Repairs on shuttle Discovery delay mission to Hubble

Launch was set for Dec. 2

engine has to be replaced

November 04, 1999|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

NASA's plans to replace failing gyroscopes aboard the Hubble Space Telescope this year are threatened by yet another delay in the launch of the space shuttle Discovery.

The space agency said yesterday that Discovery's scheduled Dec. 2 launch would be postponed to give ground crews time to replace an engine and inspect the spacecraft for faulty heat-resistant tiles.

No new launch date has been set.

Without the repairs to Hubble, one of its three remaining gyroscopes could quit working, making it impossible for controllers to reliably aim the telescope.

A delay of several days won't hurt, said Michael G. Hauser, deputy director of the Space Telescope Science Institute in Baltimore. The remaining gyros are working well.

But more significant delays, coming on top of a series of postponements since the original Oct. 14 launch date, could risk more problems for Hubble and further postpone the installation of new instruments in 2001.

"We certainly want to see it off by the 14th [of December]," Hauser said.

The space telescope has lost the use of three of its six gyroscopes. At least three functioning gyros are needed to rotate the bus-sized telescope in all three dimensions and to hold it steady during observations.

If one more fails, scientific observations will come to a halt.

"Delays increase the chances that another gyro will fail," Hauser said.

The gyroscopes now on board Hubble were installed by astronauts in December 1993, during the first Hubble servicing mission. Two of those were replaced during the second servicing mission, in February 1997.

Hauser said the repeated failures are caused by the unexpectedly rapid corrosion of fine electrical wires inside the devices.

More trouble does not seem imminent. There have been no signs of the problems that preceded past failures.

Hubble's next routine servicing mission had been set for mid-2000. But last spring, the gyroscope failures persuaded NASA to mount an emergency repair mission this fall.

In four days of spacewalks, astronauts will also replace the telescope's main computer, a fine guidance sensor, a radio transmitter and other equipment.

Discovery's Oct. 18 launch date dissolved in July when an electrical short-circuit turned up in the shuttle Columbia during liftoff.

NASA grounded the entire shuttle fleet until technicians could inspect all of them for bad wiring. That work is now done.

The Hubble mission was reset for Dec. 2. November dates were ruled out because of the approach of the Leonid meteor shower, which peaks Nov. 18.

NASA does not want astronauts working in space during the shower, while Earth plows through high-speed dust and pebbles left in the wake of the comet Tempel-Tuttle.

The annual Leonid shower is expected to be unusually heavy this year.

Yesterday, the Dec. 2 launch date was scrubbed after NASA discovered a batch of faulty tiles used to shield the shuttle from the heat of re-entry.

If any of those tiles is found on Discovery, "that would be a tile that would need to be replaced," said NASA spokesman Joel Wells.

In addition, X-rays of one of the shuttle's main engines revealed a half-inch fragment of a drill bit hidden inside a coolant chamber. Rather than fish it out, managers elected to replace the engine -- a two or three-day job.

Opportunities to launch the 10-day mission this year are dwindling. The space agency wants to launch by Dec. 14 in order to avoid flying over the Christmas holiday. And it will not fly the shuttle during the transition to the year 2000.

Repairs to make the orbiter's computers Y2K-compliant were postponed so that Discovery could fly the Hubble repair mission this fall, Hauser said. If the flight is delayed into January, those repairs would have to be completed before the shuttle could be launched again.

In the meantime, the mounting delays have pushed back the third planned Hubble servicing mission -- the one originally set for mid-2000 -- until sometime in 2001, Hauser said.

Astronomers are eager for that mission because it includes the installation of a new instrument -- the Advanced Camera for Surveys, a collaborative effort of the Johns Hopkins University and the Goddard Space Flight Center.

Astronauts will also replace failed refrigeration gear that has idled Hubble's infrared telescope.

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