Saturday, April 24, 1999 Published at 10:22 GMT 11:22 UK
Another Hubble gyroscope fails
If one more gyro fails, Hubble will go to sleep
By BBC News Online Science Editor Dr David Whitehouse
An ailing gyroscope on the Hubble Space Telescope has stopped functioning. Engineers had expected it to fail since it first started behaving erratically in January.
Normal operations of the telescope have not been affected. However it does mean that Hubble has no spare gyroscopes left.
In the past Hubble's engineers have said that if another gyroscope failed they would put the telescope into 'safe' mode to await repair. However that is now regarded as an overreaction.
The failure of Gyroscope One was detected when the gyroscope's motor current fell instantaneously to zero.
This leaves gyroscopes One, Two and Five as Hubble's only working gyroscopes.
Gyroscope Three has not been used for pointing purposes since it began behaving erratically. Because of this, its failure will have no impact on the telescopes observations.
Hubble requires three of its six gyros to control accurately the position of the telescope during science observations. This new failure means there are no available spares.
Any further gyroscope failure will cause the observatory to go into a protective safe mode that gives ground controllers complete control of the telescope, but prevents it making observations.
Hubble will be visited by a Space Shuttle crew in October during Servicing Mission 3A.
Astronauts will replace all the gyroscopes, a fine guidance sensor, a transmitter, a spare solid state recorder and a high voltage/temperature kit for protecting batteries from overheating. The crew will also install an advanced computer.
Servicing Mission 3B, to be conducted in year 2000, will install a new scientific instrument, the Advance Camera for Surveys, as well as new solar arrays and a new cooling system.
Both missions will patch over telescope skin that has degraded over the years.