In the last two days, the astronauts on the shuttle Atlantis have continued to make successful repairs to the Hubble Space Telescope. They've added a new spectrograph (more below), replaced all six of the Hubble's gyroscopes, replaced one of the two battery packs, and opened a camera that was never meant to be opened in space and replaced four circuit boards (more on this also below). The astronauts have had their share of difficulties, including stuck bolts and a new set of gyroscopes that just didn't fit (meaning that the new ones are coming back to Earth, and some refurbished gyroscopes were put in instead).
If you remember, several years ago NASA's then-Administrator Sean O'Keefe announced that, instead of a shuttle mission to repair Hubble, NASA was going to try a robot mission instead. As Julianne at Cosmic Variance noted, it's unclear that the robot would have been able to deal with the bolt and the gyroscopes (never mind replacing circuit boards). I agree that it is hard to imagine how a robot could have accomplished these tasks. Any future repairable telescopes may well be designed for robot repair, but Hubble wasn't.
Anyway, on to the exciting repairs (the batteries and gyroscopes were the most crucial, but aren't too exciting scientifically). First, the new spectrograph, which is called the Cosmic Origins Spectrograph, or COS. COS is a spectrograph (meaning it splits the light up in to component colors) that looks at ultraviolet light. One of the main science questions that COS is going to address is how much and what kinds of matter are located in between galaxies. COS will use quasars as light bulbs, and the matter in between galaxies will absorb some of the light. By careful analysis of the wavelengths of absorbed light and the amount of the absorption, astronomers can figure out where the absorbing light is, how much matter is there, and what the relative amounts of different atoms (hydrogen versus helium versus carbon, etc.) are.
But COS can do much more. For example, collaborators of mine have an approved program to take spectra of white dwarfs in the ultraviolet. So I suspect that COS will be used to study everything from the closest stars in the galaxy to the most distant galaxies, as long as they emit ultraviolet light.
The other major repair was to the Advanced Camera for Surveys, or ACS. ACS is the camera that died due to an electronics failure in January 2007, just hours after we astronomers had submitted many proposed projects to use the camera. ACS has taken many fabulous pictures, including one of the first direct pictures of a planet around another star.
ACS has three cameras: an optical imaging camera, an optical high-resolution camera, and an ultraviolet imaging camera. After the electronics problem, NASA was able to revive the ultraviolet camera using spare electronics, but not the other two cameras. Yesterday, astronauts John Grunsfeld and Drew Feustel removed more than 30 screws using a special tool (gotta keep those screws from floating away!) built just for this spacewalk. Grunsfeld removed four circuit boards from the inside of the camera and replaced them with new ones, then closed the camera back up.
Testing indicates that the repair was mostly successful. As of an hour ago, the ultraviolet camera was working (as it had been, but since all the electronics had been replaced, there was a chance it may not have worked). The wide-field camera, the main optical camera, also is working again. Unfortunately, the high-resolution camera is not working. Space Telescope engineers will keep working to try and figure out if the camera can be revived. It may be that the electronics in the camera were fried two years ago, in which case there is no saving that camera, or it may be that, with a little coaxing, the camera can be revived. But the apparent revival of the wide-field camera is very, very good news. That was Hubble's main imaging camera; and the teaming of ACS with the new WF3 will allow for some amazing images in the future. Still, let's not get too excited yet -- we won't know exactly what condition the ACS is in until it takes some new pictures. Space is hard on electronics.
Right now, the two Mikes (Mike Good and Mike Massimino) are trying to repair another broken spectrograph, the Space Telescope Imaging Spectrograph (STIS). The STIS repair will require removing 111 tiny screws that were never designed to be removed, and replacing more circuit boards. (I have enough trouble with replacing circuit boards in my home computer on Earth; I can't imagine doing it in zero-G with bulky gloves on. However, I often have my cats trying to "help" me, and I assume the astronauts will not have their pets batting around the 111 screws or pawing at the wiring.)