(I'm watching on UW2.TV as I write this, it's one of my favorite TV stations and it carries lots of NASA.TV. There are a gazillion places to see it - just search for STS-125 spacewalk - I'm watching the third one right now)
From the mission description:
...pieces of those instruments have failed in past years – not the entire instrument, but specific pieces inside of them. The crew will replace only the pieces that have failed.
But those instruments were never designed to be repaired in space. In fact, they were specifically designed not to come apart.
“When we first looked at it, we were going ‘well, maybe, maybe not,’” Ceccacci said.
Since then, the team has come up with a plan for the work that Ceccacci believes will be very successful. But it won’t be easy – the repair of the spectrograph, for instance, requires the spacewalkers to remove more than 100 screws to access a computer card they will pull out and replace.Hubble is going to be replaced, and this is the last maintenance Hubble will receive. Scott Berkun perceptively describes the importance of giving up the old to make way for the new.
The Hubble Space Telescope is seen in March 2002 with its new solar arrays after the completion of STS-109, the third Hubble servicing mission. Image: NASA
They know that in order to build whatever will replace the Hubble, they have to let go of Hubble, even if that means letting it die, so they can have the funds and resources to invest in the next thing (It’s called the Webb telescope and it’s made from Beryllium - sounds like Star Trek).More on the Webb Telescope's mirror from Technology Review. NASA's James Webb Space Telescope site is authoritative.