Hubble Servicing Mission 3A Update - Dec. 23-24 Day 4

The fourth working day in space (officially counted as Flight Day 5) is over and everything went well. The night's spacewalk was another one for the record books, 8 hours and 10 minutes, making it the third longest spacewalk in history. The longest was the third spacewalk during STS-49 in 1992 which lasted 8 hours and 29 minutes. The second longest was yesterday's spacewalk which lasted 8 hours and 15 minutes.

Astronauts Nicollier from ESA and Foale from NASA carried out the tasks - the exchange of Hubble's computer and a Fine Guidance Sensor - without any serious problems. The work with the computer began at 20:54 CET, and was finished 22:20 CET. The fine guidance sensor was in place 02:11 CET (Dec. 24), after some minutes of tension where Nicollier and Foale worked intensively on sliding the sensor on place.

Following the exchange the computer and the fine guidance sensor were "aliveness" tested. Both were found to be correctly installed. More testing will be carried out while the astronauts sleep - between 07:50 - 15:50 CET (on Dec. 24).

The new computer is a 486 model and it will be 20 times faster and have six times as much memory as the previous model - a socalled DF-224 designed in the 70's. The reason for not sending up the latest computer equipment to space is a demand for a flawless operation in the very harsh environment in space. The lifetime of the 486 is expected to exceed the 10 years Hubble itself is expected to be in operation. Despite being bombarded by cosmic and solar radiation constantly and being exposed to extreme temperatures.

There are three fine guidance sensors on Hubble. Two are all the time being used to point Hubble at the objects being observed. The third can then be used to make a special type of observations known as astrometry. Astrometry deals with the determination of precise positions and motions of stars. The next spacewalk will be this mission's third and last. It will begin at 20:40 CET.

The task are:
a.. 0:00-0:15 Set-up.
b.. 0:15-0:45 Finalizing the installation of fine guidance sensor.
c.. 0:45-2:00 Replacement of S-Band transmitter.
d.. 2:00-3:00 Replacement of Solid State Recorder.
e.. 3:15-5:30 Repair of thermal insulation.
f.. 5:30-6:00 Close-out.