By James Morgan
Science reporter, BBC News
The reboot should allow Hubble to return to full operations
The Hubble space telescope should resume science operations on Saturday, say Nasa officials.
Engineers have rebooted the computer which controls most science instruments aboard the orbiting observatory.
Hubble has been "blind" for three weeks after the failure of a command unit forced the telescope into "safe mode".
Attempts to activate a backup system stalled last week after an electrical fault, but assessments have revealed no permanent damage.
The reboot operation was resumed by switching on the "B side" of the data handling unit which deals with most of the spacecraft's scientific payload.
"If it continues to run well, science operations will resume this weekend," said Art Whipple, of Nasa's Goddard Space Flight Centre.
The telescope's Wide-field and Planetary Camera 2 is expected to be active early on Saturday morning, "and it will start doing science almost immediately", said Mr Whipple.
Another key instrument - the Advanced Camera for Surveys (ACS) - will follow "later in the week", he added.
Hubble's main flight computer put instruments in a protective safe mode on 27 September, when it detected a problem in the observatory's Science Instrument Command and Data Handling (SIC&DH) Unit.
HUBBLE SPACE TELESCOPE
Named after the great US astronomer Edwin Hubble
Launched in 1990 into a 600km-high circular orbit
Equipped with a 2.4m primary mirror and five instruments
Length: 15.9m; diameter: 4.2m; Mass: 11,110kg
All science data passes through this unit before it can be transmitted to Earth. Since it was put to sleep, Hubble has been temporarily "blind" except for one type of observation.
Over the past weeks, engineers have been working through the details of a fix that involves switching the observatory over to a "B" formatter.
But this reboot operation ground to a halt last week after two "anomalous events", one of which appears to have been an electrical fault.
"We cannot know the exact cause, of course, because we cannot get to the hardware. All we can say is that it appears to have been an electrical event," said Mr Whipple.
"Events like this are not uncommon in electrical circuits that have been turned off for a long time.
"It is possible that we may see another event of this type in the future. This is the first time we have switched these circuits on in 18 years and we will have to see how that goes."
Fortunately, the anomaly "does not appear to have done any permanent damage," he added.
"It did not blow any fuses. There was no harm done to any other instruments."
Nevertheless, the setbacks have forced Nasa to postpone a major upgrade to the telescope.
Servicing Mission 4 (SM4) was originally scheduled for 14 October, but will not now take place until February at the earliest.
Over the course of five spacewalks, astronauts will install two new instruments - Wide Field Camera 3 and the Cosmic Origins Spectrograph.
They will also repair instruments that have failed - the Advanced Camera for Surveys, the Space Telescope Imaging Spectrograph.
The faulty SIC&DH Unit will also now be replaced to ensure Hubble has a working backup system. "We will leave Hubble in a redundant state," Mr Whipple confirmed.
The servicing mission, which will use Space Shuttle Atlantis, should keep the telescope functioning at least into 2014.
The new date for the mission is currently being assessed and will be determined "by mid November", said Nasa officials.