle-to-Earth link of the communications system, failed this morning after working normally earlier in the mission. Astronaut William McArthur began troubleshooting procedures just before noon.

As one might expect, the first step called for switching the system off and then back on and to re-initialize the system. But nothing worked.

"It's yet to be determined whether the impact would be only to the television side of that dish antenna's capability or also the rendezvous radar side of it as well, which is used to assist in the rendezvous with the station," said mission control commentator Kyle Herring.

"It is not a formal requirement to have the rendezvous radar capability (to proceed with docking), though it's obviously something that's very useful to the crew on board," Herring said. "There are several different methods for providing rendezvous data to the crew during the final phases of the rendezvous and docking."

Flight controllers plan to carry out a series of overnight tests to gain insight into what might have gone wrong with the KU-band system and what, if anything, might be done to fix it.

The international space station as seen by the last shuttle visit in September. Photo: NASA

Discovery currently is scheduled to dock with the space station at 1:43 p.m. Friday as the two spacecraft sail over Ukraine. At contact, the spacecraft will be directly above 48.44 degrees north latitude and 31.37 degrees east longitude.

The docking will occur as the spacecraft sail within range of one or more Russian ground stations so Russian flight controllers can monitor telemetry from the station.

Just before docking, the station's attitude control, or stabilization, system will be taken off line to prevent any unexpected motion during the final phase of the shuttle's rendezvous and capture. The system will be re-activated after docking is complete.

During past shuttle-station dockings, the Russians have controlled the attitude control system through direct commands through Russian ground stations.

For Friday's docking, however, the Russians may send commands through a NASA S-band communications system on the station's U.S. Unity module because of expected heavy traffic on internal Russian communications links.

"The Russians are studying whether they will have the full capability of all of their satellites and ground stations ... for commanding of the station to go into free drift at the time of docking and then back into attitude after hard mate," said NASA spokesman Rob Navias.

"Because other Russian customers are using some of their satellite system resources, there is some consideration begin given to altering the docking time."

Later in the day, U.S. and Russian flight controllers agreed to keep the docking on schedule for around 1:43 p.m.

Discovery's astronauts were awakened at 8:17 a.m. to begin their first full day in space. Three rendezvous rocket firings will be carried out to fine tune the shuttle's approach to the station.

Japanese astronaut Koichi Wakata put the shuttle's 50-foot-long robot arm through its paces to make sure the space crane is healthy and ready for a full slate of space station assembly work.

Discovery's thunderous nighttime liftoff yesterday as captured in dramatic fashion by a pad camera. Photo: NASA

On Saturday, Wakata plans to use the arm to mount the 18,300-pound Z1 truss atop the U.S. Unity module. The $273 million truss houses four stabilizing gyroscopes and will serve as the temporary mounting point for a huge set of solar arrays set for delivery in December.

The robot arm also will be used to mount a new $20 million shuttle docking port on Unity's nadir, or Earth-facing, hatch. The port will be used later in the assembly sequence when the U.S. laboratory module Destiny is attached to the port currently used by visiting shuttles.

Discovery's four spacewalkers - McArthur, Leroy Chiao, Jeff Wisoff and Michael Lopez-Alegria - spent the early afternoon checking out their spacesuits to make sure they will be ready for use in four back-to-back spacewalks starting Sunday.

The astronauts also planned to check out the health of their emergency rescue jetpacks, designed to help an astronaut fly back to the shuttle should he or she ever become untethered during a spacewalk. The nitrogen gas jetpacks attach to the base of a spacewalker's backpack.

Later today, a Canadian-built computerized television system called the Orbiter Space Vision System, used to precisely position components with the robot arm, will be powered up and tested.

At 5:47 p.m., Wakata will participate in a Japanese public affairs event. One hour later, Discovery's docking system attachment ring will be extended to verify its operation and readiness for the space station linkup Friday.

The final major item on the agenda today is check out of the crew's laser rangefinders and other gear that will provide critical distance and relative velocity information during the final phase of docking.

The astronauts are scheduled to go to bed around 10:17 p.m.