Sunday, November 14, 1999 Published at 19:09 GMT
Hubble's house call
The mission involves four spacewalks
By BBC News Online Science Editor Dr David Whitehouse
Nasa has outlined its plans for the mission to repair and service the Hubble Space Telescope (HST) next month.
Hubble is now down to three working gyroscopes, the minimum with which it can operate. Without gyros, it cannot point accurately at the objects astronomers want to observe.
The launch date has not been finalised but it will be the beginning of December, possibly the 6th. The last day the mission could take-off would be 14 December.
Lead flight director, Linda Hamm, said that the minimum they were expecting to accomplish would be to leave Hubble with five working gyros.
Over the past nine months, the crew have spent dozens of six-hour sessions in a huge water tank simulating the weightlessness of space and the tricky repairs they will have to carry out in orbit.
On the first day of spacewalking, the replacement gyroscopes will be installed. They will be followed by the new computer that is much more capable than Hubble's current computer but far less powerful than the computers available in the high street.
Finally on that day, the replacement of the fine guidance sensor will take place. This instrument enables Hubble to point accurately using guide stars.
To replace the gyros, astronaut Steven Smith will have to hold onto the feet of astronaut John Grunsfield as he climbs inside Hubble's service bay.
Day two of the repair will involve spacewalks to complete the gyro changeover. A device called a voltage and temperature limiter will be installed alongside the batteries. Its function is to prevent the batteries overcharging and overheating which they could do in certain circumstances.
Throughout the spacewalks, the astronauts will have to be careful not to touch any part of the Hubble telescope other than the parts they are working on. Engineers think that its foil covering is extremely brittle and could flake off.
Finally on spacewalk day two, the astronauts will replace a reel-to-reel tape recorder with a solid-state one.
"The important things are in the first two spacewalks, after that we are doing things to get a head start on future servicing missions," said Linda Hamm,
Two more servicing missions are planned before the nominal end of Hubble's life in 2010. A mission will be sent to it in mid-2001 and another in 2003 with new solar arrays and scientific equipment.