On October 31, 2006, NASA Administrator Mike Griffin announced that there will be another servicing mission (SM4) to Hubble . Read NASA Press Release
During Servicing Mission 4 (SM4), seven astronauts will approach and enter Hubble's orbit aboard a space shuttle. The astronauts will use the shuttle's robotic arm to grapple and harness Hubble into its bay.
From there, the extensively trained astronauts will perform the maintenance and upgrade operations of SM4. This includes the procedure of removing WFPC2 from Hubble's Radial bay and replacing it with WFC3. WFC3 will maintain the same mechanical and electrical interfaces as WFPC2.
Hubble receives light, its optical system images the sky on to a focal plane that is just behind the primary mirror. After the next servicing mission, this focal plane will be shared by the five scientific instruments (WFC3, COS, ACS, NICMOS, and STIS) and three fine guidance sensors (FGS's).
The pick-off mirror of WFC3 (like WFPC2 before it) will occupy the very center portion of the HST focal plane, a section that has a relatively large field-of-view and minimal optical distortion. A large field-of-view allows for the capturing of light from a large portion of the sky, while the low distortion improves sensitivity and simplifies data analysis.