You may have heard that the Hubble Space Telescope has had an instrument glitch (CNN called Hubble "blind in one eye.") So, what's up?
Hubble, like most ground-based telescopes, has multiple cameras. Each of those cameras has a specialty. The camera that is having trouble is the "Advanced Camera for Surveys," or ACS. ACS was placed on Hubble by astronauts during the last servicing mission in 2002. It has taken most of the pretty pictures that have come out since then. Another camera, the WFPC2, is still operating, and it was responsible for many of the pictures prior to 2002. A third camera, NICMOS, is working fine. It looks at infrared light. While we try and figure out exactly what happened to ACS, these two cameras will be doing much of the work on Hubble.
What happened to ACS? That is uncertain. The gossip I hear is that most likely the power circuitry went bad. This happened to another Hubble instrument, the Space Telescope Imaging Spectrograph (STIS), several years ago. Both STIS and ACS were built with a second power supply. In the case of STIS, this backup was turned on, and the camera worked for many years before finally dying a year or so ago.
In this case, all that must be done is to change some software to startup the second power supply on ACS, and we'll remotely "flip the switch" and turn the camera on.
Doing this takes some time. The camera sent down it's telemetry (kind of like an instrumental weather report) before it automatically shut down. Engineers are looking through this to see exactly what caused the problem. If something else other than the power supply is wrong, say, a wire came loose and shorted the camera out, then turning on the second power supply could damage the telescope.
The news I hear gives the camera a better than 80% chance of coming back online in the next few weeks. If the problem is more serious, though, it may be much longer.