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More Details for 1999-12-22

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Discovery astronauts completed the two highest priority tasks of their Hubble Space Telescope servicing Wednesday with a space walk that was the second longest in history. Astronauts Steve Smith and John Grunsfeld installed six new gyroscopes and six Voltage/Temperature Improvement Kits in the telescope during their 8 hour, 15 minute spacewalk.

Working deliberately, Smith and Grunsfeld replaced three Rate Sensor Units, each containing two gyroscopes. Four of Hubble's gyroscopes had failed, making the telescope unable to point itself precisely enough to do science since Nov. 13. At least three operable gyroscopes are needed to point the telescope with the accuracy required to track its astronomical targets.

The spacewalkers also installed Voltage/Temperature Improvement Kits on wiring from Hubble's solar arrays to each of its six batteries. The kits are designed to improve control of the charging of the space telescope's 10-year-old batteries.

With Hubble latched upright in the payload bay, Smith and Grunsfeld completed all major tasks scheduled for the first of three spacewalks on three consecutive days. A few minor objectives, including applying lubricant to the door of one of the telescope's bays and taking close-up photos of the Voltage/Temperature Improvement Kits, were left undone. Flight and telescope controllers decided to cancel the photography job and schedule the 10-minute lubrication job for Thursday's space walk. The duration of the spacewalk was second only to the 8 hour, 29 minute space walk from Endeavour on STS-49 in May 1992.

A few minor problems helped account for the length of the space walk. One of the old gyroscope-containing Rate Sensor Units was a tight fit in the box designed to protect it on its return to Earth, though eventually it was placed inside and the lid closed. Another involved opening valves and removing caps on the Near Infrared Camera and Multi-Object Spectrometer, in preparation for restoring it to operation during the next Hubble Servicing mission. That task too eventually was completed.

All in all flight and telescope controllers were delighted with the accomplishments of the day.

Major tasks on Thursday's space walk by Michael Foale and Claude Nicollier include replacement of Hubble's outmoded DF-224 computer with a more modern unit 20 times faster and with six times the memory. They also will replace one of Hubble's three fine guidance sensors, used to precisely point the telescope and gather scientific data. The astronauts also may perform "get-ahead tasks," some first scheduled for a fourth space walk. That space walk was cancelled because of delays in Discovery's launch. Discovery remains in excellent condition, in an orbit with a high point of 380 statute miles and a low point of 369 miles.