The James Webb Space Telescope is intended as the successor to Hubble, which is now more than 25 years old. Hubble was last services during one of the final Space Shuttle missions, and there are no plans to make another visit. When it stops working, that will be the end of the mission. The JWST was already supposed to be in space at this point, but it’s a complicated project costing more than $8 billion to date. NASA wants to get it right, and that has led to the latest delay.