The Hubble Space Telescope (HST) uses a collection of gyroscopes to maintain it's orientation in space while it observes the Universe. It is designed to use three of these to point accurately and was launched with a total of six - three for operation and three for backup. Over the 15 years Hubble has been in orbit, several of the gyros have failed and been replaced by space shuttle crews on servicing missions. Currently, two are broken, three are operating, and one is switched off as backup. If another two fail, Hubble will not be able to point correctly and science operations will come to an end.
After the Columbia incident in 2003 NASA have decided not to send the final servicing mission to Hubble due to safety concerns (they may change their minds again, we'll have to wait and see). This means that the two dead gyros will most likely not be replaced. To try and extend the life of the HST, engineers have been working to come up with other ways to keep Hubble pointing in the right direction. Their solution is to switch off another of the gyros, leaving Hubble operating on just two, and use the fine guidence sensors to add some of the missing information.
This week Hubble begins science operations in this two gyro mode. This new mode of operation has been throughly tested on the ground using models of the telescope so the engineers are confident that it will work and not impact on the quality of science astronomers are able to perform. It should also prolong the life of the telescope allowing operations to continue into 2008 without another servicing mission.